如何在Java中将具有多个日期的地图作为值进行排序

Question

如何按多个值作为日期对HashMap条目进行排序

假设我有一个键为String且值为2个日期的Object的地图。

按对象中的两个日期进行排序

Map<String, dateWrapper> map = new HashMap<>();
map.put("user10",new dateWrapper("20180131","20181231"));    
map.put("user3",new dateWrapper("20170131","20171231")); 
map.put("user5",new dateWrapper("20160131","20161231")); 
map.put("user1",new dateWrapper("20170131","20170131")); 

**注意：上面的字符串通过使用SimpleDateFormat解析为日期

预计输出为user5，user1，user3，user10

公共类dateWrapper实现了可序列化的{

private Date date1;
private Date date2;

}
List<Map.Entry<String, dateWrapper>> dateWrapperEntries = new ArrayList<>(map.entrySet());
Trying below  this but its returning null 
Collections.sort(dateWrapperEntries, Map.Entry.comparingByValue(Comparator.comparing(dateWrapper::getDate1).thenComparing(dateWrapper::getDate2)));

private static Date convertStringToDate(String inputStr) {
    Date inputDate = null;
    java.text.SimpleDateFormat sdf = new java.text.SimpleDateFormat("yyyyMMdd");

    try {
        inputDate = sdf.parse(inputStr);
    } catch (ParseException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    return inputDate;
}

Answer 1

public static List<String> sort(Map<String, dateWrapper> map) {
    // comparator to compare two dateWrapper from Map.Entry value
    final Comparator<Map.Entry<String, dateWrapper>> comparator =
            Comparator.comparing((Function<Map.Entry<String, dateWrapper>, Date>)e -> e.getValue().getDate1())
                      .thenComparing(e -> e.getValue().getDate2());

    // convert map to entry set and compare it
    return Optional.ofNullable(map).orElse(Collections.emptyMap()).entrySet().stream()
                   .sorted(comparator)
                   .map(Map.Entry::getKey)
                   .collect(Collectors.toList());
}