The @FunctionalInterface annotation is a standard Java annotation that is used to indicate that an interface is intended to be a functional interface.

A functional interface is an interface that has only one abstract method, and is therefore suitable for use with lambda expressions and method references. By marking an interface with the @FunctionalInterface annotation, you are making it clear that the interface is intended to be used as a functional interface, and that it should have only one abstract method.

Here is an example of how to use the @FunctionalInterface annotation:

@FunctionalInterface
public interface MyInterface {

  void myMethod();

}

In this example, the MyInterface interface is annotated with @FunctionalInterface to indicate that it is intended to be used as a functional interface. The interface has only one abstract method, myMethod(), which makes it suitable for use with lambda expressions and method references.

When you use the @FunctionalInterface annotation, it is important to ensure that your interface actually has only one abstract method. If you add additional abstract methods to a functional interface, you will receive a compile-time error. By using the @FunctionalInterface annotation, you can help to ensure that your code is clean, concise, and easy to understand.