Regarding Inherited Annotation

[Ritesh Raman]

I am using following code to use Inherited Annotaion:


------TestAnnotation.java----------------------------------------------
import java.lang.annotation.Inherited;

@Inherited
public @interface TestAnnotation {
boolean isInherited() default true;
String doSomething() default "Do what?";
}

---------------------------------------------------------------------------
---------AnnotationChild.java------------------------------------------

@TestAnnotation
public class AnnotationChild {

public static void main(String [] args){

AnnotationChild ta = new AnnotationChild();
System.out.println("hello"+ta.doSomething());
}
}
--------------------------------------------------------------------------------
now i am getting compile time error.
The method doSomething() is undefined for the type AnnotationChild

i am not able to use the interface properties.
please guide where i m wrong.
Thanks
Ritesh

[Jesper de Jong]

Annotations are not on the SCJP exam, so your post does not belong in the SCJP forum. I will move it to a more appropriate forum for you.

[Rob Spoor]

Annotation methods are not inherited this way. The annotation itself is inherited, and you can retrieve it using reflection. For example:

Class<?> cls = ta.getClass();
TestAnnotation anno = cls.getAnnotation(TestAnnotation.class);
System.out.println(anno.doSomething());

However, the values are the same for all instances of the class; you can't have one instance return false for isInherited() and another true.

This example requires regular interfaces:

public interface TestInterface { 
    boolean isInherited();
    String doSomething();
} 

public class InterfaceChild implements TestInterface { 
    public boolean isInherited() { return true; }
    public String doSomething() { return "Do what?"; }

    public static void main(String [] args){ 

        AnnotationChild ta = new AnnotationChild(); 
        System.out.println("hello"+ta.doSomething()); 
    } 
}

Or, if you're lazy, use an abstract class in between:

public interface TestInterface { 
    boolean isInherited();
    String doSomething();
}

public abstract class AbstractInterface {
    public boolean isInherited() { return true; }
    public String doSomething() { return "Do what?"; }
}

 

public class InterfaceChild extends AbstractInterface { 

    public static void main(String [] args){ 

        AnnotationChild ta = new AnnotationChild(); 
        System.out.println("hello"+ta.doSomething()); 
    } 
}

I think you should read http://java.sun.com/docs/books/tutorial/java/javaOO/annotations.html, it may help you understand annotations better.

[Ritesh Raman]

Hi Rob,

Can you please explain more below line (The annotation itself is inherited).how can Annotation methods are inherited ?
How can we use '@inherited'.if possible please explain with example:

!--Annotation methods are not inherited this way. [b]The annotation itself is inherited, and you can retrieve it using reflection. For example:

i have used the below line in our code , it give NullPointerException at the line no. 3. Pease explain:

1.Class<?> cls = ta.getClass();
2.TestAnnotation anno = cls.getAnnotation(TestAnnotation.class);
3.System.out.println(anno.doSomething()); --![/b]

Thanks
Ritesh

[Rob Spoor]

Ah, right. The retention policy defaults to RetentionPolicy.CLASS. You'll need to change your annotation a bit:

import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Inherited
@Retention(RetentionPolicy.RUNTIME) // this line make sure the annotation is kept at runtime, and can be queried
public @interface TestAnnotation
{
    boolean isInherited() default true;

    String doSomething() default "Do what?";
}