objects and threads

[sylvia greene]

A thread can't invoke a wait or notify method on an object unless it owns that objects lock

I can't the concept of object here.

  class ThreadA {
 
public static void main(String [] args) {
 
      ThreadB b = new ThreadB();
      synchronized(b) {
try {
 
   System.out.println("Waiting for b to complete...");
           b.wait();
         } catch (InterruptedException e) {}
       }
        System.out.println("Total is: " + b.total);
    }
 }
 class ThreadB extends Thread {
 int total;
 
 public void run() {
     synchronized(this) {
       for(int i=0;i<100;i++) {
               total += i;
         }
       notify();
        }
     }
 } 

What is a thread and what is a object here?

Thread b = new Thread();
  

Isn't "b" both an object and thread? what's object being referred to here.

[Barry Gaunt]

Yes, in this case, b is an object and a Thread (Thread is derived from object). The wait() and notify() are being called on the shared object b The b in the main method refers to the same object as this in the run method of object b.