Java ArrayList adding values with same Id

Question

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?

Answer 1

Assuming you can use a Map: My initial thought it to use a Map<String, String> map = new HashMap();

Pseudo code:
For each (id,value pair) in resultList
   if map.get(id) exists
      add the value to the existing entry in the map
   else 
      add a new entry in the map for (id,value)

As code (NOTE: untested and might not compile, wouldn't copy & paste directly):

Map<String, String> map = new HashMap<>();
for(int i = 0; i < myList.size(); i+=2){
   String listId = resultList.get(i); //get the Id from the resultList
   String listValue = resultList.get(i+1) //get the value from the resultList
   if(map.get(listId) != null) { // if the map has this Id
      map.put(listId, map.get(listId)+ listValue); // add the Id's list value
   }
   else { // if the map doesn't have this Id
      map.put(listId, listValue) // add the entry to the map
   }
}

You can then take the resulting map and transform is back into a list. I'll let you write the code to take each entry in the map and add it to a new List. Google can be helpful here.