CODEWITHSUNDEEP
javaannotations
jukzi
jukzi

Reputation: 984

Why can java annotation contain a field or even a class?

Whats the use of fields or classes inside annotations?

public @interface Test {
    public String val = "hello"; // WHY??

    public static class MyClass {// WHY??
        public static void main(String[] args) {
            System.out.println(val);
        }
    }

    String value();
}

Upvotes: 0

Views: 118

Answers (1)

Andy Turner
Andy Turner

Reputation: 140318

Fields: because you can define fields in any interface. They're always implicitly public, static and final: in other words, to define constants in the namespace of the interface.

The answer is essentially the same for classes: it's a public, static (but not final) class. it's just a namespace in which to define the class.

To turn the question around: why shouldn't you be able to do this?

There are all sorts of syntactically valid things you can do that you might think are ill-advised. For example:

String String = "String"; String: break String;

is valid, but useless, and confusing. There are 3 different meanings of String here, 4 if you count the literal. This is far less useful than annotation members, but also allowed.

Sometimes it's effort to stop you doing things: it adds complexity to the language, and the compiler to enforce; and in the effort to prevent the odd piece of madness, you might accidentally remove useful expressivity.

Upvotes: 2

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