Testing with real instance of DisplaySystem, but mocked out SoundSystem.

IntegrationWithDisplaySystemTest.java

public class IntegrationWithDisplaySystemTest {

    @Test
    public void testDisplaySystem() {
        Injector injector = Guice.createInjector(new ComputerTestModule()
                .withInstance(DisplaySystem.class, new DisplaySystem()));

        Computer computer = injector.getInstance(Computer.class);

        // We are satisfied with no exception being thrown
        computer.showDesktop();
    }

}