Design patterns exercise

Clock.java

import java.text.SimpleDateFormat; 
import java.util.Date;

public class Clock implements Runnable { 
	public static final int TIME_24 = 0; 
	public static final int TIME_US = 1; 
	public static final int TIME_UNIX = 2; 
	private int displayType = TIME_24;

	public static void main(String[] args) { 
		Clock c = new Clock (); c.setDisplayType(TIME_24);
		new Thread(c).start();
		// After a few seconds, switch to a different display type
		try {Thread.sleep(3000);} catch (InterruptedException e) {}
		c.setDisplayType(TIME_UNIX); }

	public void setDisplayType(int dt) { 
		displayType = dt;
	}

	public void run() { 
		while (true) {
			String time = "";
			if (displayType==TIME_24) {
				time = new SimpleDateFormat("HH:mm:ss").format(new Date());
			} else if (displayType==TIME_US) {
				time = new SimpleDateFormat("hh:mm:ss a").format(new Date());
			} else if (displayType==TIME_UNIX) {
				time = String.valueOf(System.currentTimeMillis() / 1000L); 
			}
			System.out.println(time);
			try {Thread.sleep(1000);} catch (InterruptedException e) {} 
		}
	}
}