TheAlgorithms/Java · error · IllegalArgumentException

Input array cannot be null

Error message

Input array cannot be null

What it means

Thrown by SubarraySumEqualsK.countSubarrays(nums, k) when nums is null. The method uses a HashMap of prefix-sum frequencies and iterates nums with a for-each loop, so a null array would NPE during iteration without this guard. Empty arrays are allowed (returns 0).

Source

Thrown at src/main/java/com/thealgorithms/prefixsum/SubarraySumEqualsK.java:52

 * @author Ruturaj Jadhav, <a href="https://github.com/ruturajjadhav07">ruturajjadhav07</a>
 */
public final class SubarraySumEqualsK {

    private SubarraySumEqualsK() {
        // Utility class; prevent instantiation
    }

    /**
     * Counts the number of subarrays whose sum equals k.
     *
     * @param nums The input integer array.
     * @param k    The target sum.
     * @return The number of continuous subarrays summing to k.
     * @throws IllegalArgumentException if nums is null.
     */
    public static int countSubarrays(int[] nums, int k) {
        if (nums == null) {
            throw new IllegalArgumentException("Input array cannot be null");
        }

        Map<Long, Integer> prefixSumFrequency = new HashMap<>();
        prefixSumFrequency.put(0L, 1);

        long prefixSum = 0;
        int count = 0;

        for (int num : nums) {
            prefixSum += num;

            long requiredSum = prefixSum - k;
            count += prefixSumFrequency.getOrDefault(requiredSum, 0);

            prefixSumFrequency.put(prefixSum, prefixSumFrequency.getOrDefault(prefixSum, 0) + 1);
        }

        return count;

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check nums at the caller; pass new int[0] for the empty case.
2. Make the data source return empty instead of null.
3. Wrap with Objects.requireNonNull(nums) if null is a programmer error.

Example fix

// before
int c = SubarraySumEqualsK.countSubarrays(arr, k); // arr may be null

// after
int c = SubarraySumEqualsK.countSubarrays(arr == null ? new int[0] : arr, k);

int[] safe = nums == null ? new int[0] : nums;
int c = SubarraySumEqualsK.countSubarrays(safe, k);

static boolean usable(int[] a) {
    return a != null; // empty allowed, returns 0
}

try {
    int c = SubarraySumEqualsK.countSubarrays(nums, k);
} catch (IllegalArgumentException e) {
    logger.warn("Null nums in countSubarrays");
}

When it happens

Trigger: SubarraySumEqualsK.countSubarrays(null, 5), or passing an array field that was never initialized.

Common situations: A nullable int[] from a parsed request body; a service returning null on a not-found path fed into countSubarrays; test data not set up.

Related errors

• Input array cannot be null
• Input matrix cannot be null or empty
• Input array cannot be null
• Prefix sum array cannot be null
• Input array cannot be null or empty.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/959866f6ec72a820. Report an issue: GitHub.