TheAlgorithms/Java · error · IllegalArgumentException

Cannot enqueue null data

Error message

Cannot enqueue null data

What it means

Thrown by LinkedQueue.enqueue(T) as an IllegalArgumentException when the argument is null. This unbounded linked-list queue explicitly forbids null elements to disambiguate them from the empty-queue sentinel and to keep dequeue()'s return semantics unambiguous.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/queues/LinkedQueue.java:51

    /**
     * Checks if the queue is empty.
     *
     * @return true if the queue is empty, otherwise false.
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Adds an element to the rear of the queue.
     *
     * @param data the element to insert.
     * @throws IllegalArgumentException if data is null.
     */
    public void enqueue(T data) {
        if (data == null) {
            throw new IllegalArgumentException("Cannot enqueue null data");
        }

        Node<T> newNode = new Node<>(data);

        if (isEmpty()) {
            front = newNode;
        } else {
            rear.next = newNode;
        }
        rear = newNode;
        size++;
    }

    /**
     * Removes and returns the element at the front of the queue.
     *
     * @return the element at the front of the queue.
     * @throws NoSuchElementException if the queue is empty.

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check or use a default before enqueue: if (data != null) queue.enqueue(data).
2. Filter nulls at the source (e.g. Stream.filter(Objects::nonNull)) before they reach the queue.
3. Replace missing values with a sentinel/empty representation instead of null.

Example fix

// before
queue.enqueue(map.get(key)); // get may return null

// after
String v = map.get(key);
if (v != null) {
    queue.enqueue(v);
}

if (data != null) {
    queue.enqueue(data);
}

java.util.Objects.nonNull(data)

null

When it happens

Trigger: Passing a literal null to enqueue(), or passing an uninitialized field/map-lookup/Optional.orElse(null) result that evaluates to null. Any code path that fails to initialize a value before enqueuing it triggers the guard.

Common situations: Map.get() returning null for a missing key then enqueued; JSON/DB deserialization yielding null fields; default initial values that never got assigned; refactoring that removed an assignment but left the enqueue call.

Related errors

• Cannot enqueue null item.
• Queue capacity must be greater than 0
• Capacity must be greater than zero.
• Cannot insert null element
• Cannot add null element to the list

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/15f0f53e4bc05c8c. Report an issue: GitHub.