.
├── app # connects all of the modules together
│   ├── src
│   ├── build.gradle
├── common (library)
│   ├── src # common utilities and resources shared by multiple modules
│   ├── build.gradle
├── onboarding (library)
│   ├── src # onboarding specific source code & resources
│   ├── build.gradle
├── marketplace (library)
│   ├── src # marketplace specific source code & resources
│   ├── build.gradle
├── checkout (library)
│   ├── src # checkout specific source code & resources
│   ├── build.gradle
└── build.gradle

package com.ocandroid.common;

public abstract class AppCommonRouter {

    private static AppCommonRouter instance;

    // required to set instance since static methods can't be abstract
    public static void setInstance(AppCommonRouter routerInstance) {
        instance = routerInstance;
    }

    public static AppCommonRouter getInstance() {
        return instance;
    }

    public abstract void startOnboarding(@NonNull Activity activity);

    public abstract void startMarketplace(@NonNull Activity activity);

    public abstract void startCheckout(@NonNull Activity activity);
}

package com.ocandroid.app;

public class AppRouter extends AppCommonRouter {

    private static AppRouter instance = new AppRouter();

    public static AppRouter getInstance() {
        return instance;
    }

    public void startOnboarding(@NonNull Activity activity) {
        // implementation here
    }

    public void startMarketplace(@NonNull Activity activity) {
        // implementation here
    }

    public void startCheckout(@NonNull Activity activity) {
        // implementation here
    }
}

package com.ocandroid.app;

public class OCAndroidApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();
        AppCommonRouter.setInstance(AppRouter.getInstance());
        // other app creation logic
    }
}

package com.ocandroid.marketplace;

public class MarketplaceActivity extends AppCompatActivity {

    // other app methods and variables

    public void checkoutButtonClicked() {
        AppCommonRouter.startCheckout(this);
    }
}