您如何使用Java Stream API根据存储在对象内部的信息将对象列表转换为嵌套映射? [英] How do you use java stream api to convert list of objects into a nested map based on information stored inside object?
问题描述
我有一个对象List<SingleDay>
的列表,其中SingleDay
是
I have a list of objects List<SingleDay>
where SingleDay
is
class SingleDay{
private Date date;
private String County;
// otherstuff
}
我希望将此列表转换为Map<Date, Map<String, SingleDay>>
.也就是说,我希望从Date到Counties的地图都回到原始对象.
Im looking to convert this list into a Map<Date, Map<String, SingleDay>>
. That is, I want a map from Date to a map of Counties back to the original object.
例如:
02/12/2020 : { "Rockbridge": {SingleDayObject}}
如果从对象列表到地图,而不是从对象列表到嵌套地图,则我什么都无法工作,以及在网上找到的所有内容.
I have not been able to get anything to work and everything I found online if from a list of objects to a map, not a list of objects to a nested map.
基本上,我希望能够快速查询与日期和县相对应的对象.
Basically, I want to be able to quickly query the object that corresponds to the date and county.
谢谢!
推荐答案
执行以下操作:
Map<LocalDate, Map<String, SingleDay>> result = list.stream()
.collect(Collectors.toMap(SingleDay::getDate, v -> Map.of(v.getCounty(), v)));
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().plusDays(1), "Y"), new SingleDay(LocalDate.now().plusDays(2), "Z"));
Map<LocalDate, Map<String, SingleDay>> result = list.stream()
.collect(Collectors.toMap(SingleDay::getDate, v -> Map.of(v.getCounty(), v)));
// Display
result.forEach((k, v) -> System.out.println("Key: " + k + ", Value: " + v));
}
}
输出:
Key: 2020-05-27, Value: {Z=SingleDay [date=2020-05-27, County=Z]}
Key: 2020-05-26, Value: {Y=SingleDay [date=2020-05-26, County=Y]}
Key: 2020-05-25, Value: {X=SingleDay [date=2020-05-25, County=X]}
注意:我使用的是LocalDate
而不是过时的java.util.Date
.我强烈建议您使用此,以了解更多信息.
Note: I've used LocalDate
instead of outdated java.util.Date
. I highly recommend you use java.time API instead of broken java.util.Date
. Check this to learn more about it.
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