Question

I’m having a problem properly implementing generics in abstract classes that need to be overridden.

public abstract class AbstractSerachView {
    DataStore<AbstractCriteria, AbstractResults, AbstractService> resultsStore;
    public abstract DataStore<AbstractCriteria, AbstractResults, AbstractService> getResultsStore();
}

the child clases is as follows:

public class TravelSearchView extends AbstractSearchView {
    DataStore<TravelCriteria, TravelResults, TravelService> resultsStore;
    public DataStore<TravelCriteria, TravelResults, TravelService> getResultsStore() {
        return resultsStore;
    }
}

This sort of abstraction does not work at all, although it would be greatly appreciated if I could get this work in some way. What is the correct approach here? I have very little experience with generic types.

The problem is that Eclipse indicates an error in the child class: The return type is incompatible with AbstractSearchView.getResultsStore()

Furthermore, if I leave the child class get method with the 3 abstract types, and override the variable with the 3 travel types, everything is okay except for the return line, where Eclipse indicates: Type mismatch: Cannot convert from DataStore to DataStore

Answer 1

Here are your alternatives:

a) Change the superclass type signature to

    public DataStore<? extends AbstractCriteria, ? extends AbstractResults, ? extends AbstractService> getResultsStore();

b) Add the type parameters to the class as a whole:

 abstract class AbstractSearchView<
     C extends AbstractCriteria,
     R extends AbstractResults,
     S extends AbstractService>
  public abstract DataStore<C, R, S> getResultsStore();
 } 
 class TravelSearchView extends AbstractSearchView<
     TravelCriteria, TravelResults, TravelService> {
   ...
 }

The issue is that DataStore<TravelCriteria, TravelResults, TravelService> is not a subtype of DataStore<AbstractCriteria, AbstractResults, AbstractService>, even if those generic type arguments are respectively subtypes.