Java Generics and unchecked cast

Question

I'm struggling with this aspect of Generics in Java. Hopefully someone can help me see the ways.

I have a class that holds a List of objects. This code works, but I want to get rid of the cast. How can I make this more generic?

public class Executor {
    List<BaseRequest<BaseObj>> mRequests = new ArrayList<BaseRequest<BaseObj>>();

    public Executor() {
    }

    @SuppressWarnings("unchecked")
    public <T extends BaseObj> void add(final BaseRequest<T> request) {
        mRequests.add((BaseRequest<BaseObj>) request);
    }

    public void execute() {
        for (BaseRequest<BaseObj> r : mRequests) {
            // DO SOMETHING WITH r
        }
    }
}

Answer 1

In the posted snippet you need the cast because BaseRequest<? extends BaseObj> is not a subtype of BaseRequest<BaseObj>, and the cast can't be checked at runtime because of type erasure, and that's why the compiler warns you. But if you change the declaration of mRequests:

public class Executor  {

    List<BaseRequest<? extends BaseObj>> mRequests = new ArrayList<>();

    public Executor() {
    }

    public <T extends BaseObj> void add(final BaseRequest<T> request) {
        mRequests.add(request);
    }

    public void execute() {
        for (BaseRequest<? extends BaseObj> r : mRequests) {
            // DO SOMETHING WITH r
        }
    }
}

class BaseRequest<T> {}

class BaseObj {}

Let's resolve the problem step-by-step. You want to be able to call

req.add(new BaseRequest<ExtObj1>());
req.add(new BaseRequest<ExtObj2>());
req.add(new BaseRequest<ExtObj3>());

where ExtObj[1|2|3] extends BaseObj. Given the List interface:

List<T> {
  void add(T el);
}

we need to find a common supertype for BaseRequest<ExtObj1>, BaseRequest<ExtObj2> and BaseRequest<ExtObj3>. One supertype is BaseRequest<?> and another one is BaseRequest<? extends BaseObj>. I picked the second one because it's the most restrictive possible. You should know that in Java BaseRequest<ExtObj1> is not a subtype of BaseRequest<BaseObj> because generics are invariant.

Now that we have the right declaration for mRequests, finding the API for Executor.add() is straightforward. BTW, if the method body you need is really that simple, you don't even need the type parameter:

public void add(BaseRequest<? extends BaseObj> request) {
  mRequests.add(request);
}