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         


        

Answer 1

Class bar() default null;// this doesn't compile

As mentioned, the Java language specification does not allow null values in the annotations defaults.

What I tend to do is to do is to define DEFAULT_VALUE constants at the top of the annotation definition. Something like:

public @interface MyAnnotation {
    public static final String DEFAULT_PREFIX = "__class-name-here__ default";
    ...
    public String prefix() default DEFAULT_PREFIX;
}

Then in my code I do something like:

if (myAnnotation.prefix().equals(MyAnnotation.DEFAULT_PREFIX)) { ... }

In your case with a class, I would just define a marker class. Unfortunately you can't have a constant for this so instead you need to do something like:

public @interface MyAnnotation {
    public static Class DEFAULT_BAR = DefaultBar.class;
    ...
    Class bar() default DEFAULT_BAR;
}

Your DefaultFoo class would just an empty implementation so that the code can do:

if (myAnnotation.bar() == MyAnnotation.DEFAULT_BAR) { ... }

Hope this helps.