Sum content of list which has the same content

tamaramaria :

I have a sortedList from type "Details" which has the following content:

[0]=Details test0: 28.01.2020
[0]=Details test1: 1234
[0]=Details test2: abcd
[0]=Details test3: 1
[0]=Details test4: 20.6
[0]=Details test5: 20.20

[1]=Details test0: 28.01.2020
[1]=Details test1: 1234
[1]=Details test2: abcd
[1]=Details test3: 1
[1]=Details test4: 20.6
[1]=Details test5: 20.20

[2]=Details test0: 29.01.2020
[2]=Details test1: 1234
[2]=Details test2: abcd
[2]=Details test3: 1
[2]=Details test4: 20.7
[2]=Details test5: 20.20

Now the sorted list returns all this objects but I want to sum up all with the same test0,test1,test2 and sum test3,test4 and test5 (test3+test3, test4+test4 and test5+test5):

[0]=Details test0: 28.01.2020
[0]=Details test1: 1234
[0]=Details test2: abcd
[0]=Details test3: 2
[0]=Details test4: 41.2
[0]=Details test2: 40.40

[1]=Details test0: 29.01.2020
[1]=Details test1: 1234
[1]=Details test2: abcd
[1]=Details test3: 1
[1]=Details test4: 2.7
[1]=Details test5: 20.20

I'm not sure if list.stream().filter is the best option for this problem. Any ideas? two methods that should find all test0 duplicates:

    public static List<Detail> getDuplicates(final List<Detail> list) {
    return getDuplicatesMap(list).values()
        .stream()
        .filter(duplicates -> duplicates.size() > 1)
        .flatMap(Collection::stream)
        .collect(Collectors.toList());
}

private static Map<String, List<Details>> getDuplicatesMap(List<Details> list) {
    return list.stream().collect(Collectors.groupingBy(Details::test0));
}

Naman :

What you are looking for is merging duplicate entries based on a few attributes and not just identifying the duplicates. Using Collectors.toMap with a mergeFunction should help you resolve this.

public static List<Details> getDuplicates(final List<Details> list) {
    return new ArrayList<>(list.stream()
            .collect(Collectors.toMap(dtl -> 
                            Arrays.asList(dtl.getTest0(), dtl.getTest1(), dtl.getTest2()), 
                    Function.identity(), Details::mergeDuplicates))
            .values());
}

where the merge function is assumed to be using type-specific Numbers that can be added as in:

static Details mergeDuplicates(Details a, Details b) {
    return new Details(a.getTest0(), a.getTest1(), a.getTest2(),
            a.getTest3() + b.getTest3(), a.getTest4() + b.getTest4(), a.getTest5() + b.getTest5());
}