Anonymous class (anonymous class)

What is an anonymous class?

It is used when creating a class that is used only once on the spot.

Normally, you declare a class that implements an interface, create an instance of that class, and call a method, but with an anonymous class, you can declare the interface and use the method at the same time.

If you do not use anonymous classes

This time, I used a self-made interface with a method called output.

IPrint

public interface IPrint {
	void output();
}

Create a class that implements the IPrint interface and describe the processing details in the method.

PrintImpl

public class PrintImpl implements IPrint{

	@Override
	public void output() {
		System.out.println("aaa");
	}
}

Call it in the execution class.

Main

public class Main {

	public static void main(String[] args) {
		
		IPrint ip = new PrintImpl();
		ip.output();//On the console"aaa"Is output
	}
}

When using anonymous class

You can use IPrint methods in the execution class without creating a class that implements the interface.

Main

public class Main {

	public static void main(String[] args) {
		
		IPrint ip = new IPrint() {
			@Override
			public void output() {
				System.out.println("aaa");
			}
		};
		ip.output();
	}
}

In addition, you can reduce the number of lines with a lambda expression.

Main

public class Main {

	public static void main(String[] args) {
			
		IPrint ip = () -> System.out.println("xxxx");
		
		ip.output(); // "xxxx"Is output
	}
}

If you add a new method that is not in the interface, it will not be received as an interface type variable when the instance is created.

Main

public class Main3 {

	public static void main(String[] args) {
		
		new IPrint() {
			@Override
			public void output() {
				System.out.println("aaa");
			}
			
			public void output2() {
				System.out.println("bbb");
			}
		}.output2();
	}
}