Why doesn\'t Java allow interfaces to inherit annotations?

Annotation Inheritance Exclusion in Interfaces: A Deliberate Design Decision

In Java, classes do not inherit annotations from implemented interfaces. This exclusion is explicitly mentioned in the Java documentation for the Inherited annotation: annotations on interfaces have no effect.

The reasoning behind this decision is to prevent inheritance conflicts and ambiguities. Consider the following example:

<code class="java">@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@Inherited
public @interface Baz { String value(); }

interface Foo {
    @Baz("baz")
    void doStuff();
}

interface Bar {
    @Baz("phleem")
    void doStuff();
}

class Flipp {
    @Baz("flopp")
    public void doStuff() {}
}

public class MyClass extends Flipp implements Foo, Bar {}</code>

If interfaces could inherit annotations, then the doStuff method in MyClass would inherit two conflicting annotations, @Baz("baz") from Foo and @Baz("phleem") from Bar. This would create ambiguity in the system, making it unclear which annotation should be applied.

Moreover, annotations on implemented interfaces would introduce multiple inheritance problems. Since Java does not support multiple inheritance at the class level, allowing annotations from interfaces to be inherited would effectively create a form of multiple inheritance. This could lead to even more complex scenarios and potential conflicts.

Therefore, the decision to exclude annotation inheritance from interfaces was made to maintain clarity, avoid conflicts, and prevent potential issues in the implementation of the Java programming language.

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