How do you mock casts?

An occasional question that comes up on the jMock users' mailing list is how to mock casts? This is, I guess, mostly needed when interfaces are used to define capabilities of an object that are dynamically discovered by some other object in the system, somewhat like this:

public class Juicer {
    public void juice(Juiceable fruit) {
        if (fruit instanceof Peelable) ((Peelable)fruit).peel();
        fruit.juice();
    }
}

If you create a mock Juiceable object when testing the Juicer, how can you create an expectation for the cast to Peelable and make it return a mock Peelable? Casts and the instanceof operator are implemented at the language/VM level and cannot be mocked out. However, you don't need to mock them at all . You can instead define an interface in your test that can be cast to the required interface and mock that.

public class JuicerTest extends MockObjectTestCase {
    interface PeelableAndJuiceable implements Peelable, Juiceable {}

    public void testPeelsPeelableFruitBeforeJuicing() {
        PeelableAndJuiceable fruit = mock(PeelableAndJuiceable.class, "fruit");
        Juicer juicer = new Juicer();

        expects(new InThisOrder() {{
            exactly(1).of (fruit).peel();
            exactly(1).of (fruit).juice();
        }});

        juicer.juice(fruit);
    }
}

David Saff makes a good point: "[You might] find that the new interface captures a new abstraction in your code that is useful in other places. In my experience, if it's worth testing, it's often worth naming."