TheAlgorithms/Java · error · IllegalArgumentException

Input list cannot be null

Error message

Input list cannot be null

What it means

Thrown by the MaxHeap(List<HeapElement>) constructor when listElements is null. The constructor iterates the list to build the heap, so a null reference would NPE; the explicit check produces a clearer error. Note null ELEMENTS inside a non-null list are silently skipped (not an error).

Source

Thrown at src/main/java/com/thealgorithms/datastructures/heaps/MaxHeap.java:43

 * </pre>
 *
 * @author Nicolas Renard
 */
public class MaxHeap implements Heap {

    /** The internal list that stores heap elements */
    private final List<HeapElement> maxHeap;

    /**
     * Constructs a new MaxHeap from a list of elements.
     * Null elements in the input list are ignored.
     *
     * @param listElements List of HeapElement objects to initialize the heap
     * @throws IllegalArgumentException if the input list is null
     */
    public MaxHeap(List<HeapElement> listElements) {
        if (listElements == null) {
            throw new IllegalArgumentException("Input list cannot be null");
        }

        maxHeap = new ArrayList<>();

        // Safe initialization: directly add non-null elements first
        for (HeapElement heapElement : listElements) {
            if (heapElement != null) {
                maxHeap.add(heapElement);
            }
        }

        // Heapify the array bottom-up
        for (int i = maxHeap.size() / 2; i >= 0; i--) {
            heapifyDown(i + 1); // +1 because heapifyDown expects 1-based index
        }
    }

    /**

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Pass an empty list (Collections.emptyList()) instead of null when there is nothing to seed.
2. Guard the caller with a null check and default to an empty list.
3. Fix the upstream producer to never return null collections.

Example fix

// before
new MaxHeap(repository.findAll()); // returns null when empty

// after
List<HeapElement> elems = repository.findAll();
new MaxHeap(elems != null ? elems : Collections.emptyList());

List<HeapElement> elems = source != null ? source : Collections.emptyList();
new MaxHeap(elems);

When it happens

Trigger: Passing null directly; passing the result of a factory/method that returns null on empty input; an uninitialized List field defaulting to null.

Common situations: DI/bean wiring that fails to inject the list; a repository/query method returning null instead of an empty list; conditional initialization that leaves the field null.

Related errors

• Cannot insert null element
• Input list cannot be null
• Cannot insert null into the heap.
• initialCapacity < 1
• Index ${elementIndex} is out of heap range [1, ${maxHeap.siz

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/eec4cee37bc51dc6. Report an issue: GitHub.