How to use @inherited annotation in Java?

Question

I am not getting the @Inherited annotation in Java. If it automatically inherits the methods for you then if I need to implement the method in my own way then what about that ?

How does will it come to know my way of implementation ?

Plus it is said if I do not want to use this and do it rather in an old fashioned Java way I have to implement the the equals(), toString(), and the hashCode() methods of the Object class and also the annotation type method of the java.lang.annotation.Annotation class.

Why is that?

I have never implemented those even when I did not know about the @Inherited annotation and the programs used to work fine also .

Please somebody explain me from the scratch about this.

Solution

Just that there is no misunderstanding: You do ask about java.lang.annotation.Inherited. This is a annotation for annotations.It means that subclasses of annotated classes are considered having the same annotation as their superclass.

Example

Consider the following 2 Annotations:

@Inherited
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface InheritedAnnotationType {
    
}

and

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface UninheritedAnnotationType {
    
}

If three classes are annotated like this:

@UninheritedAnnotationType
class A {
    
}

@InheritedAnnotationType
class B extends A {
    
}

class C extends B {
    
}

running this code

System.out.println(new A().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println(new B().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println(new C().getClass().getAnnotation(InheritedAnnotationType.class));
System.out.println("_________________________________");
System.out.println(new A().getClass().getAnnotation(UninheritedAnnotationType.class));
System.out.println(new B().getClass().getAnnotation(UninheritedAnnotationType.class));
System.out.println(new C().getClass().getAnnotation(UninheritedAnnotationType.class));

will print a result similar to this (depending on the packages of the annotation):

null
@InheritedAnnotationType()
@InheritedAnnotationType()
_________________________________
@UninheritedAnnotationType()
null
null

As you can see UninheritedAnnotationType is not inherited but C inherits annotation InheritedAnnotationType from B.

I don't know what methods have to do with that.

Comments

[Saurabh]

this works great for classes but why not if these are interfaces? just make class A,B, C as interfaces and use C.class.getAnnotation(InheritedAnnotationType.class) and it will not work?

[fabian]

@saurabh Indeed this doesn't work for interfaces, see javadoc docs.oracle.com/javase/8/docs/api/java/lang/annotation/… : "If an Inherited meta-annotation is present on an annotation type declaration, and the user queries the annotation type on a class declaration, and the class declaration has no annotation for this type, then the class's superclass will automatically be queried for the annotation type." (The hierarchy on classes is "a line", if you consider interfaces too, it could become "a tree" resulting in inefficient lookup and/or conflicts.)

[Guillaume Husta]

We can also simplify the test with A.class.isAnnotationPresent(InheritedAnnotationType.class), which will return a boolean.

[Madbreaks]

@saurabh I know this old but because it's still relevant, the docs state: "Note that this meta-annotation type has no effect if the annotated type is used to annotate anything other than a class. Note also that this meta-annotation only causes annotations to be inherited from superclasses; annotations on implemented interfaces have no effect."