convert array list -> map using java 8

Question

i've been trying to figure out how to use java 8 streams to create a map like the one created by getCardValueCountMap method below:

public class Hand {
        private final List<Card> hand = new ArrayList<>();
        .....
        public Map<CardValue, Integer> getCardValueCountMap(){
            HashMap<CardValue, Integer> cardValueCountMap = new HashMap<>();
            for (Card card : hand) {
                final CardValue value = card.getValue();
                Integer count = cardValueCountMap.get(value);
                if (count == null){
                   cardValueCountMap.put(value, 1);
                } else {
                   cardValueCountMap.put(value, count + 1);
                }
            }
            return cardValueCountMap;
         }
....
}

_I've been trying_...doesn't show what they tried. Smells like _do-my-work-for-me_. — Savior, Apr 11 '16 at 16:53
There are like 5 duplicates in the Related tab on the right. — Savior, Apr 11 '16 at 16:58
It is the count part that i'm having trouble with hand.stream().collect(Collectors.toMap(Card::getValue, ??));.. I'm using intellij and it can't resolve getValue, plus i don't know how to get the count feature. — richs, Apr 11 '16 at 17:03
See the linked question, a great solution is to group by `Card::getValue` and count as downstream. — Tunaki, Apr 11 '16 at 17:04

Paul Boddington · Accepted Answer · 2016-04-11T17:04:14.240

2

This doesn't use streams, so isn't really an answer to your question, but in my opinion the cleanest way to do this is to take advantage of the improvements to the Map interface.

HashMap<CardValue, Integer> map = new HashMap<>();
for (Card card : hand)
    map.merge(card.getValue(), 1, Integer::sum);

edited Apr 11 '16 at 17:04

answered Apr 11 '16 at 16:58

Paul Boddington

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2

I like that. Alternatives: `hand.stream().collect(toMap(Card::getValue, c -> 1, Integer::sum));` -- `hand.stream().collect(groupingBy(Card::getValue, counting()));`. – Tunaki Apr 11 '16 at 17:10

convert array list -> map using java 8

1 Answers1