How to sort a list of lists using the Stream API?

Question

I want to output all names starting with the letter "A" and sort them in reverse alphabetical order using the Stream API. The result should look like this: Ann, Andrew, Alex, Adisson. How do I do this?

List<List<String>> list = Arrays.asList(
            Arrays.asList("John", "Ann"),
            Arrays.asList("Alex", "Andrew", "Mark"),
            Arrays.asList("Neil", "Adisson", "Bob")
    );
    List<List<String>> sortedList = list.stream()
            .filter(o -> o.startsWith("A"))
            Comparator.reverseOrder());
    System.out.println(list);

List<String> sortedList = list.stream().flatMap(List::stream).filter(s -> s.startsWith("A")).sorted(Comparator.reverseOrder()).collect(Collectors.toList()); — Eritrean
– Eritrean, Commented Jul 4, 2022 at 20:26

WJS · Accepted Answer · 2022-07-05 02:01:02Z

Since you are assigning the result to a List<List<String>> it appears you want to filter and sort each list and then print them. Here is how it might be done.

Stream the list of lists.
then stream each individual list and:
- apply the filter
- sort in reversed order using Comparator.reverseOrder
then return that as a list
and combine those lists into another list.


List<List<String>> sortedLists = list.stream()
        .map(lst -> lst.stream()
                .filter(o -> o.startsWith("A"))
                .sorted(Comparator.reverseOrder()).toList())
    .toList();
for(List<String> lst : sortedLists) {
    System.out.println(lst);
}

prints (given your data).

[Ann]
[Andrew, Alex]
[Adisson]

If, on the other hand you want to combine the lists into one applying the same requirements, then:

stream the list of lists
flatMap them to put all of the list contents into a single stream.
then apply the filter and sort methods.
and collect into a list.

List<String> sortedList = list.stream().flatMap(List::stream)
             .filter(o->o.startsWith("A"))
             .sorted(Comparator.reverseOrder())
             .toList();

for (String name : sortedList) {
    System.out.println(name);
}

prints

Ann
Andrew
Alex
Adisson

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