Question

I have simple problems with MT in java. I woudlike to synchronize acces to console. An example:
first thread write in System.out “Number one” Thread2 printl(“Number two”). I woud like to synchronize this thread writing sequentially numer in console without buffering. How do this?

Thread one
Thread two
Thread one
Thread two
...

//Code

package com.example;

public class MyThread implements Runnable{


@Override
synchronized public void run(){
    while(true){
        System.out.println("Thread first");

    }
}

}

//

package com.example;

public class MyThread2 implements Runnable {


@Override
synchronized public void run() {
    // TODO Auto-generated method stub
    System.out.println("");

}

}

//

 package com.example;

import java.util.concurrent.Semaphore;

public class Main {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

Thread th1= new Thread(new MyThread());
Thread th2= new Thread(new MyThread2());

th2.setPriority(Thread.MAX_PRIORITY);
th1.start();
th2.start();
    }

}

Answer 1

Well it’s a rather useless question I fear, because if we want a sequential ordering threads are rather useless, but oh well, here’s one solution: Assume we have N threads.

Thread 0 should write number 0, N, 2N, 3N,..

Thread 1 should write number 1, N+1, 2N+1, 3N+1

well you get the pattern. How to do this? Simple enough, we basically just need a method that waits until it’s our turn to write. The real simple solution with static objects for simplicity:

private static volatile int globalVal = 0;
private static Object lock = new Object();

public void writeInt(int val) {
    synchronized(lock) {
        while (globalVal != val) {
            try {
                lock.wait();
            }
            catch(InterruptedException e) {
            }
        }
        System.out.println(val);
        globalVal++;
        lock.notifyAll();
    }
}

the logic in the threads itself is left as an exercise for the reader.