public class RW {
    private int countReaders = 0;
     
    public synchronized void startRead () 
                        throws InterruptedException {
        while (countReaders < 0) wait ();
        countReaders++;
    }
     
    public synchronized void endRead () {
        countReaders--; 
        if (countReaders == 0) notifyAll ();
    }
    public synchronized void startWrite () 
			throws InterruptedException {
	while (countReaders != 0) wait ();
	 countReaders = -1;
    }
    
    public synchronized void endWrite () {
	countReaders = 0;
	notifyAll ();
    }
}