Java 8 count objects with multiple values

Question

Assume I have a class called Combination that consists of two fields whoose type is a simple enum value.

Examples:

new Combination(Animal.DOG, Animal.CAT),

new Combination(Animal.CAT, Animal.APE),

new Combination(Animal.MOUSE, Animal.DOG)

I have a collection of combinations and I'd like to count the total occurences of each animal so that the example output would look like:

DOG=2
CAT=2
MOUSE=1
APE=1

I already tried different approches, but I didn't found a solution yet. Is there any simple way to do this in java 8?

Thanks in advance.

you can start with for loop to get the idea of an algorithm

AdamSkywalker
– AdamSkywalker

2017-02-16 20:32:09 +00:00
Commented Feb 16, 2017 at 20:32 — AdamSkywalker
– AdamSkywalker, Commented Feb 16, 2017 at 20:32
What are the accessors for Combination?

Compass
– Compass

2017-02-16 20:33:41 +00:00
Commented Feb 16, 2017 at 20:33 — Compass
– Compass, Commented Feb 16, 2017 at 20:33
What are the different approaches that you tried?

Kayaman
– Kayaman

2017-02-16 20:35:53 +00:00
Commented Feb 16, 2017 at 20:35 — Kayaman
– Kayaman, Commented Feb 16, 2017 at 20:35

shmosel · Accepted Answer · 2017-02-16 20:36:38Z

3

Here's one way:

Map<Animal, Long> counts = combinations.stream()
        .flatMap(c -> Stream.of(c.getFirst(), c.getSecond()))
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

Alternatively:

Map<Animal, Long> counts = Stream.concat(
            combinations.stream().map(Combination::getFirst),
            combinations.stream().map(Combination::getSecond))
        .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));

answered Feb 16, 2017 at 20:36

shmosel

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