multithreading - 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 using a round robin schedule. Thats why I got:

PING PING pong pong pong PING PING PING pong pong

Now I want to use a shared variable, so every thread will know if its your turn:

public class PingPongThread extends Thread {
private String msg;
private static String turn;

public PingPongThread(String msg){
    this.msg = msg;
}
@Override
public void run() {
    while(true) {
        playTurn();
    }

}
public synchronized void playTurn(){
    if (!msg.equals(turn)){
        turn=msg;
        System.out.println(msg);
    }
}
}

Main class:

public class ThreadTest {
    public static void main(String[] args) {
        PingPongThread thread1 = new PingPongThread("PING");
        PingPongThread thread2 = new PingPongThread("pong");
        thread1.start();
        thread2.start();
    }
}

I synchronized the "turn manager" but I still get something like:

PING PING pong pong pong PING PING PING pong pong

Can someone explains what I am missing, and Why I'm not getting Ping pong... ping pong. Thanks!

Answer 1

In conclusion to my discussion with Brian Agnew, I submit this code that uses java.util.concurrent.Phaser to coordinate your ping-pong threads:

static final Phaser p = new Phaser(1);
public static void main(String[] args) {
  t("ping");
  t("pong");
}
private static void t(final String msg) {
  new Thread() { public void run() {
    while (true) {
      System.out.println(msg);
      p.awaitAdvance(p.arrive()+1);
    }
  }}.start();
}

The key difference between this solution and the one you attempted to code is that your solution busy-checks a flag, thereby wasting CPU time (and energy!). The correct approach is to use blocking methods that put a thread to sleep until it is notified of the relevant event.