Not quite the easiest way to override Dagger 2 modules for unit tests...

ModuleOverride.java

@Component(modules=MyModule.class)
@ApplicationScope
public interface MyComponent {
    void inject(MyApplication application);
}

@Module
public class MyModule {
    
    @Provides @ApplicationScope
    public String provideString() {
        return "real string";
    }
}

public class MyApplication extends Application {

    @Inject public String injectedString;

    private MyComponent component;

    public void setComponent(MyComponent component) {
        this.component = component;
        component.inject(this);
        Log.d("Injected: " + injectedString);
    }

    @Override
    public void onCreate() {
        super.onCreate();

        if (component == null) {
            setComponent(DaggerMyComponent.create());
        }
    }
}

public class MyActivityTest {
    
    MyApplication app;

    @Before
    public void setUp() {
        app = (MyApplication) getInstrumentation().getTargetContext().getApplicationContext();

        // This is the sneaky bit. Create a partial mock of the Module you want to override,
        // then just mock the provider methods you want to override.
        MyModule module = Mockito.spy(new MyModule());
        Mockito.doReturn("mocked string").when(module).provideString();

        MyComponent component = DaggerMyComponent.builder()
                .myModule(module)
                .build();

        app.setComponent(component);
    }

    @Test
    public void mock_injected_successfully() {
        assertEquals("mocked string", app.injectedString);
    }
}

... and you still need to call setComponent. You are making that method public in your production code just for testing purposes... That's bad from a design point of view.

@mheras You've got to get a different Component instance in there somehow. I've seen options using build flavors which don't involve modifying the production code, but I'm happy to be pragmatic about it. My personal feeling is "testing trumps encapsulation".

I've posted a new gist here using @tbroyer's idea: https://gist.github.com/vaughandroid/00287633102af6a66dc8