java - 如何使用 Java Stream API(groupingBy 收集器)对列表中的值进行分组？

Question

我有 Entry 的列表对象。 Entry是一个:

class Entry {
   private final Date date;
   private final String value;

   // constructor
   // getters
}

我需要按天对这些条目进行分组。例如，

2011-03-21 09:00 VALUE1
2011-03-21 09:00 VALUE2
2011-03-22 14:00 VALUE3
2011-03-22 16:00 VALUE4
2011-03-21 16:00 VALUE5

应该分组:

2011-03-21
    VALUE1
    VALUE2
    VALUE5

2011-03-22
    VALUE3
    VALUE4

我想要一个Map<Date, List<Entry>> . 如何使用 Stream API(groupingBy 收集器)获取它？

我的尝试如下:

final Map<Date, List<Entry>> entries =
        list.stream().collect(Collectors.groupingBy(request -> {
        final Calendar ogirinal = Calendar.getInstance();
        ogirinal.setTime(request.getDate());

        final Calendar cal = Calendar.getInstance();
        cal.set(Calendar.DAY_OF_MONTH, ogirinal.get(Calendar.DAY_OF_MONTH));
        cal.set(Calendar.MONTH, ogirinal.get(Calendar.MONTH));
        cal.set(Calendar.YEAR, ogirinal.get(Calendar.YEAR));

        return cal.getTime();
    }));

输出:

2011-03-21
    VALUE1
2011-03-21
    VALUE2
2011-03-22
    VALUE3
    VALUE4
2011-03-21
    VALUE5

Answer 1

您必须截断日期值，因为它们可能不同到毫秒:

import static java.time.temporal.ChronoUnit.DAYS;

final Map<Instant, List<Entry>> entries =
    list.stream().collect(Collectors.groupingBy(request -> 
        request.getDate().toInstant().truncatedTo(DAYS)));