What Are Anonymous Classes in Java and When to Use Them?

An anonymous class in Java is a class without a name that is both defined and instantiated in a single expression. Developers commonly use anonymous classes to create instances of classes or implement interfaces without writing a separate named class. This helps in writing cleaner and more concise code, especially for short-term or one-time use cases in Java programming.

Characteristics of Anonymous classes

Anonymous class is

• A local class without a name
• Define and initiated at the same time
• often used to provide short term implementations of interface or abstract class
• Mainly used in event handling thread creation or callback mechanism

Example: Example without Anonymous class

1. Greeting.java (Interface)

package com.test.rkdigital.school;
public interface Greeting {
	void sayHello();
}

2. GreetingImpl.java (Interface Implementation)

package com.test.rkdigital.school;
public class GreetingImpl implements Greeting{
	@Override
	public void sayHello() {
		// TODO Auto-generated method stub
	  System.out.println("I am from sayHello");
	}
}   

3. DemoTest.java

package com.test.rkdigital.school;

public class DemoTest {
	public static void main(String[] args) {
		Greeting obj=new GreetingImpl();
		obj.sayHello();
	}
}

Example: Example with Anonymous class

package com.test.rkdigital.school;
public interface Greeting {
	void sayHello();
}

• Defines an interface named Greeting
• It has one method sayHello() , which does not return anything and does not take any parameters .

Public class Main{
  Public static void main(String[] args){

• Start of class definition named main
• It contains the main() method , which is the entry point of any java application
• main() method where the program starts running
• String[] args allows command line argument to be passed to the program

Greeting greeting = new Greeting(){

• We are creating an object of an anonymous class that implements the greeting interfaces .
• new Greeting() start the instantiation and the {..} block defines the body of the anonymous class.
• This class does have a name and is used only once

public void sayHello(){
   System.out.println("Hello from Anonymous class");
}

• Implementation of the sayHello() method from the Greeting interface , it prints a message to the console when called .

};

• closes the anonymous class definition

1. Greeting.java (Greeting Interface)

package com.test.rkdigital.school;
public interface Greeting {
	void sayHello();
}

2. DemoTest.java

package com.test.rkdigital.school;
public class DemoTest {
	public static void main(String[] args) {
		Greeting obj=new Greeting() {
			@Override
			public void sayHello() {
				// TODO Auto-generated method stub
				System.out.println("Hello from Anonymous class Implementation");
				
			}
		};
		obj.sayHello();
	}
}