以实现一个clock为例，一个clock类里面有两个display类，分别是时针分针，通过clock实现两个display（时针，分针）的交互

一个clock工程里面一个clock包，里边两个类，一个display类，一个clock类

package clock;

public class display {
	private int value = 0;
	private int limit = 0;
	
	public display(int limit) {
		this.limit=limit;
	}
	
	public void increase()
	{
		value++;
		if(value == limit)
		{
			value = 0;
		}
	}
	
	public int getValue() {
		return value;
	}
	public static void main(String[] args) {
		display d = new display(24);
		for(;;) {
			d.increase();
			System.out.println(d.getValue());
		}

	}

}    

package clock;

public class clock {

	private display hour = new display(24);
	private display minute = new display(60);
	
	public void start() {
		while(true) {
		minute.increase();
		if(minute.getValue() == 0) {
			hour.increase();
		}
		System.out.printf("%02d:%02d\n",hour.getValue(),minute.getValue());
	 }
		}
	public static void main(String[] args) {
		clock clock = new clock();
		clock.start();

	}

}