@ControllerBuilder //This will generate a builder
public class MainController extends Controller{
  @Extra String testStringExtra;
  @Nullable @Extra int testIntExtra; //Fields annotated with @Nullable are optional
  @Extra("booleanExtraYo") boolean testBooleanExtra;

  public MainController(Bundle args) {
    super(args);
  }

  @NonNull
  @Override
  protected View onCreateView(@NonNull LayoutInflater inflater, @NonNull ViewGroup container) {
    View view = inflater.inflate(R.layout.controller_main, container, false);
    Freight.ship(this);
    return view;
  }
}

public class MainActivity extends AppCompatActivity {

  private Router router;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    ViewGroup container = (ViewGroup) findViewById(R.id.container);

    router = Conductor.attachRouter(this, container, savedInstanceState);
    if(!router.hasRootController()){
      router.setRoot(new MainControllerBuilder()
              .booleanExtraYo(false)
              .testStringExtra("Yeah this is a string")
              .testIntExtra(1)
              .asTransaction());
    }
  }
}

@ControllerBuilder(value = "Login",
                   scope = "Welcome",
                   popChangeHandler = FadeChangeHandler.class,
                   pushChangeHandler = FadeChangeHandler.class)
public class LoginController extends Controller {
    @Extra String username;
    @Nullable @Extra String password;

    public LoginController(Bundle bundle){
        super(bundle);
    }

}

public interface WelcomeNavigator extends Navigator {
  void goToLogin(final String username, @Nullable final String password);
}

public class Freight_WelcomeNavigator extends FreightNavigator implements WelcomeNavigator {
  private final Router router;

  public Freight_WelcomeNavigator(Router router) {
    this.router = router;
  }

  public void goToLogin(final String username, @Nullable final String password) {
    final LoginControllerBuilder builder = new LoginControllerBuilder();
    builder.username(username);
    builder.password(password);
    RouterTransaction rt = builder.asTransaction()
        .tag("Login");
    rt.popChangeHandler(new FadeChangeHandler());
    rt.pushChangeHandler(new FadeChangeHandler());
    router.pushController(rt);
  }
}

public class NavigatorFactory {
    private Router router;

    @Inject
    public NavigatorFactory(Router router) {
        this.router = router;
    }

    public WelcomeNavigator welcomeNavigator() {
        return new Freight_WelcomeNavigator(router);
    }
}