Java：泛型捕获错误

Question

I'm trying to make the following code work with no success

protected BackgroundTask<?> backgroundTask = null;

...

protected <T> void confirmBackgroundAction(final BackgroundTask<T> task, final T arg) {
   backgroundTask = task;
   backgroundTask.attach(AbstractWorkerActivity.this);
   backgroundTask.execute(arg);
}

Capture class looks like this:

public abstract class BackgroundTask<T> extends AsyncTask<T, Void, Long> {
...
}

With this code I'm having the following compilation error:

The method execute(capture#26-of ?...) in the type AsyncTask<capture#26-of ?,Void,Long> is not applicable for the arguments (T)

If I replace

backgroundTask.execute(arg);

with

((BackgroundTask<T>)backgroundTask).execute(arg);

I'm not having any compilation error anymore, but I have a ClassCastException at Runtime. Is there a way to pass an instance of the BackgroundTask + the argument to my function ?

Edit: I forget the code from where I call the confirmBackgroundAction() method...

confirmBackgroundAction(new SpecificBackgroundTask(), (Void) null);

Where SpecificBackgroundTask looks like

public class SpecificBackgroundTask extends BackgroundTask<Void> {
...
}

Answer 1

At compile time, the T in BackgroundTask<T> is unknown and can at most (theoretically) store 1 type T of multiple possible types with which you can call confirmBackgroundAction() .

The compiler has no way to solve this consistently. Changing T into an interface you define would fix the ambiguity.

Answer 2

Try this

class Klass {


  protected BackgroundTask<?> backgroundTask = null;

...

  protected <T> void confirmBackgroundAction(final BackgroundTask<T> task, Object arg) {
     backgroundTask = task;
     backgroundTask.attach(AbstractWorkerActivity.this);
     backgroundTask.execute( (T) arg);
  }
}