Question

I need a service lookup method that returns a compatible service. These particular services are (non-generic) subclasses of the same generic superclass, and I’d like to use generics to assure compatibility.

Classes:

• class BaseDTO
• class AaaDTO extends BaseDTO
• class BbbDTO extends BaseDTO
• interface BaseService<T extends BaseDTO>
• interface AaaService extends BaseService<AaaDTO>
• interface BbbService extends BaseService<BbbDTO>

What generics definitions must I put into the placeholders in the resolver class below? I want to express that resolve() returns a service implementation that is compatible to the given object.

public class Resolver {
    @Autowired private AaaService aaaService;
    @Autowired private BbbService bbbService;

    public <T ???> BaseService<???> resolve(T object) {
        if (object instanceof AaaDTO) {
            return (???) aaaService;
        } else if (object instanceof BbbDTO) {
            return (???) bbbService;
        } else {
            throw new IllegalArgumentException();
        }
    }
}

Simply said I want to tell the compiler that the following is safe:

BaseDTO dto = getDtoFromSomewhereElse();
resolver.resolve(dto).foo(dto);

Answer 1

If your method foo is defined in BaseServiceas follows:

public void foo(BaseDTO dto);

You could do the following:

 public <T extends BaseDTO> BaseService<T> resolve(T object) {
        if (object instanceof AaaDTO) {
            return (BaseService<T>) aaaService;
        } else if (object instanceof BbbDTO) {
            return (BaseService<T>) bbbService;
        } else {
            throw new IllegalArgumentException();
        }
    }