multithreading - Java: Thread sequential order

Question

Following code found that output results is not the order, not from small to large, how to guarantee it is order from small to large?

java code

public class TestSync {  

    /** 
     * @param args 
     */  
    public static void main(String[] args) {  
        for (int i = 0; i < 10; i++) {  
            new Thread(new Thread1()).start();  
        }  

    }  

    public int getNum(int i) {  
        synchronized (this) {  
            i++;  
        }  
        return i;  
    }  

    static class Thread1 implements Runnable {  
        static Integer value = 0;  
        @Override  
        public void run() {  
            TestSync ts = new TestSync();  
            value = ts.getNum(value);  
            System.out.println("thread1:" + value);  
        }  
    }  

}  

Answer 1

Though one wonders why this is necessary, here is one way to do it. It is not elegant but represents a minimal change to the original program:

import java.util.concurrent.*;

public class TestSync {

    public static void main(String[] args) {

    ExecutorService service = Executors.newSingleThreadExecutor();
    for (int i = 0; i < 10; i++) {
        service.submit(new Thread1());
    }

}

public int getNum(int i) {
    synchronized (this) {
        i++;
    }
    return i;
}

static class Thread1 implements Runnable {
    static Integer value = 0;
    @Override
    public void run() {
        TestSync ts = new TestSync();
        value = ts.getNum(value);
        System.out.println("thread1:" + value);
    }
}

}

Here is a version that is better. It uses an AtomicInteger for the counter (which is probably overkill in this case) to remove the unpleasant getNum() method:

import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

public class TestSync {  
    static private AtomicInteger i = new AtomicInteger(0);

    public static void main(String[] args) {  
        ExecutorService service = Executors.newSingleThreadExecutor();
        for (int i = 0; i < 10; i++) {  
            service.submit(new MyThread(i));  
        }  
        try { Thread.sleep(2*1000); } catch(Exception ex) {}
        service.shutdown();
    }  

    static class MyThread implements Runnable {  
        private int num = 0;
        public MyThread(int num) {
            this.num = num;
        }
        @Override  
        public void run() {  
            int value = i.incrementAndGet();
            System.out.println("thread # " + num + " value = " + value);  
        }  
    }  
}