Java: get List of entries from Lists nested in HashMap

Question

I have a HashMap<Foo, ArrayList<Bar>> map with the following structure:

|-- a
|   |-- a1
|   |-- a2
|   `-- a3
|
`-- b
    |-- b1
    |-- b2
    `-- b3

where a and b are objects of the type Foo and a1, a2, etc. objects of the type Bar.

What I want to have is a List<Bar> with the following structure:

|-- a1
|-- a2
|-- a3
|-- b1
|-- b2
`-- b3

Right now I have this code:

ArrayList<Bar> tempList = new ArrayList<>();
map.values().forEach(tempList::addAll);
return tempList;

But this feels a bit clumsy and inelegant. How can I achieve this using the standard Java API, preferably with java.util.Stream (or lambda expressions)?

See also http://stackoverflow.com/questions/31056919/concatenate-the-string-values-of-all-maps-in-a-list or http://stackoverflow.com/questions/35682522/convert-mapstring-navigablemaplong-collectionstring-to-liststring-usin — Tunaki, Dec 04 '16 at 17:23

score 3 · Accepted Answer · answered Dec 04 '16 at 17:16

3

List<Bar> bars = map.values()
                    .stream()
                    .flatMap(Collection::stream)
                    .collect(Collectors.toList());

answered Dec 04 '16 at 17:16

JB Nizet

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Java: get List of entries from Lists nested in HashMap

1 Answers1