Nested Classes Java

Nested class is a class within a class.A nested class is a member of its outer class.

Nested classes are divided into two categories

• static
Nested classes that are declared static are simply called static nested classes.
• non-static
Non-static nested classes are called inner classes.

Nested classes can be declared public, private, protected.

Why use nested classes

• Suppose a situation where a class A is only used by another class B, in that situation it is no point of writing two classes separately. Class A can be embedded into class B as a nested class
• Suppose a situation one of class A's variables is used by class B otherwise that variable can be declared private. In such a case it is more appropriate to nest class B inside class A to protect encapsulation

Static nested classes

Static nested classes cannot access member variables of it's outer class directly. it can use them only through an object reference.

Syntax for creating an object in static object class

OuterClass.StaticNestedClass nestedObject = new OuterClass.StaticNestedClass();

Non Static nested classes or Inner Classes

Inner classes can access member variables of it's outer class directly even their access modifiers are private.

Syntax for creating an object in an inner class

OuterClass.InnerClass innerObject = outerObject.new InnerClass();

A sample code to try out

public class A { //start of outer class
 
 private int a=100;
 private String b="host";
 
 public class B{ // start of inner class or non static nested class
  
  int e=a;//inner classes can access member variables directly even they are private 
  
 }//End of inner class or non static nested class
 
 public static class C{ //start of static nested class
  
  A ob2 = new A();
  String s =ob2.b;//static nested classes cannot access member variables directly, it should be done through an object of outer class  
  
 } //End of start of static nested class
    
    public static void main(String s[]) {
     A ob1=new A();
     
     A.B ob3=ob1.new B();//inner class object creation
     System.out.println(ob3.e);
     
     A.C ob4=new A.C();//static nested class object creation
     System.out.println(ob4.s);     
                 
    }
}//End of outer class