Error setting a default null value for an annotation's field

Question

Why am I getting an error "Attribute value must be constant". Isn't null constant???

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface SomeInterface {
    Class<? extends Foo> bar() default null;// this doesn't compile
}

Answer 1

Try this

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface SomeInterface {
    Class bar() default void.class;
}

It does not require a new class and it is already a keyword in Java that means nothing.