Mockito Spies

That is the collective noun, not the verb, although I suppose Mockito is spying...

Spies are used to make existing classes into Mockito classes. Say your class under test hooks into a Java listener, so you want some kind of dummy listener for it to hook into. You can't do this with Mockito (at least I can't find a way), but you can make your own:

//listener interface
public interface SomethingListener {
    //change that sends the event
    public void somethingChanged(EventObject e);
}

//our little dummy listener provider
public class DummyListenerProvider {

    private SwitchListener listener;

    //this is used by the class under test
    public void addSwitchListener(SwitchListener listener) {
        this.listener = listener;
    }

    //send event, saying the class is the source
    public void triggerEvent() {
        this.listener.somethingChanged(new EventObject(this));
    }

    //used by the event consumer when the event occurs
    public void switchMeOn(boolean isTrue) {
    }
}

Cool, but you still want all the nice little trinkets that come with the Mockito objects. This is where spies come in. You just wrapper the class using spy instead of mock:

DummyListenerProvider spyListenerProvider = spy(new DummyListenerProvider());

ClassUnderTest underTest = new ClassUnderTest(spyListenerProvider);

spyListenerProvider.triggerEvent();

verify(spyListenerProvider).switchMeOn(true);