TheAlgorithms/Java · error · IllegalArgumentException

Input array must not be null.

Error message

Input array must not be null.

What it means

Thrown by TwoPointers.isPairedSum when the input array is null. The two-pointer technique requires an actual array to scan from both ends; a null array would otherwise cause a NullPointerException on arr.length. This is a fast-fail precondition check converting a latent NPE into a meaningful contract violation.

Source

Thrown at src/main/java/com/thealgorithms/others/TwoPointers.java:25

 * <p>
 * Link: https://www.geeksforgeeks.org/two-pointers-technique/
 */
public final class TwoPointers {

    private TwoPointers() {
    }

    /**
     * Checks whether there exists a pair of elements in a sorted array whose sum equals the specified key.
     *
     * @param arr a sorted array of integers in ascending order (must not be null)
     * @param key the target sum to find
     * @return {@code true} if there exists at least one pair whose sum equals {@code key}, {@code false} otherwise
     * @throws IllegalArgumentException if {@code arr} is {@code null}
     */
    public static boolean isPairedSum(int[] arr, int key) {
        if (arr == null) {
            throw new IllegalArgumentException("Input array must not be null.");
        }

        int left = 0;
        int right = arr.length - 1;

        while (left < right) {
            int sum = arr[left] + arr[right];

            if (sum == key) {
                return true;
            }
            if (sum < key) {
                left++;
            } else {
                right--;
            }
        }
        return false;

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Ensure the caller passes a non-null int[] — initialize it to an empty array (new int[0]) if no elements exist.
2. When the array comes from a Map or Optional, handle the absent case before calling isPairedSum.
3. Add a null check or Objects.requireNonNull in the caller if the array origin is untrusted.

Example fix

// before
int[] arr = (int[]) map.get("pairs");
boolean found = TwoPointers.isPairedSum(arr, target);
// after
int[] arr = (int[]) map.getOrDefault("pairs", new int[0]);
boolean found = TwoPointers.isPairedSum(arr, target);

if (arr == null) {
    arr = new int[0]; // or throw a domain-specific error upstream
}
boolean found = TwoPointers.isPairedSum(arr, key);

static boolean isUsable(int[] arr) {
    return arr != null;
}

try {
    return TwoPointers.isPairedSum(arr, key);
} catch (IllegalArgumentException e) {
    // arr was null; treat as "no pair found" only if that matches domain semantics
    return false;
}

When it happens

Trigger: Calling isPairedSum(null, key), or passing an array reference that was never assigned (left null after a failed lookup or a map.get that returned null).

Common situations: Array sourced from a Map.get that returned null because the key was absent, a List.toArray() result mishandled, an unconfigured field, or test code that forgets to initialize the input.

Related errors

• Cannot insert null element
• Cannot add null element to the list
• Cannot add null element to the list
• Element cannot be null
• Input lists and result collection must not be null.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/24da8cd87352ec97. Report an issue: GitHub.