Java ArrayList adding values with same Id

yasin :

So I have a an array List that looks like this: <id,value,id,value,...>and I should find all the elements with the same id and add together there respected values into a result array list (value always comes after the id). At first I thought the problem was easy and went ahead and tried this:

        List<String> resultList = new ArrayList<>();
        for(int i = 0; i < myList.size(); i+=2){

            BigInteger currentID = new BigInteger(myList.get(i));
            BigInteger currentSum = new BigInteger(myList.get(i+1));
            String currentId = myList.get(i);
            int j = 2;

            while(j < myList.size()){
                if(currentId.equals(myList.get(j))){
                 BigInteger value2 = new BigInteger(myList.get(j+1));
                 currentSum = currentID.add(value2);
                }
                resultList.add(currentId);
                resultList.add(String.valueOf(currentSum));
                j+=2;
            }
        }

Needless to say it isn't correct, one of the many problems is that values which haven been already added together will be readded into the result array so I think they should be removed somehow in the loop. Do you guys have any suggestions how to go around this?

WJS :

If you want to use streams you can do it like this.

Given a list like the following:

      List<BigInteger> list = List.of(1,10,3,20,3,40,1,5,4,7,1,8,4,9,6,20)
           .stream().mapToLong(a -> a).mapToObj(
                  BigInteger::valueOf).collect(Collectors.toList());

First accumulate the sums in a map with the key being the id.

      Map<BigInteger, BigInteger> mapfinal =
            IntStream.iterate(0, i -> i < list.size(), i -> i += 2).mapToObj(
                  i -> new BigInteger[] { list.get(i), list.get(i + 1)
                  }).collect(Collectors.groupingBy(i -> i[0],
                        Collectors.reducing(BigInteger.ZERO,
                              a -> a[1],
                              (a, b) -> a.add(b))));

You can stop there and just use the map, or you can convert it to a list like the one you started with of alternating id/value pairs.

       List<BigInteger> listfinal = mapfinal
            .entrySet()
            .stream()
            .flatMap(e -> Stream.of(e.getKey(), e.getValue()))
            .collect(Collectors.toList());

And print the list.