Inner Class

Non static nested classes are called inner classes

Syntax

    class OuterClass {

        ...

        class InnerClass {

            ...

        }

    }

How to create an object to the inner class

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

Example

class Outer{
 void print(){
  System.out.println("I am in outer class");
 }

 class Inner{
  void print(){
   System.out.println("I am in Inner class");
  }
 }

 public static void main(String args[]){
  Outer outerObj = new Outer();
  Outer.Inner obj = outerObj.new Inner();
  obj.print();
 }
}

Output

I am in Inner class

There are two additional types of inner classes.

Those are:

    1. Local Classes

    2. Anonymous classes

Some points to Remember

1. Inner class is associated with an instance, it cannot define any static members itself. Defining static variable inside inner class causes the compiler error

class Outer{
 class Nested{
  static int a = 100;
 }
}

When you tries to compile the above program, compiler throws the below error

Outer.java:3: error: Illegal static declaration in inner class Outer.Nested
           static int a = 100;
           ^
modifier 'static' is only allowed in constant variable declarations
1 error

      

2. You can define static constants in the inner class, but not static variables.

class Outer{
 class Nested{
  static final int a = 100;
 }
}

3. Defining static methods inside inner class causes compiler error

class Outer{
 class Nested{
  static void print(){
  }
 }
}

When you tries to compile the above program, compiler throws the below error

Outer.java:3: error: Illegal static declaration in inner class Outer.Nested
         static void print(){
         ^
modifier 'static' is only allowed in constant variable declarations
1 error

     

4. An instance of InnerClass can exist only within an instance of OuterClass and has direct access to the methods and fields of its enclosing instance

Example

class Outer{
 String name;

 void setName(String name){
  this.name = name;
 }

 class Inner{
  public void print(){
   System.out.println(this.name);
   System.out.println(Outer.this.name);
  }

  String name;
       
  void setName(String name){
   this.name = name;
  }

  public String toString(){
   return name;
  }
 }

 public static void main(String args[]){
  Outer obj = new Outer();
  obj.setName("Outer Class Object");
  Outer.Inner in = obj.new Inner();
  in.setName("Inner Class Object");
  in.print();
 }
}

        

Output

Inner Class Object
Outer Class Object

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