Annotations on Interfaces?

Question

I can\'t figure out a use case for being able to annotate interfaces in Java.

Maybe someone could give me an example?

Answer 1

A use case that I am working with is javax/hibernate bean validation, we are using interfaces to help us on avoiding to define validations on every specific class.

public interface IUser {
    @NotNull Long getUserId();
    ...
}

public class WebUser implements IUser {
    private Long userId;

    @Override
    public Long getUserId(){
        return userId;
    }
    ...
}