How to use @inherited annotation in Java?

annotations, inheritance, java

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.

Problem

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.

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