Java Thread synchronization - printing out numbers in right order

Question

I'm learning how to work with threads in Java and I need some advice..

I want to print on the standard output numbers from 0..50 with the name of the thread that has done it using three threads.

I have two classes - class Counter that implements Runnable and class Main that creates and runs the threads. Counter has the variable c which is shared among the threads.

My idea was, that I increment c by 1 and then call yield() on the current thread so as the other threads would do the same. Repeat this until c reaches 50.

But it doesen't work, the numbers are printed out in wrong order. How do I fix this?

public class Counter implements Runnable {

Thread t1;
private int c = -1;

public Counter() {
}

public Counter(String name) {
    t1 = new Thread(this, name);
    t1.start();
}

@Override
public void run() {
    while (c < 50) {
        increment();
        Thread.yield();
    }
}

public void increment() {
    if (c < 50) {
        c++;
        System.out.println(Thread.currentThread().getName() + ": " + c);
    }
}
}

public class Main {

public static void main(String[] args) throws IllegalThreadStateException {
    Counter c1 = new Counter();
    Thread t1 = new Thread(c1, "Thread 1");
    Thread t2 = new Thread(c1, "Thread 2");
    Thread t3 = new Thread(c1, "Thread 3");

    t1.start();
    t2.start();
    t3.start();

}

Edit: In the end I solved it this way. Thank you all who helped me with the tough start with multithreading.

import java.util.concurrent.atomic.AtomicInteger;

public class Counter2 implements Runnable {

// you could also use simple int
private AtomicInteger c = new AtomicInteger(-1);
private static final Object syncObject = new Object();

public Counter2() {
}

@Override
public void run() {
    while (c.get() < 50) {
        synchronized (syncObject) {
            if (c.get() < 50) {
                System.out.println(Thread.currentThread().getName() + ": " + c.incrementAndGet());
            }
        }
    }
}    
}

Answer 1

Use syncrhonized section in method increment with special static object.

private static final Object syncObj = new Object();

public void increment()
{
  syncrhonized( syncObj )
  {
   c++;
   System.out.println(c);
  }
}

Or make this method synchronized via its declaration.

But it's wrong idea to store your real data in thread objects. Thread should just to manipulate with share objects but not to store them.\\ And actually I don't understand why do you start thread in

Answer 2

Make increment() synchronized in order to prevent other threads from entering the method concurrently.

In conjunction with yield() you should be able to get another thread print the next number (not always since the system might resume the thread that called yield again - see Ingo's answer - , but the order should still be the same).

synchronized increment() would mean that any thread that tries to enter that method on the same object would have to wait if another thread would have aquired the lock already by entering the method.

Answer 3

Quoting from the javadoc Thread.yield(), emphasis by me:

public static void yield()

A hint to the scheduler that the current thread is willing to yield its current use of a processor. The scheduler is free to ignore this hint .

...

It is rarely appropriate to use this method.

Answer 4

Yes your code won't work. Thread#yield() won't control the thread scheduler in the manner you desire. I"m curious what result you get. You'll probably get repeated numbers and some number that are slightly out of order.

You could use atomic integer which should remove all duplicates. But since the print statement is not atomic. You may still print your results out of order. So you should probably just synchronize the increment method. Also you don't really need yield so dump it.

If the purpose of the problem is to go from thread 1 to thread 2 to thread 3 back to thread 1, etc... Such that the results are

Thread 1:0
Thread 2:1
Thread 3:2
Thread 1:3
Thread 2:4
Thread 3:5
Thread 1:6
Thread 2:7
....

Then you'll need to lock the increment method and use wait and notifyAll. wait will cause other threads to halt processing until the current thread notifies them to start again.