Interface generics and instanceof

Question

My interface has a generic type argument. Any class which would implement the interface has to declare this type, so later users of it will know what they get back. Now, I need to do checks with instanceof on these classes to see if they are an instance of Action.

My question is: what will instanceof return if i did:

if (SomeAction instanceof Action<?>) {
    . . .

}

with this SomeAction class:

public class SomeAction implements Action<String> {
    . . .

}

Will it return true, or false? And what if I did SomeAction instanceof Action<Integer>

Answer 1

Java doesn't care about the generic type when you use instanceof. To quote the Javadocs:

Because the Java compiler erases all type parameters in generic code, you cannot verify which parameterized type for a generic type is being used at runtime

So, you can test against Action<?> or Action, but you will get a compiler error if you try to test against Action<Integer>. The error message is very informative:

Cannot perform instanceof check against parameterized type Action<Integer>. Use the form Action<?> instead since further generic type information will be erased at runtime

Answer 2

I implemented it and it returns true.

Also if you leave out generics all together, like this:

if (SomeAction instanceof Action) {
    . . .

}

If you try to actually include a generic type for Action in that if operation the code doesn't compile, with the message:

illegal generic type for instance of

for instance:

if (action instanceof Action<String>) {
            System.out.println("is Action");
} else {
            System.out.println("is not Action");
}