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LocalDate start = LocalDate.of(2020, 4, 1);
LocalDate end = LocalDate.of(2020, 5, 1);
int week = 1;

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/**
 * 获取指定时间内星期几的所有日期
 * @Author liuwenxu.com (2020-04-26)
 *
 * @param start 开始日期 2020-04-01
 * @param end   截止日期 2020-05-01
 * @param week  星期几 1
 * @return ArrayList<LocalDate> [2020-04-06, 2020-04-13, 2020-04-20, 2020-04-27]
 */
public static List getWeekInTimes(LocalDate start, LocalDate end, int week) {
    ArrayList<LocalDate> list = new ArrayList<>();

    long days = ChronoUnit.DAYS.between(start, end);
    log.info("[{}~{})之间共有：{}天", start, end, days);

    Calendar startCalender = GregorianCalendar.from(start.atStartOfDay(ZoneId.systemDefault()));
    for (int i = 0; i < days - 1; i++) {
        startCalender.add(Calendar.DATE, 1);

        if (startCalender.get(Calendar.DAY_OF_WEEK) == week + 1) {// 1代表周日，7代表周六
            list.add(
                    LocalDateTime.ofInstant(
                            startCalender.toInstant(),
                            ZoneOffset.systemDefault()
                    ).toLocalDate()
            );
        }
    }
    log.info("[{}~{})之间共有：{}个 星期{}", start, end, list.size(), week);
    return list;
}

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- [2020-04-01~2020-05-01)之间共有：30天
- [2020-04-01~2020-05-01)之间共有：4个 星期1
[2020-04-06, 2020-04-13, 2020-04-20, 2020-04-27]

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    /**
     * 获取当月的所有周末
     * @Author liuwenxu.com (2020-04-26)
     *
     * @param year  2020
     * @param month 4
     * @return 
     */
    public static List getWeekendInMonth(int year, int month) {

        List list = new ArrayList();
        Calendar calendar = Calendar.getInstance();
        calendar.set(Calendar.YEAR, year);// 不设置的话默认为当年
        calendar.set(Calendar.MONTH, month - 1);// 设置月份
        calendar.set(Calendar.DAY_OF_MONTH, 1);// 设置为当月第一天
        int daySize = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);// 当月最大天数
        for (int i = 0; i < daySize - 1; i++) {
            calendar.add(Calendar.DATE, 1);//在第一天的基础上加1
            int week = calendar.get(Calendar.DAY_OF_WEEK);
            if (week == Calendar.SATURDAY || week == Calendar.SUNDAY) {// 1代表周日，7代表周六 判断这是一个星期的第几天从而判断是否是周末
                list.add(year + "-" + month + "-" + calendar.get(Calendar.DAY_OF_MONTH));// 得到当天是一个月的第几天
            }
        }
        return list;
    }

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[2020-4-4, 2020-4-5, 2020-4-11, 2020-4-12, 2020-4-18, 2020-4-19, 2020-4-25, 2020-4-26]