design_patterns.txt

Creational patterns: how you create objects.
Structural patterns: how you compose objects.
Behavioral patterns: how you coordinate object interactions.


Creational:
Builder
Dependency Injection
Singleton


Structural:
Adapter
Facade

Behavioral :
Command
Observer
Model View Controller
Model View ViewModel

//dagger 2 & dependency injection

 is the most popular open-source dependency injection framework for Android and was developed in collaboration between Google and Square. You simply annotate a class with @Module, and populate it with @Provides methods such as the following:
@Module
public class AppModule {
  @Provides SharedPreferences provideSharedPreferences(Application app) {
    return app.getSharedPreferences("prefs", Context.MODE_PRIVATE);
  }
}
The module above creates and configures all required objects. As an additional best-practice in larger apps, you could even create multiple modules separated by function.
Then, you make a Component interface to list your modules and the classes you’ll inject:
@Component(modules = AppModule.class)
interface AppComponent {
  ...
}
The component ties together where the dependencies are coming from (the modules), and where they are going to (the injection points).
Finally, you use the @Inject annotation to request the dependency wherever you need it:
@Inject SharedPreferences sharedPreferences;
As an example, you could use this approach in an Activity and then use local storage, without any need for the Activity to know how the SharedPreferences object came to be.
Admittedly this is a simplified overview, but you can read up on the Dagger documentation for more detailed implementation details. This pattern may seem complicated and “magical” at first, but its use can help simplify your activities and fragments.