Java Annotation default values

Introduction

We can set annotation members with default values.

The default value is used when no value is set when using the annotation.

A default value is set by adding a default clause to a member's declaration.

It has this general form: 

type member() default value ;

Here, value must be of a type compatible with type.

Here is @MyAnno rewritten to include default values: 

// An annotation type declaration that includes defaults.  
@Retention(RetentionPolicy.RUNTIME)  
@interface MyAnno {  
  String str() default "DefaultStringValue";  
  int val() default 99999;  
}

The following are the four ways that we can use @MyAnno: 

@MyAnno() // both str and val default  
@MyAnno(str = "some string") // val defaults  
@MyAnno(val = 100) // str defaults  
@MyAnno(str = "Testing", val = 100) // no defaults

The following code demonstrates the use of default values in an annotation. 


import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Method;

// An annotation type declaration that includes defaults. 
@Retention(RetentionPolicy.RUNTIME)
@interface MyAnno {
  String str() default "Testing";

  int val() default 9000;
}

public class Main {

  // Annotate a method using the default values.
  @MyAnno()//from w ww. ja  v a  2s .  c  om
  public static void myMethod() {
    Main ob = new Main();

    // Obtain the annotation for this method
    // and display the values of the members.
    try {
      Class<?> c = ob.getClass();

      Method m = c.getMethod("myMethod");

      MyAnno anno = m.getAnnotation(MyAnno.class);

      System.out.println(anno.str() + " " + anno.val());
    } catch (NoSuchMethodException exc) {
      System.out.println("Method Not Found.");
    }
  }

  public static void main(String args[]) {
    myMethod();
  }
}



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