Time for action – injecting subtypes

Although creating a POJO can be an efficient way of creating simple classes, it can be limiting to have a concrete class definition scattered through the class definitions. It is a better design to use either an abstract class or an interface as the service type.

1. Create a new interface in the com.packtpub.e4.application package, called IStringService. Define the process method as abstract:

   public interface IStringService {
     public abstract String process(String string);
   }

2. Modify the existing StringService so that it implements the IStringService interface:

   public class StringService implements IStringService {
     ...
   }

3. Modify the reference in the Rainbow class to refer to the IStringService interface instead of the StringService class:

   @Inject
   private IStringService stringService;

4. Run the application, switch to the Rainbow tab, and a dependency injection fault will be shown in the host Eclipse instance's Console view:

   org.eclipse.e4.core.di.InjectionException...