题目：

定义一个时钟类(TimeDemo)，属性有：时、分、秒（默认值10时30分），方法有展示当前时间、过1秒，过1分钟，过1小时后的时分秒；定义成员方法分别对时、分、秒进行加减运算，保证运算后时间为真实时间

代码实现如下：

public class TimeDemo {
	private int hour;      //定义小时
	private int minute;    //定义分钟
	private int second;    //定义秒

	public static void main(String[] args) {
		TimeDemo timeDemo = new TimeDemo(10, 30, 00);    //定义初始时间
		timeDemo.display();
		timeDemo.addSecond(120);
		timeDemo.addMinute(120);
		timeDemo.addHour(48);
		timeDemo.subSecond(100);
		timeDemo.subMinute(100);
		timeDemo.subHour(100);
		timeDemo.display();        //打印修改后的时间
	}

	public TimeDemo(int x, int y, int z) {
		this.hour = x;
		this.minute = y;
		this.second = z;
	}

	public void display() {
		while (second > 59 || second < 0 || minute > 59 || minute < 0 || hour > 23 || hour < 0) {
			if (second > 59) {        //判断当秒为60时，分要进一位，当分为60时，小时要进一位，当小时为24时，小时要归0
				second -= 60;         
				minute++;
			}
			if (minute > 59) {
				minute -= 60;
				hour++;
			}
			if (hour > 23) {
				hour -= 24;
			}
			if (second < 0) {        //判断当秒为负数时，向分借位，当分为负数时，向小时借位，当小时为负数时，不断加24直到为正数
				second += 60;
				minute--;
			}
			if (minute < 0) {
				minute += 60;
				hour--;
			}
			if (hour < 0) {
				hour += 24;
			}
		}
		System.out.println("the time is " + this.hour + ":" + this.minute + ":" + this.second);
	}

	public void addHour(int hou) {    //加小时
		this.hour = this.hour + hou;
	}

	public void addSecond(int sec) {    //加秒
		this.second = this.second + sec;
	}

	public void addMinute(int min) {    //加分钟
		this.minute = this.minute + min;
	}

	public void subHour(int hou) {    //减小时
		this.hour = this.hour - hou;
	}

	public void subMinute(int min) {    //减分钟
		this.minute = this.minute - min;
	}

	public void subSecond(int sec) {    //减秒
		this.second = this.second - sec;
	}
}