如何按字符串类型按日历对地图进行排序？

Question

我有这个结构:

Map<String, String> map = new HashMap<String, String>();

和数据看起来像这样:

("10/11/2015", "A")
("10/12/2015", "B")
("10/13/2015", "C")
("10/14/2015", "D")
("10/15/2015", "E")

我想按日期(键)对地图进行排序,所以我的尝试是将字符串转换为Calendar类型,然后将它们移动到TreeMap中,如下所示:

DateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");
Calendar calendar = Calendar.getInstance();

TreeMap<Calendar, String> tree = new TreeMap<Calendar, String>();

for(Map.Entry<String, String> entry : map.entrySet()){
    String key = entry.getKey();
    String value =  entry.getValue();

    calendar.setTime(formatter.parse(key));
    tree.put(calendar, value);
}

但是当我打印键时,结果如下所示:

10/11/2015
10/15/2015
10/14/2015
10/12/2015
10/13/2015

我该怎么做才能正确按日期(字符串类型的日期)对地图进行排序？

Answer 1

根据您的示例,您最终应该TreeMap使用单个Calendar键,因为您使用的是相同的实例Calendar.

相反,如果您Calendar为每个条目使用了新实例,例如......

    DateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");

    Map<String, String> map = new HashMap<>();
    map.put("10/11/2015", "A");
    map.put("10/12/2015", "B");
    map.put("10/13/2015", "C");
    map.put("10/14/2015", "D");
    map.put("10/15/2015", "E");

    TreeMap<Calendar, String> tree = new TreeMap<Calendar, String>();

    for (Map.Entry<String, String> entry : map.entrySet()) {
        System.out.println(entry);
        String key = entry.getKey();
        String value = entry.getValue();

        Calendar calendar = Calendar.getInstance();
        calendar.setTime(formatter.parse(key));
        tree.put(calendar, value);
    }

    for (Calendar key : tree.keySet()) {
        System.out.println(key.getTime());
    }

它会打印更像......

Sun Oct 11 00:00:00 EST 2015
Mon Oct 12 00:00:00 EST 2015
Tue Oct 13 00:00:00 EST 2015
Wed Oct 14 00:00:00 EST 2015
Thu Oct 15 00:00:00 EST 2015

如果你使用的是Java 8(+),那么你真的应该使用新的Date/Time API,例如......

Map<LocalDate, String> tree = new TreeMap<>();

for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry);
    String key = entry.getKey();
    String value = entry.getValue();

    LocalDate ld = LocalDate.parse(key, DateTimeFormatter.ofPattern("MM/dd/yyyy"));
    tree.put(ld, value);
}

for (LocalDate key : tree.keySet()) {
    System.out.println(key);
}