向执行者提交FutureTask——为什么它有效?

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;

class  MyTask extends FutureTask<String>{
    @Override
    protected void done() {
        System.out.println("Done");
    }

    public MyTask(Runnable runnable) {
        super(runnable,null);
    }
}

public class FutureTaskTest {

    public static void main(String[] args)  {
        ExecutorService executor = Executors.newSingleThreadExecutor();
        FutureTask<String> future = new MyTask(new Runnable() {
            public void run() {
                System.out.println("Running");
            }
        });

        executor.submit(future);

        try {
            future.get();
        } catch (Exception ex ) {
            ex.printStackTrace();
        }
        executor.shutdownNow();

    }
}

public <T> Future<T> submit(Callable<T> task);
public Future<?> submit(Runnable task);

它如何知道我提交了一个FutureTask,并知道如何调用我覆盖的done()?