Question

I want to create tree structure with annotation

@Retention(RetentionPolicy.RUNTIME)
public @interface MyNode {
     String name();
     MyNode next() default null;
}

but compiler tells that it is cycle and hence it is not allowed.

I wonder why it is not allowed and how can I make something like it?

Answer 1

1. Annotations are compile-time constants.
2. Annotation members can only be compile-time constants (Strings, primitives, enums, annotations, class literals).
3. Anything that references itself can’t be a constant, so an annotation can’t reference itself.

The funny thing is: there is a part of the Java Language specification that seems to contradict this:

An annotation on an annotation type declaration is known as a
meta-annotation. An annotation type may be used to annotate its own
declaration. More generally, circularities in the transitive closure
of the “annotates” relation are permitted. For example, it is legal to
annotate an annotation type declaration with another annotation type,
and to annotate the latter type’s declaration with the former type.
(The pre-defined meta-annotation types contain several such
circularities.)
[Source]

But I get a compile error for both this (annotation references itself):

public @interface Funky {
    Funky funky();
}

and this (two annotations reference each other):

public @interface Funky {
    Monkey monkey();
}
public @interface Monkey {
    Funky funky();
}