Java generic interface hierarchy

Question

I have a class hierarchy of entities and want to create a hierarchy of service interfaces for them in Java. UI components shall then access the entity-related services through interfaces:

class BaseEntity { }
class Fruit extends BaseEntity { }
class Banana extends Fruit { }
class Apple extends Fruit { }

A UI component (which is reused in multiple places in slightly different context) needs to access the Fruit service through an interface FruitService and I want to decide during runtime in which places this will be the BananaService or AppleService service interface. I thought this would be simple using generics:

interface Service<T extends BaseEntity>
{
   List<T> getAll();
   void save (T object);
   void delete (T object);
}

// More strict interface only allowed for fruits. Referenced by UI component
interface FruitService<F extends Fruit> extends Service<Fruit> {}

// Interface only allowed for bananas
interface BananaService extends FruitService<Banana> {}

class BananaServiceImpl implements BananaService
{
   // Compiler error here because expecting Fruit type:
   @Override
   public List<Banana> getAll()
   {
   }
   ...
}

This is however giving me the following compiler error:

The return type is incompatible with Service<Fruit>.getAll()

Why does Java not recognize that the implementation has been parametrized with Banana? I would expect the generic parameter in BananaServiceImpl to be resolved to Banana as I specified in BananaService!

Answer 1

interface FruitService<F extends Fruit> extends Service<Fruit> {}

should be

interface FruitService<F extends Fruit> extends Service<F> {}

That way, you pass in the generic over to the Service