java dateutil工具类Date.add()

1则代表的是对年份操作，2是对月份操作，3是对星期操作，5是对日期操作，11是对小时操作，12是对分钟操作，13是对秒操作，14是对毫秒操作。

  public static Date addYears(Date date, int amount)  
    {  
        return add(date, 1, amount);  
    }  
  
    public static Date addMonths(Date date, int amount)  
    {  
        return add(date, 2, amount);  
    }  
  
    public static Date addWeeks(Date date, int amount)  
    {  
        return add(date, 3, amount);  
    }  
  
    public static Date addDays(Date date, int amount)  
    {  
        return add(date, 5, amount);  
    }  
  
    public static Date addHours(Date date, int amount)  
    {  
        return add(date, 11, amount);  
    }  
  
    public static Date addMinutes(Date date, int amount)  
    {  
        return add(date, 12, amount);  
    }  
  
    public static Date addSeconds(Date date, int amount)  
    {  
        return add(date, 13, amount);  
    }  
  
    public static Date addMilliseconds(Date date, int amount)  
    {  
        return add(date, 14, amount);  
    }  
  
    private static Date add(Date date, int calendarField, int amount)  
    {  
        if(date == null)  
        {  
            throw new IllegalArgumentException("The date must not be null");  
        } else  
        {  
            Calendar c = Calendar.getInstance();  
            c.setTime(date);  
            c.add(calendarField, amount);  
            return c.getTime();  
        }