Nested and inner classes

Nested and Inner Classes in Java are defined within another class and have access to the members of the containing class, including private members.

There are two main types of nested classes in Java: static inner classes and non-static inner classes.

Inner Classes (Non-Static)

Non-static inner classes are those defined within another class without the static keyword. They have access to all members of the containing class, including private members. Additionally, they have implicit access to the instance of the containing class.

public class OuterClass {
    
    private int externalData = 10;

    // Non-static inner class
    public class InnerClass {
        public void showExternalData() {
            System.out.println("External data from inner class: " + externalData);
        }
    }
    
    public static void main(String[] args) {
        OuterClass outer = new OuterClass();
        OuterClass.InnerClass inner = outer.new InnerClass();
        inner.showExternalData();
    }
}

In this example, the class OuterClass has an attribute named externalData initialized with the value 10. It contains a non-static inner class called InnerClass. The inner class has a public method named showExternalData(), which prints the value of externalData from the inner class.

The non-static inner class InnerClass has access to all members of the outer class, including the private member externalData. It contains a method showExternalData() that prints the value of externalData from the inner class.

In the main method, an instance of the outer class (OuterClass) is created called outer. Then, an instance of the inner class (InnerClass) is created using the syntax outer.new InnerClass(). Finally, the method showExternalData() is called from the instance of the inner class, printing the value of externalData from the inner class.

Static Inner Classes

Static inner classes are similar to non-static ones but are defined with the static keyword. Unlike non-static classes, they do not have access to the members of the containing class without an instance of it.

public class OuterClass {

    private static int staticData = 20;

    // Static inner class
    public static class StaticInnerClass {
        public void showStaticData() {
            System.out.println("Static data from static inner class: " + staticData);
        }
    }

    public static void main(String[] args) {
        OuterClass.StaticInnerClass staticInner = new OuterClass.StaticInnerClass();
        staticInner.showStaticData();
    }
}

The class OuterClass has a static member named staticData initialized with the value 20. It contains a static inner class named StaticInnerClass.

The static inner class StaticInnerClass does not have access to non-static members of the outer class. However, it can access static members. It contains a public method named showStaticData() that prints the value of staticData from the static inner class.

In the main method, an instance of the static inner class (StaticInnerClass) is created using the syntax OuterClass.StaticInnerClass. Then, the method showStaticData() is called from the instance of the static inner class, printing the value of staticData from the static inner class.

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