Web Applications (Java EE (JSP, Servlets), Spring, Struts..)
Mobile Apps(Android)
Microservices (Spring Boot)
Platform Independence
Build once, run anywhere
Java bytecode is the instruction set of the Java virtual machine
JDK vs JVM VS JRE
JVM (Java Virtual Machine)
runs the Java bytecode.
JRE
JVM + Libraries + Other Components (to run applets and other java applications)
JDK
JRE + Compilers + Debuggers
ClassLoader
Find and Loads Java Classes!
Three Types
System Class Loader - Loads all application classes from CLASSPATH
Extension Class Loader - Loads all classes from extension directory
Bootstrap Class Loader - Loads all the Java core files
Order of execution of ClassLoaders
JVM needs to find a class, it starts with System Class Loader.
If it is not found, it checks with Extension Class Loader.
If it not found, it goes to the Bootstrap Class Loader.
If a class is still not found, a ClassNotFoundException is thrown.
First Java Program
graph TD
A[Java Code] -->|Compiled| B(Bytecode)
B --> C{Run}
C -->|bytecode| D[Windows JVM]
D --> K[Windows Instructions]
C -->|bytecode| E[Unix JVM]
E --> L[Unix Instructions]
C -->|bytecode| F[Linux JVM]
F --> M[Linux Instructions]
C -->|bytecode| G[Any other platform JVM]
G --> N[Linux Instructions]
Notes
Every line of code we write in Java is part of something called Class. We will talk about Class
later.
First line defines a public class called HelloWorld. All the code in a class is between { and }.
When a program runs, Java should know which line of code has to be run first. public static
void main(String[] args) is the first method that is run when a program is executed.
Java, like any other programming language, is particular about syntax!!
Using Java and JavaC
There are two steps involved in running a Java Program
Compilation
Execution
Compilation
We use javac to compile java code.
Open CommandPrompt/Terminal and cd to the folder where HelloWorld.java file is present
execute the command below
You should see two files HelloWorld.java and HelloWorld.class in the folder.
HelloWorld.class contains the java bytecode
Execution
Now we can run the program using JVM
execute the command below
You should see the output "Hello World" printed in the console.
Class and Object
What is a class?
Definining an instance of a class - an object
Invoking a method on the object
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
}
javac HelloWorld.java
java HelloWorld
Variables
Value of a variable changes during the course of a program execution.
Declaring and Initializing Variables
Declaration is give a variable a name and type
Tips
Two or more variables of single type can be declared together.
Variable can be local or global. The local variables can be referenced (ie, are valid) only
within the scope of their method (or function).
All six numeric types in Java are signed.
Primitive Variables
Variables that store value.
Java defines few types like int (numbers), float(floating point numbers), char (characters).
Variables of these types store the value of the variable directly. These are not objects. These are
called primitive variables.
An example is shown below: Primitive Variables contains bits representing the value of the
variable.
Different primitive types in java are char, boolean, byte, short, int, long, double, or float. Because
of these primitive types, Java is NOT considered to be a pure objected oriented language.
Numeric Data Types
Types : byte, short, int, long, float, double
Number of bits : 8, 16, 32, 64, 32, 64
Range : -x to x-1 where x = Power(2, number of bits -1)
char Data Type
int number;
number = 5;
System.out.println(number);//5
number = number + 2;
System.out.println(number);//7
number = number + 2;
System.out.println(number);//9
TYPE variableName;
int value = 5;
Used to store characters. Size of character is 16 bits.
Examples
Reference Variables
The instance of new Animal - Animal object - is created in memory. The memory address of the
object created is stored in the dog reference variable.
Reference Variables contains a reference or a guide to get to the actual object in memory.
Puzzles
What will happen? Two objects of type Animal are created. Only one reference variable is created.
What will happen? What would happen if the same was done with primitive variables?
Identifiers
Names given to a class, method, interface, variables are called identifiers.
Legal Identifier Names
Combination of letters, numbers, $ and under-score(_)
Cannot start with a number
Cannot be a keyword
No limit on length of identifier
int i = 15;
long longValue = 1000000000000l;
byte b = (byte)254;
float f = 26.012f;
double d = 123.567;
boolean isDone = true;
boolean isGood = false;
char ch = 'a';
char ch2 = ';';
Animal dog = new Animal();
Animal dog1 = new Animal();
dog1 = new Animal();
Animal animal1 = new Animal();
Animal animal2 = new Animal();
animal1 = animal2;
Java Keywords
List of Java Keywords
Primitives DataTypes : byte,short,int,long,float,double,char,boolean
Flow Control : if, else,for,do, while, switch, case, default, break, continue,return
Exception Handling : try, catch, finally,throw,throws,assert
Modifiers : public,private,protected,final,static,native,abstract,
synchronized,transient,volatile,strictfp
Class Related : class,interface,package,extends,implements,import
Object Related : new, instanceof,super,this
Literals : true, false, null
Others : void, enum
Unused : goto,const
Literals
Any primitive data type value in source code is called Literal.
There are four types of literals:
Integer & Long
Floating Point
Boolean
Double
Literals
Integer Literals
There are 3 ways of representing an Integer Literal.
Decimal. Examples: 343, 545
Octal. Digits 0 to 7. Place 0 before a number. Examples : 070,011
Hexadecimal. Digits 0 to 9 and alphabets A to F (10-15). Case insensitive.
An integer literal by default is int.
Long Literals
All 3 integer formats: Decimal, Octal and Hexadecimal can be used to represent long by
appending with L or l.
Floating point Literals
Numbers with decimal points. Example: double d = 123.456;
To declare a float, append f. Example: float f = 123.456f;
Floating point literals are double by default.
Appending d or D at end of double literal is optional Example: double d = 123.456D;
Boolean Literals
Valid boolean values are true and false.
TRUE, FALSE or True, False are invalid.
Character Literals
Represented by single character between single quotes Example: char a = 'a'
Unicode Representation also can be used. Prefix with \u. Example: char letterA = '\u0041';
A number value can also be assigned to character. Example: char letterB = 66; Numeric
value can be from 0 to 65535;
Escape code can be used to represent a character that cannot be typed as literal. Example:
char newLine = '\n';
Puzzles
Tip - Assignment Operator
Assignment operator evaluates the expression on the right hand side and copies the value into
the variable on the left hand side.
Basic Examples
Puzzles
int eight = 010;
int nine=011;
int invalid = 089;//COMPILER ERROR! 8 and 9 are invalid in Octal
int sixteen = 0x10;
int fifteen = 0XF;
int fourteen = 0xe;
int x = 23,000;
long a = 123456789l;
long b = 0x9ABCDEFGHL;
long c = 0123456789L;
float f = 123.456;//COMPILER ERROR! A double value cannot be assigned to a
float.
boolean b = true; boolean b=false;
boolean b = TRUE;//COMPILATION ERROR
boolean b = 0; //COMPILER ERROR. This is not C Language
char ch = a;
char a = 97;
char ch1 = 66000; //COMPILER ERROR!
int value = 35;//35 is copied into 35
int squareOfValue = value * value;//value * value = 35 * 35 is stored into
squareOfValue
int twiceOfValue = value * 2;
Casting - Implicit and Explicit
Casting is used when we want to convert one data type to another.
A literal integer is by default int. Operation involving int-sized or less always result in int.
Floating point literals are by default double
Implicit Casting
Implicit Casting is done directly by the compiler.
Example : Widening Conversions i.e. storing smaller values in larger variable types.
Explicit Casting
Explicit Casting needs to be specified by programmer in code.
Example: Narrowing Conversions. Storing larger values into smaller variable types;
int a1 = 5;
int b1 = 6;
b1 = a1; // value of a1 is copied into b1
a1 = 10; // If we change a1 or b1 after this, it would not change the other
variable.. b1 will remain 6
Actor actor1 = new Actor();
actor1.setName("Actor1");
//This creates new reference variable actor1 of type Actor new Actor() on the
heap assigns the new Actor on the heap to reference variable
Actor actor2 = actor1;
actor2.setName("Actor2");
System.out.println(actor1.getName());//Actor2
byte b = 10; //byte b = (int) 10; Example below compiles because compiler
introduces an implicit cast.
short n1 = 5;
short n2 = 6;
//short sum = n1 + n2;//COMPILER ERROR
short sum = (short)(n1 + n2);//Needs an explicit cast
byte b = 5;
b += 5; //Compiles because of implicit conversion
int value = 100;
long number = value; //Implicit Casting
float f = 100; //Implicit Casting
Explicit casting would cause truncation of value if the value stored is greater than the size of
the variable.
Compound Assignment Operators
Examples : +=, -=, *=
Other Operators
Remainder(%) Operator
Remainder when one number is divided by another.
Conditional Operator
Conditional Operator is a Ternary Operator (3 Operands)
syntax : booleanCondition ? ResultIfTrue: ResultIfFalse;
long number1 = 25678;
int number2 = (int)number1;//Explicit Casting
//int x = 35.35;//COMPILER ERROR
int x = (int)35.35;//Explicit Casting
int bigValue = 280;
byte small = (byte) bigValue;
System.out.println(small);//output 24. Only 8 bits remain.
//float avg = 36.01;//COMPILER ERROR. Default Double
float avg = (float) 36.01;//Explicit Casting
float avg1 = 36.01f;
float avg2 = 36.01F; //f or F is fine
//byte large = 128; //Literal value bigger than range of variable type causes
compilation error
byte large = (byte) 128;//Causes Truncation!
int a = 5;
a += 5; //similar to a = a + 5;
a *= 10;//similar to a = a * 10;
a -= 5;//similar to a = a - 5;
a /= 5;//similar to a = a / 5;
System.out.println(10 % 4);//2
System.out.println(15 % 4);//3
System.out.println(-15 % 4);//-3
Bitwise Operators
You can work at bit level with these operators.
& is bitwise AND, | is bitwise OR, ~ is bitwise complement (negation), ^ is bitwise XOR, << is
left shift bitwise operator and >> is right shift bitwise operator.
Passing Variables to Methods
All variables , primitives and references , in Java, are passed to functions using copy-of-
variable-value.
Passing Variables to Methods : Example
Passing a primitive variable and modifying the value in a method
Passing a reference variable and modifying the value in a method
Returning a Value From Method
null is a valid return value for an object.
You can return andy type that can be implicitly coverted to return type.
You cannot return anything from a void method.
Types of Variables
Different Types of Variables: Static, Member (or instance), Local, Block
Instance Variables
Declared inside a class outside any method.
Each instance of the class would have its own values.
int age = 18;
System.out.println(
age >= 18 ? "Can Vote": "Cannot Vote");//Can Vote
age = 15;
System.out.println(
age >= 18 ? "Can Vote": "Cannot Vote");//Cannot Vote
System.out.println(25|12);//output will be 29
/*convert to binary and calculate:
00001100 (12 in decimal)
00011001 (25 in decimal)
________
00011101 (29 in decimal) */
System.out.println(25&12);//output will be 8
System.out.println(25^12);//output will be 21
Also called member value, field or property.
Local Variables
Variables declared in a method
Local Variables can only be marked with final modifier
If the name of a Local Variable is same as the name of an instance variable, it results in
shadowing.
Member Variables
Defined at class level and without keyword static.
Static Variable
Defined at class level and using keyword static.
Member Variable and Static Variable
Member Variables can be accessed only through object references.
Static Variables can be accessed through a. Class Name and b. Object Reference. It is NOT
recommended to use object reference to refer to static variables.
Example Static and Member Variables
public class StaticAndMemberVariables {
public static void main(String[] args) {
Actor actor1 = new Actor();
actor1.name = "ACTOR1";
//Actor.name //Compiler Error
//Below statement can be written as actor1.count++
//But NOT recommended.
Actor.count++;
Actor actor2 = new Actor();
actor2.name = "ACTOR2";
//Below statement can be written as actor2.count++
//But NOT recommended.
Actor.count++;
System.out.println(actor1.name);//ACTOR1
System.out.println(actor2.name);//ACTOR2
//Next 3 statements refer to same variable
System.out.println(actor1.count);//2
System.out.println(actor2.count);//2
System.out.println(Actor.count);//2
}
}
Scope of a Variable
Scope of a variable defines where (which part of code) a variable can be accessed.
Important Rules
Static Variable can be used anywhere in the class.
Member Variable can be used in any non-static method.
Local Variable can be used only in the method where it is declared.
Block Variable can be used only in the block (code between { and }) where it is declared.
Variable Scope Examples
Below code shows all these Rules in action:
class Actor {
//RULE 1: Member Variables can be accessed
//only through object references
String name;
//RULE 2:Static Variables can be accessed
//through a.Class Name and b.Object Reference
//It is NOT recommended to use object reference
//to refer to static variables.
static int count;
}
public class VariablesExample {
//RULE 1:Static Variable can be used anywhere in the class.
static int staticVariable;
//RULE 2:Member Variable can be used in any non-static method.
int memberVariable;
void method1() {
//RULE 3: method1LocalVariable can be used only in method1.
int method1LocalVariable;
memberVariable = 5;//RULE 2
staticVariable = 5;//RULE 1
//Some Code
{
//RULE 4:blockVariable can be used only in this block.
int blockVariable;
//Some Code
}
Scope Example 1
staticVariable is declared using keyword static.
It is available in the instance method method1 and static method named staticMethod.
Scope Example 2
memberVariable is declared directly in the class and does NOT use keyword static. So, it is an
instance variable.
It is available in the instance method method1 but not accessible in the static method
named staticMethod.
Scope Example 3
method1LocalVariable is declared in the method method1. So, it is a local variable.
It is available in the instance method method1 but available in any other instance or static
methods.
Scope Example 4
blockVariable is declared in a block in method1. So, it is a block variable.
It is available only in the block where it is defined.
It is not accessible any where out side the block , even in the same method.
Variable Initialization
Initialization defines the default value assigned to a variable if it is not initialized.
Important Rules
Member/Static variables are alway initialized with default values.
Default values for numeric types is 0, floating point types is 0.0, boolean is false, char is
'\u0000' and for a object reference variable is null.
Local variables are not initialized by default by compiler.
Using a local variable before initialization results in a compilation error.
Assigning a null value is a valid initialization for reference variables.
//blockVariable = 5;//COMPILER ERROR - RULE 4
}
void method2() {
//method1LocalVariable = 5; //COMPILER ERROR - RULE3
}
static void staticMethod() {
staticVariable = 5;//RULE 1
//memberVariable = 5; //COMPILER ERROR - RULE 2
}
}
Variable Initialization Examples
Lets look at an example program to understand all the rules regarding variable initialization.
Initialization Example 1
player is an instance of the class Player. It contains member variables named name and
score.
All member variables are initialized by default. Since name refers to a String i.e a reference
variable it is initialized to null. score is an int variable and hence initialized to 0.
package com.in28minutes.variables;
//RULE1:Member/Static variables are alway initialized with
//default values.Default values for numeric types is 0,
//floating point types is 0.0, boolean is false,
//char is '\u0000' and object reference variable is null.
//RULE2:Local/block variables are NOT initialized by compiler.
//RULE3 :If local variables are used before initialization,
//it would result in Compilation Error
public class VariableInitialization {
public static void main(String[] args) {
Player player = new Player();
//score is an int member variable - default 0
System.out.println(player.score);//0 - RULE1
//name is a member reference variable - default null
System.out.println(player.name);//null - RULE1
int local; //not initialized
//System.out.println(local);//COMPILER ERROR! RULE3
String value1;//not initialized
//System.out.println(value1);//COMPILER ERROR! RULE3
String value2 = null;//initialized
System.out.println(value2);//null - NO PROBLEM.
}
}
class Player{
String name;
int score;
}
Initialization Example 2
local is a local variable defined in the main method.
An attempt to access a local variable without initialization would result in a compilation
error.
Same is the case with value1 which is a String local variable.
If null is assigned to a reference variable, reference variable is considered to be assigned.
Wrapper Classes
Example 1
A wrapper class wraps (encloses) around a data type and gives it an object appearance
Wrapper: Boolean,Byte,Character,Double,Float,Integer,Long,Short
Primitive: boolean,byte,char ,double, float, int , long,short
Examples of creating wrapper classes are listed below.
Integer number = new Integer(55);//int;
Integer number2 = new Integer("55");//String
Float number3 = new Float(55.0);//double argument
Float number4 = new Float(55.0f);//float argument
Float number5 = new Float("55.0f");//String
Character c1 = new Character('C');//Only char constructor
Boolean b = new Boolean(true);
Reasons
null is a possible value
use it in a Collection
Object like creation from other types.. like String
A primitive wrapper class in the Java programming language is one of eight classes provided
in the java.lang package to provide object methods for the eight primitive types. All of the
primitive wrapper classes in Java are immutable.
Wrapper classes are final and immutable.
Creating Wrapper Classes
Integer number = new Integer(55);//int
Integer number2 = new Integer("55");//String
Float number3 = new Float(55.0);//double argument
Float number4 = new Float(55.0f);//float argument
Float number5 = new Float("55.0f");//String
Character c1 = new Character('C');//Only char constructor
//Character c2 = new Character(124);//COMPILER ERROR
Boolean b = new Boolean(true);
//"true" "True" "tRUe" - all String Values give True
Wrapper Class Utility Methods
A number of utility methods are defined in wrapper classes to create and convert them.
valueOf Methods
Provide another way of creating a Wrapper Object
xxxValue methods
xxxValue methods help in creating primitives
parseXxx methods
parseXxx methods are similar to valueOf but they return primitive values
static toString method
Look at the example of the toString static method below.
//Anything else gives false
Boolean b1 = new Boolean("true");//value stored - true
Boolean b2 = new Boolean("True");//value stored - true
Boolean b3 = new Boolean("False");//value stored - false
Boolean b4 = new Boolean("SomeString");//value stored - false
b = false;
Integer seven =
Integer.valueOf("111", 2);//binary 111 is converted to 7
Integer hundred =
Integer.valueOf("100");//100 is stored in variable
Integer integer = Integer.valueOf(57);
int primitive = integer.intValue();//57
float primitiveFloat = integer.floatValue();//57.0f
Float floatWrapper = Float.valueOf(57.0f);
int floatToInt = floatWrapper.intValue();//57
float floatToFloat = floatWrapper.floatValue();//57.0f
int sevenPrimitive =
Integer.parseInt("111", 2);//binary 111 is converted to 7
int hundredPrimitive =
Integer.parseInt("100");//100 is stored in variable
Overloaded static toString method
2nd parameter: radix
static toYyyyString methods.
Yyyy can be Hex,Binary,Octal
Wrapper Class , Auto Boxing
Boxing and new instances
Auto Boxing helps in saving memory by reusing already created Wrapper objects. However
wrapper classes created using new are not reused.
Two wrapper objects created using new are not same object.
Two wrapper objects created using boxing are same object.
Integer wrapperEight = new Integer(8);
System.out.println(Integer.
toString(wrapperEight));//String Output: 8
System.out.println(Integer
.toString(wrapperEight, 2));//String Output: 1000
System.out.println(Integer
.toHexString(wrapperEight));//String Output:8
System.out.println(Integer
.toBinaryString(wrapperEight));//String Output:1000
System.out.println(Integer
.toOctalString(wrapperEight));//String Output:10
Integer ten = new Integer(10);
ten++;//allowed. Java does the work behind the screen for us
Integer nineA = new Integer(9);
Integer nineB = new Integer(9);
System.out.println(nineA == nineB);//false
System.out.println(nineA.equals(nineB));//true
Integer nineC = 9;
Integer nineD = 9;
System.out.println(nineC == nineD);//true
System.out.println(nineC.equals(nineD));//true
String Class
A String class can store a sequence of characters. String is not a primitive in Java but a Class
in its own right.
Strings are immutable
Value of a String Object once created cannot be modified. Any modification on a String
object creates a new String object.
Note that the value of str3 is not modified in the above example. The result should be assigned to
a new reference variable (or same variable can be reused).
Where are string literals stored in memory?
All strings literals are stored in "String constant pool". If compiler finds a String literal,it checks if it
exists in the pool. If it exists, it is reused. Following statement creates 1 string object (created on
the pool) and 1 reference variable.
However, if new operator is used to create string object, the new object is created on the heap.
Following piece of code create 2 objects.
String vs StringBuffer vs StringBuilder
Immutability : String
Thread Safety : String(immutable), StringBuffer
Performance : StringBuilder (especially when a number of modifications are made.)
Example 1
String Constant Pool
All strings literals are stored in "String constant pool". If compiler finds a String literal,it
checks if it exists in the pool. If it exists, it is reused.
String str3 = "value1";
str3.concat("value2");
System.out.println(str3); //value1
String concat = str3.concat("value2");
System.out.println(concat); //value1value2
String str1 = "value";
//1. String Literal "value" - created in the "String constant pool"
//2. String Object - created on the heap
String str2 = new String("value");
Following statement creates 1 string object (created on the pool) and 1 reference variable.
However, if new operator is used to create string object, the new object is created on the
heap. Following piece of code create 2 objects.
String Method Examples
String class defines a number of methods to get information about the string content.
Get information from String
Following methods help to get information from a String.
String Manipulation methods
Most important thing to remember is a String object cannot be modified. When any of these
methods are called, they return a new String with the modified value. The original String remains
unchanged.
String str1 = "value";
//1. String Literal "value" - created in the "String constant pool"
//2. String Object - created on the heap
String str2 = new String("value");
String str = "abcdefghijk";
//char charAt(int paramInt)
System.out.println(str.charAt(2)); //prints a char - c
System.out.println("ABCDEFGH".length());//8
System.out.println("abcdefghij".toString()); //abcdefghij
System.out.println("ABC".equalsIgnoreCase("abc"));//true
//Get All characters from index paramInt
//String substring(int paramInt)
System.out.println("abcdefghij".substring(3)); //defghij
//All characters from index 3 to 6
System.out.println("abcdefghij".substring(3,7)); //defg
String s1 = new String("HELLO");
String s2 = new String("HELLO");
System.out.println(s1 == s2); // false
System.out.println(s1.equals(s2)); // true
//String concat(String paramString)
System.out.println(str.concat("lmn"));//abcdefghijklmn
String Concatenation Operator
Three Rules of String Concatenation
RULE1: Expressions are evaluated from left to right.Except if there are parenthesis.
RULE2: number + number = number
RULE3: number + String = String
Increment and Decrement Operators
Lets learn about the increment and decrement operators in Java.
Basics of Increment and Decrement Operators
Except for a minor difference ++i,i++ is similar to i = i+1 and --i,i-- is similar to i = i-1
++i is called pre-increment and i++ post increment
Increment Operators
Pre increment statement returns value after increment. Post increment statement returns value
before increment
//String replace(char paramChar1, char paramChar2)
System.out.println("012301230123".replace('0', '4'));//412341234123
//String replace(CharSequence paramCharSequence1, CharSequence
paramCharSequence2)
System.out.println("012301230123".replace("01", "45"));//452345234523
System.out.println("ABCDEFGHIJ".toLowerCase()); //abcdefghij
System.out.println("abcdefghij".toUpperCase()); //ABCDEFGHIJ
//trim removes leading and trailings spaces
System.out.println(" abcd ".trim()); //abcd
System.out.println(5 + "Test" + 5); //5Test5
System.out.println(5 + 5 + "Test"); //10Test
System.out.println("5" + 5 + "Test"); //55Test
System.out.println("5" + "5" + "25"); //5525
System.out.println(5 + 5 + "25"); //1025
System.out.println("" + 5 + 5 + "25"); //5525
System.out.println(5 + (5 + "25")); //5525
System.out.println(5 + 5 + 25); //35
Decrement Operators
Decrement Operators are similar to increment operators.
Relational Operators
Relation Operators are used to compare operands. They a always return true or false. List of
Relation Operators include <, <=, >, >=, ==, and !=.
Relation Operators Examples
Let's consider a few examples of relational operators. Let's assume a int variable named number
with a value 7.
greater than operator
greater than equal to operator
less than operator
int i = 25;
int j = ++i;//i is incremented to 26, assigned to j
System.out.println(i + " " + j);//26 26
i = 25;
j = i++;//i value(25) is assigned to j, then incremented to 26
System.out.println(i + " " + j);//26 25
i = 25;
j = --i;//i is decremented to 24, assigned to j
System.out.println(i + " " + j);//24 24
i = 25;
j = i--;//i value(25) is assigned to j, then decremented to 24
System.out.println(i + " " + j);//24 25
int number = 7;
System.out.println(number > 5);//true
System.out.println(number > 7);//false
System.out.println(number >= 7);//true
System.out.println(number < 9);//true
System.out.println(number < 7);//false
less than equal to operator
is equal to operator
NOT equal to operator
single = is assignment operator and == is comparison. Below statement uses =.
== (equals) operator
Let's look at how == equals operator works with primitives and reference variables.
Primitive Variables
Equality for Primitives only compares values
Below statement compares if a and b have same value.
Reference Variables
For reference variables, == compares if they are referring to the same object.
System.out.println(number <= 7);//true
System.out.println(number == 7);//true
System.out.println(number == 9);//false
System.out.println(number != 9);//true
System.out.println(number != 7);//false
System.out.println(number = 7);//7
int a = 5;
int b = 5;
System.out.println(a == b);//true
Integer aReference = new Integer(5);
Integer bReference = new Integer(5);
Bitwise operators
Bitwise Operators are |,&,^ and ~
Logical Operators
Logical Operators are &&, ||, |, &, ! and ^.
Short Circuit And Operator - &&
True when both operands are true.
System.out.println(aReference == bReference);//false
bReference = aReference;
//Now both are referring to same object
System.out.println(aReference == bReference);//true
int a = 5;
int b = 7;
// bitwise and
// 0101 & 0111=0101 = 5
System.out.println("a&b = " + (a & b));
// bitwise or
// 0101 | 0111=0111 = 7
System.out.println("a|b = " + (a | b));
// bitwise xor
// 0101 ^ 0111=0010 = 2
System.out.println("a^b = " + (a ^ b));
// bitwise and
// ~0101=1010
// will give 2's complement of 1010 = -6
System.out.println("~a = " + ~a);
// can also be combined with
// assignment operator to provide shorthand
// assignment
// a=a&b
a &= b;
System.out.println("a= " + a); // a = 5
Short Circuit Or Operator - ||
True when atleast one of operands are true.
Certification Tip : Logical Operators work with boolean values but not numbers.
Short circuit operators are Lazy
They stop execution the moment result is clear.
For &&, if first expression is false,result is false.
For ||, if first expression is true, the result is true.
In above 2 situations, second expressions are not executed.
Operator & and |
Logical Operators &, | are similar to &&, || except that they don't short ciruit.
They execute the second expression even if the result is decided.
Certification Tip : While & and | are very rarely used, it is important to understand them
from a certification perspective.
Operator exclusive-OR (^)
Result is true only if one of the operands is true.
System.out.println(true && true);//true
System.out.println(true && false);//false
System.out.println(false && true);//false
System.out.println(false && false);//false
System.out.println(true || true);//true
System.out.println(true || false);//true
System.out.println(false || true);//true
System.out.println(false || false);//false
//System.out.println(5 || 6);//COMPILER ERROR
int i = 10;
System.out.println(true || ++i==11);//true
System.out.println(false && ++i==11);//false
System.out.println(i);//i remains 10, as ++i expressions are not executed.
int j = 10;
System.out.println(true | ++j==11);//true
System.out.println(false & ++j==12);//false
System.out.println(j);//j becomes 12, as both ++j expressions are executed
Not Operator (!)
Result is the negation of the expression.
Arrays
TODO : Why do we need arrays?
System.out.println(true ^ false);//true
System.out.println(false ^ true);//true
System.out.println(true ^ true);//false
System.out.println(false ^ false);//false
System.out.println(!false);//true
System.out.println(!true);//false
//Declaring an Array
int[] marks;
// Creating an array
marks = new int[5]; // 5 is size of array
int marks2[] = new int[5];//Declaring and creating an array in same line.
System.out.println(marks2[0]);//New Arrays are always initialized with default
values - 0
//Index of elements in an array runs from 0 to length - 1
marks[0] = 25;
marks[1] = 30;
marks[2] = 50;
marks[3] = 10;
marks[4] = 5;
System.out.println(marks[2]);//Printing a value from array
//Printing a 1D Array
int marks5[] = { 25, 30, 50, 10, 5 };
System.out.println(marks5); //[I@6db3f829
System.out.println(
Arrays.toString(marks5));//[25, 30, 50, 10, 5]
int length = marks.length;//Length of an array: Property length
//Enhanced For Loop
for (int mark: marks) {
System.out.println(mark);
}
2D Arrays
Best way to visualize a 2D array is as an array of arrays.
Other Array Operations
//Fill array with a value
Arrays.fill(marks, 100); //All array values will be 100
//String Arrays
String[] daysOfWeek = { "Sunday", "Monday", "Tuesday", "Wednesday",
"Thursday", "Friday", "Saturday" };
int[][] matrix = { { 1, 2, 3 }, { 4, 5, 6 } };
int[][] matrixA = new int[5][6];
//Accessing elements from 2D array:
System.out.println(matrix[0][0]); //1
System.out.println(matrix[1][2]); //6
//Looping a 2D array
for (int[] array: matrix) {
for (int number: array) {
System.out.println(number);
}
}
// Printing a 2D Array
int[][] matrix3 = { { 1, 2, 3 }, { 4, 5, 6 } };
System.out.println(matrix3); //[[I@1d5a0305
System.out.println(
Arrays.toString(matrix3));
//[[I@6db3f829, [I@42698403]
System.out.println(Arrays.deepToString(matrix3));
//[[1, 2, 3], [4, 5, 6]]
System.out.println(matrix3[0]);//[I@86c347 - matrix3[0] is a 1D Array
System.out.println(Arrays.toString(matrix3[0]));//[1, 2, 3]
//Comparing Arrays
int[] numbers1 = { 1, 2, 3 };
int[] numbers2 = { 4, 5, 6 };
System.out.println(Arrays
Array of Objects
Array Certification Tips and Puzzles
.equals(numbers1, numbers2)); //false
int[] numbers3 = { 1, 2, 3 };
System.out.println(Arrays
.equals(numbers1, numbers3)); //true
// Sorting an Array
int rollNos[] = { 12, 5, 7, 9 };
Arrays.sort(rollNos);
System.out.println(Arrays.toString(rollNos));//[5, 7, 9, 12]
Person[] persons = new Person[3];
//By default, an array of 3 reference variables is created.
//The person objects are not created
System.out.println(persons[0]);//null
//Let's create the new objects
persons[0] = new Person();
persons[1] = new Person();
persons[2] = new Person();
//Creating and initializing person array in one statement
Person[] personsAgain = { new Person(),new Person(),new Person()};
//Another example
Person[][] persons2D =
{
{ new Person(),new Person(),new Person()},
{ new Person(),new Person()}
};
//You can Declare, Create and Initialize Array on same line.
int marks3[] = { 25, 30, 50, 10, 5 };
//Leaving additional comma is not a problem. (note that comma after 5)
int marks4[] = { 25, 30, 50, 10, 5, };
int marks[]; //Not Readable
If Else Condition
Conditionally execute code!
Code inside If is executed only if the condition is true.
// Basic Example
int[] runs; //Readable
//int values[5];//Compilation Error!Declaration of an Array should not include
size.
//marks = new int[];//COMPILER ERROR! Size of an array is mandatory to create
an array.
//Declaring 2D Array Examples:
int[][] matrix1; //Recommended
int[] matrix2[]; //Legal but not readable. Avoid.
//Access 10th element when array has only length 5
//Runtime Exception: ArrayIndexOutOfBoundsException
//System.out.println(marks[10]);
//Array can contain only values of same type.
//COMPILE ERROR!!
//int marks4[] = {10,15.0}; //10 is int 15.0 is float
//Cross assigment of primitive arrays is ILLEGAL
int[] ints = new int[5];
short[] shorts = new short[5];
//ints = shorts;//COMPILER ERROR
//ints = (int[])shorts;//COMPILER ERROR
//The first dimension of a 2D array is mandatory
matrixA = new int[3][];//FINE
//matrixA = new int[][5];//COMPILER ERROR
//matrixA = new int[][];//COMPILER ERROR
//Each row in a 2D Array can have a different size. This is called a Ragged
Array.
matrixA = new int[3][];//FINE
matrixA[0] = new int[3];
matrixA[0] = new int[4];
matrixA[0] = new int[5];
if(true){
System.out.println("Will be printed");
}
if(false){
System.out.println("Will NOT be printed");//Not executed
}
//Example 1
int x = 5;
if(x==5){
System.out.println("x is 5");//executed since x==5 is true
}
//Example 2
x = 6;
if(x==5){
System.out.println("x is 5");//Not executed since x==5 is false
}
//Example 3
int y = 10;
if(y==10){
System.out.println("Y is 10");//executed-condn y==10 is true
} else {
System.out.println("Y is Not 10");
}
//Example 4
y = 11;
if(y==10){
System.out.println("Y is 10");//NOT executed
} else {
System.out.println("Y is Not 10");//executed
}
//Example 5
int z = 15;
//Only one condition is executed. Rest of the conditions are skipped.
if(z==10){
System.out.println("Z is 10");//NOT executed
} else if(z==12){
System.out.println("Z is 12");//NOT executed
} else if(z==15){
System.out.println("Z is 15");//executed.
} else {
If else Puzzles
System.out.println("Z is Something Else.");//NOT executed
}
z = 18;
if(z==10){
System.out.println("Z is 10");//NOT executed
} else if(z==12){
System.out.println("Z is 12");//NOT executed
} else if(z==15){
System.out.println("Z is 15");//NOT executed
} else {
System.out.println("Z is Something Else.");//executed
}
//If else Example: without Blocks
int number = 5;
if(number < 0)
number = number + 10; //Not executed
number++; //This statement is not part of if. Executed.
System.out.println(number);//prints 6
//Puzzle 1
int k = 15;
if (k > 20) {
System.out.println(1);
} else if (k > 10) {
System.out.println(2);
} else if (k < 20) {
System.out.println(3);
} else {
System.out.println(4);
}
//Output is 2.
//Once a condition in nested-if-else is true the rest of the code is not
executed.
//Puzzle 2
int l = 15;
if(l<20)
System.out.println("l<20");
if(l>20)
System.out.println("l>20");
else
System.out.println("Who am I?");
//Output is "l<20" followed by "Who am I?" on next line. //else belong to the last if before it unless
brackets ({}) are used.
Puzzles Continued
//Puzzle 3
int m = 15;
if(m>20)
if(m<20)
System.out.println("m>20");
else
System.out.println("Who am I?");
//Nothing is printed to output.
//Code above is similar to the code snippet shown below
if(m>20) {//Condn is false. So, code in if is not executed
if(m<20)
System.out.println("m>20");
else
System.out.println("Who am I?");
}
//Puzzle 4
int x1 = 0;
//Condition in if should always be boolean
//if(x1) {} //COMPILER ERROR
//if(x1=0) {}//COMPILER ERROR. Using = instead of ==
//If else condition should be boolean
//Puzzle 5
boolean isTrue = false;
if(isTrue==true){
System.out.println("TRUE TRUE");//Will not be printed
}
if(isTrue=true){
System.out.println("TRUE");//Will be printed.
}
Switch Statement
Choose between a set of options.
From Java 6, String can be used as the switch argument.
Important Tips
There is a break statement in every case. If there is no break statement, switch continues to
execute other cases.
There is a case named default. If none of the cases match default case is executed.
Output of above switch
//Condition is isTrue=true. This is assignment. Returns true. So, code in if
is executed.
//Example 1
int number = 2;
switch (number) {
case 1:
System.out.println(1);
break;
case 2:
System.out.println(2);//PRINTED
break;
case 3:
System.out.println(3);
break;
default:
System.out.println("Default");
break;
}
// Output of above example is 2.The case which is matched is executed.
//Switch Statement Example 2 , No Breaks
number = 2;
switch (number) {
case 1:
System.out.println(1);
case 2:
System.out.println(2);
case 3:
System.out.println(3);
default:
System.out.println("Default");
}
Since there is no break after case 2, execution falls through to case 3. There is no break in case 3
as well. So, execution falls through to default.
Code in switch is executed from a matching case until a break or end of switch statement is
encountered.
Switch Statement Example 3 , Few Break's
Program Output
Case 2 matches. Since there is no code in case 2, execution falls through to case 3, executes the
println. Break statement takes execution out of the switch.
Switch Statement Example 4 , Let's Default
default is executed if none of the case's match.
2
3
Default
number = 2;
switch (number) {
case 1:
System.out.println(1);
break;
case 2:
case 3:
System.out.println("Number is 2 or 3");
break;
default:
System.out.println("Default");
break;
}
number is 2 or 3.
number = 10;
switch (number) {
case 1:
System.out.println(1);
break;
case 2:
System.out.println(2);
break;
case 3:
System.out.println(3);
break;
default:
Code Output
Switch Statement Example 5 - Default need not be Last
Output
Switch statement Example 6
Switch can be used only with char, byte, short, int, String or enum
Case value should be a compile time constant.
Loops
System.out.println("Default");
break;
}
Default
number = 10;
switch (number) {
default:
System.out.println("Default");
break;
case 1:
System.out.println(1);
break;
case 2:
System.out.println(2);
break;
case 3:
System.out.println(3);
break;
}
Default
long l = 15;
/*switch(l){//COMPILER ERROR. Not allowed.
}*/
number = 10;
switch (number) {
//case number>5://COMPILER ERROR. Cannot have a condition
//case number://COMPILER ERROR. Should be constant.
}
Loops
A loop is used to run same code again and again.
While Loop
While loop Example 2
Do While Loop
The difference between a while and a do while is that the code in do while is executed at
least once.
In a do while loop, condition check occurs after the code in loop is executed once.
Do While loop Example 1
Do While loop Example 2
int count = 0;
while(count < 5){//while this condn is true, loop is executed.
System.out.print(count);
count++;
}
//Output - 01234
count = 5;
while(count < 5){//condn is false. So, code in while is not executed.
System.out.print(count);
count++;
}//Nothing is printed to output
int count = 0;
do{
System.out.print(count);
count++;
}while(count < 5);//while this condn is true, loop is executed.
//output is 01234
count = 5;
do{
System.out.print(count);
count++;
}while(count < 5);
//output is 5
For Loop
For loop is used to loop code specified number of times.
For Loop Example 1
Syntax - For loop statement has 3 parts
Initialization => int i=0. Initialization happens the first time a for loop is run.
Condition => i<10. Condition is checked every time before the loop is executed.
Operation (Increment or Decrement usually) => i++. Operation is invoked at the start of
every loop (except for first time).
For Loop Example 2: There can be multiple statements in Initialization or Operation separated by
commas
Enhanced For Loop
Enhanced for loop can be used to loop around array's or List's.
Any of 3 parts in a for loop can be empty.
Break Statement
Break statement breaks out of a loop
Example 1
for (int i = 0; i < 10; i++) {
System.out.print(i);
}
//Output - 0123456789
for (int i = 0,j = 0; i < 10; i++,j--) {
System.out.print(j);
}
//Output - 0123456789
int[] numbers = {1,2,3,4,5};
for(int number:numbers){
System.out.print(number);
}
//Output - 12345
for (;;) {
System.out.print("I will be looping for ever..");
}
//Infinite loop => Loop executes until the program is terminated.
Break can be used in a while also.
Break statement takes execution out of inner most loop.
Labels can be used to label and refer to specific for loop in a nested for loop.
for (int i = 0; i < 10; i++) {
System.out.print(i);
if (i == 5) {
break;
}
}
//Output - 012345
//Even though the for loop runs from 0 to 10, execution stops at i==5 because
of the break statement. ÒBreak statementÓ stops the execution of the loop and
takes execution to the first statement after the loop.
int i = 0;
while (i < 10) {
System.out.print(i);
if (i == 5) {
break;
}
i++;
}
//Output - 012345
for (int j = 0; j < 2; j++) {
for (int k = 0; k < 10; k++) {
System.out.print(j + "" + k);
if (k == 5) {
break;//Takes out of loop using k
}
}
}
//Output - 000102030405101112131415
//Each time the value of k is 5 the break statement is executed.
//The break statement takes execution out of the k loop and proceeds to the
next value of j.
Continue Statement
Continue statement skips rest of the statements in the loop and starts next iteration
Continue can be used in a while also
Continue statement takes execution to next iteration of inner most loop.
outer:
for (int j = 0; j < 2; j++) {
for (int k = 0; k < 10; k++) {
System.out.print(j + "" + k);
if (k == 5) {
break outer;//Takes out of loop using j
}
}
}
//Output - 000102030405
for (int i = 0; i < 10; i++) {
if (i == 5) {
continue;
}
System.out.print(i);
}
//Output => 012346789
//Note that the output does not contain 5.
//When i==5 continue is executed. Continue skips rest of the code and goes to
next loop iteration.
//So, the print statement is not executed when i==5.
int i = 0;
while (i < 10) {
i++;
if (i == 5) {
continue;
}
System.out.print(i);
}
//Output - 1234678910
Label Example
Enum
Enum allows specifying a list of valid values (or allowed values) for a Type.
Enum Declaration
Consider the example below. It declares an enum Season with 4 possible values.
Enum Example 1
for (int j = 0; j < 2; j++) {
for (int k = 0; k < 10; k++) {
if (k == 5) {
continue;//skips to next iteration of k loop
}
System.out.print(j + "" + k);
}
}
//Output - 000102030406070809101112131416171819
//When k==5 the print statement in the loop is skipped due to continue.
//So 05 and 05 are not printed to the console.
outer:
for (int j = 0; j < 2; j++) {
for (int k = 0; k < 10; k++) {
if (k == 5) {
continue outer;//skips to next iteration of j loop
}
System.out.print(j + "" + k);
}
}
//Output - 00010203041011121314
//When k==5 is true, continue outer is called.
//So, when value of k is 5, the loop skips to the next iteration of j.
enum Season {
WINTER, SPRING, SUMMER, FALL
};
//Enum can be declared outside a class
enum SeasonOutsideClass {
WINTER, SPRING, SUMMER, FALL
};
Enum Rules
Enums can be declared in a separate class(SeasonOutsideClass) or as member of a
class(Season). Enums cannot be declared in a method.
Conversion of Enum : Function valueOf(String) is used to convert a string to enum.
public class Enum {
// Enum can be declared inside a class
enum Season {
WINTER, SPRING, SUMMER, FALL
};
public static void main(String[] args) {
/*
* //Uncommenting gives compilation error //enum cannot be created in a
* <main></main>ethod enum InsideMethodNotAllowed { WINTER, SPRING,
SUMMER, FALL };
*/
// Converting String to Enum
Season season = Season.valueOf("FALL");
// Converting Enum to String
System.out.println(season.name());// FALL
// Default ordinals of enum
// By default java assigns ordinals in order
System.out.println(Season.WINTER.ordinal());// 0
System.out.println(Season.SPRING.ordinal());// 1
System.out.println(Season.SUMMER.ordinal());// 2
System.out.println(Season.FALL.ordinal());// 3
// Looping an enum => We use method values
for (Season season1 : Season.values()) {
System.out.println(season1.name());
// WINTER SPRING SUMMER FALL (separate lines)
}
// Comparing two Enums
Season season1 = Season.FALL;
Season season2 = Season.FALL;
System.out.println(season1 == season2);// true
System.out.println(season1.equals(season2));// true
}
}
Function name() is used to find String value of an enum.
Java assigns default ordinals to an enum in order. However, it is not recommended to use
ordinals to perform logic.
Looping around an Enum - List of values allowed for an Enum can be obtained by invoking the
function values().
Comparing two Enums
Enum Example 2
//Converting String to Enum
Season season = Season.valueOf("FALL");
//Converting Enum to String
System.out.println(season.name());//FALL
//Default ordinals of enum
// By default java assigns ordinals in order
System.out.println(Season.WINTER.ordinal());//0
System.out.println(Season.SPRING.ordinal());//1
System.out.println(Season.SUMMER.ordinal());//2
System.out.println(Season.FALL.ordinal());//3
//Looping an enum => We use method values
for (Season season1: Season.values()) {
System.out.println(season1.name());
//WINTER SPRING SUMMER FALL (separate lines)
}
//Comparing two Enums
Season season1 = Season.FALL;
Season season2 = Season.FALL;
System.out.println(season1 == season2);//true
System.out.println(season1.equals(season2));//true
package com.in28minutes.java.beginners.concept.examples.enums;
public class EnumAdvanced {
// Enum with a variable,method and constructor
enum SeasonCustomized {
WINTER(1), SPRING(2), SUMMER(3), FALL(4);
// variable
private int code;
// method
public int getCode() {
return code;
}
// Constructor-only private or (default)
// modifiers are allowed
SeasonCustomized(int code) {
this.code = code;
}
// Getting value of enum from code
public static SeasonCustomized valueOf(int code) {
for (SeasonCustomized season : SeasonCustomized.values()) {
if (season.getCode() == code)
return season;
}
throw new RuntimeException("value not found");// Just for kicks
}
// Using switch statement on an enum
public int getExpectedMaxTemperature() {
switch (this) {
case WINTER:
return 5;
case SPRING:
case FALL:
return 10;
case SUMMER:
return 20;
}
return -1;// Dummy since Java does not recognize this is possible :)
}
};
public static void main(String[] args) {
SeasonCustomized season = SeasonCustomized.WINTER;
/*
* //Enum constructor cannot be invoked directly //Below line would
* cause COMPILER ERROR SeasonCustomized season2 = new
* SeasonCustomized(1);
*/
System.out.println(season.getCode());// 1
More Enum Basics
Enums can contain variables, methods, constructors. In example 2, we created a local
variable called code with a getter.
We also created a constructor with code as a parameter.
Each of the Season Type's is created by assigning a value for code.
Enum constructors can only be (default) or (private) access. Enum constructors cannot be directly
invoked.
Example below shows how we can use a switch around an enum.
System.out.println(season.getExpectedMaxTemperature());// 5
System.out.println(SeasonCustomized.valueOf(4));// FALL
}
}
//variable
private int code;
//method
public int getCode() {
return code;
}
//Constructor-only private or (default)
//modifiers are allowed
SeasonCustomized(int code) {
this.code = code;
}
WINTER(1), SPRING(2), SUMMER(3), FALL(4);
/*//Enum constructor cannot be invoked directly
//Below line would cause COMPILER ERROR
SeasonCustomized season2 = new SeasonCustomized(1);
*/
// Using switch statement on an enum
public int getExpectedMaxTemperature() {
switch (this) {
case WINTER:
return 5;
Enum Example 3
case SPRING:
case FALL:
return 10;
case SUMMER:
return 20;
}
return -1;
}
package com.in28minutes.java.beginners.concept.examples.enums;
public class EnumAdvanced2 {
// Enum with a variable,method and constructor
enum SeasonCustomized {
WINTER(1) {
public int getExpectedMaxTemperature() {
return 5;
}
},
SPRING(2), SUMMER(3) {
public int getExpectedMaxTemperature() {
return 20;
}
},
FALL(4);
// variable
private int code;
// method
public int getCode() {
return code;
}
// Constructor-only private or (default)
// modifiers are allowed
SeasonCustomized(int code) {
this.code = code;
}
public int getExpectedMaxTemperature() {
return 10;
}
};
Enum Constant Class - In the example above, take a look at how the Winter Type is declared: It
provides an overriding implementation for the getExpectedMaxTemperature method already
declared in the Enum. This feature in an Enum is called a Constant Class.
Inheritance
Inheritance allows extending a functionality of a class and also promotes reuse of existing code.
Every Class extends Object class
Every class in Java is a sub class of the class Object.
When we create a class in Java, we inherit all the methods and properties of Object class.
Create a class Actor
public static void main(String[] args) {
SeasonCustomized season = SeasonCustomized.WINTER;
System.out.println(season.getExpectedMaxTemperature());// 5
System.out.println(SeasonCustomized.FALL.getExpectedMaxTemperature());//
10
}
}
WINTER(1) {
public int getExpectedMaxTemperature() {
return 5;
}
}
String str = "Testing";
System.out.println(str.toString());
System.out.println(str.hashCode());
System.out.println(str.clone());
if(str instanceof Object){
System.out.println("I extend Object");//Will be printed
}
public class Actor {
public void act(){
System.out.println("Act");
};
}
We can extend this class by using the keyword extends . Hero class extends Actor
Since Hero extends Actor, the methods defined in Actor are also available through an instance of
Hero class.
Let's look at another class extending Actor class - Comedian.
Methods in Animal class can be executed from an instance of Comedian class.
Super class reference variable can hold an object of sub class
Object is super class of all classes. So, an Object reference variable can hold an instance of any
class.
Inheritance: IS-A Relationship
//IS-A relationship. Hero is-a Actor
public class Hero extends Actor {
public void fight(){
System.out.println("fight");
};
}
Hero hero = new Hero();
//act method inherited from Actor
hero.act();//Act
hero.fight();//fight
//IS-A relationship. Comedian is-a Actor
public class Comedian extends Actor {
public void performComedy(){
System.out.println("Comedy");
};
}
Comedian comedian = new Comedian();
//act method inherited from Actor
comedian.act();//Act
comedian.performComedy();//Comedy
Actor actor1 = new Comedian();
Actor actor2 = new Hero();
//Object is super class of all java classes
Object object = new Hero();
We should use inheritance only when there is an IS-A relationship between classes. For example,
Comedian IS-A Actor, Hero IS-A Actor are both true. So, inheritance is correct relationship
between classes.
Comedian is called a Sub Class. Actor is Super Class.
Multiple Inheritance results in a number of complexities. Java does not support Multiple
Inheritance.
We can create an inheritance chain.
Inheritance and Polymorphism
Polymorphism is defined as "Same Code" having "Different Behavior".
Example
class Dog extends Animal, Pet { //COMPILER ERROR
}
class Pet extends Animal {
}
class Dog extends Pet {
}
public class Animal {
public String shout() {
return "Don't Know!";
}
}
class Cat extends Animal {
public String shout() {
return "Meow Meow";
}
}
class Dog extends Animal {
public String shout() {
return "BOW BOW";
}
public void run(){
}
}
Execution
Puzzle and Tips - instanceof Operator in depth
instanceof operator checks if an object is of a particular type.
Animal animal1 = new Animal();
System.out.println(animal1.shout()); //Don't Know!
Animal animal2 = new Dog();//Animal reference used to store Dog object
//Reference variable type => Animal
//Object referred to => Dog
//Dog's bark method is called.
System.out.println(animal2.shout()); //BOW BOW
//Cannot invoke sub class method with super class reference variable.
//animal2.run();//COMPILE ERROR
class SuperClass {
};
class SubClass extends SuperClass {
};
interface Interface {
};
class SuperClassImplementingInteface implements Interface {
};
class SubClass2 extends SuperClassImplementingInteface {
};
class SomeOtherClass {
};
SubClass subClass = new SubClass();
Object subClassObj = new SubClass();
SubClass2 subClass2 = new SubClass2();
SomeOtherClass someOtherClass = new SomeOtherClass();
//We can run instanceof operator on the different instances created earlier.
System.out.println(subClass instanceof SubClass);//true
System.out.println(subClass instanceof SuperClass);//true
System.out.println(subClassObj instanceof SuperClass);//true
Class, Object, State and Behavior
In this tutorial, lets look at a few important object oriented concepts.
Class, Object, State and Behavior Example
System.out.println(subClass2 instanceof SuperClassImplementingInteface);//true
//instanceof can be used with interfaces as well.
//Since Super Class implements the interface, below code prints true.
System.out.println(subClass2
instanceof Interface);//true
//If the type compared is unrelated to the object, a compilation error occurs.
//System.out.println(subClass
// instanceof SomeOtherClass);//Compiler Error
//Object referred by subClassObj(SubClass)- NOT of type SomeOtherClass
System.out.println(subClassObj instanceof SomeOtherClass);//false
package com.in28minutes;
public class CricketScorer {
//Instance Variables - constitute the state of an object
private int score;
//Behavior - all the methods that are part of the class
//An object of this type has behavior based on the
//methods four, six and getScore
public void four(){
score = score + 4;
}
public void six(){
score = score + 6;
}
public int getScore() {
return score;
}
public static void main(String[] args) {
CricketScorer scorer = new CricketScorer();
scorer.six();
//State of scorer is (score => 6)
scorer.four();
//State of scorer is (score => 10)
System.out.println(scorer.getScore());
Class
A class is a Template.
In above example, class CricketScorer is the template for creating multiple objects.
A class defines state and behavior that an object can exhibit.
Object
An instance of a class.
In the above example, we create an object using new CricketScorer().
The reference of the created object is stored in scorer variable.
We can create multiple objects of the same class.
State
State represents the values assigned to instance variables of an object at a specific time.
Consider following code snippets from the above example.
The value in score variable is initially 0.
It changes to 6 and then 10.
State of an object might change with time.
Behavior
Behaviour of an object represents the different methods that are supported by it.
Above example the behavior supported is six(), four() and getScore().
toString method
toString() method in Java is used to print the content of an object.
Example
}
}
scorer.six();
//State of scorer is (score => 6)
scorer.four();
//State of scorer is (score => 10)
class Animal {
public Animal(String name, String type) {
this.name = name;
this.type = type;
To show the content of the animal object, we can override the default implementation of toString
method provided by Object class.
equals method
equals method is used to compare if two objects are having the same content.
Default implementation of equals method is defined in Object class. The implementation is
similar to == operator.
By default, two object references are equal only if they are pointing to the same object.
However, we can override equals method and provide a custom implementation to compare
the contents for an object.
}
String name;
String type;
}
Animal animal = new Animal("Tommy", "Dog");
//Output does not show the content of animal (what name? and what type?).
System.out.println(animal);//com.in28minutes.Animal@f7e6a96
//Adding toString to Animal class
class Animal {
public Animal(String name, String type) {
this.name = name;
this.type = type;
}
String name;
String type;
public String toString() {
return "Animal [name=" + name + ", type=" + type + "]";
}
}
Animal animal = new Animal("Tommy","Dog");
//Output now shows the content of the animal object.
System.out.println(animal);//Animal [name=Tommy, type=Dog]
Example
class Client {
private int id;
public Client(int id) {
this.id = id;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
}
// == comparison operator checks if the object references are pointing to the
same object.
// It does NOT look at the content of the object.
Client client1 = new Client(25);
Client client2 = new Client(25);
Client client3 = client1;
//client1 and client2 are pointing to different client objects.
System.out.println(client1 == client2);//false
//client3 and client1 refer to the same client objects.
System.out.println(client1 == client3);//true
//similar output to ==
System.out.println(client1.equals(client2));//false
System.out.println(client1.equals(client3));//true
//overriding equals method
class Client {
private int id;
public Client(int id) {
this.id = id;
}
@Override
public boolean equals(Object obj) {
Client other = (Client) obj;
if (id != other.id)
return false;
return true;
Signature of the equals method is "public boolean equals(Object obj) ".
Note that "public boolean equals(Client client)" will not override the equals method defined
in Object. Parameter should be of type Object.
The implementation of equals method checks if the id's of both objects are equal. If so, it
returns true.
Note that this is a basic implementation of equals.
Example
Any equals implementation should satisfy these properties:
Reflexive. For any reference value x, x.equals(x) returns true.
Symmetric. For any reference values x and y, x.equals(y) should return true if and only if
y.equals(x) returns true.
Transitive. For any reference values x, y, and z, if x.equals(y) returns true and y.equals(z)
returns true, then x.equals(z) must return true.
Consistent. For any reference values x and y, multiple invocations of x.equals(y) consistently
return true or consistently return false, if no information used in equals is modified.
For any non-null reference value x, x.equals(null) should return false.
Let's now provide an implementation of equals which satisfy these properties:
}
}
Client client1 = new Client(25);
Client client2 = new Client(25);
Client client3 = client1;
//both id's are 25
System.out.println(client1.equals(client2));//true
//both id's are 25
System.out.println(client1.equals(client3));//true
//Client class
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Client other = (Client) obj;
if (id != other.id)
return false;
return true;
hashCode method
HashCode's are used in hashing to decide which group (or bucket) an object should be
placed into.
A group of object's might share the same hashcode.
The implementation of hash code decides effectiveness of Hashing.
A good hashing function evenly distributes object's into different groups (or buckets).
hashCode method properties
If obj1.equals(obj2) is true, then obj1.hashCode() should be equal to obj2.hashCode()
obj.hashCode() should return the same value when run multiple times, if values of obj used
in equals() have not changed.
If obj1.equals(obj2) is false, it is NOT required that obj1.hashCode() is not equal to
obj2.hashCode(). Two unequal objects MIGHT have the same hashCode.
Example
Abstract Class
An abstract class cannot be instantiated.
}
//Client class
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
public abstract class AbstractClassExample {
public static void main(String[] args) {
//An abstract class cannot be instantiated
//Below line gives compilation error if uncommented
//AbstractClassExample ex = new AbstractClassExample();
}
}
//Abstract class can contain instance and static variables
public abstract class AbstractClassExample {
//Abstract class can contain instance and static variables
public int publicVariable;
private int privateVariable;
static int staticVariable;
}
//An Abstract method does not contain body.
//Abstract Class can contain 0 or more abstract methods
//Abstract method does not have a body
abstract void abstractMethod1();
abstract void abstractMethod2();
//Abstract method can be declared only in Abstract Class.
class NormalClass{
abstract void abstractMethod();//COMPILER ERROR
}
// Abstract class can contain fully defined non-abstract methods.
public abstract class AbstractClassExample {
//Abstract class can contain instance and static variables
public int publicVariable;
private int privateVariable;
static int staticVariable;
//Abstract Class can contain 0 or more abstract methods
//Abstract method does not have a body
abstract void abstractMethod1();
abstract void abstractMethod2();
//Abstract Class can contain 0 or more non-abstract methods
public void nonAbstractMethod(){
System.out.println("Non Abstract Method");
}
public static void main(String[] args) {
//An abstract class cannot be instantiated
//Below line gives compilation error if uncommented
//AbstractClassExample ex = new AbstractClassExample();
}
}
//Extending an abstract class
class SubClass2 extends AbstractClassExample {
void abstractMethod1() {
System.out.println("Abstract Method1");
}
void abstractMethod2() {
System.out.println("Abstract Method2");
}
Tips
Abstract Methods cannot be paired with final or private access modifiers.
A variable cannot be abstract.
Constructors
Constructor is invoked whenever we create an instance(object) of a Class. We cannot create
an object without a constructor. If we do not provide a constructor, compiler provides a
default no-argument constructor.
Constructor Example 1: Default Constructor
In the example below, there are no Constructors defined in the Animal class. Compiler provides
us with a default constructor, which helps us create an instance of animal class.
Constructor Example 2: Creating a Constructor
If we provide a constructor in the class, compiler will NOT provide a default constructor. In the
example below we provided a constructor "public Animal(String name)". So, compiler will not
provide the default constructor.
}
// A concrete sub class should implement all abstract methods.
// Below class gives compilation error if uncommented
/*
class SubClass extends AbstractClassExample {
}
*/
//An abstract sub class need not implement all abstract methods.
abstract class AbstractSubClass extends AbstractClassExample {
void abstractMethod1() {
System.out.println("Abstract Method1");
}
//abstractMethod2 is not defined.
}
public class Animal {
String name;
public static void main(String[] args) {
// Compiler provides this class with a default no-argument constructor.
// This allows us to create an instance of Animal class.
Animal animal = new Animal();
}
}
Constructor has the same name as the class and no return type. It can accept any number of
parameters.
Constructor Example 3: Provide No Argument Constructor
If we want to allow creation of an object with no constructor arguments, we can provide a no
argument constructor as well.
class Animal {
String name;
// This is called a one argument constructor.
public Animal(String name) {
this.name = name;
}
public static void main(String[] args) {
// Since we provided a constructor, compiler does not
// provide a default constructor.
// Animal animal = new Animal();//COMPILER ERROR!
// The only way we can create Animal1 object is by using
Animal animal = new Animal("Tommy");
}
}
class Animal {
String name;
public Animal() {
this.name = "Default Name";
}
// This is called a one argument constructor.
public Animal(String name) {
this.name = name;
}
public static void main(String[] args) {
// Since we provided a constructor, compiler does not
// provide a default constructor.
// Animal animal = new Animal();//COMPILER ERROR!
// The only way we can create Animal1 object is by using
Animal animal = new Animal("Tommy");
}
}
Constructor Example 4: Calling a Super Class Constructor
A constructor can invoke another constructor, or a super class constructor, but only as first
statement in the constructor. Another constructor in the same class can be invoked from a
constructor, using this({parameters}) method call. To call a super class constructor,
super({parameters}) can be used.
Both example constructors below can replace the no argument "public Animal() " constructor in
Example 3.
super() or this() should be first statements in a Constructor.
Below examples will throw a compilation error if the super or this calls are uncommented.
Constructor Example 5
Member variables/methods should not be used in constructor calls (super or this). Static
variables or methods can be used.
Constructor Example 6: Constructor cannot be directly called
A constructor cannot be explicitly called from any method except another constructor.
public Animal() {
super();
this.name = "Default Name";
}
public Animal() {
this("Default Name");
}
public Animal() {
this.name = "Default Name";
//super(), if called, should always the first statement in a constructor.
//super(); //COMPILER ERROR
}
public Animal() {
System.out.println("Creating an Animal");
//this(string), if called, should always the first statement in a
constructor.
//this("Default Name");//COMPILER ERROR
}
public Animal() {
//member variable cannot be used in a constructor call
this(name);//COMPILER ERROR since name is member variable
}
Constructor Example 7: Super Class Constructor is invoked automatically
If a super class constructor is not explicitly called from a sub class constructor, super class (no
argument) constructor is automatically invoked (as first line) from a sub class constructor.
Consider the example below:
It is almost as if super() method is invoked as the first line of every constructor. The example code
below shows how the code above behaves.
class Animal {
String name;
public Animal() {
}
public method() {
Animal();// Compiler error
}
}
class Animal {
public Animal() {
System.out.println("Animal Constructor");
}
}
class Dog extends Animal {
public Dog() {
System.out.println("Dog Constructor");
}
}
class Labrador extends Dog {
public Labrador() {
System.out.println("Labrador Constructor");
}
}
public class ConstructorExamples {
public static void main(String[] args) {
Labrador labrador = new Labrador();
}
}
//Output - Animal Constructor
Dog Constructor
Labrador Constructor
Constructor Example 8
Since a subclass constructor explicitly calls a super class constructor with no arguments, this can
cause a few compiler errors.
public Dog() makes an implicit super() call i.e. a call to Animal() (no argument) constructor. But no
such constructor is defined in Animal class.
Constructor Example 9
class Animal {
public Animal() {
super();// IMPLICIT CALL
System.out.println("Animal Constructor");
}
}
class Dog extends Animal {
public Dog() {
super();// IMPLICIT CALL
System.out.println("Dog Constructor");
}
}
class Labrador extends Dog {
public Labrador() {
super();// IMPLICIT CALL
System.out.println("Labrador Constructor");
}
}
class Animal {
String name;
public Animal(String name) {
this.name = name;
System.out.println("Animal Constructor");
}
}
class Dog extends Animal {
public Dog() { // COMPILER ERROR! No constructor for Animal()
System.out.println("Dog Constructor");
}
}
Similar example below except that the Dog no argument constructor is not provided by
programmer. However, the compiler would give the no argument constructor, which would
invoke super() method. This would again result in a compilation error.
Two ways to fix above errors. 1.Create a no arguments constructor in Animal class. 2.Make a
explicit super("Default Dog Name") call in the Dog() constructor.
Creating a super class no argument constructor
Making an explicity super call
Constructors are NOT inherited.
class Animal {
String name;
public Animal(String name) {
this.name = name;
System.out.println("Animal Constructor");
}
}
class Dog extends Animal {// COMPILER ERROR! No constructor for Animal()
}
class Animal {
String name;
public Animal() {
}
public Animal(String name) {
this.name = name;
System.out.println("Animal Constructor");
}
}
class Dog extends Animal {
public Dog() { // COMPILER ERROR! No constructor for Animal()
super("Default Dog Name");
System.out.println("Dog Constructor");
}
}
class Animal {
String name;
new Dog("Terry") is not allowed even though there is a constructor in the super class Animal with
signature public Animal(String name).
Solution is to create an explicit constructor in sub class invoking the super class constructor. Add
below constructor to Dog class.
Coupling
Coupling is a measure of how much a class is dependent on other classes. There should
minimal dependencies between classes. So, we should always aim for low coupling between
classes.
Coupling Example Problem
Consider the example below:
public Animal(String name) {
this.name = name;
System.out.println("Animal Constructor with name");
}
}
class Dog extends Animal {
}
public class ConstructorExamples {
public static void main(String[] args) {
// Dog dog = new Dog("Terry");//COMPILER ERROR
}
}
class Dog extends Animal {
public Dog() {
super("Default Dog Name");
}
}
class ShoppingCartEntry {
public float price;
public int quantity;
}
class ShoppingCart {
public ShoppingCartEntry[] items;
}
class Order {
private ShoppingCart cart;
private float salesTax;
Method orderTotalPrice in Order class is coupled heavily with ShoppingCartEntry and
ShoppingCart classes. It uses different properties (items, price, quantity) from these classes. If any
of these properties change, orderTotalPrice will also change. This is not good for Maintenance.
Coupling Example Solution
Consider a better implementation with lesser coupling between classes below: In this
implementation, changes in ShoppingCartEntry or CartContents might not affect Order class at
all.
public Order(ShoppingCart cart, float salesTax) {
this.cart = cart;
this.salesTax = salesTax;
}
// This method know the internal details of ShoppingCartEntry and
// ShoppingCart classes. If there is any change in any of those
// classes, this method also needs to change.
public float orderTotalPrice() {
float cartTotalPrice = 0;
for (int i = 0; i < cart.items.length; i++) {
cartTotalPrice += cart.items[i].price
* cart.items[i].quantity;
}
cartTotalPrice += cartTotalPrice * salesTax;
return cartTotalPrice;
}
}
class ShoppingCartEntry
{
float price;
int quantity;
public float getTotalPrice()
{
return price * quantity;
}
}
class CartContents
{
ShoppingCartEntry[] items;
public float getTotalPrice()
{
float totalPrice = 0;
for (ShoppingCartEntry item:items)
Cohesion
Cohesion is a measure of how related the responsibilities of a class are. A class must be
highly cohesive i.e. its responsibilities (methods) should be highly related to one another.
Cohesion Example Problem
Example class below is downloading from internet, parsing data and storing data to database.
The responsibilities of this class are not really related. This is not cohesive class.
{
totalPrice += item.getTotalPrice();
}
return totalPrice;
}
}
class Order
{
private CartContents cart;
private float salesTax;
public Order(CartContents cart, float salesTax)
{
this.cart = cart;
this.salesTax = salesTax;
}
public float totalPrice()
{
return cart.getTotalPrice() * (1.0f + salesTax);
}
}
class DownloadAndStore{
void downloadFromInternet(){
}
void parseData(){
}
void storeIntoDatabase(){
}
void doEverything(){
downloadFromInternet();
parseData();
storeIntoDatabase();
}
Cohesion Example Solution
This is a better way of approaching the problem. Different classes have their own responsibilities.
Encapsulation
Encapsulation is hiding the implementation of a Class behind a well defined interfaceÓ.
Encapsulation helps us to change implementation of a class without breaking other code.
Encapsulation Approach 1
In this approach we create a public variable score. The main method directly accesses the score
variable, updates it.
Example Class
Let's use the CricketScorer class.
}
class InternetDownloader {
public void downloadFromInternet() {
}
}
class DataParser {
public void parseData() {
}
}
class DatabaseStorer {
public void storeIntoDatabase() {
}
}
class DownloadAndStore {
void doEverything() {
new InternetDownloader().downloadFromInternet();
new DataParser().parseData();
new DatabaseStorer().storeIntoDatabase();
}
}
public class CricketScorer {
public int score;
}
Encapsulation Approach 2
In this approach, we make score as private and access value through get and set methods.
However, the logic of adding 4 to the score is performed in the main method.
Example Class
Let's use the CricketScorer class.
Encapsulation Approach 3
In this approach - For better encapsulation, the logic of doing the four operation also is moved to
the CricketScorer class.
Example Class
public static void main(String[] args) {
CricketScorer scorer = new CricketScorer();
scorer.score = scorer.score + 4;
}
public class CricketScorer {
private int score;
public int getScore() {
return score;
}
public void setScore(int score) {
this.score = score;
}
}
public static void main(String[] args) {
CricketScorer scorer = new CricketScorer();
int score = scorer.getScore();
scorer.setScore(score + 4);
}
public class CricketScorer {
private int score;
public void four() {
score += 4;
}
}
Let's use the CricketScorer class.
Encapsulation Example
In terms of encapsulation Approach 3 > Approach 2 > Approach 1. In Approach 3, the user of
scorer class does not even know that there is a variable called score. Implementation of Scorer
can change without changing other classes using Scorer.
Interface
An interface defines a contract for responsibilities (methods) of a class. Let's look at a few
examples of interfaces.
Defining an Interface
An interface is declared by using the keyword interface. Look at the example below: Flyable is an
interface.
An interface can contain abstract methods -- NOT TRUE ANY MORE
In the above example, fly method is abstract since it is only declared (No definition is provided).
Implementing an Interface
We can define a class implementing the interface by using the implements keyword. Let us look
at a couple of examples:
Example 1
Class Aeroplane implements Flyable and implements the abstract method fly().
public static void main(String[] args) {
CricketScorer scorer = new CricketScorer();
scorer.four();
}
//public abstract are not necessary
public abstract interface Flyable {
//public abstract are not necessary
public abstract void fly();
}
public class Aeroplane implements Flyable{
@Override
public void fly() {
System.out.println("Aeroplane is flying");
}
}
Example 2
Using the Interface and Implementation
The interface classes can directly be instantiated and stored in the class reference variables
An interface reference variable can hold objects of any implementation of interface.
Variables in an interface
Variables in an interface are always public, static, final. Variables in an interface cannot be
declared private.
Methods in an interface
Interface methods are by default public and abstract. A concrete default method (fully defined
method) can be created in an interface. Consider the example below:
public class Bird implements Flyable{
@Override
public void fly() {
System.out.println("Bird is flying");
}
}
Bird bird = new Bird();
bird.fly();//Bird is flying
Aeroplane aeroplane = new Aeroplane();
aeroplane.fly();//Aeroplane is flying
Flyable flyable1 = new Bird();
Flyable flyable2 = new Aeroplane();
interface ExampleInterface1 {
//By default - public static final. No other modifier allowed
//value1,value2,value3,value4 all are - public static final
int value1 = 10;
public int value2 = 15;
public static int value3 = 20;
public static final int value4 = 25;
//private int value5 = 10;//COMPILER ERROR
}
Extending an Interface
An interface can extend another interface. Consider the example below:
Class implementing SubInterface1 should implement both methods - method3 and
method1(from ExampleInterface1) An interface cannot extend a class.
A class can implement multiple interfaces. It should implement all the method declared in all
Interfaces being implemented.
interface ExampleInterface1 {
//By default - public abstract. No other modifier allowed
void method1();//method1 is public and abstract
//private void method6();//COMPILER ERROR!
}
interface SubInterface1 extends ExampleInterface1{
void method3();
}
/* //COMPILE ERROR IF UnCommented
//Interface cannot extend a Class
interface SubInterface2 extends Integer{
void method3();
}
*/
interface ExampleInterface2 {
void method2();
}
class SampleImpl implements ExampleInterface1,ExampleInterface2{
/* A class should implement all the methods in an interface.
If either of method1 or method2 is commented, it would
result in compilation error.
*/
public void method2() {
System.out.println("Sample Implementation for Method2");
}
public void method1() {
System.out.println("Sample Implementation for Method1");
}
}
Interface , Things to Remember
A class should implement all the methods in an interface, unless it is declared abstract. A Class
can implement multiple interfaces. No new checked exceptions can be thrown by
implementations of methods in an interface.
Method Overloading
A method having the same name as another method (in same class or a sub class) but
having different parameters is called an Overloaded Method.
Method Overloading Example 1
doIt method is overloaded in the below example:
Method Overloading Example 2
Overloading can also be done from a sub class.
Overloading - Other Rules
An overloaded method should have different arguments than the original method. It can also
have a different return type. A method cannot be overloaded just by only changing the return
type. Overloaded methods are always treated as if they are different methods altogether.
Overloading does not put any restrictions on access modifiers or exceptions thrown from the
method. Overloaded method invocation is based on the Type of the Reference variable. It is NOT
based on the object it refers to.
Java Example
Constructors
public HashMap(int initialCapacity, float loadFactor)
public HashMap() {
public HashMap(int initialCapacity)
Methods
class Foo{
public void doIt(int number){
}
public void doIt(String string){
}
}
class Bar extends Foo{
public void doIt(float number){
}
}
public boolean addAll(Collection<? extends E> c)
public boolean addAll(int index, Collection<? extends E> c)
Rules
Method Overriding
Creating a Sub Class Method with same signature as that of a method in SuperClass is called
Method Overriding.
Method Overriding Example 1:
Let's define an Animal class with a method shout.
Let's create a sub class of Animal , Cat - overriding the existing shout method in Animal.
bark method in Cat class is overriding the bark method in Animal class.
Java Example
HashMap public int size() overrides AbstractMap public int size()
Example
Overriding Method Cannot have lesser visibility
Overriding method cannot have lesser visibility than the Super Class method. Consider these two
examples
Example 1
public class Animal {
public String bark() {
return "Don't Know!";
}
}
class Cat extends Animal {
public String bark() {
return "Meow Meow";
}
}
publicMethod in SubClass can only be declared as public. Keyword protected, private or (default)
instead of public would result in Compilation Error.
Example 2
defaultMethod in SuperClass is declared with default access. Any method overriding it can have
access default or greater. So default, protected and public are fine. Overriding method cannot be
private.
Overriding method cannot throw new Checked Exceptions
Consider the example below:
class SuperClass{
public void publicMethod(){
}
}
class SubClass{
//Cannot reduce visibility of SuperClass Method
//So, only option is public
public void publicMethod() {
}
}
class SuperClass{
void defaultMethod(){
}
}
class SubClass{
//Can be overridden with public,(default) or protected
//private would give COMPILE ERROR!
public void defaultMethod(){
}
}
publicMethod() in SuperClass throws FileNotFoundException. So, the SubClass publicMethod()
can throw FileNotFoundException or any sub class of FileNotFoundException. It can also not
throw an Exception (as in the example). But, it cannot throw any new Exception. For example,
Òpublic void publicMethod() throws IOExceptionÓ would cause compilation error.
Other Overriding Rules
A Sub Class can override only those methods that are visible to it. Methods marked as static or
final cannot be overridden. You can call the super class method from the overriding method
using keyword super.
Overriding and Polymorphism Example
Overridden method invocation is based on the object referred to. It is not based on the Type of
the Reference variable. This is called Polymorphism. Consider the example below:
class SuperClass{
public void publicMethod() throws FileNotFoundException{
}
}
class SubClass{
//Cannot throw bigger exceptions than Super Class
public void publicMethod() /*throws IOException*/ {
}
}
class Animal{
public void bark(){
System.out.println("Animal Bark");
}
}
class Dog extends Animal{
public void bark(){
System.out.println("Dog Bark");
}
}
public class PolymorphismExample {
public static void main(String[] args) {
Animal[] animals = {new Dog(),new Animal()};
animals[0].bark();//Dog bark
animals[1].bark();//Animal bark
}
animals[0] contains a reference to Dog Object. When animals[0].bark() method is called method
in Dog class is invoked even though the type of reference variable is Animal. animals[1] contains a
reference to Animal Object. When animals[1].bark() method is called method in Animal class is
invoked.
Covariant Returns
A sub class is considered to be of same type as its super class. So, in interfaces or abstract class, it
is fine to provide implementations using the Sub Class Types as Return Types.
(com.in28minutes.SameType)
Class Modifiers
Let us learn about a few Java Class Modifiers.
Access Modifiers
Access modifier for a class can be public or (default), It cannot be private or protected.
Non-access modifiers
strictfp, final, abstract modifiers are valid on a class.
Class Access Modifiers
Lets learn about a few Java Class Access Modifiers.
public class modifier
A public class is visible to all other classes.
}
public class PublicClass{
}
class DefaultClass{
}
protected class Error{//COMPILER ERROR
}
private class Error{//COMPILER ERROR
}
default class modifier
A class is called a Default Class is when there is no access modifier specified on a class. Default
classes are visible inside the same package only. Default access is also called Package access.
Default Class Modifier Examples
Default Access Class Example
package com.in28minutes.classmodifiers.defaultaccess.a;
Another Class in Same Package: Has access to default class
Class in Different Package: NO access to default class
Method and Variable Access Modifiers
Method and variable access modifiers can be public, protected, private or (default)
Two Access Modifier Questions
When we talk about access modifiers, we would discuss two important questions
/* No public before class. So this class has default access*/
class DefaultAccessClass {
//Default access is also called package access
}
package com.in28minutes.classmodifiers.defaultaccess.a;
public class AnotherClassInSamePackage {
//DefaultAccessClass and AnotherClassInSamePackage
//are in same package.
//So, DefaultAccessClass is visible.
//An instance of the class can be created.
DefaultAccessClass defaultAccess;
}
package com.in28minutes.classmodifiers.defaultaccess.b;
public class ClassInDifferentPackage {
//Class DefaultAccessClass and Class ClassInDifferentPackage
//are in different packages (*.a and *.b)
//So, DefaultAccessClass is not visible to ClassInDifferentPackage
//Below line of code will cause compilation error if uncommented
//DefaultAccessClass defaultAccess; //COMPILE ERROR!!
}
Is Accessible through reference/instance variable?
We create an instance of the class and try to access the variables and methods declared in the
class.
Is Accessible through Inheritance?
Can we access the super class variables and methods from a Sub Class?
Important Access Things to Remember
A sub class trying to access through reference/instance variables, will have the same access as a
normal class (non sub class). Access modifiers cannot be applied to local variables
Access Modifiers Example
Let's consider the following class with variables and methods declared with all 4 access modifiers:
ExampleClass example = new ExampleClass();
example.publicVariable = 5;
example.publicMethod();
public class SubClass extends ExampleClass {
void subClassMethod(){
publicVariable = 5;
protectedVariable = 5;
}
}
package com.in28minutes.membermodifiers.access;
public class ExampleClass {
int defaultVariable;
public int publicVariable;
private int privateVariable;
protected int protectedVariable;
void defaultMethod(){
}
public void publicMethod(){
}
private void privateMethod(){
}
Method Access Modifiers
Let's discuss about access modifiers in order of increasing access.
private
a. Private variables and methods can be accessed only in the class they are declared. b. Private
variables and methods from SuperClass are NOT available in SubClass.
default or package
a. Default variables and methods can be accessed in the same package Classes. b. Default
variables and methods from SuperClass are available only to SubClasses in same package.
protected
a. Protected variables and methods can be accessed in the same package Classes. b. Protected
variables and methods from SuperClass are available to SubClass in any package
public
a. Public variables and methods can be accessed from every other Java classes. b. Public variables
and methods from SuperClass are all available directly in the SubClass
Access Modifier Example: Class in Same Package
Look at the code below to understand what can be accessed and what cannot be.
protected void protectedMethod(){
}
}
package com.in28minutes.membermodifiers.access;
public class TestClassInSamePackage {
public static void main(String[] args) {
ExampleClass example = new ExampleClass();
example.publicVariable = 5;
example.publicMethod();
//privateVariable is not visible
//Below Line, uncommented, would give compiler error
//example.privateVariable=5; //COMPILE ERROR
//example.privateMethod();
example.protectedVariable = 5;
example.protectedMethod();
Access Modifier Example: Class in Different Package
Look at the code below to understand what can be accessed and what cannot be.
Access Modifier Example: Sub Class in Same Package
Look at the code below to understand what can be accessed and what cannot be.
example.defaultVariable = 5;
example.defaultMethod();
}
}
package com.in28minutes.membermodifiers.access.different;
import com.in28minutes.membermodifiers.access.ExampleClass;
public class TestClassInDifferentPackage {
public static void main(String[] args) {
ExampleClass example = new ExampleClass();
example.publicVariable = 5;
example.publicMethod();
//privateVariable,privateMethod are not visible
//Below Lines, uncommented, would give compiler error
//example.privateVariable=5; //COMPILE ERROR
//example.privateMethod();//COMPILE ERROR
//protectedVariable,protectedMethod are not visible
//Below Lines, uncommented, would give compiler error
//example.protectedVariable = 5; //COMPILE ERROR
//example.protectedMethod();//COMPILE ERROR
//defaultVariable,defaultMethod are not visible
//Below Lines, uncommented, would give compiler error
//example.defaultVariable = 5;//COMPILE ERROR
//example.defaultMethod();//COMPILE ERROR
}
}
package com.in28minutes.membermodifiers.access;
public class SubClassInSamePackage extends ExampleClass {
void subClassMethod(){
publicVariable = 5;
publicMethod();
Access Modifier Example: Sub Class in Different Package
Look at the code below to understand what can be accessed and what cannot be.
Final modifier
Let's discuss about Final modifier in Java.
Final class cannot be extended
Consider the class below which is declared as final.
//privateVariable is not visible to SubClass
//Below Line, uncommented, would give compiler error
//privateVariable=5; //COMPILE ERROR
//privateMethod();
protectedVariable = 5;
protectedMethod();
defaultVariable = 5;
defaultMethod();
}
}
package com.in28minutes.membermodifiers.access.different;
import com.in28minutes.membermodifiers.access.ExampleClass;
public class SubClassInDifferentPackage extends ExampleClass {
void subClassMethod(){
publicVariable = 5;
publicMethod();
//privateVariable is not visible to SubClass
//Below Line, uncommented, would give compiler error
//privateVariable=5; //COMPILE ERROR
//privateMethod();
protectedVariable = 5;
protectedMethod();
//privateVariable is not visible to SubClass
//Below Line, uncommented, would give compiler error
//defaultVariable = 5; //COMPILE ERROR
//defaultMethod();
}
}
Below class will not compile if uncommented. FinalClass cannot be extended.
Example of Final class in Java is the String class. Final is used very rarely as it prevents re-use of
the class.
Final methods cannot be overriden.
Consider the class FinalMemberModifiersExample with method finalMethod which is declared as
final.
Any SubClass extending above class cannot override the finalMethod().
Final variable values cannot be changed.
Once initialized, the value of a final variable cannot be changed.
Final arguments value cannot be modified.
Consider the example below:
final public class FinalClass {
}
/*
class ExtendingFinalClass extends FinalClass{ //COMPILER ERROR
}
*/
public class FinalMemberModifiersExample {
final void finalMethod(){
}
}
class SubClass extends FinalMemberModifiersExample {
//final method cannot be over-riddent
//Below method, uncommented, causes compilation Error
/*
final void finalMethod(){
}
*/
}
final int finalValue = 5;
//finalValue = 10; //COMPILER ERROR
Other Non access Modifiers
A class cannot be both abstract and final
strictfp
This modifier can be used on a class and a method. This (strictfp) cannot be used on a variable.
IEEE standard for floating points would be followed in the method or class where strictfp modifier
is specified.
volatile
Volatile can only be applied to instance variables. A volatile variable is one whose value is always
written to and read from "main memory". Each thread has its own cache in Java. The volatile
variable will not be stored on a Thread cache.
native
Can be applied only to methods. These methods are implemented in native languages (like C)
Static Variables and Methods
Static variables and methods are class level variables and methods. There is only one copy of
the static variable for the entire Class. Each instance of the Class (object) will NOT have a
unique copy of a static variable. Let's start with a real world example of a Class with static
variable and methods.
Static Variable/Method , Example
count variable in Cricketer class is static. The method to get the count value getCount() is also a
static method.
void testMethod(final int finalArgument){
//final argument cannot be modified
//Below line, uncommented, causes compilation Error
//finalArgument = 5;//COMPILER ERROR
}
public class Cricketer {
private static int count;
public Cricketer() {
count++;
}
static int getCount() {
return count;
}
public static void main(String[] args) {
4 instances of the Cricketer class are created. Variable count is incremented with every instance
created in the constructor.
Static Variables and Methods Example 2
Example class below explains all the rules associated with access static variables and static
methods.
Cricketer cricketer1 = new Cricketer();
Cricketer cricketer2 = new Cricketer();
Cricketer cricketer3 = new Cricketer();
Cricketer cricketer4 = new Cricketer();
System.out.println(Cricketer.getCount());//4
}
}
public class StaticModifierExamples {
private static int staticVariable;
private int instanceVariable;
public static void staticMethod() {
//instance variables are not accessible in static methods
//instanceVariable = 10; //COMPILER ERROR
//Also, this Keyword is not accessible. this refers to object.
//static methods are class methods
//static variables are accessible in static methods
staticVariable = 10;
}
public void instanceMethod() {
//static and instance variables are accessible in instance methods
instanceVariable = 10;
staticVariable = 10;
}
public static void main(String[] args) {
//static int i =5; //COMPILER ERROR
StaticModifierExamples example = new StaticModifierExamples();
//instance variables and methods are only accessible through object references
example.instanceVariable = 10;
example.instanceMethod();
//StaticModifierExamples.instanceVariable = 10;//COMPILER ERROR
//StaticModifierExamples.instanceMethod();//COMPILER ERROR
In a static method, instance variables are not accessible. Keyword this is also not accessible.
However static variables are accessible.
In instance methods, both static and instance variables are accessible.
Instance variables and methods are only accessible through object references.
Static variables and methods are accessible through object references and Class Name.
//static variables and methods are accessible through object references and
Class Name.
example.staticVariable = 10;
example.staticMethod();
StaticModifierExamples.staticVariable = 10;
StaticModifierExamples.staticMethod();
}
}
public static void staticMethod() {
//instance variables are not accessible in static methods
//instanceVariable = 10; //COMPILER ERROR
//Also, this Keyword is not accessible. this refers to object.
//static methods are class methods
//static variables are accessible in static methods
staticVariable = 10;
}
public void instanceMethod() {
instanceVariable = 10;
staticVariable = 10;
}
example.instanceVariable = 10;
example.instanceMethod();
//StaticModifierExamples.instanceVariable = 10;//COMPILER ERROR
//StaticModifierExamples.instanceMethod();//COMPILER ERROR
example.staticVariable = 10;
example.staticMethod();
StaticModifierExamples.staticVariable = 10;
StaticModifierExamples.staticMethod();
It is always recommended to use Class Name to access a static variable or method. This is
because static methods are class level methods. It is not appropriate to use instance references
to call static methods (even though it compiles and works).
Static methods cannot be overridden
Consider the example below:
When code below is run, static method in Animal is executed. Static method invocation is based
on the type of reference variable. It does not depend on the type of object referred to.
Local variables cannot be declared as static
Example below:
Class Contents
Let's discuss what a Java class can contain and what it cannot.
Section
Java Source File Rules A Java Source File can contain a.0 or 1 Public Classes b.0 or 1 or More Non
Public Classes
Order should be
1.
Package Statement
2.
Imports
3.
Class Declarations
class Animal{
static void StaticMethod(){
System.out.println("Animal Static Method");
}
}
class Dog extends Animal{
static void StaticMethod(){
System.out.println("Dog Static Method");
}
}
Animal animal = new Dog();
animal.StaticMethod();//Animal Static Method
public static void main(String[] args) {
//static int i =5; //COMPILER ERROR
}
Comments can be anywhere in the file. If there is a public class, file name should the (name of
public class) + ".java". If name of public class is Scorer, name of file should be Scorer.java. If there
is no public class, there are no restrictions on file name.
Example Class
Nested Class
Nested Classes are classes which are declared inside other classes.
Nested Class Example
Consider the following example:
Inner Class
/* Comments Anywhere*/
package com.in28minutes.classcontent;
class DefaultClass1{
}
/* Comments Anywhere*/
class DefaultClass2{
}
/* Comments Anywhere*/
public class PublicClass1 {
}
/* Cannot have another Public Class.
public class PublicClass2 {
}
*/
class OuterClass {
public class InnerClass {
}
public static class StaticNestedClass {
}
public void exampleMethod() {
class MethodLocalInnerClass {
};
}
}
Generally the term inner class is used to refer to a non-static class declared directly inside
another class. Consider the example of class named InnerClass.
Static Inner Class
A class declared directly inside another class and declared as static. In the example above, class
name StaticNestedClass is a static inner class.
Method Inner Class
A class declared directly inside a method. In the example above, class name
MethodLocalInnerClass is a method inner class.
Inner Class
Inner Class is a very important Java Concept. Let's learn about it in this tutorial.
Inner Class Example
Consider the following Example:
class OuterClass {
private int outerClassInstanceVariable;
public class InnerClass {
private int innerClassVariable;
public int getInnerClassVariable() {
return innerClassVariable;
}
public void setInnerClassVariable(
int innerClassVariable) {
this.innerClassVariable = innerClassVariable;
}
public void privateVariablesOfOuterClassAreAvailable() {
outerClassInstanceVariable = 5; // we can access the value
System.out.println("Inner class ref is " + this);
//Accessing outer class reference variable
System.out.println("Outer class ref is " + OuterClass.this);
}
}
public void createInnerClass(){
//Just use the inner class name to create it
InnerClass inner = new InnerClass();
}
Inner class cannot be directly instantiated.
To create an Inner Class you need an instance of Outer Class.
Creating an Inner Class instance in outer class
Consider the method createInnerClass from the example above: This method shows how to
create an inner class instance.
Instance variables of Outer Class are available in inner class
Consider the method privateVariablesOfOuterClassAreAvailable from InnerClass declared above:
Static Inner Nested Class
}
public class InnerClassExamples {
public static void main(String[] args) {
// COMPILER ERROR! You cannot create an inner class on its own.
// InnerClass innerClass = new InnerClass();
OuterClass example = new OuterClass();
// To create an Inner Class you need an instance of Outer Class
OuterClass.InnerClass innerClass = example.new InnerClass();
}
}
// InnerClass innerClass = new InnerClass(); //Compiler Error
OuterClass example = new OuterClass();
OuterClass.InnerClass innerClass = example.new InnerClass();
public void createInnerClass(){
//Just use the inner class name to create it
InnerClass inner = new InnerClass();
}
public void privateVariablesOfOuterClassAreAvailable() {
outerClassInstanceVariable = 5; // we can access the value
System.out.println("Inner class ref is " + this);
//Accessing outer class reference variable
System.out.println("Outer class ref is " + OuterClass.this);
}
Let's learn about Static Inner Nested Class in this Java tutorial.
Static Inner Nested Class Example
Consider the example below:
class OuterClass {
private int outerClassInstanceVariable;
public static class StaticNestedClass {
private int staticNestedClassVariable;
public int getStaticNestedClassVariable() {
return staticNestedClassVariable;
}
public void setStaticNestedClassVariable(
int staticNestedClassVariable) {
this.staticNestedClassVariable = staticNestedClassVariable;
}
public void privateVariablesOfOuterClassAreNOTAvailable() {
// outerClassInstanceVariable = 5; //COMPILE ERROR
}
}
}
public class InnerClassExamples {
public static void main(String[] args) {
// Static Nested Class can be created without needing to create its
// parent. Without creating NestedClassesExample, we created
// StaticNestedClass
OuterClass.StaticNestedClass staticNestedClass1 = new
OuterClass.StaticNestedClass();
staticNestedClass1.setStaticNestedClassVariable(5);
OuterClass.StaticNestedClass staticNestedClass2 = new
OuterClass.StaticNestedClass();
staticNestedClass2.setStaticNestedClassVariable(10);
// Static Nested Class member variables are not static. They can have
// different values.
System.out.println(staticNestedClass1
.getStaticNestedClassVariable()); //5
System.out.println(staticNestedClass2
.getStaticNestedClassVariable()); //10
}
}
Creating Static Nested Class
Static Nested Class can be created without needing to create its parent. Without creating
NestedClassesExample, we createdStaticNestedClass.
Member variables are not static
Static Nested Class member variables are not static. They can have different values.
Outer class instance variables are not accessible.
Instance variables of outer class are not available in the Static Class.
Method Inner Class
Let us learn about Method Inner Class in this tutorial.
Method Inner Class Example
Consider the example below: MethodLocalInnerClass is declared in exampleMethod();
OuterClass.StaticNestedClass staticNestedClass1 = new
OuterClass.StaticNestedClass();
System.out.println(staticNestedClass1
.getStaticNestedClassVariable()); //5
System.out.println(staticNestedClass2
.getStaticNestedClassVariable()); //10
public void privateVariablesOfOuterClassAreNOTAvailable() {
// outerClassInstanceVariable = 5; //COMPILE ERROR
}
class OuterClass {
private int outerClassInstanceVariable;
public void exampleMethod() {
int localVariable;
final int finalVariable = 5;
class MethodLocalInnerClass {
public void method() {
//Can access class instance variables
System.out
.println(outerClassInstanceVariable);
//Cannot access method's non-final local variables
//localVariable = 5;//Compiler Error
System.out.println(finalVariable);//Final variable is fine..
Method inner class is not accessible outside the method
Look at the commented code below exampleMethod. MethodLocalInnerClass can be instantiated
only in the method where it is declared.
Method inner class can access class instance variables
Method inner class cannot access method's non-final local variables
Variable Arguments
Variable Arguments allow calling a method with different number of parameters.
Variable Arguments Example
}
}
//MethodLocalInnerClass can be instantiated only in this method
MethodLocalInnerClass m1 = new MethodLocalInnerClass();
m1.method();
}
//MethodLocalInnerClass can be instantiated only in the method where it is
declared
//MethodLocalInnerClass m1 = new MethodLocalInnerClass();//COMPILER ERROR
}
//Can access class instance variables
System.out.println(outerClassInstanceVariable);
//Cannot access method's non-final local variables
//localVariable = 5;//Compiler Error
System.out.println(finalVariable);//Final variable is fine..
//int(type) followed ... (three dot's) is syntax of a variable argument.
public int sum(int... numbers) {
//inside the method a variable argument is similar to an array.
//number can be treated as if it is declared as int[] numbers;
int sum = 0;
for (int number: numbers) {
sum += number;
}
return sum;
}
public static void main(String[] args) {
Variable Arguments Syntax
Data Type followed ... (three dot's) is syntax of a variable argument.
Inside the method a variable argument is similar to an array. For Example: number can be treated
in below method as if it is declared as int[] numbers;
Variable Argument: only Last Parameter
Variable Argument should be always the last parameter (or only parameter) of a method. Below
example gives a compilation error
Variable Argument of Type Custom Class
Even a class can be used a variable argument. In the example below, bark method is overloaded
with a variable argument method.
VariableArgumentExamples example = new VariableArgumentExamples();
//3 Arguments
System.out.println(example.sum(1, 4, 5));//10
//4 Arguments
System.out.println(example.sum(1, 4, 5, 20));//30
//0 Arguments
System.out.println(example.sum());//0
}
public int sum(int... numbers) {
public int sum(int... numbers) {
int sum = 0;
for (int number: numbers) {
sum += number;
}
return sum;
}
public int sum(int... numbers, float value) {//COMPILER ERROR
}
Exception Handling
In this tutorial, let's understand the need for exception handling and learn how to handle
exceptions.
Example without Exception Handling
Let's first look an example without exception handling. Method main throws an exception
because toString method is invoked on a null object.
Output of above program is
Exception Example 2 , Propagation of an Exception
In this example, main invokes method1, which invokes method2 which throws a
NullPointerException. Check the output of this program.
class Animal {
void bark() {
System.out.println("Bark");
}
void bark(Animal... animals) {
for (Animal animal: animals) {
animal.bark();
}
}
}
public static void main(String[] args) {
String str = null;
str.toString();
}
Exception in thread "main" java.lang.NullPointerException at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.main(ExceptionHand
lingExample1.java:6)
public static void main(String[] args) {
method1();
}
private static void method1() {
method2();
}
private static void method2() {
String str = null;
str.toString();
Look at the stack trace. Exception which is thrown in method2 is propagating to method1 and
then to main. This is because there is no exception handling in all 3 methods - main, method1
and method2
Exception Example 3: Execution of method stopped
Look at the example below: A println method call is added after every method call.
Note that none of the lines with text "Line after Exception - " are executed. If an exception occurs,
lines after the line where exception occurred are not executed. Since all three methods main,
method1() and method2() do not have any Exception Handling, exception propagates from
method2 to method1 to main.
}
//Output - Exception in thread "main" java.lang.NullPointerException at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method2(ExceptionH
andlingExample1.java:15)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method1(ExceptionH
andlingExample1.java:10)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.main(ExceptionHand
lingExample1.java:6)
public static void main(String[] args) {
method1();
System.out.println("Line after Exception - Main");
}
private static void method1() {
method2();
System.out.println("Line after Exception - Method 1");
}
private static void method2() {
String str = null;
str.toString();
System.out.println("Line after Exception - Method 2");
}
//Output - Exception in thread "main" java.lang.NullPointerException
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method2(ExceptionH
andlingExample1.java:18)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method1(ExceptionH
andlingExample1.java:12)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.main(ExceptionHand
lingExample1.java:7)
Exception Handling Example 4: Try catch block
Let's add a try catch block in method2
Output
Since Exception Handling is added in the method method2, the exception did not propogate to
method1. You can see the "Line after Exception - **" in the output for main, method1 since they
are not affected by the exception thrown. Since the exception was handled in method2, method1
and main are not affected by it. This is the main essence of exception handling. However, note
that the line after the line throwing exception in method2 is not executed.
Few important things to remember from this example. 1.If exception is handled, it does not
propogate further. 2.In a try block, the lines after the line throwing the exception are not
executed.
Exception Handling Example 5: Need for Finally
Consider the example below: In method2, a connection is opened. However, because of the
exception thrown, connection is not closed. This results in unclosed connections.
public static void main(String[] args) {
method1();
System.out.println("Line after Exception - Main");
}
private static void method1() {
method2();
System.out.println("Line after Exception - Method 1");
}
private static void method2() {
try {
String str = null;
str.toString();
System.out.println("Line after Exception - Method 2");
} catch (Exception e) {
// NOT PRINTING EXCEPTION TRACE- BAD PRACTICE
System.out.println("Exception Handled - Method 2");
}
}
Exception Handled - Method 2
Line after Exception - Method 1
Line after Exception - Main
package com.in28minutes.exceptionhandling;
class Connection {
Output
Connection that is opened is not closed. Because an exception has occurred in method2,
connection.close() is not run. This results in a dangling (un-closed) connection.
void open() {
System.out.println("Connection Opened");
}
void close() {
System.out.println("Connection Closed");
}
}
public class ExceptionHandlingExample1 {
// Exception Handling Example 1
// Let's add a try catch block in method2
public static void main(String[] args) {
method1();
System.out.println("Line after Exception - Main");
}
private static void method1() {
method2();
System.out.println("Line after Exception - Method 1");
}
private static void method2() {
try {
Connection connection = new Connection();
connection.open();
// LOGIC
String str = null;
str.toString();
connection.close();
} catch (Exception e) {
// NOT PRINTING EXCEPTION TRACE- BAD PRACTICE
System.out.println("Exception Handled - Method 2");
}
}
}
Connection Opened
Exception Handled - Method 2
Line after Exception - Method 1
Line after Exception - Main
Exception Handling Example 6 - Finally
Finally block is used when code needs to be executed irrespective of whether an exception is
thrown. Let us now move connection.close(); into a finally block. Also connection declaration is
moved out of the try block to make it visible in the finally block.
Output
Connection is closed even when exception is thrown. This is because connection.close() is called
in the finally block. Finally block is always executed (even when an exception is thrown). So, if we
want some code to be always executed we can move it to finally block.
Code in finally is NOT executed only in two situations. If exception is thrown in finally. If JVM
Crashes in between (for example, System.exit()).
finally is executed even if there is a return statement in catch or try
private static void method2() {
Connection connection = new Connection();
connection.open();
try {
// LOGIC
String str = null;
str.toString();
} catch (Exception e) {
// NOT PRINTING EXCEPTION TRACE - BAD PRACTICE
System.out.println("Exception Handled - Method 2");
} finally {
connection.close();
}
}
Connection Opened
Exception Handled - Method 2
Connection Closed
Line after Exception - Method 1
Line after Exception - Main
private static void method2() {
Connection connection = new Connection();
connection.open();
try {
// LOGIC
String str = null;
str.toString();
return;
} catch (Exception e) {
Exception Handling Syntax
Let's look at a few quirks about Exception Handling syntax.
try without a catch is allowed
Output:
Try without a catch is useful when you would want to do something (close a connection) even if
an exception occurred without handling the exception.
Try without both catch and finally is not allowed.
Below method would give a Compilation Error!! (End of try block)
// NOT PRINTING EXCEPTION TRACE - BAD PRACTICE
System.out.println("Exception Handled - Method 2");
return;
} finally {
connection.close();
}
}
private static void method2() {
Connection connection = new Connection();
connection.open();
try {
// LOGIC
String str = null;
str.toString();
} finally {
connection.close();
}
}
Connection Opened
Connection Closed
Exception in thread "main" java.lang.NullPointerException at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method2(ExceptionH
andlingExample1.java:33) at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.method1(ExceptionH
andlingExample1.java:22) at
com.in28minutes.exceptionhandling.ExceptionHandlingExample1.main(ExceptionHand
lingExample1.java:17)
Exception Handling Hierarchy
Throwable is the highest level of Error Handling classes.
Below class definitions show the pre-defined exception hierarchy in Java.
Below class definitions show creation of a programmer defined exception in Java.
Errors
Error is used in situations when there is nothing a programmer can do about an error. Ex:
StackOverflowError, OutOfMemoryError.
Exception
Exception is used when a programmer can handle the exception.
Un-Checked Exception
RuntimeException and classes that extend RuntimeException are called unchecked exceptions.
For Example: RuntimeException,UnCheckedException,UnCheckedException2 are unchecked or
RunTime Exceptions. There are subclasses of RuntimeException (which means they are
subclasses of Exception also.)
Checked Exception
private static void method2() {
Connection connection = new Connection();
connection.open();
try {
// LOGIC
String str = null;
str.toString();
}//COMPILER ERROR!!
}
//Pre-defined Java Classes
class Error extends Throwable{}
class Exception extends Throwable{}
class RuntimeException extends Exception{}
//Programmer defined classes
class CheckedException1 extends Exception{}
class CheckedException2 extends CheckedException1{}
class UnCheckedException extends RuntimeException{}
class UnCheckedException2 extends UnCheckedException{}
Other Exception Classes (which don't fit the earlier definition). These are also called Checked
Exceptions. Exception, CheckedException1,CheckedException2 are checked exceptions. They are
subclasses of Exception which are not subclasses of RuntimeException.
Throwing RuntimeException in method
Method addAmounts in Class AmountAdder adds amounts. If amounts are of different currencies
it throws an exception.
Output
class Amount {
public Amount(String currency, int amount) {
this.currency = currency;
this.amount = amount;
}
String currency;// Should be an Enum
int amount;// Should ideally use BigDecimal
}
// AmountAdder class has method addAmounts which is throwing a
RuntimeException
class AmountAdder {
static Amount addAmounts(Amount amount1, Amount amount2) {
if (!amount1.currency.equals(amount2.currency)) {
throw new RuntimeException("Currencies don't match");
}
return new Amount(amount1.currency, amount1.amount + amount2.amount);
}
}
public class ExceptionHandlingExample2 {
public static void main(String[] args) {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR", 5));
}
}
Exception in thread "main" java.lang.RuntimeException: Currencies don't match
at
com.in28minutes.exceptionhandling.AmountAdder.addAmounts(ExceptionHandlingExam
ple2.java:17)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample2.main(ExceptionHand
lingExample2.java:28)
Exception message shows the type of exception(java.lang.RuntimeException) and the string
message passed to the RuntimeException constructor("Currencies don't match");
Throwing Exception (Checked Exception) in method
Let us now try to change the method addAmounts to throw an Exception instead of
RuntimeException. It gives us a compilation error.
All classes that are not RuntimeException or subclasses of RuntimeException but extend
Exception are called CheckedExceptions. The rule for CheckedExceptions is that they should be
handled or thrown. Handled means it should be completed handled - i.e. not throw out of the
method. Thrown means the method should declare that it throws the exception
Throws Exception Example
Let's look at how to declare throwing an exception from a method.
Look at the line "static Amount addAmounts(Amount amount1, Amount amount2) throws
Exception". This is how we declare that a method throws Exception. This results in compilation
error in main method. This is because Main method is calling a method which is declaring that it
might throw Exception. Main method again has two options a. Throw b. Handle
Code with main method throwing the exception below
class AmountAdder {
static Amount addAmounts(Amount amount1, Amount amount2) {
if (!amount1.currency.equals(amount2.currency)) {
throw new Exception("Currencies don't match");// COMPILER ERROR!//
Unhandled exception type Exception
}
return new Amount(amount1.currency, amount1.amount + amount2.amount);
}
}
class AmountAdder {
static Amount addAmounts(Amount amount1, Amount amount2) throws Exception
{
if (!amount1.currency.equals(amount2.currency)) {
throw new Exception("Currencies don't match");
}
return new Amount(amount1.currency, amount1.amount + amount2.amount);
}
}
public static void main(String[] args) throws Exception {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR", 5));
}
Output
Handling an Exception
main can also handle the exception instead of declaring throws. Code for it below.
Output
Custom Defined Exception Classes
For the scenario above we can create a customized exception, CurrenciesDoNotMatchException.
If we want to make it a Checked Exception, we can make it extend Exception class. Otherwise, we
can extend RuntimeException class.
Extending Exception Class
No we can change the method addAmounts to throw CurrenciesDoNotMatchException - even the
declaration of the method changed.
Exception in thread "main" java.lang.Exception: Currencies don't match
at
com.in28minutes.exceptionhandling.AmountAdder.addAmounts(ExceptionHandlingExam
ple2.java:17)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample2.main(ExceptionHand
lingExample2.java:28)
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR",5));
} catch (Exception e) {
System.out.println("Exception Handled in Main");
}
}
Exception Handled in Main
class CurrenciesDoNotMatchException extends Exception{
}
main method needs to be changed to catch: CurrenciesDoNotMatchException
Output:
Let's change main method to handle Exception instead of CurrenciesDoNotMatchException
Output:
There is no change in output from the previous example. This is because Exception catch block
can catch Exception and all subclasses of Exception.
class AmountAdder {
static Amount addAmounts(Amount amount1, Amount amount2)
throws CurrenciesDoNotMatchException {
if (!amount1.currency.equals(amount2.currency)) {
throw new CurrenciesDoNotMatchException();
}
return new Amount(amount1.currency, amount1.amount + amount2.amount);
}
}
public class ExceptionHandlingExample2 {
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR",
5));
} catch (CurrenciesDoNotMatchException e) {
System.out.println("Exception Handled in Main" + e.getClass());
}
}
}
Exception Handled in Mainclass
com.in28minutes.exceptionhandling.CurrenciesDoNotMatchException
public class ExceptionHandlingExample2 {
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR",5));
} catch (Exception e) {
System.out.println("Exception Handled in Main" + e.getClass());
}
}
}
Exception Handled in Mainclass
com.in28minutes.exceptionhandling.CurrenciesDoNotMatchException
Extend RuntimeException
Let's change the class CurrenciesDoNotMatchException to extend RuntimeException instead of
Exception
Output:
Change methods addAmounts in AmountAdder to remove the declaration " throws
CurrenciesDoNotMatchException"
No compilation error occurs since RuntimeException and subclasses of RuntimeException are not
Checked Exception's. So, they don't need to be handled or declared. If you are interested in
handling them, go ahead and handle them. But, java does not require you to handle them.
Remove try catch from main method. It is not necessary since CurrenciesDoNotMatchException is
now a RuntimeException.
Output:
Multiple Catch Blocks
Now, let's add two catch blocks to the main
class CurrenciesDoNotMatchException extends RuntimeException{
}
Exception Handled in Mainclass
com.in28minutes.exceptionhandling.CurrenciesDoNotMatchException
public class ExceptionHandlingExample2 {
public static void main(String[] args) {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR", 5));
}
}
Exception in thread "main"
com.in28minutes.exceptionhandling.CurrenciesDoNotMatchException at
com.in28minutes.exceptionhandling.AmountAdder.addAmounts(ExceptionHandlingExam
ple2.java:21)
at
com.in28minutes.exceptionhandling.ExceptionHandlingExample2.main(ExceptionHand
lingExample2.java:30)
Output:
We can have two catch blocks for a try. Order of Handling of exceptions: a. Same Class b. Super
Class.
Specific Exceptions before Generic Exceptions
Specific Exception catch blocks should be before the catch block for a Generic Exception. For
example, CurrenciesDoNotMatchException should be before Exception. Below code gives a
compilation error.
Catch block handles only specified Exceptions (and sub types)
A catch block of type ExceptionType can only catch types ExceptionType and sub classes of
ExceptionType. For Example: Let us change the main method code as shown below. Main method
throws a NullPointerException.
public class ExceptionHandlingExample2 {
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR",
5));
} catch (CurrenciesDoNotMatchException e) {
System.out.println("Handled CurrenciesDoNotMatchException");
} catch (Exception e) {
System.out.println("Handled Exception");
}
}
}
Handled CurrenciesDoNotMatchException
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("DOLLAR",
5));
} catch (Exception e) { // COMPILER ERROR!!
System.out.println("Handled Exception");
} catch (CurrenciesDoNotMatchException e) {
System.out.println("Handled CurrenciesDoNotMatchException");
}
}
Since NullPointerException is not a sub class of CurrenciesDoNotMatchException it wouldn't be
handled by the catch block. Instead a NullPointerException would be thrown out by the main
method.
Exception Handling Best Practices
In all above examples we have not followed an Exception Handling good practice(s). Never
Completely Hide Exceptions. At the least log them. printStactTrace method prints the entire stack
trace when an exception occurs. If you handle an exception, it is always a good practice to log the
trace.
Console
Console is used to read input from keyboard and write output.
Getting a Console reference
Console utility methods
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("RUPEE",
5));
String string = null;
string.toString();
} catch (CurrenciesDoNotMatchException e) {
System.out.println("Handled CurrenciesDoNotMatchException");
}
}
//Output : Exception in thread "main" java.lang.NullPointerException at
com.in28minutes.exceptionhandling.ExceptionHandlingExample2.main(ExceptionHand
lingExample2.java:34)
public static void main(String[] args) {
try {
AmountAdder.addAmounts(new Amount("RUPEE", 5), new Amount("RUPEE",
5));
String string = null;
string.toString();
} catch (CurrenciesDoNotMatchException e) {
System.out.println("Handled CurrenciesDoNotMatchException");
e.printStackTrace();
}
}
//Console console = new Console(); //COMPILER ERROR
Console console = System.console();
Password doesn't show what is being entered
Format or Printf
Format or Printf functions help us in printing formatted output to the console.
Format/Printf Examples
Let's look at a few examples to quickly understand printf function.
In the simplest form, string to be formatted starts with % followed by conversion indicator => b -
boolean c - char d - integer f - floating point s - string.
Other Format/Printf Examples
console.printf("Enter a Line of Text");
String text = console.readLine();
console.printf("Enter a Password");
char[] password = console.readPassword();
console.format("\nEntered Text is %s", text);
System.out.printf("%d", 5);//5
System.out.printf("My name is %s", "Rithu");//My name is Rithu
System.out.printf("%s is %d Years old", "Rithu", 5);//Rithu is 5 Years old
//Prints 12 using minimum 5 character spaces.
System.out.printf("|%5d|", 12); //prints | 12|
//Prints 1234 using minimum 5 character spaces.
System.out.printf("|%5d|", 1234); //prints | 1234|
//In above example 5 is called width specifier.
//Left Justification can be done by using -
System.out.printf("|%-5d|", 12); //prints |12 |
//Using 0 pads the number with 0's
System.out.printf("%05d", 12); //prints 00012
//Using , format the number using comma's
System.out.printf("%,d", 12345); //prints 12,345
//Using ( prints negative numbers between ( and )
System.out.printf("%(d", -12345); //prints (12345)
System.out.printf("%(d", 12345); //prints 12345
For floating point numbers, precision can be specified after dot(.). Below example uses a
precision of 2, so .5678 gets changed to .57
An error in specifying would give a RuntimeException. In below example a string is passed to %d
argument.
String Buffer & String Builder
StringBuffer and StringBuilder are used when you want to modify values of a string
frequently. String Buffer class is thread safe where as String Builder is NOT thread safe.
String Buffer Examples
String Builder Examples
//Using + prints + or - before the number
System.out.printf("%+d", -12345); //prints -12345
System.out.printf("%+d", 12345); //prints +12345
System.out.printf("%5.2f", 1234.5678); //prints 1234.57
System.out.printf("%5d","Test");
//Throws java.util.IllegalFormatConversionException
//To change the order of printing and passing of arguments, argument index can
be used
System.out.printf("%3$.1f %2$s %1$d", 123, "Test", 123.4); //prints 123.4 Test
123
//format method has the same behavior as printf method
System.out.format("%5.2f", 1234.5678);//prints 1234.57
StringBuffer stringbuffer = new StringBuffer("12345");
stringbuffer.append("6789");
System.out.println(stringbuffer); //123456789
//All StringBuffer methods modify the value of the object.
Similar functions exist in StringBuffer also.
Method Chaining
All functions also return a reference to the object after modifying it.This allows a concept called
method chaining.
Date
Date is no longer the class Java recommends for storing and manipulating date and time.
Most of methods in Date are deprecated. Use Calendar class instead. Date internally
represents date-time as number of milliseconds (a long value) since 1st Jan 1970.
Creating Date Object
Manipulating Date Object
Lets now look at adding a few hours to a date object. All date manipulation to date needs to be
done by adding milliseconds to the date. For example, if we want to add 6 hour, we convert 6
hours into millseconds. 6 hours = 6 * 60 * 60 * 1000 milliseconds. Below examples shows specific
code.
StringBuilder sb = new StringBuilder("0123456789");
//StringBuilder delete(int startIndex, int endIndexPlusOne)
System.out.println(sb.delete(3, 7));//012789
StringBuilder sb1 = new StringBuilder("abcdefgh");
//StringBuilder insert(int indext, String whatToInsert)
System.out.println(sb1.insert(3, "ABCD"));//abcABCDdefgh
StringBuilder sb2 = new StringBuilder("abcdefgh");
//StringBuilder reverse()
System.out.println(sb2.reverse());//hgfedcba
StringBuilder sb3 = new StringBuilder("abcdefgh");
System.out.println(sb3.reverse().delete(5, 6).insert(3, "---"));//hgf---edba
//Creating Date Object
Date now = new Date();
System.out.println(now.getTime());
Formatting Dates
Formatting Dates is done by using DateFormat class. Let's look at a few examples.
Formatting Dates with a locale
Format Date's using SimpleDateFormat
Let's look at a few examples of formatting dates using SimpleDateFormat.
Date date = new Date();
//Increase time by 6 hrs
date.setTime(date.getTime() + 6 * 60 * 60 * 1000);
System.out.println(date);
//Decrease time by 6 hrs
date = new Date();
date.setTime(date.getTime() - 6 * 60 * 60 * 1000);
System.out.println(date);
//Formatting Dates
System.out.println(DateFormat.getInstance().format(
date));//10/16/12 5:18 AM
System.out.println(DateFormat.getDateInstance(
DateFormat.FULL, new Locale("it", "IT"))
.format(date));//martedÒ 16 ottobre 2012
System.out.println(DateFormat.getDateInstance(
DateFormat.FULL, Locale.ITALIAN)
.format(date));//martedÒ 16 ottobre 2012
//This uses default locale US
System.out.println(DateFormat.getDateInstance(
DateFormat.FULL).format(date));//Tuesday, October 16, 2012
System.out.println(DateFormat.getDateInstance()
.format(date));//Oct 16, 2012
System.out.println(DateFormat.getDateInstance(
DateFormat.SHORT).format(date));//10/16/12
System.out.println(DateFormat.getDateInstance(
DateFormat.MEDIUM).format(date));//Oct 16, 2012
System.out.println(DateFormat.getDateInstance(
DateFormat.LONG).format(date));//October 16, 2012
System.out.println(new SimpleDateFormat("yy-MM-dd")
Default Locale
Calendar
Calendar class is used in Java to manipulate Dates. Calendar class provides easy ways to add
or reduce days, months or years from a date. It also provide lot of details about a date
(which day of the year? Which week of the year? etc.)
Calendar is abstract
Calendar class cannot be created by using new Calendar. The best way to get an instance of
Calendar class is by using getInstance() static method in Calendar.
Calendar set day, month and year
Setting day, month or year on a calendar object is simple. Call the set method with appropriate
Constant for Day, Month or Year. Next parameter is the value.
.format(date));//12-10-16
System.out
.println(new SimpleDateFormat("yy-MMM-dd")
.format(date));//12-Oct-16
System.out.println(new SimpleDateFormat(
"yyyy-MM-dd").format(date));//2012-10-16
//Parse Dates using DateFormat
Date date2 = DateFormat.getDateInstance(
DateFormat.SHORT).parse("10/16/12");
System.out.println(date2);//Tue Oct 16 00:00:00 GMT+05:30 2012
//Creating Dates using SimpleDateFormat
Date date1 = new SimpleDateFormat("yy-MM-dd")
.parse("12-10-16");
System.out.println(date1);//Tue Oct 16 00:00:00 GMT+05:30 2012
Locale defaultLocale = Locale.getDefault();
System.out.println(defaultLocale
.getDisplayCountry());//United States
System.out.println(defaultLocale
.getDisplayLanguage());//English
//Calendar calendar = new Calendar(); //COMPILER ERROR
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.DATE, 24);
calendar.set(Calendar.MONTH, 8);//8 - September
calendar.set(Calendar.YEAR, 2010);
Calendar get method
Let's get information about a particular date - 24th September 2010. We use the calendar get
method. The parameter passed indicates what value we would want to get from the calendar ,
day or month or year or .. Few examples of the values you can obtain from a calendar are listed
below.
Calendar - Modify a Date
We can use the calendar add and roll methods to modify a date. Calendar add method can be
used to find a date 5 days or 5 months before the date by passing a ,5 i.e. a negative 5.
Roll method
Roll method will only the change the value being modified. YEAR remains unaffected when
MONTH is changed, for instance.
Creating calendar: Example 2
Formatting Calendar object.
Done by getting the date using calendar.getTime() and using the usual formatting of dates.
System.out.println(calendar.get(Calendar.YEAR));//2010
System.out.println(calendar.get(Calendar.MONTH));//8
System.out.println(calendar.get(Calendar.DATE));//24
System.out.println(calendar.get(Calendar.WEEK_OF_MONTH));//4
System.out.println(calendar.get(Calendar.WEEK_OF_YEAR));//39
System.out.println(calendar.get(Calendar.DAY_OF_YEAR));//267
System.out.println(calendar.getFirstDayOfWeek());//1 -> Calendar.SUNDAY
calendar.add(Calendar.DATE, 5);
System.out.println(calendar.getTime());//Wed Sep 29 2010
calendar.add(Calendar.MONTH, 1);
System.out.println(calendar.getTime());//Fri Oct 29 2010
calendar.add(Calendar.YEAR, 2);
System.out.println(calendar.getTime());//Mon Oct 29 2012
calendar.roll(Calendar.MONTH, 5);
System.out.println(calendar.getTime());//Mon Mar 29 2012
Calendar gregorianCalendar = new GregorianCalendar(
2011, 7, 15);
System.out.println(DateFormat.getInstance().format(
calendar.getTime()));//3/29/12 11:39 AM
Number Format
Number format is used to format a number to different locales and different formats.
Format number Using Default locale
Format number using locale
Formatting a number using Netherlands locale
Formatting a number using Germany locale
Formatting a Currency using Default locale
Format currency using locale
Setting maximum fraction digits for a float
Parsing using NumberFormat
Parsing a float value using number format
Parsing only number value using number format
System.out.println(NumberFormat.getInstance().format(321.24f));//321.24
System.out.println(NumberFormat.getInstance(new
Locale("nl")).format(4032.3f));//4.032,3
System.out.println(NumberFormat.getInstance(Locale.GERMANY).format(4032.3f));/
/4.032,3
System.out.println(NumberFormat.getCurrencyInstance().format(40324.31f));//$40
,324.31
formatting a Currency using Netherlands locale
System.out.println(NumberFormat.getCurrencyInstance(new
Locale("nl")).format(40324.31f));//? 40.324,31
numberFormat numberFormat = NumberFormat.getInstance();
System.out.println(numberFormat.getMaximumFractionDigits());//3
numberFormat.setMaximumFractionDigits(5);
System.out.println(numberFormat.format(321.24532f));//321.24533
System.out.println(numberFormat.parse("9876.56"));//9876.56
Collection Interfaces
Arrays are not dynamic. Once an array of a particular size is declared, the size cannot be
modified. To add a new element to the array, a new array has to be created with bigger size
and all the elements from the old array copied to new array. Collections are used in
situations where data is dynamic. Collections allow adding an element, deleting an element
and host of other operations. There are a number of Collections in Java allowing to choose
the right Collection for the right context. Before looking into Collection classes, let's take a
quick look at all the important collection interfaces and the operations they allow.
Collection Interface
Most important methods declared in the collection interface are the methods to add and remove
an element. add method allows adding an element to a collection and delete method allows
deleting an element from a collection. size() methods returns number of elements in the
collection. Other important methods defined as part of collection interface are shown below.
List Interface
List interface extends Collection interface. So, it contains all methods defined in the Collection
interface. In addition, List interface allows operation specifying the position of the element in the
Collection. Any implementation of the List interface would maintain the insertion order. When a
new element is inserted, it is inserted at the end of the list of elements. We can also use the void
numberFormat.setParseIntegerOnly(true);
System.out.println(numberFormat.parse("9876.56"));//9876
interface Collection<E> extends Iterable<E>
{
boolean add(E paramE);
boolean remove(Object paramObject);
int size();
boolean isEmpty();
void clear();
boolean contains(Object paramObject);
boolean containsAll(Collection<?> paramCollection);
boolean addAll(Collection<? extends E> paramCollection);
boolean removeAll(Collection<?> paramCollection);
boolean retainAll(Collection<?> paramCollection);
Iterator<E> iterator();
//A NUMBER OF OTHER METHODS AS WELL..
}
add(int paramInt, E paramE); method to insert an element at a specific position. We can also set
and get the elements at a particular index in the list using corresponding methods.
Other important methods are listed below:
Map Interface
First and foremost, Map interface does not extend Collection interface. So, it does not inherit any
of the methods from the Collection interface. A Map interface supports Collections that use a key
value pair. A key-value pair is a set of linked data items: a key, which is a unique identifier for
some item of data, and the value, which is either the data or a pointer to the data. Key-value pairs
are used in lookup tables, hash tables and configuration files. A key value pair in a Map interface
is called an Entry. Put method allows to add a key, value pair to the Map.
Get method allows to get a value from the Map based on the key.
Other important methods are shown below:
interface List<E> extends Collection<E>
{
boolean addAll(int paramInt, Collection<? extends E> paramCollection);
E get(int paramInt);
E set(int paramInt, E paramE);
void add(int paramInt, E paramE);
E remove(int paramInt);
int indexOf(Object paramObject);
int lastIndexOf(Object paramObject);
ListIterator<E> listIterator();
ListIterator<E> listIterator(int paramInt);
List<E> subList(int paramInt1, int paramInt2);
}
V put(K paramK, V paramV);
V get(Object paramObject);
interface Map<K, V>
{
int size();
boolean isEmpty();
boolean containsKey(Object paramObject);
boolean containsValue(Object paramObject);
Set Interface
Set Interface extends Collection Interface. Set interface only contains the methods from the
Collection interface with added restriction that it cannot contain duplicates.
SortedSet Interface
SortedSet Interface extends the Set Interface. So, it does not allow duplicates. In addition, an
implementation of SortedSet interface maintains its elements in a sorted order. It adds
operations that allow getting a range of values (subSet, headSet, tailSet).
Important Operations listed below:
V get(Object paramObject);
V put(K paramK, V paramV);
V remove(Object paramObject);
void putAll(Map<? extends K, ? extends V> paramMap);
void clear();
Set<K> keySet();
Collection<V> values();
Set<Entry<K, V>> entrySet();
boolean equals(Object paramObject);
int hashCode();
public static abstract interface Entry<K, V>
{
K getKey();
V getValue();
V setValue(V paramV);
boolean equals(Object paramObject);
int hashCode();
}
}
// Unique things only - Does not allow duplication.
// If obj1.equals(obj2) then only one of them can be in the Set.
interface Set<E> extends Collection<E>
{
}
SortedMap Interface
SortedMap interface extends the Map interface. In addition, an implementation of SortedMap
interface maintains keys in a sorted order. Methods are available in the interface to get a ranges
of values based on their keys.
Queue Interface
Queue Interface extends Collection interface. Queue Interface is typically used for
implementation holding elements in order for some processing.
Queue interface offers methods peek() and poll() which get the element at head of the queue.
The difference is that poll() method removes the head from queue also. peek() would keep head
of the queue unchanged.
public interface SortedSet<E> extends Set<E> {
SortedSet<E> subSet(E fromElement, E toElement);
SortedSet<E> headSet(E toElement);
SortedSet<E> tailSet(E fromElement);
E first();
E last();
Comparator<? super E> comparator();
}
public interface SortedMap<K, V> extends Map<K, V> {
Comparator<? super K> comparator();
SortedMap<K, V> subMap(K fromKey, K toKey);
SortedMap<K, V> headMap(K toKey);
SortedMap<K, V> tailMap(K fromKey);
K firstKey();
K lastKey();
}
Iterator interface
Iterator interface enables us to iterate (loop around) a collection. All collections define a method
iterator() that gets an iterator of the collection. hasNext() checks if there is another element in the
collection being iterated. next() gets the next element.
Collections
Collections can only hold Objects - not primitives.
ArrayList
ArrayList implements the list interface. So, ArrayList stores the elements in insertion order (by
default). Element's can be inserted into and removed from ArrayList based on their position. Let's
look at how to instantiate an ArrayList of integers.
Code like below is permitted because of auto boxing. 5 is auto boxed into Integer object and
stored in ArrayList.
Add method (by default) adds the element at the end of the list.
ArrayList of String Example
Below example shows how to create and use a String ArrayList. ArrayList can have duplicates
(since List can have duplicates). size() method gets number of elements in the ArrayList.
contains(Object) method checks if an element is present in the arraylist.
interface Queue<E> extends Collection<E>
{
boolean offer(E paramE);
E remove();
E poll();
E element();
E peek();
}
public interface Iterator<E> {
boolean hasNext();
E next();
}
List<Integer> integers = new ArrayList<Integer>();
Integers.add(5);//new Integer(5)
List<String> arraylist = new ArrayList<String>();
Iterating around a list
Other ArrayList (List) methods
indexOf() function - returns index of element if element is found. Negative number otherwise.
get() function - get value at specified index.
remove() function
remove() function has two variations.
//adds at the end of list
arraylist.add("Sachin");//[Sachin]
//adds at the end of list
arraylist.add("Dravid");//[Sachin, Dravid]
//adds at the index 0
arraylist.add(0, "Ganguly");//[Ganguly, Sachin, Dravid]
//List allows duplicates - Sachin is present in the list twice
arraylist.add("Sachin");//[ Ganguly, Sachin, Dravid, Sachin]
System.out.println(arraylist.size());//4
System.out.println(arraylist.contains("Dravid"));//true
Iterator<String> arraylistIterator = arraylist
.iterator();
while (arraylistIterator.hasNext()) {
String str = arraylistIterator.next();
System.out.println(str);//Prints the 4 names in the list on separate
lines.
}
//example1 - value is present
System.out.println(arraylist.indexOf("Dravid"));//2
//example2 - value is not present
System.out.println(arraylist.indexOf("Bradman"));//-1
System.out.println(arraylist.get(1));//Sachin
Sorting Collections
List of Objects of a Custom Class
Consider the following class Cricketer.
Let's now try to sort a list containing objects of Cricketer class.
//Using the object as parameter
//Dravid is removed from the list
arraylist.remove("Dravid");//[Ganguly, Sachin, Sachin]
//Using index as argument.
//Object at index 1 (Sachin) is removed
arraylist.remove(1);//[Ganguly, Sachin]
List<String> numbers = new ArrayList<String>();
numbers.add("one");
numbers.add("two");
numbers.add("three");
numbers.add("four");
System.out.println(numbers);//[one, two, three, four]
//Strings - By Default - are sorted alphabetically
Collections.sort(numbers);
System.out.println(numbers);//[four, one, three, two]
class Cricketer{
int runs;
String name;
public Cricketer(String name, int runs) {
super();
this.name = name;
this.runs = runs;
}
@Override
public String toString() {
return name + " " + runs;
}
}
We get a compiler error since Cricketer class does not implement Comparable interface. We were
able to sort numbers in earlier example because String class implements Comparable. Let's make
the Cricketer class implement the Comparable Interface.
Now let's try to sort the cricketers.
Other option to sort collections is by creating a separate class which implements Comparator
interface. Example below:
List<Cricketer> cricketers = new ArrayList<Cricketer>();
cricketers.add(new Cricketer("Bradman", 9996));
cricketers.add(new Cricketer("Sachin", 14000));
cricketers.add(new Cricketer("Dravid", 12000));
cricketers.add(new Cricketer("Ponting", 11000));
System.out.println(cricketers);
//[Bradman 9996, Sachin 14000, Dravid 12000, Ponting 11000]
//Cricketer class does not implement Comparable Interface
//Collections.sort(cricketers); //COMPILER ERROR
class Cricketer implements Comparable<Cricketer> {
//OTHER CODE/PROGRAM same as previous
//compareTo takes an argument of the same type of the class
//compareTo returns -1 if this < that
// 1 if this > that
// 0 if this = that
@Override
public int compareTo(Cricketer that) {
if (this.runs > that.runs) {
return 1;
}
if (this.runs < that.runs) {
return -1;
}
return 0;
}
}
Collections.sort(cricketers);
System.out.println(cricketers);
//[Bradman 9996, Ponting 11000, Dravid 12000, Sachin 14000]
class DescendingSorter implements Comparator<Cricketer> {
//compareTo returns -1 if cricketer1 < cricketer2
Let's now try to sort the previous defined collection:
Convert List to Array
There are two ways. First is to use toArray(String) function. Example below. This creates an array
of String's
Other is to use toArray() function. Example below. This creates an array of Objects.
// 1 if cricketer1 > cricketer2
// 0 if cricketer1 = cricketer2
//Since we want to sort in descending order,
//we should return -1 when runs are more
@Override
public int compare(Cricketer cricketer1,
Cricketer cricketer2) {
if (cricketer1.runs > cricketer2.runs) {
return -1;
}
if (cricketer1.runs < cricketer2.runs) {
return 1;
}
return 0;
}
}
Collections
.sort(cricketers, new DescendingSorter());
System.out.println(cricketers);
//[Sachin 14000, Dravid 12000, Ponting 11000, Bradman 9996]
List<String> numbers1 = new ArrayList<String>();
numbers1.add("one");
numbers1.add("two");
numbers1.add("three");
numbers1.add("four");
String[] numbers1Array = new String[numbers1.size()];
numbers1Array = numbers1.toArray(numbers1Array);
System.out.println(Arrays.toString(numbers1Array));
//prints [one, two, three, four]
Object[] numbers1ObjArray = numbers1.toArray();
System.out.println(Arrays
.toString(numbers1ObjArray));
//[one, two, three, four]
Convert Array to List
Other List interface implementations
Other classes that implement List interface are Vector and LinkedList.
Vector
Vector has the same operations as an ArrayList. However, all methods in Vector are synchronized.
So, we can use Vector if we share a list between two threads and we would want to them
synchronized.
LinkedList
Linked List extends List and Queue.Other than operations exposed by the Queue interface,
LinkedList has the same operations as an ArrayList. However, the underlying implementation of
Linked List is different from that of an ArrayList. ArrayList uses an Array kind of structure to store
elements. So, inserting and deleting from an ArrayList are expensive operations. However, search
of an ArrayList is faster than LinkedList. LinkedList uses a linked representation. Each object holds
a link to the next element. Hence, insertion and deletion are faster than ArrayList. But searching
is slower.
Set Interface
HashSet, LinkedHashSet and TreeSet implement the Set interface. Let's look at examples of
these collection classes.
HashSet
HashSet implements set interface. Sets do not allow duplicates. HashSet does not support
ordering.
HashSet Example
Let's try to add Sachin to the Set now. Sachin is Duplicate. So will not be added. returns false.
String values[] = { "value1", "value2", "value3" };
List<String> valuesList = Arrays.asList(values);
System.out.println(valuesList);//[value1, value2, value3]
Set<String> hashset = new HashSet<String>();
hashset.add("Sachin");
System.out.println(hashset);//[Sachin]
hashset.add("Dravid");
System.out.println(hashset);//[Sachin, Dravid]
LinkedHashSet
LinkedHashSet implements set interface and exposes similar operations to a HashSet. Difference
is that LinkedHashSet maintains insertion order. When we iterate a LinkedHashSet, we would get
the elements back in the order in which they were inserted.
TreeSet
TreeSet implements Set, SortedSet and NavigableSet interfaces.TreeSet is similar to HashSet
except that it stores element's in Sorted Order.
Notice that the list is sorted after inserting Dravid.
Notice that the list is sorted after inserting Ganguly.
Objects that are inserted into a TreeSet should be comparable.
TreeSet - NavigableSet interface examples 1
TreeSet implements this interface. Let's look at an example with TreeSet. Note that elements in
TreeSet are sorted.
hashset.add("Sachin");//returns false since element is not added
System.out.println(hashset);//[Sachin, Dravid]
Set<String> treeSet = new TreeSet<String>();
treeSet.add("Sachin");
System.out.println(treeSet);//[Sachin]
//Alphabetical order
treeSet.add("Dravid");
System.out.println(treeSet);//[Dravid, Sachin]
treeSet.add("Ganguly");
System.out.println(treeSet);//[Dravid, Ganguly, Sachin]
//Sachin is Duplicate. So will not be added. returns false.
treeSet.add("Sachin");//returns false since element is not added
System.out.println(treeSet);//[Dravid, Ganguly, Sachin]
TreeSet<Integer> numbersTreeSet = new TreeSet<Integer>();
numbersTreeSet.add(55);
numbersTreeSet.add(25);
numbersTreeSet.add(35);
numbersTreeSet.add(5);
numbersTreeSet.add(45);
NavigableSet interface has following methods. Lower method finds the highest element lower
than specified element. Floor method finds the highest element lower than or equal to specified
element. Corresponding methods for finding lowest number higher than specified element are
higher and ceiling. A few examples using the Set created earlier below.
NavigableSet subSet,headSet,tailSet Methods in TreeSet
All the three methods - subSet,headSet,tailSet - are inclusive with Lower Limit and NOT inclusive
with higher limit. In the sub set below, Lower Limit is inclusive - 25 included. Higher limit is not
inclusive - 55 excluded.
In the sub set below, Higher limit not inclusive - 55 excluded.
//Find the highest number which is lower than 25
System.out.println(numbersTreeSet.lower(25));//5
//Find the highest number which is lower than or equal to 25
System.out.println(numbersTreeSet.floor(25));//25
//Find the lowest number higher than 25
System.out.println(numbersTreeSet.higher(25));//35
//Find the lowest number higher than or equal to 25
System.out.println(numbersTreeSet.ceiling(25));//25
TreeSet<Integer> exampleTreeSet = new TreeSet<Integer>();
exampleTreeSet.add(55);
exampleTreeSet.add(25);
exampleTreeSet.add(105);
exampleTreeSet.add(35);
exampleTreeSet.add(5);
System.out.println(exampleTreeSet);//[5, 25, 35, 55, 105]
//Get sub set with values >=25 and <55
SortedSet<Integer> subTreeSet = exampleTreeSet
.subSet(25, 55);
System.out.println(subTreeSet);//[25, 35]
//Get sub set with values <55
SortedSet<Integer> headTreeSet = exampleTreeSet
.headSet(55);
System.out.println(headTreeSet);//[5, 25, 35]
//Get sub set with values >=35
SortedSet<Integer> tailTreeSet = exampleTreeSet
.tailSet(35);
All the sub set methods - subSet,headSet,tailSet - return dynamic sub sets. When original set is
modified (addition or deletion), corresponding changes can affect the sub sets as well.
Let's now insert a value 30 into the exampleTreeSet. Remember that subTreeSet, headTreeSet,
tailTreeSet are sub sets of exampleTreeSet.
30 is in the range of subTreeSet and headTreeSet. So, it is printed as part of exampleTreeSet,
subTreeSet and headTreeSet.
System.out.println(tailTreeSet);//[35, 55, 105]
//In the sub set, Lower limit inclusive - 35 included.
//Get sub set with value >=25 and <=55 (both inclusive parameters - true)
SortedSet<Integer> subTreeSetIncl = exampleTreeSet
.subSet(25, true, 55, true);
System.out.println(subTreeSetIncl);//[25, 35, 55]
//Get sub set with value >25 and <55 (both inclusive parameters - false)
SortedSet<Integer> subTreeSetNotIncl = exampleTreeSet
.subSet(25, false, 55, false);
System.out.println(subTreeSetNotIncl);//[35]
//Get sub set with values <=55. Inclusive set to true.
SortedSet<Integer> headTreeSetIncl = exampleTreeSet
.headSet(55, true);
System.out.println(headTreeSetIncl);//[5, 25, 35, 55]
//Get sub set with values >35. Inclusive set to false.
SortedSet<Integer> tailTreeSetNotIncl = exampleTreeSet
.tailSet(35, false);
System.out.println(tailTreeSetNotIncl);//[55, 105]
System.out.println(exampleTreeSet);//[5, 25, 35, 55, 105]
System.out.println(subTreeSet);//[25, 35]
System.out.println(headTreeSet);//[5, 25, 35]
System.out.println(tailTreeSet);//[35, 55, 105]
exampleTreeSet.add(30);
System.out.println(exampleTreeSet);//[5, 25, 30, 35, 55, 105]
System.out.println(subTreeSet);//[25, 30, 35]
System.out.println(headTreeSet);//[5, 30, 25, 35]
System.out.println(tailTreeSet);//[35, 55, 105]
65 is printed as part of exampleTreeSet and tailTreeSet. Key thing to remember is that all the sub
sets are dynamic. If you modify the original set,the sub set might be affected.
TreeSet - NavigableSet interface methods - pollFirst, pollLast and more
pollFirst returns the first element and removes it from the set.
pollLast returns the last element and removes it from the set.
Map Interface
Let's take a look at different implementations of the Map interface.
HashMap
HashMap implements Map interface , there by supporting key value pairs.
HashMap Example
//Let's now add 65 to the set
exampleTreeSet.add(65);
System.out.println(exampleTreeSet);//[5, 25, 30, 35, 55, 65, 105]
System.out.println(subTreeSet);//[25, 30, 35]
System.out.println(headTreeSet);//[5, 30, 25, 35]
System.out.println(tailTreeSet);//[35, 55, 65, 105]
TreeSet<Integer> treeSetOrig = new TreeSet<Integer>();
treeSetOrig.add(55);
treeSetOrig.add(25);
treeSetOrig.add(35);
treeSetOrig.add(5);
System.out.println(treeSetOrig);//[5, 25, 35, 55]
//descendingSet method returns the tree set in reverse order
TreeSet<Integer> treeSetDesc = (TreeSet<Integer>) treeSetOrig
.descendingSet();
System.out.println(treeSetDesc);//[55, 35, 25, 5]
System.out.println(treeSetOrig);//[5, 25, 35, 55]
System.out.println(treeSetOrig.pollFirst());//5
System.out.println(treeSetOrig);//[25, 35, 55]
//In above example element 5 is removed from the set and also removed from the
tree set.
System.out.println(treeSetOrig);//[25, 35, 55]
System.out.println(treeSetOrig.pollLast());//55
System.out.println(treeSetOrig);//[25, 35]
Hash Map Methods
get method gets the value of the matching key.
If existing key is reused, it would replace existing value with the new value passed in.
TreeMap
TreeMap is similar to HashMap except that it stores keys in sorted order. It implements
NavigableMap interface and SortedMap interfaces along with the Map interface.
We will now insert a Cricketer with key dravid. In sorted order,dravid comes before sachin. So, the
value with key dravid is inserted at the start of the Map.
Map<String, Cricketer> hashmap = new HashMap<String, Cricketer>();
hashmap.put("sachin",
new Cricketer("Sachin", 14000));
hashmap.put("dravid",
new Cricketer("Dravid", 12000));
hashmap.put("ponting", new Cricketer("Ponting",
11500));
hashmap.put("bradman", new Cricketer("Bradman",
9996));
System.out.println(hashmap.get("ponting"));//Ponting 11500
//if key is not found, returns null.
System.out.println(hashmap.get("lara"));//null
//In the example below, an entry with key "ponting" is already present.
//Runs are updated to 11800.
hashmap.put("ponting", new Cricketer("Ponting",
11800));
//gets the recently updated value
System.out.println(hashmap.get("ponting"));//Ponting 11800
Map<String, Cricketer> treemap = new TreeMap<String, Cricketer>();
treemap.put("sachin",
new Cricketer("Sachin", 14000));
System.out.println(treemap);
//{sachin=Sachin 14000}
treemap.put("dravid",
new Cricketer("Dravid", 12000));
System.out.println(treemap);
//{dravid=Dravid 12000, sachin=Sachin 14000}
We will now insert a Cricketer with key ponting. In sorted order, ponting fits in between dravid
and sachin.
NavigableMap Interface Examples (TreeMap) Set I
Let's look at an example with TreeMap. Note that keys in TreeMap are sorted.
lowerKey method finds the highest key lower than specified key. floorKey method finds the
highest key lower than or equal to specified key. Corresponding methods for finding lowest key
higher than specified key are higher and ceiling. A few examples using the Map created earlier
below.
treemap.put("ponting", new Cricketer("Ponting",
11500));
System.out.println(treemap);
//{dravid=Dravid 12000, ponting=Ponting 11500, sachin=Sachin 14000}
treemap.put("bradman", new Cricketer("Bradman",
9996));
System.out.println(treemap);
//{bradman=Bradman 9996, dravid=Dravid 12000, ponting=Ponting 11500,
sachin=Sachin 14000}
TreeMap<Integer, Cricketer> numbersTreeMap = new TreeMap<Integer, Cricketer>
();
numbersTreeMap.put(55, new Cricketer("Sachin",
14000));
numbersTreeMap.put(25, new Cricketer("Dravid",
12000));
numbersTreeMap.put(35, new Cricketer("Ponting",
12000));
numbersTreeMap.put(5,
new Cricketer("Bradman", 9996));
numbersTreeMap
.put(45, new Cricketer("Lara", 10000));
//Find the highest key which is lower than 25
System.out.println(numbersTreeMap.lowerKey(25));//5
//Find the highest key which is lower than or equal to 25
System.out.println(numbersTreeMap.floorKey(25));//25
//Find the lowest key higher than 25
System.out.println(numbersTreeMap.higherKey(25));//35
//Find the lowest key higher than or equal to 25
System.out.println(numbersTreeMap.ceilingKey(25));//25
NavigableMap Interface Examples (TreeMap) Set II
Methods similar to subSet,headSet,tailSet (of TreeSet) are available in TreeMap as well. They are
called subMap, headMap, tailMap.They have the similar signatures and results as the
corresponding TreeSet Methods. They are inclusive with Lower Limit and NOT inclusive with
higher limit - unless the (optional) inclusive flag is passed. The resultant sub map's are dynamic. If
original map get modified, the sub map might be affected as well.
NavigableMap Interface Examples (TreeMap) Set III
Consider the next set of method examples below:
descendingMap method returns the tree set in reverse order.
TreeMap<Integer, Cricketer> exampleTreeMap = new TreeMap<Integer, Cricketer>
();
exampleTreeMap.put(55, new Cricketer("Sachin",
14000));
exampleTreeMap.put(25, new Cricketer("Dravid",
12000));
exampleTreeMap.put(5,
new Cricketer("Bradman", 9996));
exampleTreeMap
.put(45, new Cricketer("Lara", 10000));
//Lower limit (5) inclusive, Uppper Limit(25) NOT inclusive
System.out.println(exampleTreeMap.subMap(5, 25));//{5=Bradman 9996}
System.out.println(exampleTreeMap.headMap(30));
//{5=Bradman 9996, 25=Dravid 12000}
System.out.println(exampleTreeMap.tailMap(25));
//{25=Dravid 12000, 45=Lara 10000, 55=Sachin 14000}
TreeMap<Integer, Cricketer> treeMapOrig = new TreeMap<Integer, Cricketer>();
treeMapOrig.put(55, new Cricketer("Sachin", 14000));
treeMapOrig.put(25, new Cricketer("Dravid", 12000));
treeMapOrig.put(5, new Cricketer("Bradman", 9996));
treeMapOrig.put(45, new Cricketer("Lara", 10000));
System.out.println(treeMapOrig);
//{5=Bradman 9996, 25=Dravid 12000, 45=Lara 10000, 55=Sachin 14000}
NavigableMap<Integer, Cricketer> treeMapDesc = treeMapOrig
.descendingMap();
System.out.println(treeMapDesc);
//{55=Sachin 14000, 45=Lara 10000, 25=Dravid 12000, 5=Bradman 9996}
pollFirstEntry returns the first entry in the map and removes it from the map.
pollLastEntry returns the last entry from the map and removes it from the map.
PriorityQueue
PriorityQueue implements the Queue interface.
PriorityQueue Example
Adding an element into priority queue - offer method
Peek method examples
System.out.println(treeMapOrig);
//{5=Bradman 9996, 25=Dravid 12000, 45=Lara 10000, 55=Sachin 14000}
System.out.println(treeMapOrig.pollFirstEntry());//5=Bradman 9996
System.out.println(treeMapOrig);
//{25=Dravid 12000, 45=Lara 10000, 55=Sachin 14000}
//In above example element 5 is removed from the set and also removed from the
tree set.
System.out.println(treeMapOrig);
//{25=Dravid 12000, 45=Lara 10000, 55=Sachin 14000}
System.out.println(treeMapOrig.pollLastEntry());//55=Sachin 14000
System.out.println(treeMapOrig);
//{25=Dravid 12000, 45=Lara 10000}
//Using default constructor - uses natural ordering of numbers
//Smaller numbers have higher priority
PriorityQueue<Integer> priorityQueue = new PriorityQueue<Integer>();
priorityQueue.offer(24);
priorityQueue.offer(15);
priorityQueue.offer(9);
priorityQueue.offer(45);
System.out.println(priorityQueue);//[9, 24, 15, 45]
//peek method get the element with highest priority.
System.out.println(priorityQueue.peek());//9
//peek method does not change the queue
System.out.println(priorityQueue);//[9, 24, 15, 45]
//poll method gets the element with highest priority.
System.out.println(priorityQueue.poll());//9
//peek method removes the highest priority element from the queue.
Priority Queue and Comparator
We can create priority queue using a comparator class i.e. custom defined priority.
Collections static methods
Generics
Generics are used to create Generic Classes and Generic methods which can work with
different Types(Classes).
Need for Generics , Example
Consider the class below:
System.out.println(priorityQueue);//[24, 15, 45]
//This comparator gives high priority to the biggest number.
Comparator reverseComparator = new Comparator<Integer>() {
public int compare(Integer paramT1,
Integer paramT2) {
return paramT2 - paramT1;
}
};
PriorityQueue<Integer> priorityQueueDesc = new PriorityQueue<Integer>(
20, reverseComparator);
priorityQueueDesc.offer(24);
priorityQueueDesc.offer(15);
priorityQueueDesc.offer(9);
priorityQueueDesc.offer(45);
//45 is the largest element. Our custom comparator gives priority to highest
number.
System.out.println(priorityQueueDesc.peek());//45
static int binarySearch(List, key) //Can be used only on sorted list
static int binarySearch(List, key, Comparator)
static void reverse(List)//Reverse the order of elements in a List.
static Comparator reverseOrder();
//Return a Comparator that sorts the reverse of the collection current sort
sequence.
static void sort(List)
static void sort(List, Comparator)
MyList can be used to store a list of Strings only.
To store integers, we need to create a new class. This is problem that Generics solve. Instead of
hard-coding String class as the only type the class can work with, we make the class type a
parameter to the class.
Generics Example
Let's replace String with T and create a new class.
Note the declaration of class:
class MyList {
private List<String> values;
void add(String value) {
values.add(value);
}
void remove(String value) {
values.remove(value);
}
}
MyList myList = new MyList();
myList.add("Value 1");
myList.add("Value 2");
class MyListGeneric<T> {
private List<T> values;
void add(T value) {
values.add(value);
}
void remove(T value) {
values.remove(value);
}
T get(int index) {
return values.get(index);
}
}
class MyListGeneric<T>
Instead of T, We can use any valid identifier If a generic is declared as part of class declaration, it
can be used any where a type can be used in a class - method (return type or argument), member
variable etc. For Example: See how T is used as a parameter and return type in the class
MyListGeneric. Now the MyListGeneric class can be used to create a list of Integers or a list of
Strings
Generics Restrictions
In MyListGeneric, Type T is defined as part of class declaration. Any Java Type can be used a type
for this class. If we would want to restrict the types allowed for a Generic Type, we can use a
Generic Restrictions. Consider the example class below:
In declaration of the class, we specified a constraint "T extends Number". We can use the class
MyListRestricted with any class extending Number - Float, Integer, Double etc.
String not valid substitute for constraint "T extends Number".
MyListGeneric<String> myListString = new MyListGeneric<String>();
myListString.add("Value 1");
myListString.add("Value 2");
MyListGeneric<Integer> myListInteger = new MyListGeneric<Integer>();
myListInteger.add(1);
myListInteger.add(2);
class MyListRestricted<T extends Number> {
private List<T> values;
void add(T value) {
values.add(value);
}
void remove(T value) {
values.remove(value);
}
T get(int index) {
return values.get(index);
}
}
MyListRestricted<Integer> restrictedListInteger = new
MyListRestricted<Integer>();
restrictedListInteger.add(1);
restrictedListInteger.add(2);
Generic Method Example
A generic type can be declared as part of method declaration as well. Then the generic type can
be used anywhere in the method (return type, parameter type, local or block variable type).
Consider the method below:
The method can now be called with any Class type extend Number.
Generics and Collections Example 1
Consider the classes below:
Let's create couple of Arrays and Lists as shown below:
Let's create a couple of static methods as shown below:
//MyListRestricted<String> restrictedStringList =
//new MyListRestricted<String>();//COMPILER ERROR
static <X extends Number> X doSomething(X number){
X result = number;
//do something with result
return result;
}
Integer i = 5;
Integer k = doSomething(i);
class Animal {
}
class Dog extends Animal {
}
Animal[] animalsArray = { new Animal(), new Dog() };
Dog[] dogsArray = { new Dog(), new Dog() };
List<Animal> animalsList = Arrays.asList(animalsArray);
List<Dog> dogsList = Arrays.asList(dogsArray);
Array method can be called with Animal[] or Dog[]
List method works with List. Gives compilation error with List.
Summary : List not compatible with List even thought Dog extends Animal. However, Dog[] is
compatible with Animal[].
Generics and Collections Example 2 - extends
Consider the methods below:
Method declared with List<? extends Animal> compiles with both List and List
Generics and Collections Example 3 - Super
Method declared with List<? super Dog> compiles with both List and List.
static void doSomethingArray(Animal[] animals) {
//do Something with Animals
}
static void doSomethingList(List<Animal> animals) {
//do Something with Animals
}
doSomethingArray(animalsArray);
doSomethingArray(dogsArray);
doSomethingList(animalsList);
//List<Dog> not compatible with List<Animal>
//doSomethingList(dogsList);//COMPILER ERROR
static void doSomethingListModified(List<? extends Animal> animals) {
//Adding an element into a list declared with ? is prohibited.
//animals.add(new Animal());//COMPILER ERROR!
//animals.add(new Dog());//COMPILER ERROR!
}
doSomethingListModified(animalsList);
doSomethingListModified(dogsList);
static void doSomethingListModifiedSuper(List<? super Dog> animals) {
//Adding an element into a list declared with ? is prohibited.
//animals.add(new Animal());//COMPILER ERROR!
//animals.add(new Dog());//COMPILER ERROR!
}
List of any super class of Dog is fine. List of any Subclass of Dog is not valid parameter.
Generics and Collections Example 4 , extends with interface
Below method can be called with a List declared with any type implementing the interface
Serializable.
Generics and Collections , Few more Examples and Rules
A method declared with List can only be called with a List declared with type Object. None of the
other classes are valid. A method declared with List<?> can be called with a List of any type. //A
method declared with List<? extends Object> can be called with a List of any type - since all
classes are sub classes of Object. ? can only be used in Declaring a type. Cannot be used as part
of definition.
Generics and Collection , Compatibility with old code
Consider the method below: It is declared to accept a Generic ArrayList. The method adds a string
to the arraylist.
Consider the code below:
doSomethingListModifiedSuper(animalsList);
doSomethingListModifiedSuper(dogsList);
static void doSomethingListInterface(List<? extends Serializable> animals)
{
//Adding an element into a list declared with ? is prohibited.
//animals.add(new Animal());//COMPILER ERROR!
//animals.add(new Dog());//COMPILER ERROR!
}
List<? extends Animal> listAnimals = new ArrayList<Dog>(); //COMPILES
//List<?> genericList = new ArrayList<? extends Animal>(); //COMPILER ERROR
static void addElement(ArrayList something){
something.add(new String("String"));
}
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(5);
numbers.add(6);
addElement(numbers);
We are passing a ArrayList to a method accepting ArrayList as parameter. We are trying to a add a
string to it as well, inside the method. Compiling this class would give a warning: javac gives
warning because multiplyNumbersBy2(ArrayList) is invoked with a Specific ArrayList and in the
method addElement an element is added to ArrayList.
To get more details run javac specifying the parameter Xlint:unchecked.
Files
Let us first look at the File class which helps us to create and delete files and directories. File
class cannot be used to modify the content of a file.
File Class
Create a File Object
File basic Methods
Check if the file exists.
if file does not exist creates it and returns true. If file exists, returns false.
Getting full path of file.
Renaming a file
//com/rithus/generics/GenericsExamples.java uses unchecked or unsafe
operations.
//Recompile with -Xlint:unchecked for details.
javac -Xlint:unchecked com/rithus/generics/GenericsExamples.java
//com/rithus/generics/GenericsExamples.java:21: warning: [unchecked]
//unchecked call to add(E) as a member of the raw type java.util.ArrayList
//something.add(new String("String"));
// ^
File file = new File("FileName.txt");
System.out.println(file.exists());
System.out.println(file.createNewFile());
System.out.println(file.getAbsolutePath());
System.out.println(file.isFile());//true
System.out.println(file.isDirectory());//false
File Class - Directory
A File class in Java represents a file and directory.
Print full directory path
This does not create the actual file.
Actual file is created when we invoke createNewFile method.
Print the files and directories present in the folder.
Creating a directory
Creating a file in a new directory
Read and write from a File
File fileWithNewName = new File("NewFileName.txt");
file.renameTo(fileWithNewName);
//There is no method file.renameTo("NewFileName.txt");
File directory = new File("src/com/rithus");
System.out.println(directory.getAbsolutePath());
System.out.println(directory.isDirectory());//true
File fileInDir = new File(directory,"NewFileInDirectory.txt");
System.out.println(fileInDir.createNewFile()); //true - First Time
System.out.println(Arrays.toString(directory.list()));
File newDirectory = new File("newfolder");
System.out.println(newDirectory.mkdir());//true - First Time
File notExistingDirectory = new File("notexisting");
File newFile = new File(notExistingDirectory,"newFile");
//Will throw Exception if uncommented: No such file or directory
//newFile.createNewFile();
System.out.println(newDirectory.mkdir());//true - First Time
Implementations of Writer and Reader abstract classes help us to write and read (content of)
files. Writer methods - flush, close, append (text) Reader methods - read, close (NO FLUSH) Writer
implementations - FileWriter,BufferedWriter,PrintWriter Reader implementations -
FileReader,BufferedReader
FileWriter and FileReader
FileWriter and FileReader provide basic file writing and reading operations. Let's write an
example to write and read from a file using FileReader and FileWriter.
FileWriter Class
We can write to a file using FileWriter class.
Write a string to a file using FileWriter
FileWriter Constructors
FileWriter Constructors can accept file(File) or the path to file (String) as argument. When a writer
object is created, it creates the file - if it does not exist.
FileReader Class
File Reader can be used to read entire content from a file at one go.
Read from file using FileReader
//FileWriter helps to write stuff into the file
FileWriter fileWriter = new FileWriter(file);
fileWriter.write("How are you doing?");
//Always flush before close. Writing to file uses Buffering.
fileWriter.flush();
fileWriter.close();
FileWriter fileWriter2 = new FileWriter("FileName.txt");
fileWriter2.write("How are you doing Buddy?");
//Always flush before close. Writing to file uses Buffering.
fileWriter2.flush();
fileWriter2.close();
FileReader Constructors
FileReader constructors can accept file(File) or the path to file (String) as argument.
BufferedWriter and BufferedReader
BufferedWriter and BufferedReader provide better buffering in addition to basic file writing
and reading operations. For example, instead of reading the entire file, we can read a file line
by line. Let's write an example to write and read from a file using BufferedReader and
BufferedWriter.
BufferedWriter Class
BufferedWriter class helps writing to a class with better buffering than FileWriter.
BufferedWriter Constructors
BufferedWriter Constructors only accept another Writer as argument
Using BufferedWriter class
FileReader fileReader = new FileReader(file);
char[] temp = new char[25];
//fileReader reads entire file and stores it into temp
System.out.println(fileReader.read(temp));//18 - No of characters Read from
file
System.out.println(Arrays.toString(temp));//output below
//[H, o, w, , a, r, e, , y, o, u, , d, o, i, n, g, ?, , , , , ,]
fileReader.close();//Always close anything you opened:)
FileReader fileReader2 = new FileReader("FileName.txt");
System.out.println(fileReader2.read(temp));//24
System.out.println(Arrays.toString(temp));//output below
FileWriter fileWriter3 = new FileWriter("BufferedFileName.txt");
BufferedWriter bufferedWriter = new BufferedWriter(fileWriter3);
bufferedWriter.write("How are you doing Buddy?");
bufferedWriter.newLine();
bufferedWriter.write("I'm Doing Fine");
//Always flush before close. Writing to file uses Buffering.
bufferedWriter.flush();
bufferedWriter.close();
fileWriter3.close();
BufferedReader Class
BufferedReader helps to read the file line by line
BufferedReader Constructors
BufferedReader Constructors only accept another Reader as argument.
BufferedReader , Reading a file
PrintWriter
PrintWriter provides advanced methods to write formatted text to the file. It supports printf
function.
PrintWriter constructors
PrintWriter constructors supports varied kinds of arguments , File, String (File Path) and Writer.
PrintWriter , Write to a file
Other than write function you can use format, printf, print, println functions to write to
PrintWriter file.
Reading the file created using BufferedReader
FileReader fileReader3 = new FileReader("BufferedFileName.txt");
BufferedReader bufferedReader = new BufferedReader(fileReader3);
String line;
//readLine returns null when reading the file is completed.
while((line=bufferedReader.readLine()) != null){
System.out.println(line);
}
PrintWriter printWriter = new PrintWriter("PrintWriterFileName.txt");
//writes "My Name" to the file
printWriter.format("%15s", "My Name");
printWriter.println(); //New Line
printWriter.println("Some Text");
//writes "Formatted Number: 4.50000" to the file
printWriter.printf("Formatted Number: %5.5f", 4.5);
printWriter.flush();//Always flush a writer
printWriter.close();
Serialization
Serialization helps us to save and retrieve the state of an object.
Serialization and De-Serialization - Important methods
Serialization => Convert object state to some internal object representation. De-Serialization =>
The reverse. Convert internal representation to object.
Two important methods 1.ObjectOutputStream.writeObject() // serialize and write to file
2.ObjectInputStream.readObject() // read from file and deserialize
Implementing Serializable Interface
To serialize an object it should implement Serializable interface. In the example below, Rectangle
class implements Serializable interface. Note that Serializable interface does not declare any
methods to be implemented.
Serializing an object - Example
Below example shows how an instance of an object can be serialized. We are creating a new
Rectangle object and serializing it to a file Rectangle.ser.
FileReader fileReader4 = new FileReader("PrintWriterFileName.txt");
BufferedReader bufferedReader2 = new BufferedReader(fileReader4);
String line2;
//readLine returns null when reading the file is completed.
while((line2=bufferedReader2.readLine()) != null){
System.out.println(line2);
}
class Rectangle implements Serializable {
public Rectangle(int length, int breadth) {
this.length = length;
this.breadth = breadth;
area = length * breadth;
}
int length;
int breadth;
int area;
}
FileOutputStream fileStream = new FileOutputStream("Rectangle.ser");
ObjectOutputStream objectStream = new ObjectOutputStream(fileStream);
objectStream.writeObject(new Rectangle(5, 6));
objectStream.close();
De-serializing an object , Example
Below example show how a object can be deserialized from a serialized file. A rectangle object is
deserialized from the file Rectangle.ser
Serialization , Transient variables
Area in the previous example is a calculated value. It is unnecessary to serialize and deserialize.
We can calculate it when needed. In this situation, we can make the variable transient. Transient
variables are not serialized. (transient int area;)
If you run the program again, you would get following output
Note that the value of rectangle.area is set to 0. Variable area is marked transient. So, it is not
stored into the serialized file. And when de-serialization happens area value is set to default value
i.e. 0.
Serialization , readObject method
FileInputStream fileInputStream = new FileInputStream("Rectangle.ser");
ObjectInputStream objectInputStream = new ObjectInputStream(
fileInputStream);
Rectangle rectangle = (Rectangle) objectInputStream.readObject();
objectInputStream.close();
System.out.println(rectangle.length);// 5
System.out.println(rectangle.breadth);// 6
System.out.println(rectangle.area);// 30
//Modified Rectangle class
class Rectangle implements Serializable {
public Rectangle(int length, int breadth) {
this.length = length;
this.breadth = breadth;
area = length * breadth;
}
int length;
int breadth;
transient int area;
}
System.out.println(rectangle.length);// 5
System.out.println(rectangle.breadth);// 6
System.out.println(rectangle.area);// 0
We need to recalculate the area when the rectangle object is deserialized. This can be achieved by
adding readObject method to Rectangle class. In addition to whatever java does usually while
deserializing, we can add custom code for the object.
When an object of Rectangle class is de-serialized, Java invokes the readObject method. The area
is recalculated in this method. If we run the program again, we get the calculated area value back.
Remember that area is not part of the serialized file. It is re-calculated in the readObject method.
Serialization , writeObject method
We can also write custom code when serializing the object by adding the writeObject method to
Rectange class. writeObject method accepts an ObjectOutputStream as input parameter. To the
writeObject method we can add the custom code that we want to run during Serialization.
If you run the above program again, you would get following output
Serializing an Object chain
Objects of one class might contain objects of other classes. When serializing and de-serializing,
we might need to serialize and de-serialize entire object chain. Look at the class below. An object
of class House contains an object of class Wall.
private void readObject(ObjectInputStream is) throws IOException,
ClassNotFoundException {
// Do whatever java does usually when de-serialization is called
is.defaultReadObject();
// In addition, calculate area also
area = this.length * this.breadth;
}
System.out.println(rectangle.length);// 5
System.out.println(rectangle.breadth);// 6
System.out.println(rectangle.area);// 30
private void writeObject(ObjectOutputStream os) throws IOException {
//Do whatever java does usually when serialization is called
os.defaultWriteObject();
}
System.out.println(rectangle.length);//5
System.out.println(rectangle.breadth);//6
System.out.println(rectangle.area);//30
class House implements Serializable {
public House(int number) {
super();
this.number = number;
House implements Serializable where Wall doesn't.
Let's run this example program:
This is because Wall is not serializable. Two solutions are possible. Make Wall transient => wall will
not be serialized. This causes the wall object state to be lost. Make Wall implement Serializable =>
wall object will also be serialized and the state of wall object along with the house will be stored.
Serialization Program 1: Make Wall transient
}
Wall wall;
int number;
}
class Wall{
int length;
int breadth;
int color;
}
public class SerializationExamples2 {
public static void main(String[] args)
throws IOException {
FileOutputStream fileStream = new FileOutputStream(
"House.ser");
ObjectOutputStream objectStream = new ObjectOutputStream(
fileStream);
House house = new House(10);
house.wall = new Wall();
//Exception in thread "main" java.io.NotSerializableException:
//com.in28minutes.serialization.Wall
objectStream.writeObject(house);
objectStream.close();
}
}
//Output:
//Exception in thread "main" java.io.NotSerializableException:
com.in28minutes.serialization.Wall
// at java.io.ObjectOutputStream.writeObject0(Unknown Source)
// at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
Serialization Program 2: Make Wall implement Serializable
With both these programs, earlier main method would run without throwing an exception.
If you try de-serializing, In Example2, state of wall object is retained whereas in Example1, state of
wall object is lost.
Serialization and Initialization
When a class is Serialized, initialization (constructor's, initializer's) does not take place. The state
of the object is retained as it is.
Serialization and inheritance
However in the case of inheritance ( a sub class and super class relationship), interesting things
happen. Let's consider the example code below:
Hero class extends Actor and Hero class implements Serializable. However, Actor class does not
implement Serializable.
class House implements Serializable {
public House(int number) {
super();
this.number = number;
}
transient Wall wall;
int number;
}
class Wall implements Serializable {
int length;
int breadth;
int color;
}
class Actor {
String name;
public Actor(String name) {
super();
this.name = name;
}
Actor() {
name = "Default";
}
}
Let's run the code below:
Code executes successfully but after de-serialization, the values of super class instance variables
are not retained. They are set to their initial values. When subclass is serializable and superclass
is not, the state of subclass variables is retained. However, for the super class, initialization
(constructors and initializers) happens again.
Serialization and Static Variables
Static Variables are not part of the object. They are not serialized.
Threads
Threads allow Java code to run in parallel. Let's first understand the need for threading and
class Hero extends Actor implements Serializable {
String danceType;
public Hero(String name, String danceType) {
super(name);
this.danceType = danceType;
}
Hero() {
danceType = "Default";
}
}
FileOutputStream fileStream = new FileOutputStream("Hero.ser");
ObjectOutputStream objectStream = new ObjectOutputStream(fileStream);
Hero hero = new Hero("Hero1", "Ganganam");
// Before -> DanceType:Ganganam Name:Hero1
System.out.println("Before -> DanceType:" + hero.danceType + " Name:"
+ hero.name);
objectStream.writeObject(hero);
objectStream.close();
FileInputStream fileInputStream = new FileInputStream("Hero.ser");
ObjectInputStream objectInputStream = new ObjectInputStream(
fileInputStream);
hero = (Hero) objectInputStream.readObject();
objectInputStream.close();
// After -> DanceType:Ganganam Name:Default
System.out.println("After -> DanceType:" + hero.danceType + " Name:"
+ hero.name);
then look into how to create a thread and what is synchronization?
Need for Threads
We are creating a Cricket Statistics Application. Let's say the steps involved in the application are
STEP I: Download and Store Bowling Statistics => 60 Minutes STEP II: Download and Store Batting
Statistics => 60 Minutes STEP III: Download and Store Fielding Statistics => 15 Minutes STEP IV:
Merge and Analyze => 25 Minutes Steps I, II and III are independent and can be run in parallel to
each other. Run individually this program takes 160 minutes. We would want to run this program
in lesser time. Threads can be a solution to this problem. Threads allow us to run STEP I, II and III
in parallel and run Step IV when all Steps I, II and III are completed.
Need for Threads Example
Below example shows the way we would write code usually , without using Threads.
downloadAndStoreBowlingStatistics starts only after downloadAndStoreBattingStatistics
completes execution. downloadAndStoreFieldingStatistics starts only after
downloadAndStoreBowlingStatistics completes execution. What if I want to run them in parallel
without waiting for the others to complete? This is where Threads come into picture.
Creating a Thread Class
Creating a Thread class in Java can be done in two ways. Extending Thread class and
implementing Runnable interface. Let's create the BattingStatisticsThread extending Thread class
and BowlingStatisticsThread implementing Runnable interface.
Creating a Thread By Extending Thread class
Thread class can be created by extending Thread class and implementing the public void run()
method. Look at the example below: A dummy implementation for BattingStatistics is provided
which counts from 1 to 1000.
ThreadExamples example = new ThreadExamples();
example.downloadAndStoreBattingStatistics();
example.downloadAndStoreBowlingStatistics();
example.downloadAndStoreFieldingStatistics();
example.mergeAndAnalyze();
class BattingStatisticsThread extends Thread {
//run method without parameters
public void run() {
for (int i = 0; i < 1000; i++)
System.out
.println("Running Batting Statistics Thread "
+ i);
}
}
Creating a Thread by Implementing Runnable interface
Thread class can also be created by implementing Runnable interface and implementing the
method declared in Runnable interface Òpublic void run()Ó. Example below shows the Batting
Statistics Thread implemented by implementing Runnable interface.
Running a Thread
Running a Thread in Java is slightly different based on the approach used to create the thread.
Thread created Extending Thread class
When using inheritance, An object of the thread needs be created and start() method on the
thread needs to be called. Remember that the method that needs to be called is not run() but it is
start().
Thread created implementing RunnableInterface.
Three steps involved.
1.
Create an object of the BowlingStatisticsThread(class implementing Runnable).
2.
Create a Thread object with the earlier object as constructor argument.
3.
Call the start method on the thread.
Thread Example , Complete Program
Let's consider the complete example using all the snippets of code created above.
class BowlingStatisticsThread implements Runnable {
//run method without parameters
public void run() {
for (int i = 0; i < 1000; i++)
System.out
.println("Running Bowling Statistics Thread "
+ i);
}
}
BattingStatisticsThread battingThread1 = new BattingStatisticsThread();
battingThread1.start();
BowlingStatisticsThread battingInterfaceImpl = new BowlingStatisticsThread();
Thread battingThread2 = new Thread(
battingInterfaceImpl);
battingThread2.start();
public class ThreadExamples {
public static void main(String[] args) {
class BattingStatisticsThread extends Thread {
Output:
// run method without parameters
public void run() {
for (int i = 0; i < 1000; i++)
System.out.println("Running Batting Statistics Thread " + i);
}
}
class BowlingStatisticsThread implements Runnable {
// run method without parameters
public void run() {
for (int i = 0; i < 1000; i++)
System.out.println("Running Bowling Statistics Thread " + i);
}
}
BattingStatisticsThread battingThread1 = new BattingStatisticsThread();
battingThread1.start();
BowlingStatisticsThread battingInterfaceImpl = new BowlingStatisticsThread();
Thread battingThread2 = new Thread(battingInterfaceImpl);
battingThread2.start();
}
}
Running Batting Statistics Thread 0
Running Batting Statistics Thread 1
..
..
Running Batting Statistics Thread 10
Running Bowling Statistics Thread 0
..
..
Running Bowling Statistics Thread 948
Running Bowling Statistics Thread 949
Running Batting Statistics Thread 11
Running Batting Statistics Thread 12
..
..
Running Batting Statistics Thread 383
Running Batting Statistics Thread 384
Running Bowling Statistics Thread 950
Running Bowling Statistics Thread 951
..
Running Bowling Statistics Thread 998
Running Bowling Statistics Thread 999
Discussion about Thread Example
Above output shows sample execution of the thread. The output will not be the same with every
run. We can notice that Batting Statistics Thread and the Bowling Statistics Threads are
alternating in execution. Batting Statistics Thread runs upto 10, then Bowling Statistics Thread
runs upto 949, Batting Statistics Thread picks up next and runs up to 384 and so on. There is no
usual set pattern when Threads run (especially when they have same priority , more about this
later..). JVM decides which Thread to run at which time. If a Thread is waiting for user input or a
network connection, JVM runs the other waiting Threads.
Thread Synchronization
Since Threads run in parallel, a new problem arises. i.e. What if thread1 modifies data which is
being accessed by thread2? How do we ensure that different threads don't leave the system in an
inconsistent state? This problem is usually called Thread Synchronization Problem. Let's first look
at an example where this problem can occur.
Example Program:
Consider the SpreadSheet class below. It consists of three cells and also a method setAndGetSum
which sets the values into the cell and sums them up.
Running Batting Statistics Thread 385
..
Running Batting Statistics Thread 998
Running Batting Statistics Thread 999
class SpreadSheet {
int cell1, cell2, cell3;
int setandGetSum(int a1, int a2, int a3) {
cell1 = a1;
sleepForSomeTime();
cell2 = a2;
sleepForSomeTime();
cell3 = a3;
sleepForSomeTime();
return cell1 + cell2 + cell3;
}
void sleepForSomeTime() {
try {
Thread.sleep(10 * (int) (Math.random() * 100));
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
Serial Run
Let's first run the above example in a serial way and see what the output would be.
Output would contain a multiple of 6 always because we are calling with i, i2 and i3 and summing
up. So, the result should generally be i*6 with i running from 0 to 3.
Example Program using Threads
Let's now run the SpreadSheet class in a Thread. Example Code below:
We are creating 2 instances of the Thread using the interface , one and two. And start method is
invoked to run the thread. Both threads share the instance of SpreadSheet class , spreadsheet.
Output
public static void main(String[] args) {
SpreadSheet spreadSheet = new SpreadSheet();
for (int i = 0; i < 4; i++) {
System.out.print(spreadSheet.setandGetSum(i, i * 2, i * 3) + " ");
}
}
//Output - 0 6 12 18
public class ThreadExampleSynchronized implements Runnable {
SpreadSheet spreadSheet = new SpreadSheet();
public void run() {
for (int i = 0; i < 4; i++) {
System.out.print(
spreadSheet.setandGetSum(i,i * 2, i * 3)
+ " ");
}
}
public static void main(String[] args) {
ThreadExampleSynchronized r = new ThreadExampleSynchronized();
Thread one = new Thread(r);
Thread two = new Thread(r);
one.start();
two.start();
}
}
FIRST RUN : 0 1 6 9 12 15 18 18
SECOND RUN : 0 3 6 6 12 15 18 18
THIRD RUN : 0 3 6 9 12 15 18 18
Output Discussion What we see is that different runs have different results. That's expected with
threads. What is not expected is to see numbers like 1, 9, 15 and 3 in the output. We are
expecting to see multiples of 6 in the output(as in the earlier serial run) but we see numbers
which are not multiples of 6. Why is this happening? This is a result of having two threads run in
parallel without synchronization. Consider the code in the setAndGetSum method.
After setting the value to each cell, there is a call for the Thread to sleep for some time. After
Thread 1 sets the value of cell1, it goes to Sleep. So, Thread2 starts executing. If Thread 2 is
executing Òreturn cell1 + cell2 + cell3;Ó, it uses cell1 value set by Thread 1 and cell2 and cell3
values set by Thread 2. This results in the unexpected results that we see when the method is run
in parallel. What is explained is one possible scenario. There are several such scenarios possible.
The way you can prevent multiple threads from executing the same method is by using the
synchronized keyword on the method. If a method is marked synchronized, a different thread
gets access to the method only when there is no other thread currently executing the method.
Let's mark the method as synchronized:
Output of the program now is Ò0 0 6 6 12 12 18 18Ó. This is expected output , all numbers are
multiples of 6.
Threads & Synchronized Keyword
A method or part of the method can be marked as synchronized. JVM will ensure that there is
only thread running the synchronized part of code at any time. However, thread synchronization
is not without consequences. There would be a performance impact as the rest of threads wait
for the current thread executing a synchronized block. So, as little code as possible should be
marked as synchronized.
Synchronized method Example
int setandGetSum(int a1, int a2, int a3) {
cell1 = a1;
sleepForSomeTime();
cell2 = a2;
sleepForSomeTime();
cell3 = a3;
sleepForSomeTime();
return cell1 + cell2 + cell3;
}
synchronized int setandGetSum(int a1, int a2, int a3) {
cell1 = a1;
sleepForSomeTime();
cell2 = a2;
sleepForSomeTime();
cell3 = a3;
sleepForSomeTime();
return cell1 + cell2 + cell3;
}
Synchronized block Example
All code which goes into the block is synchronized on the current object.
Synchronized static method Example
Static synchronized block Example
Static blocks are synchronized on the class.
Static and non static synchronized methods and blocks
Static methods and block are synchronized on the class. Instance methods and blocks are
synchronized on the instance of the class i.e. an object of the class. Static synchronized methods
and instance synchronized methods don't affect each other. This is because they are
synchronized on two different things.
States of a Thread
Different states that a thread can be in are defined the class State.
NEW;
RUNNABLE;
RUNNING;
BLOCKED/WAITING;
TERMINATED/DEAD; Let's consider the example that we discussed earlier.
Example Program
synchronized void synchronizedExample1() {
//All code goes here..
}
void synchronizedExample2() {
synchronized (this){
//All code goes here..
}
}
synchronized static int getCount(){
return count;
}
static int getCount2(){
synchronized (SynchronizedSyntaxExample.class) {
return count;
}
}
States of a Thread - Examples
A thread is in NEW state when an object of the thread is created but the start method is not yet
called. At the end of line 1, battingThread1 is in NEW state. A thread is in RUNNABLE state when it
is eligible to run, but not running yet. (A number of Threads can be in RUNNABLE state. Scheduler
selects which Thread to move to RUNNING state). In the above example, sometimes the Batting
Statistics thread is running and at other time, the Bowling Statistics Thread is running. When
Batting Statistics thread is Running, the Bowling Statistics thread is ready to run. It's just that the
scheduler picked Batting Statistics thread to run at that instance and vice-versa. When Batting
Statistics thread is Running, the Bowling Statistics Thread is in Runnable state (Note that the
Bowling Statistics Thread is not waiting for anything except for the Scheduler to pick it up and run
it). A thread is RUNNING state when it's the one that is currently , what else to say, Running. A
thread is in BLOCKED/WAITING/SLEEPING state when it is not eligible to be run by the Scheduler.
Thread is alive but is waiting for something. An example can be a Synchronized block. If Thread1
LINE 1: BattingStatisticsThread battingThread1 = new
BattingStatisticsThread();
LINE 2: battingThread1.start();
LINE 3: BowlingStatisticsThread battingInterfaceImpl = new
BowlingStatisticsThread();
LINE 4: Thread battingThread2 = new Thread(battingInterfaceImpl);
LINE 5:battingThread2.start();
//Output - Running Batting Statistics Thread 0
Running Batting Statistics Thread 1
..
..
Running Batting Statistics Thread 10
Running Bowling Statistics Thread 0
..
..
Running Bowling Statistics Thread 948
Running Bowling Statistics Thread 949
Running Batting Statistics Thread 11
Running Batting Statistics Thread 12
..
..
Running Batting Statistics Thread 383
Running Batting Statistics Thread 384
Running Bowling Statistics Thread 950
Running Bowling Statistics Thread 951
..
Running Bowling Statistics Thread 998
Running Bowling Statistics Thread 999
Running Batting Statistics Thread 385
..
Running Batting Statistics Thread 998
Running Batting Statistics Thread 999
enters synchronized block, it blocks all the other threads from entering synchronized code on the
same instance or class. All other threads are said to be in Blocked state. A thread is in
DEAD/TERMINATED state when it has completed its execution. Once a thread enters dead state, it
cannot be made active again.
Thread Priority
Scheduler can be requested to allot more CPU to a thread by increasing the threads priority. Each
thread in Java is assigned a default Priority 5. This priority can be increased or decreased (Range 1
to 10). If two threads are waiting, the scheduler picks the thread with highest priority to be run. If
all threads have equal priority, the scheduler then picks one of them randomly. Design programs
so that they don't depend on priority.
Thread Priority Example
Consider the thread example declared below:
Priority of thread can be changed by invoking setPriority method on the thread.
Java also provides predefined constants Thread.MAX_PRIORITY(10), Thread.MIN_PRIORITY(1),
Thread.NORM_PRIORITY(5) which can be used to assign priority to a thread.
Thread Join method
Join method is an instance method on the Thread class. Let's see a small example to understand
what join method does. Let's consider the thread's declared below: thread2, thread3, thread4
Let's say we would want to run thread2 and thread3 in parallel but thread4 can only run when
thread3 is finished. This can be achieved using join method.
Join method example
Look at the example code below:
class ThreadExample extends Thread {
public void run() {
for (int i = 0; i < 1000; i++)
System.out
.println( this.getName() + " Running "
+ i);
}
}
ThreadExample thread1 = new ThreadExample();
thread1.setPriority(8);
ThreadExample thread2 = new ThreadExample();
ThreadExample thread3 = new ThreadExample();
ThreadExample thread4 = new ThreadExample();
thread3.join() method call force the execution of main method to stop until thread3 completes
execution. After that, thread4.start() method is invoked, putting thread4 into a Runnable State.
Overloaded Join method
Join method also has an overloaded method accepting time in milliseconds as a parameter.
In above example, main method thread would wait for 2000 ms or the end of execution of
thread4, whichever is minimum.
Thread , Static methods
Thread yield method
Yield is a static method in the Thread class. It is like a thread saying " I have enough time in the
limelight. Can some other thread run next?". A call to yield method changes the state of thread
from RUNNING to RUNNABLE. However, the scheduler might pick up the same thread to run
again, especially if it is the thread with highest priority. Summary is yield method is a request
from a thread to go to Runnable state. However, the scheduler can immediately put the thread
back to RUNNING state.
Thread sleep method
sleep is a static method in Thread class. sleep method can throw a InterruptedException. sleep
method causes the thread in execution to go to sleep for specified number of milliseconds.
Thread and Deadlocks
Let's consider a situation where thread1 is waiting for thread2 ( thread1 needs an object whose
synchronized code is being executed by thread1) and thread2 is waiting for thread1. This
situation is called a Deadlock. In a Deadlock situation, both these threads would wait for one
another for ever.
Deadlock Example
Consider the example classes below: Resource represents any resource that you need access to.
A network connection, database connection etc. Operation represents an operation that can be
done on these resources. Let's say that Operation need two resources, resource1 and resource2
and offer two operations method1 and method2. Look at the program below:
Thread3.start();
thread2.start();
thread3.join();//wait for thread 3 to complete
System.out.println("Thread3 is completed.");
thread4.start();
Thread4.join(2000);
class Resource {
Method1 executes some code on resource1 first and then executes some code on resource2.
Method2 does the reverse. We use the sleep method call to simulate the fact that these operation
could take some time to complete. Let's create two threads sharing the above operation using the
code below: Threads one and two now share object operation. The thread code runs both
operations method1 and method2.
}
class SomeOperation {
Resource resource1 = new Resource();
Resource resource2 = new Resource();
void method1() throws InterruptedException {
synchronized (resource1) {
Thread.sleep(1000);
//code using resource1
synchronized (resource2) {
//code using resource2
}
}
}
void method2() throws InterruptedException {
System.out.println(Thread.currentThread().getName()
+ "is in method2");
synchronized (resource2) {
//code using resource2
Thread.sleep(1000);
synchronized (resource1) {
//code using resource1
}
}
}
}
public class ThreadDeadlock implements Runnable {
SomeOperation operation = new SomeOperation();
@Override
public void run() {
try {
operation.method1();
operation.method2();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
When executed this program just hangs, because of a deadlock. To make what is happening
behind the screens more clear, Let's add in a few sysout's in the Operation class.
Output:
public static void main(String[] args) {
ThreadDeadlock r = new ThreadDeadlock();
Thread one = new Thread(r);
Thread two = new Thread(r);
one.start();
two.start();
}
}
class SomeOperation {
Resource resource1 = new Resource();
Resource resource2 = new Resource();
void method1() throws InterruptedException {
synchronized (resource1) {
System.out.println("Method1 - got resource1");
Thread.sleep(1000);
//code using resource1
System.out.println("Method1 - waiting for resource2");
synchronized (resource2) {
System.out.println("Method1 - got resource2");
//code using resource2
}
}
}
void method2() throws InterruptedException {
synchronized (resource2) {
System.out.println("Method2 - got resource2");
//code using resource2
Thread.sleep(1000);
System.out.println("Method2 - waiting for resource1");
synchronized (resource1) {
System.out.println("Method2 - got resource1");
//code using resource1
}
}
}
}
Now we have two threads waiting for resources held by one another. This results in a deadlock.
Thread - wait, notify and notifyAll methods
Consider the example below: Calculator thread calculates two values: Sum upto Million and Sum
upto 10 Million. Main program uses the output of sum upto million.
Example 1
Output
Method1 - got resource1
Method1 - waiting for resource2
Method1 - got resource2
Method1 - got resource1
Method2 - got resource2
Method1 - waiting for resource2
Method2 - waiting for resource1
HANGSÉÉÉÉÉÉÉÉ
class Calculator extends Thread {
long sumUptoMillion;
long sumUptoTenMillion;
public void run() {
calculateSumUptoMillion();
calculateSumUptoTenMillion();
}
private void calculateSumUptoMillion() {
for (int i = 0; i < 1000000; i++) {
sumUptoMillion += i;
}
}
private void calculateSumUptoTenMillion() {
for (int i = 0; i < 10000000; i++) {
sumUptoTenMillion += i;
}
}
}
public class ThreadWaitAndNotify {
public static void main(String[] args) {
Calculator thread = new Calculator();
thread.start();
System.out.println(thread.sumUptoMillion);
}
}
Output printed is 0. This is because the thread has not finished calculating the value of
sumUptoMillion when the main method prints the value to the output. We have to find a way to
stop the main method from running until sumUptoMillion is calculated. One option is to use the
join method. But, join method would make the main method wait until both the operations
(sumUptoMillion and sumUptoTenMillion) are completed. But, we want main method to wait only
for sumUptoMillion. We can achieve this using wait and notify methods. wait and notify methods
can only be used in a synchronized context.
Example with wait and notify methods
0
package com.in28minutes.threads;
class Calculator extends Thread {
long sumUptoMillion;
long sumUptoTenMillion;
public void run() {
synchronized (this) {
calculateSumUptoMillion();
notify();
}
calculateSumUptoTenMillion();
}
private void calculateSumUptoMillion() {
for (int i = 0; i < 1000000; i++) {
sumUptoMillion += i;
}
System.out.println("Million done");
}
private void calculateSumUptoTenMillion() {
for (int i = 0; i < 10000000; i++) {
sumUptoTenMillion += i;
}
System.out.println("Ten Million done");
}
}
public class ThreadWaitAndNotify {
public static void main(String[] args) throws InterruptedException {
Calculator thread = new Calculator();
synchronized(thread){
thread.start();
thread.wait();
}
Output
Wait method example
Below snippet shows how wait is used in earlier program. wait method is defined in the Object
class. This causes the thread to wait until it is notified.
Notify method example
Below snippet shows how notify is used in earlier program. notify method is defined in the Object
class. This causes the object to notify other waiting threads.
A combination of wait and notify methods make the main method to wait until the sum of million
is calculated. However, not the main method does not wait for Sum of Ten Million to be
calculated.
notifyAll method
If more than one thread is waiting for an object, we can notify all the threads by using notifyAll
method.
Assert
Assertions are introduced in Java 1.4. They enable you to validate assumptions. If an assert
fails (i.e. returns false), AssertionError is thrown (if assertions are enabled). assert is a
keyword in java since 1.4. Before 1.4, assert can be used as identifier.
System.out.println(thread.sumUptoMillion);
}
}
Million done
499999500000
Ten Million done
synchronized(thread){
thread.start();
thread.wait();
}
synchronized (this) {
calculateSumUptoMillion();
notify();
}
Thread.notifyAll();
Assert Details
To compile code using 1.3 you can use the command below javac -source 1.3 OldCode.java =>
assert can be used as identifier with -source 1.4,1.5,5,1.6,6 => assert cannot be used as identifier
Assertions can easily be enabled and disabled. Assertions are disabled by default.
Enabling Assertions
Enable assertions: java -ea com.in28minutes.AssertExamples (OR) java -enableassertions
com.in28minutes.AssertExamples
Disable Assertions
Disable assertions: java -da com.in28minutes.AssertExamples (OR) java -disableassertions
com.in28minutes.AssertExamples
Enable Assertions in specific packages
Selectively enable assertions in a package only java -ea:com.rithus
Selectively enable assertions in a package and its subpackages only java -ea:com.in28minutes...
Enable assertions including system classes
java -ea -esa
Basic assert condition example
Basic assert is shown in the example below
Assert with debugging information: Example
If needed - debugging information can be added to an assert. Look at the example below.
private int computerSimpleInterest(int principal,float interest,int years){
assert(principal>0);
return 100;
}
private int computeCompoundInterest(int principal,float interest,int years){
//condition is always boolean
//second parameter can be anything that converts to a String.
assert(principal>0): "principal is " + principal;
return 100;
}
public static void main(String[] args) {
AssertExamples examples = new AssertExamples();
System.out.println(examples.computerSimpleInterest(-1000,1.0f,5));
}
Asserts - Not for validation
Assertions should not be used to validate input data to a public method or command line
argument. IllegalArgumentException would be a better option.
In public method, only use assertions to check for cases which are never supposed to happen.
Garbage Collection
Garbage Collection is a name given to automatic memory management in Java. Aim of
Garbage Collection is to Keep as much of heap available (free) for the program as possible.
JVM removes objects on the heap which no longer have references from the heap.
Garbage Collection Example
Let's say the below method is called from a function.
An object of the class GregorianCalendar is created on the heap by the first line of the function
with one reference variable calendar.
After the function ends execution, the reference variable calendar is no longer valid. Hence, there
are no references to the object created in the method.
JVM recognizes this and removes the object from the heap. This is called Garbage Collection.
When is Garbage Collection run?
Garbage Collection runs at the whims and fancies of the JVM (it isn't as bad as that). Possible
situations when Garbage Collection might run are 1.when available memory on the heap is low
2.when cpu is free
Garbage Collection , Important Points
Programmatically, we can request (remember it's just a request - Not an order) JVM to run
Garbage Collection by calling System.gc() method.
JVM might throw an OutOfMemoryException when memory is full and no objects on the heap are
eligible for garbage collection.
finalize() method on the objected is run before the object is removed from the heap from the
garbage collector. We recommend not to write any code in finalize();
Initialization Blocks
Initialization Blocks - Code which runs when an object is created or a class is loaded
Types of Initialization Blocks
void method(){
Calendar calendar = new GregorianCalendar(2000,10,30);
System.out.println(calendar);
}
There are two types of Initialization Blocks Static Initializer: Code that runs when a class is loaded.
Instance Initializer: Code that runs when a new object is created.
Static Initializer
Code within static{ and } is called a static initializer. This is run only when class is first loaded.
Only static variables can be accessed in a static initializer.
Example Output
Even though three instances are created static initializer is run only once.
Instance Initializer Block
Let's look at an example
public class InitializerExamples {
static int count;
int i;
static{
//This is a static initializers. Run only when Class is first loaded.
//Only static variables can be accessed
System.out.println("Static Initializer");
//i = 6;//COMPILER ERROR
System.out.println("Count when Static Initializer is run is " + count);
}
public static void main(String[] args) {
InitializerExamples example = new InitializerExamples();
InitializerExamples example2 = new InitializerExamples();
InitializerExamples example3 = new InitializerExamples();
}
}
Static Initializer
Count when Static Initializer is run is 0
public class InitializerExamples {
static int count;
int i;
{
//This is an instance initializers. Run every time an object is created.
//static and instance variables can be accessed
System.out.println("Instance Initializer");
i = 6;
count = count + 1;
System.out.println("Count when Instance Initializer is run is " + count);
}
Code within instance initializer is run every time an instance of the class is created.
Example Output
Java Bean Conventions
When is a java class called a bean? Let's find an answer to this question.
Java Bean Example
Consider the code example below:
public static void main(String[] args) {
InitializerExamples example = new InitializerExamples();
InitializerExamples example1 = new InitializerExamples();
InitializerExamples example2 = new InitializerExamples();
}
}
Instance Initializer
Count when Instance Initializer is run is 1
Instance Initializer
Count when Instance Initializer is run is 2
Instance Initializer
Count when Instance Initializer is run is 3
public class JavaBeansStandards {
private String name;
private boolean good;
public String getName() {
return name;
}
public boolean isGood() {
return good;
}
public void setName(String name) {
this.name = name;
}
public void setGood(boolean isGood) {
this.good = isGood;
}
Private Member Variables
Good practice is to have all member variables in a class declared as private.
Naming setter and getter methods
To modify and access values of properties we use setter and getter methods. Getters and
setters should be public.
Getters should not have any arguments passed.
Setters should take one argument (the property value) with same type as the return value of
getter.
Non boolean getter name should be (get + PropertyName)
boolean getter name can be (get + PropertyName) or (is + PropertyName)
All setters should be named (set + PropertyName)
Examples
Listener naming conventions
Methods to register/add a listener should use prefix "add" and suffix "Listener". They should
accept 1 parameter - the listener object to be added.
public void addSomeListener(MyListener listener){
}
public void removeSomeListener(MyListener listener){
}
}
private String name;
private boolean good;
public String getName() {
return name;
}
public boolean isGood() {
return good;
}
public void setName(String name) {
this.name = name;
}
public void setGood(boolean isGood) {
this.good = isGood;
}
Example
Methods to de-register/remove a listener should use prefix "remove" and suffix "Listener". They
should accept 1 parameter - the listener object to be removed
Example
Regular Expressions
Regular Expressions make parsing, scanning and splitting a string very easy. We will first look
at how you can evaluate a regular expressions in Java , using Patter, Matcher and Scanner
classes. We will then look into how to write a regular expression.
Regular Expression in Java , Matcher and Pattern Example
Code below shows how to execute a regular expression in java.
Matcher class
Matcher class has the following utility methods: find(), start(), group(). find() method returns true
until there is a match for the regular expression in the string. start() method gives the starting
index of the match. group() method returns the matching part of the string.
Examples
Let's run this method with a regular expression to search for Ò12Ó in the string Ò122345612Ó.
Output
public void addSomeListener(MyListener listener){
}
public void removeSomeListener(MyListener listener){
}
private static void regex(String regex, String string) {
Pattern p = Pattern.compile(regex);
Matcher m = p.matcher(string);
List<String> matches = new ArrayList<String>();
while (m.find()) {
matches.add(m.start() + "<" + m.group() + ">");
}
System.out.println(matches);
}
regex("12", "122345612");
Output shows the matching strings 12, 12. Also, shown in the output is the starting index of each
match. First 12 is present starting at index 0. The next 12 in the string starts at index 7.
Creating Regular Expressions for Java
Let's test regular expressions by using the method we created earlier: regex().
Simple Regular Expressions
Search for 12 in the string
Certain characters escaped by \ have special meaning in regular expressions. For example, /s
matches a whitespace character. Remember that to represent \ in a string, we should prepend \
to it. Let us see a few examples below.
Space character - \s
Digit - \d
Word character (letter, digits or underscore) - \w
Square brackets are used in regular expressions to search for a range of characters. Few
examples below. look for a,b,c,d,1,2,3,4 =>Note that this does not look for capital A,B,C,D
Regular Expressions , Multiple Characters
is used in regular expression to look for 1 or more characters. For example a+ looks for 1 or
more character a's.
[0<12>, 7<12>]
regex("12", "122345612");//[0<12>, 7<12>]
System.out.println("\\");//prints \ (only one slash)
regex("\\s", "12 1234 123 ");//[2< >, 7< >, 11< >]
regex("\\d", "12 12");//[0<1>, 1<2>, 3<1>, 4<2>]
regex("\\w", "ab 12 _");//[0<a>, 1<b>, 3<1>, 4<2>, 6<_>]
regex("[a-d1-4]", "azbkdm 15AB");//[0<a>, 2<b>, 4<d>, 7<1>]
regex("[a-dA-D]", "abyzCD");//[0<a>, 1<b>, 4<C>, 5<D>]
regex("a+", "aaabaayza");//[0<aaa>, 4<aa>, 8<a>]
Look for one or more characters from a to z (only small case).
Regular Expressions , Look for Repetitions
Regular expressions can be joined together to look for a combination. a+b+ looks 1 or more a's
and 1 or more b's next to each other. Notice that only a's or only b's do not match.
* - 0 or more repetitions.
Below expression looks for 1 or more a's followed by 0 or more b's followed by 1 or more c's. abc
=> match. ac=> match (since we used * for b). ab => does not match.
? - 0 or 1 repetitions.
a+b*c? looks for 1 or more a's followed by 0 or more b's followed by 0 or 1 c's. a => matches. ab
=> matches. abc=>matches. abcc => does not match (only 0 or 1 c's)
^a looks for anything other than a
[^abcd]+a looks for anything which is not a or b or c or d, repeated 0 or more times, followed by a
. matches any character
a.c looks for Ôa' followed by any character followed by Ôc'. abc => match abbc => no match (.
matches 1 character only)
Greedy Regular Expressions
a+ matches a, aa,aaa,aaaa, aaaaa. If you look at the output of the below expression, it matches
the biggest only aaaaa. This is called greedy behaviour. similar behavior is shown by *.
regex("[a-z]+", "abcZ2xyzN1yza");//[0<abc>, 5<xyz>, 10<yza>]
//0123456789012
regex("a+b+", "aabcacaabbbcbb");//[0<aab>, 6<aabbb>]
regex("a+b*c+", "abcdacdabdbc");//[0<abc>, 4<ac>]
regex("a+b*c?", "adabdabcdabccd");//[0<a>, 2<ab>, 5<abc>, 9<abc>]
regex("[^a]+", "bcadefazyx");//[0<bc>, 3<def>, 7<zyx>]
regex("[^abcd]+a", "efgazyazyzb");//[0<efga>, 4<zya>]
regex("a.c", "abca ca!cabbc");//[0<abc>, 3<a c>, 6<a!c>]
You can make + reluctant (look for smallest match) by appending ?
Similarly *? is reluctant match for the greedy * If you want to look for characters . or * in a regular
expression, then you should escape them. Example: If I want to look for ...(3 dots), we should use
... To represent ... as string we should put two 's instead of 1.
Regular Expression using Scanner class
Below code shows how Scanner class can be used to execute regular expression. findInLine
method in Scanner returns the match , if a match is found. Otherwise, it returns null.
Example
Tokenizing
Tokenizing means splitting a string into several sub strings based on delimiters. For example,
delimiter ; splits the string ac;bd;def;e into four sub strings ac, bd, def and e. Delimiter can in
itself be any of the regular expression(s) we looked at earlier. String.split(regex) function
takes regex as an argument.
Example method for Tokenizing
regex("a+", "aaaaab");//[0<aaaaa>]
regex("a+?", "aaaaab");//[0<a>, 1<a>, 2<a>, 3<a>, 4<a>]
regex("\\.\\.\\.", "...a....b...c");//[0<...>, 4<...>, 9<...>]
private static void regexUsingScanner(String regex,
String string) {
Scanner s = new Scanner(string);
List<String> matches = new ArrayList<String>();
String token;
while ((token = s.findInLine(regex)) != null) {
matches.add(token);
}
;
System.out.println(matches);
}
regexUsingScanner("a+?", "aaaaab");//[a, a, a, a, a]
Example:
Tokenizing using Scanner Class
Example:
Scanner Class: Other Functions
Scanner has more useful functions other than just looking for a delimiter
Example:
private static void tokenize(String string,String regex) {
String[] tokens = string.split(regex);
System.out.println(Arrays.toString(tokens));
}
tokenize("ac;bd;def;e",";");//[ac, bd, def, e]
private static void tokenizeUsingScanner(String string,String regex) {
Scanner scanner = new Scanner(string);
scanner.useDelimiter(regex);
List<String> matches = new ArrayList<String>();
while(scanner.hasNext()){
matches.add(scanner.next());
}
System.out.println(matches);
}
tokenizeUsingScanner("ac;bd;def;e",";");//[ac, bd, def, e]
private static void lookForDifferentThingsUsingScanner(
String string) {
Scanner scanner = new Scanner(string);
while(scanner.hasNext()){
if(scanner.hasNextBoolean()){
System.out.println("Found Boolean:" + scanner.nextBoolean());
} else if(scanner.hasNextInt()){
System.out.println("Found Integer:" + scanner.nextInt());
} else {
System.out.println("Different thing:" + scanner.next());
}
}
}
lookForDifferentThingsUsingScanner("true false 12 3 abc true 154");
Output
Expressions
Where are objects created? Where are strings created?
TODO
replace with java at start of code
//Getters and Setters are eliminated to keep the example short
Java SE vs ME vs EE
Check for long lines which are cut off in pdf
Notes from Venkat's Talk
Designed for a different world - PEnguins
OOPS - Terrible - 1970s
FP - Lambda calculus-1929
Complexity from problems vs Complexity from solutions
Structured Programming - One Start One Exit????
Functional Programming
Assignment LEss
Statements vs Expressions
Pure Function - No side effects and Zero dependencies that change
Referential Transparency
Pure Functions
idempotent
referenctial transparency
memorizable
easier to testt
may be lazily evaluated
Higher order function
take/create/return object vs take/create/return function
Java Future - imperative + oops -> functional + oops
//Found Boolean:true
//Found Boolean:false
//Found Integer:12
//Found Integer:3
//Different thing:abc
//Found Boolean:true
//Found Integer:154
Stream - Does not evaluate the function on all the data. It takes a collection of
functions (fusing - intermediate operations combined) and executes them on each
piece of data..
Functional Pipeline, Function
"Coolection pipeline pattern" -> Martin Fowler
Lambda - Stateless
Closure has State
Code is Liability not an asset!
Exception Handling - Promise
Reactive Programming
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