Some Coding Challenges: Implementing a Lock.

The challenge is to implement a lock in Java by only using synchronized, wait, notify as constructs.

The lock shall work as follows:

var lock = new Lock();

private void doSomething(){
  lock.lock();
  // no other thread shall be able to go here in the meantime
  lock.unlock();
}

A solution written in Java:

class Lock {
      boolean isLocked = false;

      public synchronized void lock() {
        while (isLocked) {
          try {
            wait();
          } catch (InterruptedException e) {
            // neglect
          }
        }
        isLocked = true;
      }

      public synchronized void unlock() {
        isLocked = false;
        notify();
      }
}

You can use this like so:

var task = new Runnable() {
  Lock countLock = new Lock();
  int count = 0;

  @Override
  public void run() {
    while (true) {
      countLock.lock();
      if (count < 1000) {
        count += 1;
        System.out.println(Thread.currentThread().getName() + " :" + count);
      } else {
        countLock.unlock();
        break;
      }
      countLock.unlock();
    }
    System.out.println("finished " + Thread.currentThread().getName());
  }
};

(new Thread(task)).start();
(new Thread(task)).start();