How to create optional parameters for own annotations?

Question

Following is the annotation code

public @interface ColumnName {
   String value();
   String datatype();
 }

I would like to make datatype an optional parameter, for example

@ColumnName(value="password")

should be a valid code.

score 141 · Accepted Answer · edited Dec 19 '13 at 08:47

141

Seems like the first example in the official documentation says it all ...

/**
 * Describes the Request-For-Enhancement(RFE) that led
 * to the presence of the annotated API element.
 */
public @interface RequestForEnhancement {
    int    id();
    String synopsis();
    String engineer() default "[unassigned]"; 
    String date()     default "[unimplemented]"; 
}

edited Dec 19 '13 at 08:47

Steve Chambers

37,270
24
156
208

answered Aug 19 '10 at 09:33

Riduidel

22,052
14
85
185

2

i only looked at the tutorials.ie why i couldnt find this.may i know the difference between unassigned and unimplemented – Biju CD Aug 19 '10 at 10:23
17

There is not difference. They're just String values. They could be "peter" and "paul". – sparkyspider Jan 21 '12 at 06:49
9

And what if my params are `Class>`s? – Aetherus May 03 '16 at 03:57
2

In that case, Java's type system still applies. For example `Class> proxy() default Object.class` – Michael Miller Feb 27 '19 at 21:49
4

Thanks! The official documentation may have the example, but this SO post was the top hit on my Goggle search so your response and the link are helpful! – cptully Nov 03 '21 at 20:56

score 40 · Answer 2 · answered Aug 19 '10 at 09:35

40

To make it optional you can assign it a default value like that:

public @interface ColumnName {
   String value();
   String datatype() default "String";
 }

Then it doesn't need to be specified when using the Annotation.

answered Aug 19 '10 at 09:35

Johannes Wachter

2,715
17
17

Marian Klühspies · Answer 3 · 2022-12-19T13:51:27.340

For custom types you can do something like this

public @interface SomeAnnotation {

  Class<? extends SomeInterface> yourCustomType() default SomeNullInterface.class;
  
}

/**
 * Your custom type
 */
public interface SomeInterface {

}

/**
 * Your fake null value type
 */
public interface SomeNullInterface extends SomeInterface {

}

And somewhere in your code you can then check for null like this

if(!yourAnnotation.yourCustomType().isAssignableFrom(SomeNullInterface.class)){
  //your value is null
}

How to create optional parameters for own annotations?

3 Answers3