java获取当月的第一天和最后一天,获取本周的第一天和最后一天

    /**
     * 获取指定日期所在周的第一天和最后一天,用下划线连接
     * @param dataStr
     * @return
     * @throws ParseException
     */
    public static String getFirstAndLastOfMonth(String dataStr,String dateFormat,String resultDateFormat) throws ParseException {
        //获取当前月第一天：
        Calendar c = Calendar.getInstance();
        c.setTime(new SimpleDateFormat(dateFormat).parse(dataStr));
        c.add(Calendar.MONTH, 0);
        c.set(Calendar.DAY_OF_MONTH,1);//设置为1号,当前日期既为本月第一天
        String first = new SimpleDateFormat(resultDateFormat).format(c.getTime());
        System.out.println("===============first:"+first);

        //获取当前月最后一天
        Calendar ca = Calendar.getInstance();
        ca.set(Calendar.DAY_OF_MONTH, ca.getActualMaximum(Calendar.DAY_OF_MONTH));
        String last = new SimpleDateFormat(resultDateFormat).format(ca.getTime());
        System.out.println("===============last:"+last);
        return first+"_"+last;
    }

    /**
     * 每周的第一天和最后一天
     * @param dataStr
     * @param dateFormat
     * @param resultDateFormat
     * @return
     * @throws ParseException
     */
    public static String getFirstAndLastOfWeek(String dataStr,String dateFormat,String resultDateFormat) throws ParseException {
        Calendar cal = Calendar.getInstance();
        cal.setTime(new SimpleDateFormat(dateFormat).parse(dataStr));
        int d = 0;
        if (cal.get(Calendar.DAY_OF_WEEK) == 1) {
            d = -6;
        } else {
            d = 2 - cal.get(Calendar.DAY_OF_WEEK);
        }
        cal.add(Calendar.DAY_OF_WEEK, d);
        // 所在周开始日期
        String data1 = new SimpleDateFormat(resultDateFormat).format(cal.getTime());
        cal.add(Calendar.DAY_OF_WEEK, 6);
        // 所在周结束日期
        String data2 = new SimpleDateFormat(resultDateFormat).format(cal.getTime());
        return data1 + "_" + data2;

    }

使用场景,计算日活跃,周活跃,月活跃是的日期计算