Java 8 Mapping a list of maps grouping by a key

Question

I have the following: List<Map<String, Object>>. I want to obtain a Map<String, List<Object> using streams. The key of this map will be the key, that it is repeated in the list. Basically, is a one-to-many, this would be an example of the data:

[
    { id: "a", name: "AAA"}
    { id: "a", name: "BBB"}
    { id: "b", name: "XXX"}
    { id: "b", name: "YYY"}
]

And I would like to obtain:

{
    a: ["AAA", "BBB"],
    b: ["XXX", "YYY"]
}

So, this new map is grouping by id from the previous list.

IMO use object instead of Map due to it would be simple the process. class MyObject{ String id; String name;} and use following code to achieve final result: list.stream() .collect(Collectors.groupingBy(MyObject::getId, Collectors.mapping(MyObject::getName, Collectors.toList()))); — Hadi
– Hadi, Commented Apr 1, 2019 at 16:23

fps · Accepted Answer · 2019-04-01 17:56:32Z

Use Collectors.groupingBy along with Collectors.mapping:

Map<String, List<Object>> result = listOfMaps.stream()
    .collect(Collectors.groupingBy(map -> map.get("id").toString(),
                                   Collectors.mapping(map -> map.get("name"),
                                                      Collectors.toList())));

Collectors.groupingBy creates the map you want, while Collectors.mapping adapts a collector into another one by applying a mapping function to each element of the stream. (In this case, it collects to a list by first transforming each map of the stream to the value mapped to the "name" key).

If needed, the code above might be modified to skip maps that don't have an entry with the "id" key:

Map<String, List<Object>> result = listOfMaps.stream()
    .filter(map -> map.get("id") != null))
    .collect(Collectors.groupingBy(map -> map.get("id").toString(),
                                   Collectors.mapping(map -> map.get("name"),
                                                      Collectors.toList())));

Collectives™ on Stack Overflow

Java 8 Mapping a list of maps grouping by a key

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related