Find Max of Multiple Lists

Question

I'm pretty new to java streams and am trying to determine how to find the max from each list, in a list of lists, and end with a single list that contains the max from each sublist.

I can accomplish this by using a for loop and stream like so:

// databaseRecordsLists is a List<List<DatabaseRecord>>

List<DatabaseRecord> mostRecentRecords = new ArrayList<>();

for (List<DatabaseRecord> databaseRecords : databaseRecordsLists) {
            mostRecentRecords.add(databaseRecords.stream()
                    .max(Comparator.comparing(DatabaseRecord::getTimestamp))
                    .orElseThrow(NoSuchElementException::new));
        }

I've looked into the flatMap api, but then I'll only end up with a single map of all DatabaseRecord objects, where I need a max from each individual list.

Any ideas on a cleaner way to accomplish this?

Answer 1

You don't need flatMap. Create a Stream<List<DatabaseRecord>>, and map each List<DatabaseRecord> of the Stream to the max element. Then collect all the max elements into the output List.

List<DatabaseRecord> mostRecentRecords =
    databaseRecordsLists.stream()
                        .map(list -> list.stream()
                                         .max(Comparator.comparing(DatabaseRecord::getTimestamp))
                                         .orElseThrow(NoSuchElementException::new))
                        .collect(Collectors.toList());