Concatenating every other array element

Question

I am learning how to use Java 8 streams. How can I do the following using streams instead of a for loop:

public static void clump(ArrayList strList)
{
    for(int i = 0; i < strList.size() - 1; i++)
    {
        String newStr = "(" + strList.get(i) + " "
                + strList.get(i + 1) + ")";

        strList.set(i, newStr);
        strList.remove(i + 1);
    }
}

Answer 1

Use an IntStream between 0 and half the list's size, and multiply the elements by 2:

List<String> joined = 
    IntStream.range(0, strList.size() / 2)
        .mapToObj(i -> "(" + strList.get(2*i) + " "
                + strList.get(2*i + 1) + ")")
        .collect(Collectors.toList());

This puts the joined elements into a new list. To get rid of the un-joined elements from the original list, and prepend the joined elements, we can use a subList, which is a mutable view of the list:

List<String> subList = strList.subList(0, strList.size() / 2 * 2);
subList.clear();
subList.addAll(joined);

Ideone Demo

---

An alternative to the sublist stuff would be to handle the last element in the stream too:

List<String> joined = 
    IntStream.range(0, (1 + strList.size()) / 2)
        .mapToObj(i ->
            (2*i + 1) < strList.size()
                ? "(" + strList.get(2*i) + " " + strList.get(2*i + 1) + ")"
                : strList.get(2*i))
        .collect(Collectors.toList());

Ideone demo