java custom annotation: make an attribute optional

Question

I defined my own custom annotation

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType();
}

how, if at all, can I make the attribute optional

Thanks; I don't know that attribute is mandatory to declare if 'default' is not provided :) — Kanagavelu Sugumar, Nov 26 '13 at 12:32

score 106 · Accepted Answer · edited Sep 29 '15 at 04:14

106

You can provide a default value for the attribute:

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType() default Object.class;
}

edited Sep 29 '15 at 04:14

Sam Liao

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answered Aug 31 '09 at 15:17

Dan Dyer

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found it before you, but I think it is fair to select your answer – flybywire Aug 31 '09 at 15:26
4

@flybywire How can you say that you found it before Dan as you answered at 15:18 while he answered at 15:17 ? – redochka Mar 15 '12 at 14:19
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Maybe Dan types faster :-) – Tom Bartel Sep 19 '12 at 06:43
Link is broken. Is this the right replacement? http://docs.oracle.com/javase/tutorial/java/annotations/declaring.html – Craig P. Motlin May 29 '13 at 22:38
5

FYI for anyone else passing an array of Strings to their annotation. I found I was able to use `String [] names() default {}` to pass an empty array by default. – Marvo Jun 22 '15 at 20:28

score 6 · Answer 2 · answered Aug 31 '09 at 15:18

6

Found it. It can't be optional, but a default can be declared like this:

@Target(value={ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface MyCustomAnnotation  {
    Class<?> myType() default String.class;
}

If no default can make sense as "empty" value then that is a problem.

answered Aug 31 '09 at 15:18

flybywire

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If you need a default class, and you might actually need String.class as a value, consider using java.util.Void.class instead (as you can't instantiate or really use it in any other way) – Ajax Sep 22 '12 at 14:50

score 1 · Answer 3 · answered Oct 13 '13 at 07:06

For Optional attribute you need to provide default value for that attribute you can provide default value using "default" keyword.

Note : For only one attribute you can use attribute name as value. If you use your attribute name as value you can directly pass value like this @MyCustomAnnotation(true) instead of @MyCustomAnnotation(myType = true).

See this example for more details

java custom annotation: make an attribute optional

3 Answers3