Java Generics Unchecked Cast Warning: abstract class A<T extends A>

Question

Yet another Java Generics unchecked cast question coming up:

We have something like the following:

public abstract class A<T extends A> {

    public final T doSomething() {
        return (T)this; //unchecked cast A<T> to T
    }
}

So, is this unchecked cast something we can surpress or is there really a case where this can fail?

Answer 1

Yes, it absolutely can fail. Example:

public class B extends A<B> {
    // empty
}

public class C extends A<B> {
    // empty
}

public class AnotherClass {
    public void someMethod() {
        A<B> c = new C();
        B b = c.doSomething();  /* ClassCastException */
    }
}

Note that @awksp's suggested change to ...

abstract class A<T extends A<T>> { /* ... */ }

... does not rescue this example. With that change, the type safety diagnostic in A.doSomething() remains the only one, and of course AnotherClass.someMethod() still throws ClassCastException .

Answer 2

is there really a case where this can fail?

Sure. Here's an example:

class A<T extends A<?>> {
    @SuppressWarnings("unchecked")
    T get() {
        return (T) this;
    }
}

class B<T extends A<T>> extends A<T> {

}

class C<T extends B<T>> extends A<T> {

}

Now C's method will return a C, but C isn't a legal substitution for T extends B<T> , since C doesn't extend B.