Combine two ArrayList while summing by a common value

Question

Having a object that has 3 properties:

Item
    String date;
    int value1;
    int value2;

I have first ArrayList<Item> listA which contains:

2017-01-18, 0, 0
2017-01-17, 0, 0
2017-01-16, 0, 0
2017-01-15, 0, 0

Second ArrayList<Item> listB which contains:

2017-01-18, 7, 3
2017-01-15, 4, 0

I want to combine both lists, into a final one, having the values summed by same date

2017-01-18, 7, 3
2017-01-17, 0, 0
2017-01-16, 0, 0
2017-01-15, 4, 0

Combine them both with `addAll()` then just implement the `Comparable` interface with your logic to sort by date, then `Collections.sort(yourCombinedList)`, or create a Implentation of the `Comparator` interface - http://stackoverflow.com/a/14452176/4252352 — Mark, Jan 18 '17 at 17:59
The main issue for me is adding the value1 and value2 for same date — Alin, Jan 18 '17 at 18:02
I see - misunderstood, you want to aggregate duplicate dates — Mark, Jan 18 '17 at 18:07

Leandro Borges Ferreira · Accepted Answer · 2017-01-19T17:10:25.397

0

Try this:

public static void sumList(List<Item> list1, List<Item> list2){
    List<Item> bigger;
    List<Item> smaller;

    if(list1.size() > list2.size()){
        bigger = list1;
        smaller = list2;
    } else{
        bigger = list2;
        smaller = list1;
    }

    for(int i = 0; i < bigger.size(); i++){
        for(int j = 0; j < smaller.size(); j++){
            if(bigger.get(i).date.equals(smaller.get(j).date)){
            bigger.get(i).value1 = bigger.get(i).value1 + smaller.get(j).value1

            bigger.get(i).value2 = bigger.get(i).value2 + smaller.get(j).value2
            }
        }
    }
}

happy coding!

edited Jan 19 '17 at 17:10

answered Jan 18 '17 at 18:03

Leandro Borges Ferreira

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This is something that I was trying to avoid... but seems like the only solution... – Alin Jan 18 '17 at 18:04
Is it ok to use Java 8 for you? As I can see this question is tagged for Android... So you cannot use the Java 8 methods for List... You could have a better solution using Kotlin... – Leandro Borges Ferreira Jan 18 '17 at 18:10
I need to use it on android – Alin Jan 18 '17 at 18:17
Is Kotlin an option? – Leandro Borges Ferreira Jan 18 '17 at 18:31
No, just plain android studio tools – Alin Jan 18 '17 at 18:45
Yeah... Without using foreEach I don't see a much better way than this... – Leandro Borges Ferreira Jan 18 '17 at 18:50
I haven't tried your code yet, but probably the `if(bigger` should be `if(smaller.get`. Also, shouldn't on `for` loop, be another loop to check for all other list's items? – Alin Jan 19 '17 at 13:46
I didn't understand the if bigger and if smaller... It looks ok for me, maybe I am missing something. I am basically iterating the smaller list and summing in the bigger one when they are equals. I iterate all the smaller itens, so I don't see the need for another loop. – Leandro Borges Ferreira Jan 19 '17 at 14:40
I was refering to `if(bigger.get(i).date.equals(bigger.get(i).date)){` Isn't that always true? – Alin Jan 19 '17 at 15:58
Oh, it's true! Sorry about that. I edited the answer. – Leandro Borges Ferreira Jan 19 '17 at 16:02
Still unclear, because a date in small list can be in another position in big list. For instance 2017-01-15 is position 4 in big list and position 1 in small list. So comparing same position in both lists isn't going to be always true. Or maybe I am missing something in your logic. – Alin Jan 19 '17 at 16:59
Sorry man... I was wrong again. I edited the answer once again. – Leandro Borges Ferreira Jan 19 '17 at 17:11

score 0 · Answer 2 · answered Jan 18 '17 at 18:17

try this

List<Item> firstList = new ArrayList<Item>();
List<Item> secondList = new ArrayList<Item>();

HashMap<String, Item> hashMap = new HashMap<String, Item>();
for(Item item: firstList) {
    hashMap.put(item.date, item);
}
for(Item item: secondList) {
    Item tempItem = hashMap.get(item.date);
    if(tempItem != null) {
        tempItem.add(item);
        hashMap.put(tempItem.date, tempItem);
    } else {
        hashMap.put(item.date, item);
    }
}

and your item class be like

class Item {
    String date;
    int value1;
    int value2;

    public void add(Item item) {
        this.value1 = this.value1 + item.value1;
        this.value2 = this.value2 + item.value2;
    }
}

Combine two ArrayList while summing by a common value

2 Answers2