Is there a Java 8 equivalent of the collate method in grails?

Question

I'm trying to break a list into a list of lists. In groovy I can easily do this:

def letters = 'a'..'g'

assert letters.collate(3) == [['a', 'b', 'c'], ['d', 'e', 'f'], ['g']]

Is there an equivalent in Java 8? I've looked into Collectors, but it seems a bit complicated. I really just want to group the items in the list into x.

Answer 1

You may look into the Partition API for lists by Guava :

public static <T> java.util.List<java.util.List<T>> partition(java.util.List<T> list, int size)

Returns consecutive sublists of a list, each of the same size (the final list may be smaller).

Answer 2

How about this?

char start = 'a';
char last = 'g';
int n = 3;

List<Character> letters = IntStream.rangeClosed(start, last)
                                   .mapToObj(it -> (char) it)
                                   .collect(toList());

List<List<Character>> result = IntStream.range(0, (letters.size() + n - 1) / n)
               .map(i -> i * n)
               .mapToObj(i -> letters.subList(i, Math.min(i + n, letters.size())))
               .collect(toList());

OR

List<List<Character>> result = IntStream.range(0, letters.size()).boxed().
          collect(collectingAndThen(
               groupingBy(i -> i / n, mapping(letters::get, toList())),
               map -> new ArrayList<>(map.values())
          ));