private static class WorkerThread extends Thread {

        private final ProgressBarHandler progressBarHandler;
        private final Semaphore[] semaphores;
        private final int index;
        private int progress;

        public WorkerThread(final ProgressBarHandler progressBarHandler, final Semaphore[] semaphores, final int index) {
            this.progressBarHandler = progressBarHandler;
            this.semaphores = semaphores;
            this.index = index;
        }

        /**
         * We use Semaphores here to coordinate the threads because the Semaphore in java is not 'fully-bracketed',
         * which means the thread to release a permit does not have to be the one that has acquired
         * the permit in the first place.
         * We can utilise this feature of Semaphore to let one thread to release a permit for the next thread.
         */
        @Override
        public void run() {
            final Semaphore currentSemaphore = semaphores[index];
            final Semaphore nextSemaphore = semaphores[(index + 1) % semaphores.length];

            try {
                while (true) {
                    currentSemaphore.acquire();

                    sleep(1000); // we use sleep call to mock some lengthy work.
                    Message message = progressBarHandler.obtainMessage();
                    message.arg1 = (progress += 10);
                    progressBarHandler.sendMessage(message);

                    nextSemaphore.release();

                    if (progress == 100) {
                        progress = 0;
                    }
                }

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