Regarding starting the threads on a condition

Question

In an interview today I was asked a question I could not solve:

Let's say there are three threads named T1 ,T2 and T3. Now T1 will print the numbers from 1 to 5, T2 will print the numbers from 6 to 10 and T3 will print the numbers from 10 to 15. The three threads have been started by say t1.start(), t2.start()and t3.start(), but the T3 thread should be executed first, then the T1 thread, and lastly the T2 thread should be executed.

Please advise on possible approaches to this problem. As per my research there is the concept of Thread.join, or cyclic barriers. Please advise on the best approach, and if possible please show a small piece of code for better understanding.

Answer 1

I agree that this question isn't great, but as you do want some sort of answer...

This prints out 10-15, 1-5, 6-10, as per your requirement (T3 then T1 then T2) (btw - should T3 actually print from 11 to 15?):

public class Test {
  static class Printer implements Runnable {
    private final int from;
    private final int to;
    private Thread joinThread;

    Printer(int from, int to, Thread joinThread) {
      this.from = from;
      this.to = to;
      this.joinThread = joinThread;
    }

    @Override
    public void run() {
      if(joinThread != null) {
        try {
          joinThread.join();
        } catch (InterruptedException e) { /* ignore for test purposes */ }
      }
      for (int i = from; i <= to; i++) {
        System.out.println(i);
      }
    }
  }
  public static void main(String[] args) throws InterruptedException {

    Thread T3 = new Thread(new Printer(10, 15, null));
    Thread T1 = new Thread(new Printer(1, 5, T3));
    Thread T2 = new Thread(new Printer(6, 10, T1));
    T1.start();
    T2.start();
    T3.start();
  }
}

CountdownLatch is also a good alternative here perhaps.

Answer 2

There is no real best approach. They were most likely, looking to see how you attempt to answer the question.

When dealing with vague questions like this, I would first try to really determine what they are looking for. Do they need to execute in order? printing out the numbers in sequence, or does t3 need to print (10-15) while the other two threads wait. Or do they just need to start in the order directed and print out whenever they get to it. Reading the question as its stated seems to be just having each thread print out a defined sequence, after starting them in a defined order. That says nothing about blocking. You could just make a wrapper class holding a priority, stick them in a ordered queue, and iterate through the collection starting each thread.

I think that they may also have been looking for you to say something about the fact that it doesnt make sense to use multiple threads (assuming they want the numbers sequential) in this case, since regardless of anything else two of the three threads are doing nothing. It would be faster to only use one thread. Then add something about it might be more efficient to use some sort executor service where each thread could pull the next number to write out of a common sorted resource. then you could go into ways to protect the queue to ensure that only one is removed/written at a time. There are a lot of ways to do this, and a lot of issues with each. It also might have been good to mention something about the benefits/costs of blocking vs non-blocking as well. Since there are many different ways to do this, it probably makes sense to just familiarize yourself with the Concurrencys api.

The Concurrency util classes in Java are pretty well documented and in most cases give useful examples. This is a good place to start. Try taking a look at Semaphores,Phasers, or A Count Down Latch for this. Though, in practice Executors or The Fork/Join Framework might be worth looking into a little more.

Sorry about all that rambling but the point that I am trying to make is that there is no one best way to answer something like this. I think what this question was really about was trying to see your approach. Asking for code example will not really help you that much. Just take some time and sit down and read through some good tutorials.