How does one Convert a List to Map with Java-8 Streams where Key is the ListValue and the Value is the amount of values of that specific list

Question

This is my list:

List<Card> cards;

my Java-8 stream where i want to create the Map

Map<String, Integer> cardsMap = cards.stream().collect(Collectors.groupingBy(Card::getCardValue, amount of cards that are grouped));

This obviously doesn't work but i am clueless of how i would do it otherwise.

Possible duplicate of [Java 8 List into Map](http://stackoverflow.com/questions/20363719/java-8-listv-into-mapk-v) — PM 77-1, Jan 15 '17 at 23:43
as far as i can see it doesn't describe how i would make the value a count of all the grouped items — sansactions, Jan 15 '17 at 23:45
sure, gona try to make it, didn't make it since i am trying to get familliar with java8 — sansactions, Jan 15 '17 at 23:47

score 7 · Accepted Answer · answered Jan 16 '17 at 00:02

Is this what you meant?

    Map<String, Long> cardsMap = cards
            .stream()
            .collect(Collectors.groupingBy(Card::getCardValue, Collectors.counting()));

It will give you a map from card values to counts of cards with that value in your original list. For example, if you have:

    List<Card> cards = Arrays.asList(new Card("4"), new Card("8"), new Card("4"));

(and I know I’ve probably reduced your Card() constructor), the above will map "4" to 2 and "8" to 1.

just found it myself, but yes that was basically how one should do it, thanks — sansactions, Jan 16 '17 at 00:15

score 2 · Answer 2 · edited Dec 24 '19 at 23:56

2

This is how to do it:

Map<String, Long> cardsMap = cards.stream().collect(Collectors.groupingBy(e -> e.getCardValue(), Collectors.counting()));

edited Dec 24 '19 at 23:56

Austin

answered Jan 16 '17 at 00:14

sansactions

2 Answers2