How to paginate a list of objects in Java 8?

Question

Given a java.util.List with n elements and a desired page size m, I want to transform it to a map containing n/m+n%m elem

Answer 1

As noted in the comments this also works if the list is not a natural sequence of integers. You would have to use a generated IntStream then and refer to the elements in list by index.

List list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);

Map map = IntStream
    .range(0, list.size())
    .boxed()
    .collect(groupingBy(
        i -> i / 3, //no longer i-1 because we start with 0
        mapping(i -> list.get((int) i).toString(), joining(","))
        ));

//result: {0="1,2,3", 1="4,5,6", 2="7,8,9", 3="10"}

We start with an IntStream representing the indices of the list.

groupingBy groups the elements by some classifier. In your case it groups x elements per page.

mapping applies a mapping function to the elements and collects them afterwards. The mapping is necessary because joiningonly accepts CharSequence. joining itself joins the elements by using an arbitrary delimiter.