Some Coding Challenges: Implementing a Countdown Latch.

The task is to implement a so called CountdownLatch in Java. It will be initialized with an integer count and provides two methods void await() and void countDown(). Threads that call countDown trigger the count to decrease be 1 each time. Threads calling the await are blocked until the count has reached the value 0.

A possible implementation could look like so:

  class CountdownLatch {
      private int count;

      public CountdownLatch(int count) {
        this.count = count;
      }

      public synchronized void await() {
        while (this.count > 0) { // verify count to still be positive
          try {
            wait();
          } catch (InterruptedException e) {
            break;
          }
        }
      }

      public synchronized void countDown() {
        count -= 1;
        notifyAll(); // notify awaiters to check condition
      }
  }

This can be seen in action like so:

var cl = new CountdownLatch(5);

// schedule 5 threads that count-down the latch
IntStream.range(0, 5)
        .mapToObj(i -> new Runnable() {
          @Override
          public void run() {
            System.out.println("Task " + i + " is running.");
            try {
              Thread.sleep(1000);
            } catch (InterruptedException e) {…