How do you use java stream api to convert list of objects into a nested map based on information stored inside object?

问题

I have a list of objects List<SingleDay> where SingleDay is

class SingleDay{ 
      private Date date;
      private String County;

   // otherstuff
}

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.

For example:
02/12/2020 : { "Rockbridge": {SingleDayObject}}

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.

Basically, I want to be able to quickly query the object that corresponds to the date and county.

Thanks!

回答1:

Do it as follows:

Map<LocalDate, Map<String, SingleDay>> result = list.stream()
                .collect(Collectors.toMap(SingleDay::getDate, v -> Map.of(v.getCounty(), v)));

Demo:

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));
    }
}

Output:

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]}

Note: I've used LocalDate instead of outdated java.util.Date.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.

回答2:

A slightly more readable data structure would be making a Map<SingleDay, SingleDay>. Might seem redundant but that will allow you to easily find your objects.

Map<SingleDay, SingleDay> myMap = new HashMap();
list.forEach(day -> myMap.put(day, day));

Then to get one you can just do

SingleDay day = mMap.get(myDay);

Very important to not forget to override the equals() method of SingleDay, based on what you consider as equal.

来源：https://stackoverflow.com/questions/62005476/how-do-you-use-java-stream-api-to-convert-list-of-objects-into-a-nested-map-base

标签

java

date

java-stream