Java Thread Ping Pong example

Question

I\'m trying to understand thread basics, and as a first example I create two thread that write a String on the stdout. As I know the scheduler allows to execute the threads

Answer 1

One of the possible implementations:

public class PingPongDemo {

    private static final int THREADS = 2;

    private static int nextIndex = 0;

    private static String getMessage(int index) {
        return index % 2 == 0 ? "ping" : "pong";
    }

    public static void main(String[] args) throws Throwable {
        var lock = new ReentrantLock();

        var conditions = new Condition[THREADS];
        for (int i = 0; i < conditions.length; i++) {
            conditions[i] = lock.newCondition();
        }

        for (int i = 0; i < THREADS; i++) {
            var index = i;

            new Thread(() -> {
                lock.lock();
                try {
                    while (true) {
                        System.out.println(getMessage(index));
                        try {
                            Thread.sleep(1000);
                        } catch (InterruptedException e) {
                            throw new RuntimeException(e);
                        }

                        nextIndex = (nextIndex + 1) % THREADS;

                        conditions[nextIndex].signal();

                        while (nextIndex != index) {
                            conditions[index].awaitUninterruptibly();
                        }
                    }
                } finally {
                    lock.unlock();
                }
            }).start();

            if (index < THREADS - 1) {
                lock.lock();
                try {
                    while (nextIndex != (index + 1)) {
                        conditions[index + 1].awaitUninterruptibly();
                    }
                } finally {
                    lock.unlock();
                }
            }
        }
    }

}

Here, we're effectively making round-robin output.

Answer 2

Each instance of the PingPongThread is synchronising on itself and not on a shared resource. In order to control the message passing you'll need to synchronise on a shared resource (e.g. your turn variable ?)

However I don't think this is really going to work. I think you should check out wait() and notify() to do this (if you want to understand the threading primitives). See this for an example.