wait() in a while (true) loop to simulate a Clock in Java

Im trying to simulate a digital clock in Java by using wait() and notify(). If i run the following Code (well, actually a other very simlue GUI Class to display the Clock) i get a Illegal Monitor Exception. Im new to Java, so probably Ive used wait() the wrong way....

public class JDigitalClock extends JLabel implements Runnable {

private static final long serialVersionUID = 1L;
private boolean stopped = false;
DateTimeFormatter dtf;
LocalDateTime now;

public JDigitalClock() {
	dtf = DateTimeFormatter.ofPattern("HH:mm:ss");
	now = LocalDateTime.now();
	setText(dtf.format(now));
	setFont(new Font("Sans Serif", Font.PLAIN, 70));
	setHorizontalAlignment(SwingConstants.CENTER);
}

@Override
public void run() {
	while (true) {
		sleep();
		while (getStopped())
			try {
				wait();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		now = LocalDateTime.now();
		setText(dtf.format(now));
	}

}

public void setStopped(boolean stopped) {
	this.stopped = stopped;
	if (!stopped)
		notify();
}

public boolean getStopped() {
	return this.stopped;
}

public void sleep() {
	try {
		Thread.sleep(1000);
	} catch (InterruptedException e) {
		e.printStackTrace();
	}
}