Java 流 : Grouping and counting by multiple fields

Question

我有以下对象:

class Event {
private LocalDateTime when;
private String what;

public Event(LocalDateTime when, String what) {
  super();
  this.when = when;
  this.what = what;
}

public LocalDateTime getWhen() {
  return when;
}

public void setWhen(LocalDateTime when) {
  this.when = when;
}

public String getWhat() {
  return what;
}

public void setWhat(String what) {
  this.what = what;
}

我需要按年/月 (yyyy-mm) 和事件类型进行汇总，然后进行计数。例如下面的列表

List<Event> events = Arrays.asList(
  new Event(LocalDateTime.parse("2017-03-03T09:01:16.111"), "EVENT1"),
  new Event(LocalDateTime.parse("2017-03-03T09:02:11.222"), "EVENT1"),
  new Event(LocalDateTime.parse("2017-04-03T09:04:11.333"), "EVENT1"), 
  new Event(LocalDateTime.parse("2017-04-03T09:04:11.333"), "EVENT2"),
  new Event(LocalDateTime.parse("2017-04-03T09:06:16.444"), "EVENT2"),
  new Event(LocalDateTime.parse("2017-05-03T09:01:26.555"), "EVENT3")
);

应该产生以下结果:

Year/Month  Type  Count
2017-03     EVENT1    2  
2017-04     EVENT1    1
2017-04     EVENT2    2
2017-04     EVENT3    1

知道我是否可以(如果可以，如何)使用 Streams API 实现它？

Answer 1

如果你不想创建一个新的键类，正如 assylias 所建议的，你可以做一个双 groupingBy

Map<YearMonth,Map<String,Long>> map = 
     events.stream()
           .collect(Collectors.groupingBy(e -> YearMonth.from(e.getWhen()),
                    Collectors.groupingBy(x -> x.getWhat(), Collectors.counting()))
                   );

...后跟嵌套打印

map.forEach((k,v)-> v.forEach((a,b)-> System.out.println(k + " " +  a + " " + b)));

这打印

2017-05 EVENT3 1
2017-04 EVENT2 2
2017-04 EVENT1 1
2017-03 EVENT1 2

---

编辑:我注意到日期的顺序与 OP 的预期解决方案相反。使用 groupingBy 的 3 参数版本，您可以指定一个排序的映射实现

Map<YearMonth,Map<String,Long>> map = 
     events.stream()
           .collect(Collectors.groupingBy(e -> YearMonth.from(e.getWhen()), TreeMap::new, 
                    Collectors.groupingBy(x -> x.getWhat(), Collectors.counting()))
                   );

现在打印相同的 map.forEach(...)

2017-03 EVENT1 2
2017-04 EVENT2 2
2017-04 EVENT1 1
2017-05 EVENT3 1