Default method return value in Java

Question

While working with annotations I stumbled accross the following piece of code (it's the Hibernate @NotNull annotation):

@Target(value = {ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.PARAMETER})
@Retention(value = RetentionPolicy.RUNTIME)
@Documented
@Constraint(validatedBy = {})
public @interface NotNull {

    @Target(value = {ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.PARAMETER})
    @Retention(value = RetentionPolicy.RUNTIME)
    @Documented
    public @interface List {

        public NotNull[] value();
    }

    public String message() default "{javax.validation.constraints.NotNull.message}";

    public Class<?>[] groups() default {};

    public Class<? extends Payload>[] payload() default {};
}

I was wondering about the default keyword/construct in the method definition, which I've never seen before. As I understood, it lets you define a default value for this method (or annotation property).

Now I was trying to apply this construct to a normal interface, but it failed. This would fail to compile:

public interface DefaultTest {
    public String test() default "value";
}

But this would work:

public @interface DefaultTest {
    public String test() default "value";
}

So my question is: Is the default keyword annotation-specific? And if yes, what speaks against using this construct in normal interface definitions?

Answer 1

It can only be used for annotations. If you want a default return value for an interface you need to use an abstract class instead.

public abstract class DefaultTest {

  public String test() {
    return "value";
  }

}

The reason they don't have default values for interfaces is because you could inherit multiple interfaces and what would happen if each interface had a different default value?

Answer 2

in interfaces methods have to written. got it.