[oguz]

Hi everyone,
i'm new to forums and java threads.
As i was studying threads, i encountered the two sample classes below:

Code:

class NewThread implements Runnable {
	String name; // name of thread

	Thread t;

	boolean suspendFlag;

	NewThread(String threadname) {
		name = threadname;
		t = new Thread(this, name);
		System.out.println("New thread: " + t);
		suspendFlag = false;
		t.start(); // Start the thread
	}

	// This is the entry point for thread.
	public void run() {
		try {
			for (int i = 15; i > 0; i--) {
				System.out.println(name + ": " + i);
				Thread.sleep(200);
				synchronized (this) {
					while (suspendFlag) {
						wait();
					}
				}
			}
		} catch (InterruptedException e) {
			System.out.println(name + " interrupted.");
		}
		System.out.println(name + " exiting.");
	}

	void mysuspend() {
		suspendFlag = true;
	}

	synchronized void myresume() {
		suspendFlag = false;
		notify();
	}
}

Code:

class SuspendResume {
	public static void main(String args[]) {
		NewThread ob1 = new NewThread("One");
		NewThread ob2 = new NewThread("Two");
		try {
			Thread.sleep(1000);
			ob1.mysuspend();
			System.out.println("Suspending thread One");
			Thread.sleep(1000);
			ob1.myresume();
			System.out.println("Resuming thread One");
			ob2.mysuspend();
			System.out.println("Suspending thread Two");
			Thread.sleep(1000);
			ob2.myresume();
			System.out.println("Resuming thread Two");
		} catch (InterruptedException e) {
			System.out.println("Main thread Interrupted");
		}
		// wait for threads to finish
		try {
			System.out.println("Waiting for threads to finish.");
			ob1.t.join();
			ob2.t.join();
		} catch (InterruptedException e) {
			System.out.println("Main thread Interrupted");
		}
		System.out.println("Main thread exiting.");
	}
}

and there are some points i don't understant.
First why are we using synchronized as we are calling wait(). I don't see any way there could be parallel usege.
Second, when i remove the synchronized keyword from the code i get java.lang.IllegalMonitorStateException. After this if i also remove the lines with ob2 i don't get any exception. How come removing ob2 affects ob1 and prevents the program from throwing exception ?

As i said before, i'm new to threads, and if you could help me i would really appriciate it.