Compare elements within arraylist of Object using java stream

Question

I have list of Object . I want to compare elements of the list and perform operation if the are equal on a condition.

class A{ String abc; int pqr; }

I have List items with me "items": [ { "pqr": 2, "abc": 1994 }, { "pqr": 2, "abc": 1994 }, { "pqr": 1, "abc": 64 } ]

Now i want my output list as "items": [ { "pqr": 4, "abc": 1994 }, { "pqr": 1, "abc": 64 } ] i/e comparing th elements on the basis of abc and if its is same add its pqr value and remove the duplicate.

How can i do this using stream or efficiently?

Answer 1

As mentioned by Ralf you can use groupingBy to get a Map with abc as key and the sum of pqr as value:

Map<String, Integer> groupBy = list.stream()
    .collect(Collectors.groupingBy(Container::getAbc, Collectors.summingInt(Container::getPqr)));
System.out.println(groupBy);

produces {1994=4, 64=1}. If you need to convert this to a List you have to iterate over this map:

List<Container> list = groupBy.entrySet()
    .stream()
    .map(e -> new Container(e.getKey(), e.getValue()))
    .collect(Collectors.toList());
System.out.println(list);

produces (with overwritten toString) [A{abc=1994, pqr=4}, A{abc=64, pqr=1}]