I am looking to merge the lists in a list if the 1st index match in Java8. I have a list of lists

[[ABC, 1.0, 4.0, 4.0], [ABC, 2.0, 4.0, 4.0], [ABC, 1.0, 72.0, 72.0], [XYZ, 1.0, 36.0, 36.0], [XYZ, 2.0, 16.0, 16.0]]

And I want the result as below in an effective way like using stream in java. Result should be

[[ABC, 4.0, 116.0, 116.0], [XYZ, 3.0, 52.0, 52.0]]

Could anyone help please?

CodePudding user response：

In this case Collectors.toMap should be used with the key selected as the first index, and a merge function to sum the rest elements.

Example:

List<List<Object>> input = ...; 
List<List> merged = new ArrayList<>(input.stream()
    .collect(Collectors.toMap(
        list -> list.get(0),
        list -> list,
        (l1, l2) -> Stream.concat(
            Stream.of(l1.get(0))),
            IntStream.range(1, l1.size())
                .mapToObj(i -> (Double)l1.get(i)   (Double)l2.get(i))
        )
        .collect(Collectors.toList()),
        LinkedHashMap::new  // keep insertion order
    ))
    .values() // Collection<List<Object>>
);

However, the input lists seem to be a mix of String and double data and they should be better converted into a class/record.

CodePudding user response：

1. For each list in the list of lists
2. If first value is not in hashmap add to hashmap using first value as key
3. If key is already in hashmap merge with list already in hashmap
4. Create new list of lists based on hashmap entry set

Extra credit: use LinkedHashMap to preserve order of appearance or TreeMap if you want it in alphabetical order