What is the syntax for annotation type elements?

Question

How do I read/understand the following statement in Java?

Class<?>[] groups() default {};

Class < ? extends Payload>[] payload() default {};

I think I can understand them individually but I don't know if I get good sense of what it means in its entirety.

Individually:

1. Class<?> means any class and Class< ? extends Payload> means any class that extends the Payload class
2. [ ] seems to refer to any array of classes. Is that correct?
3. groups() and payload() are method names.
4. default{} Use this when there is no implementation?

I am really not sure how to understand the above statements? Any help would be very much appreciated.

Answer 1

default {} --> Use this when there is no implementation?

In an Annotation definition you can specify a default value for a parameter. The {} after default is an array literal for an empty array. You could also set it to some non empty value. This also works for other objects too. eg the Data annotation from lombok where a default string is declared:

public @interface Data {
    String staticConstructor() default "";
}

Class means any class and "Class< ? extends Payload>" means any class that extends the Payload class

Yes

[ ] --> Seems to refer to any array of classes. Is that correct?

Yes

groups() and payload() are method names.

They are the name of the parameter used in annotation declaration, as well as the name of the getter methods for those values. eg you could define:

@Data(staticConstructor = "of") class Foobar {}

And later you could get the value by using the getter method created:

Data dataAnnotation = Foobar.class.getAnnotation(Data.class);
String staticConstructor = dataAnnotation.staticConstructor();

Note that you will not actually be able to retrieve the value for an annotation in runtime unless the annotation definition is also annotated with @Retention(RetentionPolicy.RUNTIME)