java中的计数器如何使用wait()和notify()？

Question

I have two threads that increment a counter and I want that they do it in an alternating manner, meaning that first thread one accesses the counter object, then thread two, thread one again... I was told that this can be achieved with wait() and notify() but I can't figure out how to use them properly.

class counter {
   int val;
   public counter() {
      val = 0;
   }
   public incr() {
      val++;
   }
}

Answer 1

One solution could look like that:

public class ThreadA implements Runnable {
   counter count;
   int activeThreads;
   int id;
   int iterations;
   public Thread A(int activeThreads, int id, counter count, int iterations) {
      this.activeThreads = activeThreads;
      this.id = id;
      this.count = count;
      this.iterations = iterations;
   }
   @Override
   public void run() {
        for (int i = 0; i < iterations; i++) {
            synchronized (count) {
            /* achieving alternating manner by working in the
               corresponding modulo ring, assuming that the id's
               are chosen accordingly (0, 1, 2, 3, ...)
            */
                while (counter.value() % activeThreads != id) {
                    try {
                        counter.wait();
                    } catch (InterruptedException e) {
                        System.out.printf("Thread with id %d got
                        interrupted\n", id); 
                    }
                }

                counter.increment();                
                counter.notifyAll();
            }
        }
    }
}