Java泛型导致问题

Question

My IDE is complaining that "NCM_Callable cannot be converted to Callable<ReturnInterface<? extends Object>> on this line this.ph.submitCallable(new NCM_Callable(this, new DeviceGroup(this.deviceGroupNames.get(i)), ph)); In the "fetchDevices()" method

I just want to be able to pass Callables to my ecs that returns a ReturnInterface containing any type of object.

I suspect there is something wrong with my usage of <> generic definitions, but I can't seem to figure out what it is. Any help would be appreciated.

  @Override
public void fetchDevices() throws Exception {
    System.out.println("[NCM_Fetcher]fetchingDevices()");
    for (int i = 0; i < this.deviceGroupNames.size(); i++) {
        System.out.println("[NCM_Fetcher]fetchingDevices().submitting DeviceGroup Callable " + i+ " of "+this.deviceGroupNames.size());
        this.ph.submitCallable(new NCM_Callable(this, new DeviceGroup(this.deviceGroupNames.get(i)), ph));
    }
    this.ph.execute();//See progressBarhelper below
}

ProgressBarHelper: I have a strange error at "ecs.submit()". From what I've read, it seems like I may need a helper method? How do I fix?

public class ProgressBarHelper extends SwingWorker<Void, ReturnInterface> implements ActionListener {
ExecutorService pool = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

protected CompletionService<ReturnInterface<?>> ecs;

public final void submitCallable(Callable<? extends ReturnInterface<?>> c) {
    //create a map for this future
    ecs.submit(c);
    this.callables.add(c);//Error here is Callable<CAP#1 cannot be converted to Callable<ReturnInterface<?>>
    System.out.println("[ProgressBarHelper]submitted");

}
}

And Finally, the NCM_Callable class with its Generics.

public class NCM_Callable implements Callable<ReturnInterface<ResultSet>>, ReturnInterface<ResultSet> {

Answer 1

As far as I could check, the problematic line is ecs.submit :

public final void submitCallable(Callable<? extends ReturnInterface<?>> c) {
    // create a map for this future
    ecs.submit(c); // Error here is Callable<CAP#1 cannot be converted to Callable<ReturnInterface<?>>
   this.callables.add(c); // this is fine on my machine
}

The problem is that java generics is invariant . There's a workaround for this: you can replace the wildcard with a named, bound type :

public final <T extends ReturnInterface<?>> void // this is how you name and bind the type
   submitCallable(Callable<T> c) { // here you are referring to it
    // create a map for this future
    ecs.submit((Callable<ReturnInterface<?>>) c); // here you need to cast. 
    ...
}

The casting is okay in runtime due to type erasure . But then be very careful on what you do with those Callables .

Finally, this is how you can invoke this function:

private static class ReturnInterfaceImpl implements ReturnInterface<String>  {

};

public void foo() {
    Callable<ReturnInterfaceImpl> c = new Callable<ReturnInterfaceImpl>() {
        @Override
        public ReturnInterfaceImpl call() throws Exception {
            return null;
        }
    };
    submitCallable(c);
}