Re: Yet another generics question: Needs unchecked conversion to conform to ...

Michel T. wrote:


You have not and in general cannot verify that the object is of that
type. The signature is wrong. For instance, in your example ServiceX is
not a Service<ServiceComponent> but it is a Service<? extends
ServiceComponent>.

Generally a good move when using reflection and generics, is to add a
layer of indirection, so you don't have to mix the two.

So instead of loading Service<? extends ServiceComponent>, load a
ServiceProvider:

interface ServiceProvider {
     <T extends ServiceComponent> Service<T> find(
         Class<T> componentClass
     );
}

class ServiceProviderX implements ServiceProvider {
     @SuppressWarnings("unchecked")
     <T extends ServiceComponent> Service<T> find(
         Class<T> componentClass
     ) {
          if (componentClass != ServiceComponentX.class) {
              throw new SomeException();
          }
          return (Service<T>)new ServiceX();
     }
}

I think you are stuck with the unchecked warning there, but at least the
code is now sound. Alternatively, add an asSubclass-like method to Service.

Really, we want a Service interface for each type of ServiceComponent. So:

public interface ServiceComponent {
}
public interface Service<T extends ServiceComponent> {
     T createServiceComponent();
     void doSomethingWith(T component);
}
public interface XServiceComponent implements ServiceComponent {
}
public interface XService extends Service<XServiceComponent> {
}
class XServiceComponentImpl implements XServiceComponent {
}
public class XServiceImpl implements XService {
     public XServiceComponent createServiceComponent() {
         return new XServiceComponentImpl();
     }
     public void doSomethingWith(XServiceComponent component) {
         ...
     }
}
....
     public <T extends Service<?>> find(
          Class<T> clazz, String serviceClassName
     ) throws
         NullPointerException,
         LinkageError,
         ExceptionInInitializerError,
         ClassNotFoundException,
         ClassCastException,
         NoSuchMethodException,
         SecurityException,
         InstantiationException,
         IllegalAccessException,
         IllegalArgumentException,
         InvocationTargetException
     {
         ...
         return clazz.cast(service);
     }
....
  // Or: ServiceX service = find(
         Service<XServiceComponent> service = find(
              XService.class, className
         );

Tom Hawtin