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Java Single Member Annotations



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A single-member annotation contains only one member. It works like a normal annotation except that it allows a shorthand form of specifying the value of the member. When only one member is present, you can simply specify the value for that member when the annotation is applied, you do not need to specify the name of the member. However, in order to use this shorthand, the name of the member must be value.

Java Single-Member Annotations Example

Following is an example program that creates and uses a single-member annotation :

/* Java Program Example - Java Single-Member Annotation */

import java.lang.annotation.*;
import java.lang.reflect.*;

/* a single-member annotation */
@Retention(RetentionPolicy.RUNTIME)
@interface MySingle {
	int value();       // this variable name must be value
}

class Single {
	
	/* annotate a method using a single-member annotation */
	@MySingle(10)
	public static void myMethod() {
		Single obj = new Single();
		
		try {
			Method m = obj.getClass().getMethod()("myMethod");
			
			MySingle anno = m.getAnnotation(MySingle.class);
			
			System.out.println(anno.value());      // displays 10
			
		} catch(NoSuchMethodException exc) {
			System.out.println("Method not found..!!");
		}
	}
	
	public static void main(String args[])
	{
		myMethod();
	}
}

As expected, this program displays the value 10. In the program, @MySingle is used to annotate the myMethod(), as shown here :

@MySingle(10)

Notice that value = need not be specified.

You can use single-value syntax when applying an annotation that has other members, but those other members must all have the default values. For example, here the value abc is added, with a default value of zero :

@interface SomeAnnotat {
	int value();
	int abc() default 0;
}

In cases in which you want to use the default for abc, you can apply @SomeAnnotat, as shown next, by simply specifying the value of value by using the single-member syntax.

@SomeAnnotat(88)

In this case, abc defaults to zero, and value gets the value 88. Of course, to specify a different value for abc requires that both members be explicitly named, as shown here:

@SomeAnnotat(value = 88, abc = 99)

Remember, whenever you are using a single-member annotation, the name of that member must be value.


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