Inner classes in java

• Class inside a class is known as nested class.
• We will use nested classes to group classes and interfaces at one place for readability and maintainability.

1. package com.instanceofjava;
2. public class A{ 
3.  class B{
4. // inner class
5. }
6. }

• Inner class is a part of nested class. Non-static nested classes are known as inner classes. 
• Nested classes are two types
• Non-static nested class(inner class)
  
  1. Member inner class
  2. Anonymous inner class
  3. Local inner class
• Static nested class.

Member inner class:

•  A class is defined within a class and outside of methods of that class known as member inner class.

1. package instanceofjavaforus;
2. public class A{ 
3.  class B{
4. public void show(){
5. }
6. }
7. public static void main(String args[]){
8. A a= new A();
9. a.B b= a.new B();
10. b.show();
11. }
12. }

 

Anonymous inner class:

• Anonymous inner class is a inner class which does not have a name and whose instance is created at the time creating class itself.
• Anonymous inner classes are somewhat different from normal classes in its creation.
• There are two ways to create in two ways.
  1. Using class.
  2. Using interface.

Using class:

1. package instanceofjavaforus;
2. public class A{ 
3. public void show(){
4. }
5. public static void main(String args[]){
6. A a= new A(){
7.  public void display(){
8. System.out.println('anonymous class method');
9. }
10. };
11. a.display();
12. }
13. }

  Using interface:

1. package instanceofjavaforus;
2. public interface A{ 
3.  public void show();
4. }

1. package instanceofjavaforus;
2. public class Demo{ 
3. public static void main(String args[]){
4. A a= new A(){
5.  public void show(){
6. System.out.println('anonymous class method');
7. }
8. };
9. a.show();
10. }
11. }

 

Local inner class:

• A class which is defined inside a method of another class known as local inner class

1. package instanceofjavaforus;
2. public class Demo{ 
3. void create(){
4. class A{
5. void show(){
6. System.out.println("local inner class method");
7. }
8. }
9. A a= new A();
10. a.show();
11. }
12. public static void main(String args[]){
13. Demo obj= new Demo();
14.  obj.create();
15. }
16. }

 

Static nested class:

• A static nested class is a class which is defined inside a class as static.

1. package instanceofjavaforus;
2. public class Outer{ 
3.   static class inner{
4. public void show(){
5. System.out.println("static inner class method");
6. }
7. }
8. public static void main(String args[]){
9. Outer.inner in= new Outer.innner();
10. in.show();
11. }
12. }