How to collect in Map<String, ArrayList<String>> in list of fields in list of object?

Question

I have a AssistantList and need to map to Map of (Key - Assistant Login, Value - ArrayList of its Supervisor)

I used below code to map using for loop.

How can I use the Stream API to achieve the same functionality?

for(Assistant assistant: assistantList)
{
    for(Supervisor supervisor: assistant.getSupervisorList())
    {
            map.computeIfAbsent(assistant.getLogin(), 
                    k->new ArrayList<>()).add(supervisor.getLogin());
    }
}

I think current way is more readable. BTW java stream version like this: assistantList.stream() .collect(groupingBy(Assistant::getLogin, Collectors.mapping(Assistant::getSupervisorList, Collectors.collectingAndThen(Collectors.toList(), lists -> lists.stream().flatMap(List::stream).map(Supervisor::getLogin).collect(Collectors.toList()))))); — Hadi
– Hadi, Commented Aug 12, 2021 at 15:29
I’d rather use for(Assistant assistant: assistantList) { List<…> l = map.computeIfAbsent(assistant.getLogin(), k->new ArrayList<>()); for(Supervisor supervisor: assistant.getSupervisorList()) l.add(supervisor.getLogin()); } — Holger
– Holger, Commented Aug 12, 2021 at 16:00

udalmik · Accepted Answer · 2021-08-13 08:10:03Z

2

I think you can use combination of groupBy for grouping and flatMapping for merging the supervisors:

import static java.util.stream.Collectors.flatMapping;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toList;

Map<String, List<String>> grouped = assistantList.stream()
        .collect(groupingBy(
                Assistant::getLogin,
                flatMapping(
                        assistant -> assistant.getSupervisorList()
                                .stream()
                                .map(Supervisor::getLogin),
                        toList())
        ));
    }
}

Also I think Map<String, Set<String>> could work better, to remove possible supervisor duplicate values.

flatMapping is Java 9+ collector, for Java 8 you can check this and this answers on possible alternatives.

edited Aug 13, 2021 at 8:10

answered Aug 12, 2021 at 15:22

udalmik

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3 Comments

Hadi Over a year ago

java 8 version like this:

assistantList.stream()                 .collect(groupingBy(Assistant::getLogin,                         Collectors.mapping(Assistant::getSupervisorList,                                 Collectors.collectingAndThen(Collectors.toList(), lists ->                                         lists.stream().flatMap(List::stream).map(Supervisor::getLogin).collect(Collectors.toList())))));

Holger Over a year ago

I’d rather use the flatMapping implementation shown at the end of this answer in Java 8.

udalmik Over a year ago

Thx, looks good, I added ref to this answer as well

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