how to execute join(); function on mainThread?

Question

I am trying to execute join(); function on main thread TO get a main thread object I have used a reference from Thread.currentThread(); by the following code but it often gives me a NullPointerException as if mainthread has not been intialized:

    public class Main{

     MyThread t1 = new MyThread();

    public static void main(String[] args)  {
      t1.mainthread = Thread.currentThread();

    MyThread t = new MyThread();
    t.start();

    for (int i = 0; i<10 ; i++) 
        System.out.println("main thread");

    }

    }

The child thread classs :

     public class MyThread extends Thread {

     Thread mainthread ;    

@Override
public void run() {

    for (int i = 0; i<10 ; i++) 
        System.out.println("child thread ");
    try {
        mainthread.join();
    } catch (InterruptedException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

}

It often gives me a NullPointerException as if mainthread has not been intialized

Have a look up this https://stackoverflow.com/questions/18479771/java-multithreading-concept-and-join-method — ThuongLe, Dec 03 '19 at 04:01

score 1 · Answer 1 · answered Dec 03 '19 at 04:10

1

That's because you haven't.

In your main, you run t1.mainthread =, referring to static field t1.Then later you create another instance of MyThread, which means it has its own version of the mainthread variable, and you haven't set that one.

answered Dec 03 '19 at 04:10

rzwitserloot

85,357
5
51
72

score 0 · Answer 2 · answered Dec 03 '19 at 04:05

0

Corrected code.You do not join in the childclass usign join.Instead in main you join method to your thread will join the the thread to main

public class Main {
    MyThread t1 = new MyThread();

    public static void main(String[] args) throws InterruptedException {

        MyThread t = new MyThread();
        t.start();

        for (int i = 0; i < 10; i++)
            System.out.println("main thread");

        // This will join your thread with main thread
        t.join();

    }
}

class MyThread extends Thread {

    Thread mainthread;

    @Override
    public void run() {

        for (int i = 0; i < 10; i++) {
            System.out.println("child thread ");
        }

    }
}

answered Dec 03 '19 at 04:05

tarun

218
2
11

I want to make child thread wait until execution of main thread not the opposite – Dec 03 '19 at 04:17
@Esraa Salama : Threads ussually have coordinator and worker concept where worker does processing aka child thread andmain thread aka coordinator waits for child thread to complete its processing/task. Its generally like that.Cannot come up with a scenario where yur code will be helpful. Also using static with threads is risky affair and not a good practice. – tarun Dec 03 '19 at 04:31

how to execute join(); function on mainThread?

2 Answers2