Java中使用Calendar进行获取指定时间，使用SimpleDateFormat进行格式化转换

public class DateTranslate {

    
    public static Date getMonthFirstDay(Date date, Integer sep) {
 Calendar cal = Calendar.getInstance();
 cal.setTime(getThisWeekMonday(date));
 cal.add(Calendar.MONTH, sep);
 cal.set(Calendar.DAY_OF_MONTH,1);

 return cal.getTime();
    }

    
    public static Date getLastWeekMonday(Date date) {
 Calendar cal = Calendar.getInstance();
 cal.setTime(getThisWeekMonday(date));
 cal.add(Calendar.DATE, -7);
 return cal.getTime();
    }

    
    public static Date getLastDay(Date date) {
 Calendar cal = Calendar.getInstance();
 cal.setTime(date);
 cal.add(Calendar.DATE, -1);
 return cal.getTime();
    }

    
    public static Date getThisWeekMonday(Date date) {
 Calendar cal = Calendar.getInstance();
 cal.setTime(date);
 // 获得当前日期是一个星期的第几天
 int dayWeek = cal.get(Calendar.DAY_OF_WEEK);
 if (1 == dayWeek) {
     cal.add(Calendar.DAY_OF_MONTH, -1);
 }
 // 设置一个星期的第一天，按中国的习惯一个星期的第一天是星期一
 cal.setFirstDayOfWeek(Calendar.MONDAY);
 // 获得当前日期是一个星期的第几天
 int day = cal.get(Calendar.DAY_OF_WEEK);
 // 根据日历的规则，给当前日期减去星期几与一个星期第一天的差值
 cal.add(Calendar.DATE, cal.getFirstDayOfWeek() - day);
 return cal.getTime();
    }
}

SimpleDateFormat formatString = new SimpleDateFormat("yyyyMMddHHmmss");
System.out.println(formatString.parse("20191129123456")); // 字符串转时间
// 格式化时间 
String nowTime = formatString.format(new Date() );  // 时间转字符串