TheAlgorithms/Java · error · IllegalArgumentException

Input array must not be null

Error message

Input array must not be null

What it means

Thrown by ShuffleArray.shuffle when the input int[] arr is null. shuffle performs an in-place Fisher–Yates shuffle indexing arr.length-1 down to 1, so a null array would NPE inside the loop. The guard rejects null explicitly with a clear message. An empty or single-element array is allowed (the loop simply does nothing).

Source

Thrown at src/main/java/com/thealgorithms/misc/ShuffleArray.java:32

 *
 * This class provides a static method to shuffle an array in place.
 *
 * @author Rashi Dashore (https://github.com/rashi07dashore)
 */
public final class ShuffleArray {

    private ShuffleArray() {
    }

    /**
     * Shuffles the provided array in-place using the Fisher–Yates algorithm.
     *
     * @param arr the array to shuffle; must not be {@code null}
     * @throws IllegalArgumentException if the input array is {@code null}
     */
    public static void shuffle(int[] arr) {
        if (arr == null) {
            throw new IllegalArgumentException("Input array must not be null");
        }

        Random random = new Random();
        for (int i = arr.length - 1; i > 0; i--) {
            int j = random.nextInt(i + 1);
            swap(arr, i, j);
        }
    }

    /**
     * Swaps two elements in an array.
     *
     * @param arr the array
     * @param i   index of first element
     * @param j   index of second element
     */
    private static void swap(int[] arr, int i, int j) {
        if (i != j) {

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the array is non-null before shuffling (instantiate to an empty array if empty).
2. Fix upstream methods to return empty arrays rather than null.
3. Add a null guard at the call site that skips shuffling for null.

Example fix

// before
int[] deck = loadDeck(); // may be null
ShuffleArray.shuffle(deck);

// after
int[] deck = loadDeck();
if (deck == null) deck = new int[0];
if (deck.length > 1) ShuffleArray.shuffle(deck);

Objects.requireNonNull(arr, "arr");
if (arr.length > 1) ShuffleArray.shuffle(arr);

static boolean isShuffleable(int[] a) {
    return a != null && a.length > 1;
}

When it happens

Trigger: Calling ShuffleArray.shuffle(null), or passing an int[] variable that was never assigned / returned null from a loader. Common when shuffling a list converted via stream().mapToInt(...).toArray() on an empty/null source that yielded null.

Common situations: Uninitialized array field, a helper that returns null on empty input instead of an empty array, or a refactor that dropped the assignment.

Related errors

• Invalid input
• Array cannot be null
• Cannot insert null into the heap.
• Cannot insert null element
• Cannot add null element to the list

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/bdacd9b863452ffb. Report an issue: GitHub.