Question

I’ve following class

class A{

private int id =0;
   public void setMessageId(Message m) {
      synchronized(m) {
        m.setId(id++);
      }
   }
}

Can two threads with same instance of class A with DIFFERENT instances of Message enter into the synchronized block at same time and set same id to different instances of message?

Answer 1

Yes. Since the block is synchronized on the message, two threads using two different message instances can enter the synchronized block concurrently, and both get the same ID.

Make the method synchronized, and everything will be fine. Or use an AtomicInteger.