Returning a generic Interface with concrete implementation in java

Question

I have a generic Interface which is implemented by a class, that I want to return in a generic method. Interface:

public interface IWorker<T extends Object, K extends Object> {  
public K doWork(T o);
}

Implementation:

public class WorkerImpl implements IWorker<String, List<Serializable>>
{
   @Override
   public List<Serializable> doWork(String s)
   {
       return ...
   }
}

ActionCoordinator Interface for the generic method returning the implementation:

public interface IActionCoordinator
{
    public <T extends Serializable, K extends Serializable> IWorker<T, K> getAction(T  request);
}

ActionCoordinator implementation:

public class ActionCoordinatorImpl implements IActionCoordinator
{
    @Override
    public <T extends Serializable, K extends Serializable> IWorker<T, K> getAction(final T requ)
    {
        return (IWorker<T,K>)new WorkerImpl();
    }
}

Problem: In eclipse this will work, but doing a maven build with the JDK 1.6.0_35 doesn't and says "inconvertible types". I can get around with that:

public class ActionCoordinatorImpl implements IActionCoordinator
{
    @Override
     public <T extends Serializable, K extends Serializable> IWorker<T, K> getAction(final T requ)
    {
        Object temp = new WorkerImpl();
        return (IWorker<T,K>)temp;
    }
}

But that it's not supposed to be, that wouldn't be type-safe at all.

Some ideas would be nice. Thanks in advance...

EDIT: What works for me now is the following: I changed all the generic Ts and Ks to be Serializable. Thats all what I needed to restrict the WorkerImpl to be. the actual caller still needs an IWorker<T,K> but IWorker<Serializable,Serializable> fits and works...thanks everyone, but I still wonder why eclipse is not saying anything...

Answer 1

The problem is not that maven won't compile, it's that Eclipse isn't complaining about an unsafe cast: You probably have an option turned off in your Eclipse preferences and are doing a strict compile in maven.

Your WorkerImpl is unsuitable to return from your factory method in ActionCoordinatorImpl , because there's no guarantee that the Serializable passed to its doWork() method will be a String - it just has to be any Serializable .

Also, you could simplify your code by changing IWorker from

public interface IWorker<T extends Object, K extends Object>

to

public interface IWorker<T, K>

Since they are equivalent (everything extends Object)

Answer 2

你是否正确设置了maven的源/目标 ？

Answer 3

Your WorkerImpl only implements IWorker<String, List<Serializable>> . You then try to make it an IWorker<T extends Serializable, K extends Serializable> in ActionCoordinatorImpl which is not correct and should be reported as an error.

It would be difficlt to recommend a fix until you are clear what you want to do.

Answer 4

new WorkerImpl() yields IWorker<String, List<Serializable>> , how are you going to convert it to IWorker<T,K> ? And where do you think compiler should get the type K ? Will you always pass it explicitly when invoking getAction ? Then you have to pass type T also, but it is also set by the parameter requ , and the invocation would look ugly.