java Nested Classes ~ GNIITHELP

A class within another class is known as Nested class. The scope of the nested is bounded by the scope of its enclosing class.

---

Static Nested Class

A satic nested class is the one that has static modifier applied. Because it is static it cannot refer to non-static members of its enclosing class directly. Because of this restriction static nested class is seldom used.

---

Non-static Nested class

Non-static Nested class is most important type of nested class. It is also known as Inner class. It has access to all variables and methods of Outer class and may refer to them directly. But the reverse is not true, that is, Outer class cannot directly access members of Inner class.

One more important thing to notice about an Inner class is that it can be created only within the scope of Outer class. Java compiler generates an error if any code outside Outer class attempts to instantiate Inner class.

---

Example of Inner class

class Outer
{
 public void display()
 {
  Inner in=new Inner();
  in.show();
 }

 class Inner
 {
  public void show()
  {
   System.out.println("Inside inner");
  } 
 }
}

class Test
{
 public static void main(String[] args)
 {
  Outer ot=new Outer();
  ot.display();
 }
}

Output :

Inside inner

---

Example of Inner class inside a method

class Outer
{
 int count;
 public void display()
 {
  for(int i=0;i<5;i++)
  {
   class Inner       //Inner class defined inside for loop
   {        
    public void show()  
    {
     System.out.println("Inside inner "+(count++));
    } 
   }
   Inner in=new Inner();
   in.show();
  }
 }
}

class Test
{
 public static void main(String[] args)
 {
  Outer ot=new Outer();
  ot.display();
 }
}

Output :

Inside inner 0
Inside inner 1
Inside inner 2
Inside inner 3
Inside inner 4

---

Example of Inner class instantiated outside Outer class

class Outer
{
 int count;
 public void display()
 {
  Inner in=new Inner();
  in.show();
 }

 class Inner
 {
  public void show()
  {
   System.out.println("Inside inner "+(++count));
  } 
 }
}

class Test
{
 public static void main(String[] args)
 {
  Outer ot=new Outer();
  Outer.Inner in= ot.new Inner();
  in.show();
 }
}

Output :

Inside inner 1

---

Annonymous class

A class without any name is called Annonymous class.

interface Animal
{
 void type();
}
public class ATest {
 public static void main(String args[])
 {
  Animal an = new Animal(){         //Annonymous class created
  public void type()
  {
   System.out.println("Annonymous animal"); 
  }
 };
  an.type();      
 }
}

Output :

Annonymous animal

Here a class is created which implements Animal interace and its name will be decided by the compiler. This annonymous class will provide implementation of type() method.