TheAlgorithms/Java · error · IllegalArgumentException

Invalid input

Error message

Invalid input

What it means

Thrown by CountDistinctElementsInWindow.countDistinct(int[], int) when arr is null, empty, k <= 0, or k > arr.length. A single guard collapses four distinct precondition failures into one message because all of them make the sliding-window count undefined. The method needs at least one valid window to produce output.

Source

Thrown at src/main/java/com/thealgorithms/slidingwindow/CountDistinctElementsInWindow.java:26

 *
 * @see <a href="https://www.geeksforgeeks.org/count-distinct-elements-in-every-window-of-size-k/">Reference</a>
 */
public final class CountDistinctElementsInWindow {

    private CountDistinctElementsInWindow() {
    }

    /**
     * Returns an array where each element is the count of distinct
     * elements in the corresponding window of size k.
     *
     * @param arr the input array
     * @param k   the window size
     * @return array of distinct element counts per window
     */
    public static int[] countDistinct(int[] arr, int k) {
        if (arr == null || arr.length == 0 || k <= 0 || k > arr.length) {
            throw new IllegalArgumentException("Invalid input");
        }

        int n = arr.length;
        int[] result = new int[n - k + 1];
        Map<Integer, Integer> freqMap = new HashMap<>();

        for (int i = 0; i < k; i++) {
            freqMap.merge(arr[i], 1, Integer::sum);
        }
        result[0] = freqMap.size();

        for (int i = k; i < n; i++) {
            freqMap.merge(arr[i], 1, Integer::sum);

            int outgoing = arr[i - k];

            Integer count = freqMap.get(outgoing);
            if (count != null) {

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Validate arr != null && arr.length > 0 && k >= 1 && k <= arr.length before calling.
2. When computing k from a ratio (e.g. size * p), clamp to at least 1 and at most arr.length.
3. Document the single-message guard at your API boundary so callers know which condition failed via separate checks.

Example fix

// before
int[] counts = CountDistinctElementsInWindow.countDistinct(arr, k);

// after
if (arr == null || arr.length == 0) return new int[0];
if (k < 1 || k > arr.length) throw new IllegalArgumentException("k out of range: " + k);
int[] counts = CountDistinctElementsInWindow.countDistinct(arr, k);

if (arr == null || arr.length == 0) return new int[0];
if (k < 1 || k > arr.length) throw new IllegalArgumentException("k must be in [1, arr.length]");

public static boolean isValidWindow(int[] arr, int k) { return arr != null && arr.length > 0 && k >= 1 && k <= arr.length; }

When it happens

Trigger: Calling countDistinct(null, k); countDistinct(new int[0], k); countDistinct(arr, 0); countDistinct(arr, -1); countDistinct(arr, arr.length + 1) where the window is larger than the array.

Common situations: Window size k read from config and left at 0/default; array loaded from a stream that produced no elements; k derived from a fraction that rounds to 0 for small arrays.

Related errors

• IPv4 address is empty.
• Input cannot be null or empty
• Input cannot be null or empty
• Input cannot be null
• Unit cannot be null.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/debb5d1714962f0e. Report an issue: GitHub.