Java - generic classes hierarchy and generic methods overloading

Question

I have a (non abstract) generic base class A<L,M> , an abstract generic sub class B<K, L, M> extends A<L,M> and a (non abstract) class C extends B<Integer, Integer, Void> :

public class A<L,M> { }

public abstract class B<K, L, M> extends A<L,M> { }

public class C extends B<Integer, Integer, Void> { }

I have a utils class that has a few methods, two are relevant here:

public static <K, L, M> void doSomething(B<K, L, M> b) {...}
public static <L, M>    void doSomething(A<L, M>    a) {...}

To be clear, both methods have the same name.
If I call doSomething(c); (where C c ) it goes as expected to the 1st method.
My issue is with the following code (at another class that uses the utils):

private void doSomethingMoreComplex(A a) { // a is actually C
    Utils.doSomething(a);
}

So here a is actually C but it goes to the second method, I am not sure if I made a mistake in the code or this is actually the expected behavior.

Answer 1

or this is actually the expected behavior.

Yes, it's the expected behavior.

The compiler chooses which method is run, and it doesn't know that it is a B (because you've said it's an A ). So it doesn't know if the first overload is safe to invoke, but it knows the second overload is.

Answer 2

This is the expected behavior. The runtime type of a is inconsequential, as this method resolution is done in compile time. Since doSomething(B) cannot be applied to an argument of type A , the method is resolved to doSomething(A) .

You could handle this yourself by explicitly downcasting a :

private void doSomethingMoreComplex(A a) { // a is actually C
    if (a instanceof B) {
        Utils.doSomething((B) a);
    } else {
        Utils.doSomething(a);
    }
}

... but, well, in a word - yuck.

The more idiomatic way to address this would be to have doSomething as a method of A and override ( not overload ) it in B .