How can I cancel a task of ExecutorService on click of a button?

Question

I have a list of items on which user can click for download. For which I am having this code:

//Maintained this in my ListAdapter class so that if user clicks on cancel download i can do:
// tasks.get(info.getFieldID()).cancel(true);

public static Map<String, Future<Object>> tasks = new HashMap<String, Future<Object>>();
    Future f = downloadExecutor.submit(new Callable<Object>() {
         // do downloading.
    }
    tasks.put(key,f);

I am trying to cancel this the respective download when user clicks on cancel by calling tasks.get(key).cancel(true);

But my task is not getting cancelled.

Few questions:

1. Am I doing anything wrong by maintaining the list of future objects in the adapter class ?

2. How should i get a task to get cancelled when user clicks on a button?

3. Also how do i know that the task has been cancelled?

I have read about ExecutorService but I am little confused about the implementation.

Answer 1

You are really asking several questions together. The logic behind your problem appears to be connecting a physical action (button press) with an executable action (cancel task).

In simple terms, you seem to recognize the need to store a reference to the task in order to cancel it. In pseudocode (because your particular implementation may differ), you want something like:

Button myButton = (Button)findViewById(R.id.mybutton);
myButton.setOnClickListener(new OnClickListener() {
    @Override
    onClick(View v) {
        tasks
             // retrieve the proper task based on the button
             .get(oneKey)
             // once you have the task, cancel it
             .cancel(true);
    }
});

In the code above, each button will be associated with whichever task you choose, which you can do through hard-coding or passing variables.

Using the hashtable, retrieve the task you want. Then use its cancel method to cancel it.