TheAlgorithms/Java · error · IllegalArgumentException

Array cannot be null

Error message

Array cannot be null

What it means

Thrown by SentinelLinearSearch.find(T[], T) when array == null. The sentinel technique writes a sentinel into the last array slot, which dereferences the array reference, so a null array would cause a NullPointerException. The guard converts it into a clear IllegalArgumentException.

Source

Thrown at src/main/java/com/thealgorithms/searches/SentinelLinearSearch.java:49

 *
 * @author TheAlgorithms Contributors
 * @see LinearSearch
 * @see SearchAlgorithm
 */
public class SentinelLinearSearch implements SearchAlgorithm {
    /**
     * Performs sentinel linear search on the given array.
     *
     * @param array the array to search in
     * @param key the element to search for
     * @param <T> the type of elements in the array, must be Comparable
     * @return the index of the first occurrence of the key, or -1 if not found
     * @throws IllegalArgumentException if the array is null
     */
    @Override
    public <T extends Comparable<T>> int find(T[] array, T key) {
        if (array == null) {
            throw new IllegalArgumentException("Array cannot be null");
        }

        if (array.length == 0) {
            return -1;
        }

        if (key == null) {
            return findNull(array);
        }

        // Store the last element
        T lastElement = array[array.length - 1];

        // Place the sentinel (search key) at the end
        array[array.length - 1] = key;

        int i = 0;
        // Search without bound checking since sentinel guarantees we'll find the key

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check the array before calling find() and return -1 or an empty result.
2. Initialize fields to empty arrays rather than null.
3. Use Optional or @NonNull annotations to make nullability explicit at the type level.

Example fix

// before
int idx = new SentinelLinearSearch().find(arr, key);

// after
if (arr == null) return -1;
int idx = new SentinelLinearSearch().find(arr, key);

if (array == null) return -1;

public static <T> boolean isSearchable(T[] array) { return array != null; }

When it happens

Trigger: Calling find(null, key); passing a field that was never initialized; receiving null from a loader or Map.get() and forwarding it.

Common situations: Optional fields left null; data sources that return null instead of an empty array; legacy code using null as a sentinel for 'no data'.

Related errors

• Key must not be null.
• The input array cannot be null
• Input array must not be null.
• IPv4 address is empty.
• Input cannot be null or empty

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/5256061e3604a9dc. Report an issue: GitHub.