Use cases for implementing annotations

Question

What are valid use cases for implementing annotations?

When designing primarily annotation based configuration systems I occasionally need to create classes which i

Answer 1

I use it when I created an annotation and want to make its use optional, by providing a default when the annotation is omitted. This could occur when your library introduces a new annotation and you want your library to remain backwards compatible.

In this example, BeanB is written in source code against an older version of your library, so you want to use a default when you identify such a class.

@Id
class BeanA {}

// No annotation
class BeanB {}

The default implementation;

private static final Id DEFAULT_ID = new Id() {

    @Override
    public IDType value() {
        return IDType.LOCAL;
    }

    @Override
    public Class annotationType() {
        return Id.class;
    }
};

Processing;

Id beanId = (bean.getClass().isAnnotationPresent(Id.class))
    ? bean.getClass().getAnnotation(Id.class)
    : DEFAULT_ID;