计算时间第几周的算法

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;

public class CalendarTest {
public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
cal.setTime(new Date());
cal.set(Calendar.DAY_OF_MONTH, 23);
System.out.println("星期" + (cal.get(Calendar.DAY_OF_WEEK)-1));
Date date = calculateByWeeksOfMonday(cal.getTime(), 3);
SimpleDateFormat f = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
System.out.println(f.format(date));
}

/**
* 计算返回离当前时间(包含当前时间周)近几周的星期一的时间,精确到当天时间
* @param date　时间
* @param amount　近几周
* @return
*/
private static Date calculateByWeeksOfMonday(Date date,int amount){
Calendar cal = Calendar.getInstance();
cal.setTime(date);
int dayOfWeek =  cal.get(Calendar.DAY_OF_WEEK)-1; //返回当前周的星期几
amount += -1;//减去当前周
int days = amount*7;//７天为一个周
switch (dayOfWeek) {
case 0://星期日
days+=6;
break;
case 1://星期一
days+=0;
break;
case 2://星期二
days+=1;
break;
case 3://星期三
days+=2;
break;
case 4://星期四
days+=3;
break;
case 5://星期五
days+=4;
break;
case 6://星期六
days+=5;
break;
default:
break;
}
cal.add(Calendar.DAY_OF_MONTH, -days);//设置距离当前周的近几周的星期一的时间差
cal.set(Calendar.HOUR_OF_DAY, 0);
cal.set(Calendar.MINUTE, 0);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
return cal.getTime();
}
}