04. Interface injection

###Adding objects to a container using interfaces

In the previous section we already saw the most basic way of adding objects to a container by calling

container.add(MyClass.class)

In practice, our object's collaborators are not only concrete classes. In a well designed system we usually have interfaces and different implementations of the interfaces are used for production and testing. Let's take a look at the following example.

public class ReservationService {

    private ReservationRepository repository;

    public ReservationService(ReservationRepository repository) {
        this.repository = repository;
    }

    public Confirmation reserve(Room room) {
        // some logic using our repository
    }
} 

A reservation service is using a ReservationRepository as a collaborator. ReservationRepository is an interface.

public interface ReservationRepository { 
    public Confirmation reserve(Room room); 
}

A reservation repository has two implementations: database-based and in-memory.

public interface DBReservationRepository implements ReservationRepository { 
    public Confirmation reserve(Room room) { // goes to a database }
}

public interface InMemoryReservationRepository implements ReservationsRepository {
    public Confirmation reserv(Room room) { // saves reservations in memory }
} 

In our production code we'd like to use a database-based implementation.

Container container = new SimpleContainer();
container.add(ReservationService.class);
container.add(ReservationRepository.class, DBReservationRepository.class); 

But in tests we'd like to use an in-memory representation.

Container container = new SimpleContainer();
container.add(ReservationService.class);
container.add(ReservationRepository.class, InMemoryReservationRepository.class); 

As you can see, in Yadic you can add objects to the container using an interface name. Actually, you can think of Yadic as a map with class/interface name keys and values being objects of a given type. To retrieve the object back form a container, you have to use an interface name, not a concrete class.

ReservationRepository repository = container.get(ReservationRepository.class);