Inner classes in java

Inner classes in java

In java inner class which is defined inside another class is called nested class or non-static inner class. For instantiating the inner class or nested class object we first need to create the outer class object. We can access outer class member’s variables insides of inner class just like normal members variables of class.

We cannot declare static method or static variable inside the non-static inner class. If we try to declare it shows error The field or static variable cannot be declared static in a non-static inner type.

Below is syntax of the inner class.

public class OuterClass

{

     // inner class or non-static nested class

     class InnerClass

     {

           //..      

     }   

}

A benefit of nested class is we can make our code more encapsulated so lets check below is an example of how you can declare inner classes in Java.

public class OuterClass

{

     String country = "India";

     static String language = "java";

            

     class InnerClass

     {

           public void display(){

                System.out.println(language);

                System.out.println(country);

           }

     }

    

     public static void main(String[] args) {

           // create object of Outer class

           OuterClass outerCls = new OuterClass();

          

// create an object of inner class using outer class object refrance

           OuterClass.InnerClass innerCls = outerCls.new InnerClass();

           innerCls.display();

     }

}

Output

java

India

Inner class present in the outer class, so we must instantiate the outer class first, in order to instantiate the inner class.

Else you can write the method in outer class which can create the object of inner class and call that method, Check below example for more details.

public class OuterClass

{

     String country = "India";

     static String language = "java";

            

     class InnerClass{

           public void display(){

                System.out.println(language);

                System.out.println(country);

           }

     }

     public void initiateClass(){

           InnerClass innerCls = new InnerClass();

           innerCls.display();

     }

    

     public static void main(String[] args) {

           OuterClass outerCls = new OuterClass();

           outerCls.initiateClass();

     }

}

Output

java

India

If the outer class member variable is re-declared in the inner class then for accessing the member variable of outer class we need to use this keyword with outer class name like OuterClass.this.variableName.

Important Note: Inner classes are members of the outer class, so you can apply any access modifiers like private, protected to your inner class which is not possible in normal outer classes.

Click here for more details of static inner class in java ...

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