获取某周前的礼拜一

package com.poi.examples;    import java.text.SimpleDateFormat;  import java.util.*;    public class PrevWeek {       public static void main(String[] args) {              SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");              Date dt = prevWeek(new Date(), 7);              System.out.println(sdf.format(dt));          }                 public static Date prevWeek(Date dt, int week) {              Calendar cal = Calendar.getInstance();              cal.setTime(dt);              cal.add(Calendar.DAY_OF_MONTH, -week * 7 + (9-cal.get(Calendar.DAY_OF_WEEK)));              return cal.getTime();          }  }