How to merge duplicated property value and sum their other property - java

I have list of this object, wanted to sum duplicated item's contrast and remove duplicates.

public class ContrastItem {
    private String range;
    private Long contrast;
}

            [{
                "contrast": 3,
                "range": "00:00:00-00:59:59"
            },
            {
                "contrast": 1,
                "range": "00:00:00-00:59:59"
            },
            {
                "contrast": 0,
                "range": "01:00:00-01:59:59"
            },
            {
                "contrast": 0,
                "range": "01:00:00-01:59:59"
            }]

Expected:

            [{
                "contrast": 4,
                "range": "00:00:00-00:59:59"
            },
            {
                "contrast": 0,
                "range": "01:00:00-01:59:59"
            }]

I have tried stream mapping the list with groupby and summingLong but could't wrap around my head to solve it.(don't know what i'm doing)

mylist.stream()
.collect(Collectors.groupingBy(
ContrastItem::getRange,Collectors.summingLong(ContrastItem::getContrast)))
.entrySet().stream().map().collect(Collectors.toList());

Your approach is quite close,

itemList.stream()
        .collect(Collectors.groupingBy
        (
            ContrastItem::getRange,
            Collectors.summingLong(ContrastItem::getContrast)
        ));

this code creates a map {01:00:00-01:59:59=0, 00:00:00-00:59:59=4}

then you need to convert to ContrastItem list

public static void main(String[] args) {
    List<ContrastItem> itemList = new ArrayList<>();
    itemList.add(new ContrastItem("00:00:00-00:59:59", 3L));
    itemList.add(new ContrastItem("00:00:00-00:59:59", 1L));
    itemList.add(new ContrastItem("01:00:00-01:59:59", 0L));
    itemList.add(new ContrastItem("01:00:00-01:59:59", 0L));

    List<ContrastItem> collect = itemList.stream()
            .collect(Collectors.groupingBy(
                    ContrastItem::getRange,
                    Collectors.summingLong(ContrastItem::getContrast)))
            .entrySet().stream()
            .map(entry -> new ContrastItem(entry.getKey(), entry.getValue()))
            .collect(Collectors.toList());
    System.out.println(collect);
}

You can definitely do it with Stream and it looks not very complicated. groupingBy + reducing is what you need.

Collection<ContrastItem> items = contrastItems.stream()
   .collect(Collectors.groupingBy(
      ContrastItem::getRange,
      Collectors.reducing(new ContrastItem(0L, ""),
         (one, two) -> new ContrastItem(one.getContrast() + two.getContrast(),
                                        two.getRange()))))
   .values();