我在 Java 中有一个包含一组随机日期的数组:

{ January 20 2015, February 12 2015, February 20 2015, June 21 2015, July 12 2015, July 28 2015, July 30 2015, September 24 2015, December 31 2015 }

如何按月将此数组拆分为多个数组？

我愿意

{ {January 20 2015}, {February 12 2015, February 20 2015}, {June 21 2015}, {July 12 2015, July 28 2015, July 30 2015}, {September 24 2015}, {December 31 2015} }

我可以遍历整个数组并检查下一个日期是否仍在同一个月内，如果是，则将其添加到子数组。但是我想知道是否有更简洁或更有效的方法。

编辑:

此外，我需要按年和月排序，例如，2014 年 1 月 15 日和 2015 年 1 月 23 日不应合并。

这是我想出的一个方法，但它看起来不是很有效:

private void splitListByMonth(){
    ArrayList<ArrayList<Homework>> mainArrayList = new ArrayList<>();
    ArrayList<String> titleList = new ArrayList<>();

    Calendar calendar = Calendar.getInstance();
    SimpleDateFormat dateFormat = new SimpleDateFormat("MMMM yyy");
    for(Homework homework:mList){
        calendar.setTimeInMillis(homework.getDate());
        String monthString = dateFormat.format(calendar.getTime());

        if(titleList.contains(monthString)){
            int index = titleList.indexOf(monthString);
            mainArrayList.get(index).add(homework);
        } else {
            titleList.add(monthString);
            int index = titleList.indexOf(monthString);
            mainArrayList.get(index).add(homework);
        }
    }
    Log.d("Tag",""+titleList);
    Log.d("Tag",""+mainArrayList);
}

最佳答案

你在正确的轨道上，但将年/月字符串化是一种缓慢的方式，只需跟踪年和月:

@SuppressWarnings("null")
private static List<List<Date>> splitByMonth(Date ... dates) {
    List<List<Date>> datesByMonth = new ArrayList<>();
    List<Date> monthList = null;
    int currYear = 0, currMonth = -1;
    Calendar cal = Calendar.getInstance();
    for (Date date : dates) {
        cal.setTime(date);
        if (cal.get(Calendar.YEAR) != currYear || cal.get(Calendar.MONTH) != currMonth) {
            monthList = new ArrayList<>();
            datesByMonth.add(monthList);
            currYear = cal.get(Calendar.YEAR);
            currMonth = cal.get(Calendar.MONTH);
        }
        monthList.add(date);
    }
    return datesByMonth;
}

注意参数必须预先排序。在这一点上，问题和评论有点不清楚。

测试代码

public static void main(String[] args) throws Exception {
    // Build list of all dates
    String[] txtDates = { "January 20 2015", "February 12 2015", "February 20 2015", "June 21 2015",
            "July 12 2015", "July 28 2015", "July 30 2015", "September 24 2015", "December 31 2015",
            "January 15 2014", "January 15 2015" };
    SimpleDateFormat fmt = new SimpleDateFormat("MMMM d yyyy");
    Date[] allDates = new Date[txtDates.length];
    for (int i = 0; i < txtDates.length; i++)
        allDates[i] = fmt.parse(txtDates[i]);

    // Sort dates, then split them by month
    Arrays.sort(allDates);
    List<List<Date>> datesByMonth = splitByMonth(allDates);

    // Print result
    for (List<Date> dates : datesByMonth) {
        StringBuilder buf = new StringBuilder();
        for (Date date : dates) {
            if (buf.length() != 0)
                buf.append(", ");
            buf.append(fmt.format(date));
        }
        System.out.println(buf);
    }
}

输出

January 15 2014
January 15 2015, January 20 2015
February 12 2015, February 20 2015
June 21 2015
July 12 2015, July 28 2015, July 30 2015
September 24 2015
December 31 2015

关于Java按月份将月份数组排序为多个数组，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/32752476/