How static and nonstatic method synchronizes if both of them are using same

Question

class ThreadA
     public static void main(String args[])
          ThreadB b=new ThreadB();
          b.setName("MyThread");
          b.start();
     synchronized(b){
          try{
               System.out.println(" wating for b to complete...");
               b.wait();     
          catch(InterruptedException e)
          System.out.println(b.total);
class ThreadB extends Thread
     int total;     
     public void run()
     System.out.println(Thread.currentThread().getName());
     synchronized (this)
          for(int i=0;i<10;i++)
               total+=i;
     notify();
in this example both the threads : ThreadB and main thread are trying to use sameobject i.e b
then when ThreadB will get started and immediately synchronize b i.e get a lock on b then how main will get
the lock and again synchronize b. How the main thread and ThreadB will run simultaneously ...please clear this .
the output of this program is :-
waiting for b to complete
45

Answer 1

I reckon the main thread is getting the lock first. If it weren't it would never finish. The println at the start of the ThreadB probably delays the thread enough to enable the main thread to win the race mostly. So the main thread gets the lock and then it goes into wait(), which releases the lock and b does it's loop, calls notify() and finishes, releasing the lock. Now the main thread, having been released from wait takes the lock back, then winds up.
What's wrong with this code is that if b got the lock first it would call notify() before the main thread called wait(), which would do nothing and and main thread would stick on wait indefinitely.
Wait and notify should always be used in conjuction with some kind of flag which says whether the thread needs to wait or is the condition to continue already met.