Question

I want to make a thread, which runs, computes something with the data i give it, and returns a few values, or an object. The thread is a part of a Swing GUI.

My question: How can I make a method that runs when I make the thread, and returns an object (or whatever I want it to return)?

My code:

private void nextTurn () {
    // do something
    if (turn == white) {
        try {
            Engine e = new Engine(); // Engine is implemented by runnable
            e.start();
            Move m = e.getBestMove (board);
            // thread should work, next code should be excecuted immediately
        }
        catch (Exception e) {}
    }

    // end of Main class
}

This is the first time I am working with Threads, and I know you should avoid them if possible, but I need it this time for my GUI.
The info on the Oracle site on Threads did not help me out. I am able to make a program with multiple Threads that runs indefinately, but I can’t make it work with functions.

Answer 1

Since this is with a Swing GUI, consider using a SwingWorker object which creates a background thread (all the code run in the doInBackground method), and then can return a final result and/or interim results. Information on how to use this is well documented in the tutorials here:

Concurrency in Swing

SwingWorkers have property change support and thus will allow listeners to observe its state (as a SwingWorker.StateValue) via a PropertyChangeListener. This is one way your program can determine that the thread has completed its processing, get the returned result and go from there.

On an unrelated note, this isn’t in your production code is it?:

catch (Exception e) {}

If so, you will likely want to fix this as ignored exceptions can bite you in the tail big time.

e.g.,

  if (turn == white) {
     try {
        final SwingWorker<Move, Void> mySwingWorker = new SwingWorker<Move, Void>() {
           @Override
           protected Move doInBackground() throws Exception {
              Engine e = new Engine(); // Engine is implemented by runnable
              e.start();
              Move m = e.getBestMove(board);                  
              return m;
           }
        };

        mySwingWorker.addPropertyChangeListener(new PropertyChangeListener() {
           public void propertyChange(PropertyChangeEvent evt) {
              if (StateValue.DONE == mySwingWorker.getState()) {
                 try {
                    Move m = mySwingWorker.get();

                    // TODO: insert code to run on the EDT after move determined

                 } catch (InterruptedException e) {
                    e.printStackTrace();
                 } catch (ExecutionException e) {
                    e.printStackTrace();
                 }
              }
           }
        });
        mySwingWorker.execute();

     } catch (Exception e) {
        e.printStackTrace();
     }
  }