Anonymous class in Java

In Java, an anonymous class is a class that is defined and instantiated in a single expression, without a name. Anonymous classes are used to create one-time use objects that implement or extend a particular class or interface.

Here’s an example of an anonymous class in Java:

public class HelloWorld {
    public void sayHello() {
        System.out.println("Hello World!");
    }

    public static void main(String[] args) {
        HelloWorld hw = new HelloWorld() {
            public void sayHello() {
                System.out.println("Hello from anonymous class!");
            }
        };

        hw.sayHello();
    }
}

In this example, we define a class HelloWorld with a method sayHello(). Then, in the main() method, we create an anonymous class that extends HelloWorld and overrides the sayHello() method. We create an instance of the anonymous class and call the sayHello() method, which prints “Hello from anonymous class!” to the console.

Here are some rules to keep in mind when working with anonymous classes in Java:

1. Anonymous classes can only extend one class or implement one interface.
2. Anonymous classes cannot define constructors, since they have no name and cannot be explicitly called.
3. Anonymous classes can define instance variables and static variables, but these variables can only be accessed within the anonymous class.
4. Anonymous classes can access final variables from the enclosing scope.
5. Anonymous classes can only access non-final variables from the enclosing scope if they are effectively final, meaning that they are not modified anywhere in the enclosing scope.

Overall, anonymous classes can be useful in Java when you need to create a one-time use object that implements or extends a particular class or interface. By defining and instantiating the class in a single expression, you can create an object without needing to create a named subclass or implement a separate interface.