Different Output with Join method

Question

I read some tutorials and even already asked question here

But I am confused again with join method. What I know Java cannot guarantee the order of execution of threads under normal circumstances.

what i read using join() ,it makes sure that as soon as a thread calls join,the current thread will not execute unless the thread you have called join is finished.

My example which I tried

public class ThreadTest1 extends Thread{

    @Override
    public void run() {
        super.run();
        for(int i=0; i<10; i++)
        {
            System.out.println(i + "   :"+ Thread.currentThread().getName());
        }
    }

    public static void main(String[] args) {

        System.out.println("Thread is getting started");
        ThreadTest1 th0= new ThreadTest1();
        th0.start();
        ThreadTest1 th1= new ThreadTest1();
        th1.start();
        try {
            th1.join(10000);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}

Outputs:

First time output
0   :Thread-1
1   :Thread-1
2   :Thread-1
0   :Thread-0
3   :Thread-1
1   :Thread-0
4   :Thread-1
2   :Thread-0
5   :Thread-1
3   :Thread-0
6   :Thread-1
4   :Thread-0
7   :Thread-1
8   :Thread-1
9   :Thread-1
5   :Thread-0
6   :Thread-0
7   :Thread-0
8   :Thread-0
9   :Thread-0

Second time output:
0   :Thread-1
0   :Thread-0
1   :Thread-1
1   :Thread-0
2   :Thread-1
2   :Thread-0
3   :Thread-1
3   :Thread-0
4   :Thread-1
4   :Thread-0
5   :Thread-1
5   :Thread-0
6   :Thread-1
6   :Thread-0
7   :Thread-1
7   :Thread-0
8   :Thread-1
8   :Thread-0
9   :Thread-1
9   :Thread-0

Why both outputs are different.

I used join() with th1 object. when th0 is running and jvm found th1 thread So thread-1 should be finished first then Thread-0.

Why its is printing Thread-1, Thread-0 without any sequence. If outputs will be like this then what is the use of join() methd?

Can anyone please explain with my outputs.

That `th2.start();` should be `th1.start();`, correct? Same thing for th/th0. — uraimo, Jun 04 '15 at 09:20

uraimo · Accepted Answer · 2015-06-04T09:28:15.157

1

The problem here is that both threads start independently and start printing stuff, what happens between th0.start and th1.start is not deterministic.

Also, it is the Main System Thread of your application that is waiting for th1 to complete, th0 and th1 execute their run method independently from each other, that's why every time you get a differente sequence.

edited Jun 04 '15 at 09:28

answered Jun 04 '15 at 09:22

uraimo

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score 0 · Answer 2 · answered Jun 04 '15 at 09:27

can you please try by using below...

 try {
        th1.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

Actually, by using join(10000) method we are saying to wait atmost 10000ms for the other thread... if other thread did not finish within 10000ms then execution of current thread starts.

The use of join() does not specify the time it waits until other thread completes the execution...

Different Output with Join method

2 Answers2