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Nested classes in Java:

Static Nested Classes: Behave like regular static members of the class. They do not have access to non-static members of the outer class. Created without an instance of the outer class.

public class Outer {
    private static String staticMsg = "Static message";
    private String instanceMsg = "Instance message";

    static class StaticNested {
        void displayStatic() {
            // Access to static members of the outer class
            System.out.println(staticMsg);
            // No access to non-static members of the outer class
            // System.out.println(instanceMsg); // Compilation error
        }
    }
}

Inner Classes: Associated with an instance of the outer class. Have full access to all (static and non-static) members of the outer class, including private ones. Can be declared private, protected, package-private, or public.

public class Outer {
    private static String staticMsg = "Static message";
    private String instanceMsg = "Instance message";

    class Inner {
        void display() {
            // Access to static and non-static members of the outer class
            System.out.println(staticMsg);
            System.out.println(instanceMsg);
        }
    }
}

Local Classes: Declared within a block (method, constructor, or static initializer). Only available within that block. Can refer to final or effectively final local variables and parameters, as well as all members of the outer class.

public class Outer {
    private String instanceMsg = "Instance message";

    void methodWithLocalClass() {
        final String localVar = "Local variable"; // or effectively final

        class Local {
            void display() {
                System.out.println(instanceMsg);
                System.out.println(localVar);
            }
        }

        Local localObj = new Local();
        localObj.display();
    }
}

Anonymous Classes: Classes without a name. Used for declaring and creating an instance of a class that implements an interface or extends a class, simultaneously. Useful for creating small, one-time objects.

public class Outer {
    void methodWithAnonymousClass() {
        // Anonymous class implementing Runnable
        Runnable myRunnable = new Runnable() {
            @Override
            public void run() {
                System.out.println("Running from anonymous class");
            }
        };
        new Thread(myRunnable).start();

        // Anonymous class extending Thread
        Thread myThread = new Thread() {
            @Override
            public void run() {
                System.out.println("Running from anonymous class extending Thread");
            }
        };
        myThread.start();
    }
}

Comparison of nested class types:

Class type	Declaration location	Access to outer class members	Is an instance of the outer class needed for creation	Access modifiers
Static nested class	Inside the class, with the `static` keyword.	Only to static members.	No	All
Inner class	Inside the class (non-static).	To all (static and non-static) members.	Yes	All
Local class	Inside a block (methods, constructors, initializers).	To all outer class members + `final` or effectively `final` local variables and parameters.	Yes	No (`final` or `abstract` implicitly)
Anonymous class	Inside a block (expression).	To all outer class members + `final` or effectively `final` local variables and parameters.	Yes	No

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