List

Question

I have List<Person> where Person is as below.

class Person {

    String personId;
    LocalDate date;
    String type;

    // getters & setters

}

I'm trying to convert this to List<Person> to Map<String, Map<LocalDate,List<Person>>> where outer map's key is personId and inner map's key is date and I couldn't figure out how to achieve this. Thus far have tried something like below. Open to Java 8 solutions as well.

Map<String,Map<LocalDate,List<Person>>> outerMap = new HashMap<>();
Map<LocalDate,List<Person>> innerMap = new HashMap<>();

for(Person p : list) {
    List<Person> innerList = new ArrayList<>();
    innerList.add(p);
    innerMap.put(p.getDate(), innerList);
    outerMap.put(p.getPersonId(), innerMap);
}

You should explain your trail of thought. Why is `innerList` declared inside the loop? Why `innerMap` declared outside? Why `outerMap` is declared outside as well? — M. Prokhorov, Apr 12 '18 at 14:48
Possible duplicate of [Convert a Nested List to nested Map](https://stackoverflow.com/questions/48379078/convert-a-nested-list-to-nested-map) — Hadi J, Apr 12 '18 at 14:55

score 17 · Accepted Answer · answered Apr 12 '18 at 14:48

17

list.stream()
    .collect(Collectors.groupingBy(
        Person::getPersonId,
        Collectors.groupingBy(
            Person::getDate
)));

answered Apr 12 '18 at 14:48

Eugene

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score 7 · Answer 2 · answered Apr 12 '18 at 15:32

This answer shows how to do it with streams and this other one how to do it with a traditional iterative approach. Here's yet another way:

Map<String, Map<LocalDate, List<Person>>> outerMap = new HashMap<>();
list.forEach(p -> outerMap
        .computeIfAbsent(p.getPersonId(), k -> new HashMap<>()) // returns innerMap
        .computeIfAbsent(p.getDate(), k -> new ArrayList<>())   // returns innerList
    .add(p)); // adds Person to innerList

This uses Map.computeIfAbsent to create a new innerMap and put it in the outerMap if not present (the key being personId), and also to create a new innerList and put it in the innerMap if not present (the key being date). Finally, the Person is added to the innerList.

Glad to know about `Map.computeIfAbsent`. Thanks Federico! – Ram Apr 12 '18 at 16:06 — Ram, Apr 12 '18 at 16:06

score 3 · Answer 3 · edited Apr 12 '18 at 14:52

Your for loop organization should be changed in such a way that it tries getting an existing List<Person> from the nested map before proceeding with creation of the new list. This is a two-step process:

Map<String,Map<LocalDate,List<Person>>> outerMap = new HashMap<>();
for(Person p : list) {
    Map<LocalDate,List<Person>> innerMap = outerMap.get(p.getPersonId());
    if (innerMap == null) {
        innerMap = new HashMap<>();
        outerMap.put(p.getPersonId(), innerMap);
    }
    List<Person> innerList = innerMap.get(p.getDate());
    if (innerList == null) {
        innerList = new ArrayList<>();
        innerMap.put(p.getDate(), innerList);
    }
    innerList.add(p);
}

3 Answers3