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）和事件类型汇总，然后计算。例如以下列表

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

我是否可以通过Streams API实现这一目标（如果是，如何）？

Answer 1

如果您不想定义自己的密钥，可以groupBy两次。结果是相同的，但格式略有不同：

 System.out.println(events.stream()
            .collect(Collectors.groupingBy(e -> YearMonth.from(e.getWhen()),
                    Collectors.groupingBy(Event::getWhat, Collectors.counting()))));

结果是：

 {2017-05={EVENT3=1}, 2017-04={EVENT2=2, EVENT1=1}, 2017-03={EVENT1=2}}

Answer 2

如果您不想按照assylias的建议创建新的密钥类，则可以执行双groupingBy

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

...后面是嵌套打印

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

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，您可以指定已排序的地图实现

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

Answer 3

你可以创建一个＆＃34;键＆＃34;包含年/月和事件类型的类：

class Group {
  private YearMonth ym;
  private String type;

  public Group(Event e) {
    this.ym = YearMonth.from(e.getWhen());
    this.type = e.getWhat();
  }

  //equals, hashCode, toString etc.
}

然后，您可以使用该密钥对事件进行分组：

Map<Group, Long> result = events.stream()
                .collect(Collectors.groupingBy(Group::new, Collectors.counting()));
result.forEach((k, v) -> System.out.println(k + "\t" + v));

输出：

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

Answer 4

final DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM");
    Stream.of(
            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")
            ).collect(Collectors.groupingBy(event -> 
               dateTimeFormatter.format(event.getWhen()),
               Collectors.groupingBy(Event::getWhat, counting())))
             .forEach((whenDate,v) -> v.forEach((whatKey,counter) -> 
                System.out.println(whenDate+ " "+ whatKey+" "+counter)));

无需使用Arrays.asList（）方法来获取流。直接使用Stream.of（）方法获取流。

<强>输出：

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

Answer 5

我们可以在POJO中创建方法，其中包含用于分组的字段列表，如下所示

public String getWhenAndWhat() {
    return YearMonth.from(when) + ":" + what; //you can use delimiters like ':','-',','
}

和流代码

System.out.println(events.stream()
            .collect(Collectors.groupingBy(Event::getWhenAndWhat, Collectors.counting())));

输出将是：

{2017-05：EVENT3 = 1，2017-04：EVENT1 = 1，2017-04：EVENT2 = 2,2017-03：EVENT1 = 2}

Java Stream：按多个字段分组和计数

5 个答案: