Java Inner Classes

In Java, an inner class is a class defined within another class. There are four types of inner classes in Java:

1. Non-static nested class (also known as an inner class)
2. Static nested class
3. Local class
4. Anonymous class

Here is an example of a non-static nested class (inner class) in Java:

public class OuterClass {
    private int outerVar;

    public void outerMethod() {
        InnerClass innerObj = new InnerClass();
        innerObj.innerMethod();
    }

    public class InnerClass {
        private int innerVar;

        public void innerMethod() {
            outerVar = 10;
            innerVar = 20;
            System.out.println("Inner class method: outerVar = " + outerVar + ", innerVar = " + innerVar);
        }
    }

public class Main {
    public static void main(String[] args) {
        OuterClass outerObj = new OuterClass();
        outerObj.outerMethod(); // Output: "Inner class method: outerVar = 10, innerVar = 20"
    }
}

}

In this example, OuterClass has an inner class called InnerClass. The InnerClass can access the members (fields and methods) of the OuterClass, including private members.

To create an instance of the InnerClass, we first need to create an instance of the OuterClass, and then use that instance to create an instance of the InnerClass. We can then call the innerMethod() of the InnerClass.

When we run the main() method, we create an instance of OuterClass and call its outerMethod(). Within outerMethod(), we create an instance of InnerClass and call its innerMethod(). The output shows that the InnerClass is able to access the outerVar variable of the OuterClass.

Non-static nested classes are useful for organizing code and making it more readable. They can also help to encapsulate implementation details and reduce the number of classes in a package.

 

In Java, a static nested class is a nested class that is declared with the static keyword. It is a class that is a static member of its outer class, and can be accessed using the name of the outer class. Here's an example of a static nested class in Java:

public class OuterClass {
    private static int outerVar = 10;

    public static class StaticNestedClass {
        private int innerVar;

        public void innerMethod() {
            innerVar = 20;
            System.out.println("Static nested class method: outerVar = " + outerVar + ", innerVar = " + innerVar);
        }
    }
public class Main {
    public static void main(String[] args) {
        OuterClass.StaticNestedClass staticObj = new OuterClass.StaticNestedClass();
        staticObj.innerMethod(); // Output: "Static nested class method: outerVar = 10, innerVar = 20"
    }
}


}

When we run the main() method, we create an instance of StaticNestedClass using the syntax OuterClass.StaticNestedClass. We can then call the innerMethod() of the StaticNestedClass.

In Java, a local class is a class that is defined inside a method. Local classes have access to the variables and parameters of the enclosing method, but can only be accessed from within that method. Here's an example of a local class in Java:

public class OuterClass {
    private int outerVar = 10;

    public void outerMethod() {
        int localVar = 20;

        class LocalClass {
            private int innerVar;

            public void innerMethod() {
                innerVar = 30;
                System.out.println("Local class method: outerVar = " + outerVar + ", localVar = " + localVar + ", innerVar = " + innerVar);
            }
        }

        LocalClass localObj = new LocalClass();
        localObj.innerMethod(); // Output: "Local class method: outerVar = 10, localVar = 20, innerVar = 30"
    }
public class Main {
    public static void main(String[] args) {
        OuterClass outerObj = new OuterClass();
        outerObj.outerMethod(); // Output: "Local class method: outerVar = 10, localVar = 20, innerVar = 30"
    }
}

}

 

In this example, OuterClass has a method called outerMethod(), which contains a local class called LocalClass. The LocalClass can access the variables outerVar and localVar of the outerMethod(), as well as its own innerVar variable.

To create an instance of the LocalClass, we first need to call the outerMethod(), which will then create an instance of the LocalClass. We can then call the innerMethod() of the LocalClass.

 

In Java, an anonymous class is a class that is defined and instantiated at the same time, without giving it a name. Anonymous classes are often used for creating one-time use classes that implement an interface or extend a class. Here's an example of an anonymous class in Java:

public class OuterClass {
    private int outerVar = 10;

    public void outerMethod() {
        Interface innerObj = new Interface() {
            private int innerVar;

            @Override
            public void interfaceMethod() {
                innerVar = 20;
                System.out.println("Anonymous class method: outerVar = " + outerVar + ", innerVar = " + innerVar);
            }
        };

        innerObj.interfaceMethod(); // Output: "Anonymous class method: outerVar = 10, innerVar = 20"
    }
}

interface Interface {
    void interfaceMethod();
}

public class Main {
    public static void main(String[] args) {
        OuterClass outerObj = new OuterClass();
        outerObj.outerMethod(); // Output: "Anonymous class method: outerVar = 10, innerVar = 20"
    }
}

Anonymous classes are useful when we need to create a small, one-time use class that implements an interface or extends a class. They can help to reduce the amount of code we need to write, and can make the code more concise and readable.