Threads and 1.4 exam

[Mansi Dave]

Hi everyone,
Do we need to know Thread.join() for the 1.4 exam?
Thanks.
Mansi

[Barkat Mardhani]

I am assuming so. I read from the feedback from those who took 1.4 that test included 8-10 heavy-duty thread questions....

[Ron Newman]

There's not much to know about join. It blocks waiting for the thread you're joining to finish running.

[Timothy Toe]

class B implements Runnable {
  public void run () {
    System.out.println(Thread.currentThread().getName() + " begin");
    for(int i=1; i<11; i++){
      System.out.print(Thread.currentThread().getName() + i);
      try {
        Thread.sleep(500);
      }catch (InterruptedException e){
        System.out.println(Thread.currentThread().getName() + " thread's sleep interrupted !");
      }
    }
    System.out.println("\n" + Thread.currentThread().getName() + " end");
  }
}
public class TestJoin {
  public static void main (String args[]) {
    System.out.println(Thread.currentThread().getName() + " begin");
    Thread t = new Thread(new B(), "T");
    t.start();
    try {
      System.out.println("main will now wait for T to finish.");
      t.join(); //Waits for this thread to die.
    }catch(InterruptedException e){
      System.out.println("main thread's wait to join after T interrupted !");
    }
    System.out.println(Thread.currentThread().getName() + " end");
  }
}  

This program shows the working of the ThreadObject.join() method. The join() method forces a thread to wait for another thread to die before it is allowed to continue executing. Or, put in another way, thread A may call thread B’s join() method if A wants to stop executing and wait for thread B to finish before A continues.
Without t.join() – notice that t’s count is after main’s begin and end. This is because both the main and t threads execute independent of each other and since t sleeps, the main thread has plenty of CPU time to finish executing.

----
main begin
main end
T begin
T1T2T3T4T5T6T7T8T9T10
T end
----

With t.join() - The output shows that T’s counting activity is between main’s begin and end. Now main waits for t to die 1st before continuing – because main calls t.join(). By calling this method, main tells the t thread that “I will join after you !”

----
main begin
main will now wait for T to finish.
T begin
T1T2T3T4T5T6T7T8T9T10
T end
main end
----

Some extra experimenting :
Call t.join() before calling t.start() – This means main is telling t that it wants to join at the end of t’s execution EVEN BEFORE t STARTED EXECUTING. t.join() will have no effect on main. Maybe t replies : “you may continue since I have not even started executing… do not wait for me.” Join only makes main wait if t is already started.

------
main begin
main end
T begin
T1T2T3T4T5T6T7T8T9T10
T end
------