Print integers from 1 to 10 with only 5 Threads in a specific order [duplicate]

Question

I have recently started with multi-threading in Java

I have an issue solving a problem where I have got only 5 Threads ranging from T1, T2,...T5.

The task is to print numbers from 1 to 10 in the following order.

T1 -> 1
T2 -> 2
T3 -> 3
T4 -> 4
T5 -> 5
T1 -> 6
T2 -> 7
T3 -> 8
T4 -> 9
T5 -> 10

I tried solving it with this piece of code.

public static void main(String[] args) throws InterruptedException {
    Counter counter = new Counter();
    Thread[] tArray = new Thread[] { new Thread(counter, "T1"), new Thread(counter, "T2"),
            new Thread(counter, "T3"), new Thread(counter, "T4"), new Thread(counter, "T5") };
    for (int i = 0; i < 10; i++) {
        if (i < 5) {
            tArray[i].start();
            tArray[i].join();
        } else {
            tArray[i - 5] = new Thread(counter, "T" + ((i - 5) + 1)); //Instantiating new Thread which is not allowed.
            tArray[i - 5].start();
            tArray[i - 5].join();
        }
    }
}

public class Counter implements Runnable {

    private int count = 0;

    @Override
    public synchronized void run() {
       System.out.println(Thread.currentThread().getName() + " -> " + ++count);
    }

}

But since only 5 threads are allowed my solution is not accepted since I am also instantiating new Thread in the else block of the for loop.

Any help would be highly appreciated.

Answer 1

You need to arrange interaction between threads. The most natural way for thread interaction is setting up blocking queues which connect threads. Queues can be independent objects, or belong to particular threads. In your case, you need to make a circle of 5 threads.

class CountPrinter extends Thread {
   String name;
   ArrayBlockingQueue<Integer> inp = new ArrayBlockingQueue<>();
   CountPrinter next;

   public void run() {
      for (;;)
         int n = inp.take();
         if (n == 11) {// only 10 numbers must be printed
            next.inp.put(11);
            return;
         }
         System.out.println(name+"->"+n);
         next.inp.put(n+1);
      }
   }
}

After creating a thread and before its start, you need to assign the fields name and next. I believe you can program this yourself. Also, the first thread must be provided with the initial value 1.