Typesafe enum in Java

Question

Not sure whether the title is misleading, but requirement is below.

I need to use a string value as input to a custom annotation. When use an enum value, the IDE gives

java attribute value must be constant.

@test("test") // works

@test(Const.myEnum.test.toString()) //java attribute value must be constant

I read about the importance of the string value being immutable. Is it possible to achive through enum (not the public static final String hack).

thanks.

Answer 1

Enums can be used in annotations. You should do it like this:

@test(Const.myEnum.test)

assuming you have defined an enum like this:

package Const;

public enum myEnum {
    test;
}

and the annotation like this:

public @interface test {
    myEnum value();
}

Answer 2

There shouldn't be any problem using an enum, the issue might be with how you are declaring it or the annotation. Here is an example that compiles without any issue.

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.METHOD })
public @interface MyAnnotation {

    MyEnum value();

    public enum MyEnum {
        ONE, TWO, THREE, FOUR
    }
}

public class AnnotationTest {

    @MyAnnotation(MyEnum.ONE)
    public void someMethod() {
        //...
    }

}