Submitting FutureTasks to an Executor - why does it work?
java, oop
Solution
From the executor's point of view, you've submitted a `Runnable` task. The `run` method of this task (implemented by `FutureTask`) is what calls `done` at the appropriate time. The executor doesn't make any direct call to `done`.
Problem
I have the following test code. ``` 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(); } } ``` This works fine - the overridden 'done' methond in MyTask is called when the task is done. But how does the executor know how to call that ? The executor only have these submit methods: ``` public <T> Future<T> submit(Callable<T> task); public Future<?> submit(Runnable task); ``` Internally it seems 'submit' wraps the callable/runnable in a new FutureTask(). As far as the executor is concerned I've submitted a Runnable or Callable - from what I gather from these 2 signatures. How does it know I submitted a FutureTask and know how to call my overridden done() ?