class Reader extends Thread {
    Calculator c;

    public Reader(Calculator calc) {
	c = calc;
	start();   //from ctor
    }

    public void run() {
	synchronized(c) {
	    try {
		System.out.println("Waiting  for calculation...");
		c.wait();
	    } catch (InterruptedException e) {}
	}
	System.out.println("Total is " + c.total);
    }

    public static void main(String[] args) {
	Calculator calculator = new Calculator();
	//	new Reader(calculator).start();  //some number of them
	new Reader(calculator);   //start is in ctor
	new Reader(calculator);
	calculator.start();
    }
}

class Calculator extends Thread {
    int total;

    public void run() {
	synchronized(this) {
	    for (int i=0; i<100; i++)
		total += i;
	    notifyAll();
	    //	    notify();    //wake one.  others will wake when this ends
	}
	//while (true)     //but what if this thread never ends?  yes
	// ;
	try {
	    Thread.sleep(3000); //or goes to sleep for a while
	} catch (InterruptedException e) {}
    }
}