需求：给定任意一个月，如何按照中国周的习惯，把一个月分成四个时间段
（1）以自然周为划分依据
（2）不能跨月
（3）把首尾自然周，天数较少的合并到其最近的自然周里面
（4）最后结果应该是吧一个月分成四个时间段，并给出每段的起止日期
大概需求就如上所述，不废话，上代码：（以封装好在main()函数里，直接copy即可，记得导相应的包和新建实体类）
public class one_month_four_week {
public static void main(String[] args) {
int year = 2017;
int month = 10;//改成你要的年月，运行即可查看结果
Calendar cal = Calendar.getInstance();
cal.set(Calendar.YEAR, year);
cal.set(Calendar.MONTH, month - 1);// -1才正确
int days = cal.getActualMaximum(Calendar.DATE);// Calendar.DAY_OF_MONTH也对
System.out.println("首先，计算这个月有" + days + "天");
String[] weekDays = { "星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六" };// 国外一周起始
int[] chinaWeek = { 7, 1, 2, 3, 4, 5, 6 };// 对应成中国一周的第几天
SimpleDateFormat str_date_format = new SimpleDateFormat("yyyy-MM-dd");
List<EachDayAttr> monthDay = new ArrayList<>();
int week_of_month = 1;
for (int i = 1; i <= days; i++) {
EachDayAttr oneDay = new EachDayAttr();
oneDay.setWhich(i);
// 计算这个月的每一天，对应星期几
String dayStr = year + "-" + month + "-" + i;
Date daysDate = null;
try {
daysDate = str_date_format.parse(dayStr);
} catch (ParseException e) {
e.printStackTrace();
}
cal.setTime(daysDate);
int w1 = cal.get(Calendar.DAY_OF_WEEK) - 1; // 指示一个星期中的第几天
int which_day_chinaWeek = chinaWeek[w1];
System.out.println("该月的第"+ i +"天，该星期（外国习惯）的第"+ w1 +"天："+ weekDays[w1] +">>对应中国周的第"+ which_day_chinaWeek +"天！");
//关键：该月每一天对应，中国周里面的第几周
if (which_day_chinaWeek == 1) {// 星期一，中国习惯里新的一周的开始
week_of_month++;
oneDay.setWeek(week_of_month);
} else {
oneDay.setWeek(week_of_month);
}
monthDay.add(oneDay);
}
// 分组：按中国习惯分组
Map<Integer, List<EachDayAttr>> weekGroup = new TreeMap<>();
for (EachDayAttr oneDay : monthDay) {
if (weekGroup.containsKey(oneDay.getWeek())) {
weekGroup.get(oneDay.getWeek()).add(oneDay);
} else {
List<EachDayAttr> addGroup = new ArrayList<>();
addGroup.add(oneDay);
weekGroup.put(oneDay.getWeek(), addGroup);
}
}
// 再分组：合并成四周
if (weekGroup.size() == 4) {
// weekGroup不做处理
} else if (weekGroup.size() == 5) {
if (weekGroup.get(1).size() < weekGroup.get(5).size()) {
weekGroup.get(2).addAll(weekGroup.get(1));
weekGroup.remove(1);
} else {
weekGroup.get(4).addAll(weekGroup.get(5));
weekGroup.remove(5);
}
} else if (weekGroup.size() == 6) {
weekGroup.get(2).addAll(weekGroup.get(1));
weekGroup.get(5).addAll(weekGroup.get(6));
weekGroup.remove(1);
weekGroup.remove(6);
}
// 吧weekGroup按天数排序，取最小为开始日期，最大为结束日期即可
for (Entry<Integer, List<EachDayAttr>> entry : weekGroup.entrySet()) {
Collections.sort(entry.getValue(), new Comparator<EachDayAttr>() {
public int compare(EachDayAttr day1, EachDayAttr day2) {
return day1.getWhich() - day2.getWhich();
}
});
}
// 最后从分好组、排好序的weekGroup中取值封装
// weekGroup-->entry中健为整数，而值为list<?>集合
Map<Integer, String[]> rtResult = new HashMap<>();
int signWeek = 1;
for (Entry<Integer, List<EachDayAttr>> entry : weekGroup.entrySet()) {
String[] addResult = new String[2];
int firstDay = entry.getValue().get(0).getWhich();
int lastDay = entry.getValue().get(entry.getValue().size() - 1).getWhich();
addResult[0] = year + "-" + month + "-" + firstDay;
addResult[1] = year + "-" + month + "-" + lastDay;
rtResult.put(signWeek, addResult);
signWeek++;
}
// 至此，输出看下结果
for (Entry<Integer, String[]> entry : rtResult.entrySet()) {
System.out.println("----该月第"+ entry.getKey() +"周-----");
System.out.println("开始日期：" + entry.getValue()[0]);
System.out.println("结束日期：" + entry.getValue()[1]);
}
}
}总结：上面方法。涉及知识点其实挺多，也是日常项目中遇到的比较多的，比如Calendar这个出来日期的类，还有分组排序、map的遍历等；
加上自定义的实体类，强迫症的小伙伴可以看下：
public class EachDayAttr {
private int which;
private int week;
private int flag;
//get和set方法省略
}最后。附上SQL大神直接用sql是怎么把一个月按中国周分的（这里一个月最多会有六周，为了更清楚，笔者有改动，但该sql精华不变）
select min(dt) 一周开始, max(dt) 一周结束, count(flag) 该周天数, ROW_NUMBER() over(order by 1) as 周数
from (
select level rn,
trunc(to_date(201707,'yyyymm'), 'mm') + level - 1 dt,
max(decode (to_char(trunc(to_date(201707,'yyyymm'), 'mm') + level - 1, 'd'), 2, level, 0)) over(order by level) flag
from dual
connect by level <= last_day(trunc(to_date(201707,'yyyymm'))) - trunc(to_date(201707,'yyyymm'), 'mm') + 1
)
group by flag
order by min(rn)
这是在mybatis里面实现的，传入的参数是整形的年月，比如int yyyymm = 201707；