来自线程的方法调用在哪里运行？

Question

This is the code I wish to write

 void longRunningMethod() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                //a time consuming task
                updateUi();
            }
        }).start();
    }
    public void updateUi(){
         Collections.sort(playerArrayList, Collections.reverseOrder(new Comparator<Player>() {
                    @Override
                    public int compare(Player p1, Player p2) {
                        return Integer.valueOf(p1.getScore()).compareTo(p2.getScore());
                    }
                }));
         adapter.notifyDataSetChanged();
         //some more code here
    }

Firstly : When I call updateUi() from thread, will it run on the thread from where it was called (ie thread of longRunningMethod() ) or will it run in the main thread?

Secondly : as in the code, I am sorting an arraylist of custom objects, Is it safe to run it in the main thread? Or should I use another thread?

I know that about task implementation with completion Listener but I want to run the updateUi method on different thread just to be on safer side.

Answer 1

Any action performed within a thread, including the methods you call, will still be executed in the running thread, and not in the Main thread. Therefore the way you are implementing your updateUI method will make it to still run within the executing thread.

In your specific case, it seems you wish to notify the UI via your Adapter, so you may do as next:

  public void updateUi(){
    Collections.sort(playerArrayList, Collections.reverseOrder(new Comparator<Player>() {
        @Override
        public int compare(Player p1, Player p2) {
            return Integer.valueOf(p1.getScore()).compareTo(p2.getScore());
        }
    }));

    // Notify the UI
    final Runnable runnable = () -> adapter.notifyDataSetChanged();
    new Handler(Looper.getMainLooper()).post(runnable);
}

where the UI gets notified with the next two lines:

final Runnable runnable = () -> adapter.notifyDataSetChanged();
new Handler(Looper.getMainLooper()).post(runnable);

As a side note, if you are only sorting, and the list is quite small which can be completed in a few milliseconds, then probably using a thread isn't a good choice, and you may sort in the UI. Or, if on the contrary, is actually a large list which could block the UI while sorting, then placing such logic in a thread and notify the UI when done is the correct implementation.