Java: How to use Thread.join

Question

I'm new to threads. How can I get t.join to work, whereby the thread calling it waits until t is done executing?

This code would just freeze the program, because the thread is waiting for itself to die, right?

public static void main(String[] args) throws InterruptedException {
 Thread t0 = new Thready();
 t0.start();

}

@Override
public void run() {
 for (String s : info) {
  try {
   join();
   Thread.sleep(1000);
  } catch (InterruptedException e) {
   e.printStackTrace();
  }
  System.out.printf("%s %s%n", getName(), s);
 } 
}

What would I do if I wanted to have two threads, one of which prints out half the info array, then waits for the other to finish before doing the rest?

Answer 1

Use something like this:

public void executeMultiThread(int numThreads)
   throws Exception
{
    List threads = new ArrayList();

    for (int i = 0; i < numThreads; i++)
    {
        Thread t = new Thread(new Runnable()
        {
            public void run()
            {
                // do your work
            }
        });

        // System.out.println("STARTING: " + t);
        t.start();
        threads.add(t);
    }

    for (int i = 0; i < threads.size(); i++)
    {
        // Big number to wait so this can be debugged
        // System.out.println("JOINING: " + threads.get(i));
        ((Thread)threads.get(i)).join(1000000);
    }

Answer 2

You have to call the join method on the other Thread.
Something like:

@Override
public void run() {
    String[] info = new String[] {"abc", "def", "ghi", "jkl"};

    Thread other = new OtherThread();
    other.start();

    for (int i = 0; i < info.length; i++) {
        try {
            if (i == info.length / 2) {
                other.join();    // wait for other to terminate
            }
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s %s%n", getName(), info[i]);
    }       
}

Answer 3

With otherThread being the other thread, you can do something like this:

@Override
public void run() {
    int i = 0;
    int half = (info.size() / 2);

    for (String s : info) {
        i++;
        if (i == half) {
        try {
            otherThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s %s%n", getName(), s);
        Thread.yield(); //Give other threads a chance to do their work
    }       
}

The Java-tutorial from Sun: http://java.sun.com/docs/books/tutorial/essential/concurrency/join.html