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；在测试类中实现过10秒，过10分，过10小时后的时间展示

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;
  }
}