Question

Consider this code:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Bar {
    Foo foo() default FooImpl.FooConstant;
}

Compiler error:

annotation value not of an allowable type

If I replace Foo with FooImpl the code is accepted.

What’s the reason for this behavior?

Answer 1

If I replace Foo with FooImpl the code is accepted.

I would be very surprised if this compiled, unless FooImpl is an enum.

Annotation members may only contain the following:

• primitive type
• String
• Class literal
• annotation
• enum item
• or 1-dimensional arrays of any of the above

It is a compile-time error if the return type of a method declared in
an annotation type is any type other than one of the following: one of
the primitive types, String, Class and any invocation of Class, an
enum type (§8.9), an annotation type, or an array (§10) of one of the
preceding types. It is also a compile-time error if any method
declared in an annotation type has a signature that is
override-equivalent to that of any public or protected method declared
in class Object or in the interface annotation.Annotation.

Source: JLS