Mixed arrays and lists

Mixed collections and arrays — the exam loves asking for both in one method:

// array in, list out
public static ArrayList<Integer> positives(int[] a) {
    ArrayList<Integer> out = new ArrayList<Integer>();
    for (int x : a) {
        if (x > 0) {
            out.add(x);
        }
    }
    return out;
}

Four conversion facts that decide correctness:

• int[] and ArrayList<Integer> are different types; neither auto-converts. Box manually (out.add(x) boxes the int).
• list.remove(Integer.valueOf(5)) removes the VALUE 5; list.remove(5) removes INDEX 5. The exam tests this distinction.
• Removing while iterating forward skips the element after the removed one — iterate backwards or use an index-corrected loop.
• a.length vs list.size() — one is a field, one is a method.

When a spec says "return a new list", never mutate the input.