如何基于特定值进行聚合并在给定对象列表的情况下创建嵌套地图? [英] How to aggregate based on certain value and create nested map given a list of objects?
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问题描述
我有一个List<SingleDay>
,其中SingleDay
是:
class SingleDay{
private Date date;
private String County;
// otherstuff
}
我想通过以下转换将其转换为Map<LocalDate, Map<String,SingleDay>>
:
I would like to convert this into a Map<LocalDate, Map<String,SingleDay>>
by the following transformation:
Key: 2020-05-27, Value: {'County1'=SingleDay [date=2020-05-27, County='County1'],
'County2'=SingleDay [date=2020-05-27, County='County2'],
'County3'=SingleDay [date=2020-05-27, County='County3']
}
Key: 2020-05-28, Value: {'County4'=SingleDay [date=2020-05-28, County='County4'],
'County5'=SingleDay [date=2020-05-28, County='County5'],
'County3'=SingleDay [date=2020-05-28, County='County3']
}
Note, this is a question that arose from my previous question : How do you use java stream api to convert list of objects into a nested map based on information stored inside object?
推荐答案
执行以下操作:
Map<LocalDate, Map<String, SingleDay>> result = list.stream()
.collect(Collectors.groupingBy(SingleDay::getDate, Collectors.toMap(SingleDay::getCounty, e -> e)));
演示:
import java.time.LocalDate;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
class SingleDay {
private LocalDate date;
private String County;
public SingleDay(LocalDate date, String county) {
this.date = date;
County = county;
}
public LocalDate getDate() {
return date;
}
public String getCounty() {
return County;
}
@Override
public String toString() {
return "SingleDay [date=" + date + ", County=" + County + "]";
}
// otherstuff
}
public class Main {
public static void main(String[] args) {
List<SingleDay> list = List.of(
new SingleDay(LocalDate.now(), "X"),
new SingleDay(LocalDate.now(), "Y"),
new SingleDay(LocalDate.now(), "Z"),
new SingleDay(LocalDate.now().plusDays(1), "A"),
new SingleDay(LocalDate.now().plusDays(1), "B"),
new SingleDay(LocalDate.now().plusDays(1), "C"));
Map<LocalDate, Map<String, SingleDay>> result = list.stream()
.collect(Collectors.groupingBy(SingleDay::getDate, Collectors.toMap(SingleDay::getCounty, e -> e)));
// Display
result.forEach((k, v) -> System.out.println("Key: " + k + ", Value: " + v));
}
}
输出:
Key: 2020-05-27, Value: {A=SingleDay [date=2020-05-27, County=A], B=SingleDay [date=2020-05-27, County=B], C=SingleDay [date=2020-05-27, County=C]}
Key: 2020-05-26, Value: {X=SingleDay [date=2020-05-26, County=X], Y=SingleDay [date=2020-05-26, County=Y], Z=SingleDay [date=2020-05-26, County=Z]}
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