What causes "Queue is empty, cannot remove element" in TheAlgorithms/Java?

Guard with isEmpty() before remove() and skip or wait when empty.

TheAlgorithms/Java · error · IllegalStateException

Queue is empty, cannot remove element

Error message

Queue is empty, cannot remove element

What it means

Thrown by Queue.remove() as an IllegalStateException when isEmpty() (nItems == 0). The array ring buffer cannot return a front element it does not hold, so removal from an empty queue is rejected. This is the standard FIFO underflow guard.

Solutions

Guard with isEmpty() before remove() and skip or wait when empty.
Drive removal loops by nItems/size rather than an external counter that can overshoot.
Catch IllegalStateException when an empty queue is an expected branch.

Example fix

// before
T v = queue.remove(); // throws when empty

// after
T v = null;
if (!queue.isEmpty()) {
    v = queue.remove();
}

Defensive patterns

Strategy: validation

Validate before calling

if (!queue.isEmpty()) {
    T v = queue.remove();
}

Type guard

null

Try / catch

null

Prevention

isEmpty()-guard every remove().
Use the queue's own size/empty state to bound drain loops.
Wrap remove to return Optional so callers handle the empty case in the type.

When it happens

Trigger: Calling remove() before any insert(), or calling remove() more times than insert(). Also after the queue is fully drained, any further remove() throws.

Common situations: Consumer outpacing producer; loop bounds larger than the number of inserts; draining logic with an extra pop after exhaustion; request handler removing from a shared queue with no pending work.

Related errors

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/56abaf38e09892b9. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/datastructures/queues/Queue.java:69

        if (isFull()) {
            return false;
        }
        rear = (rear + 1) % maxSize;
        queueArray[rear] = element;
        nItems++;
        return true;
    }

    /**
     * Removes and returns the element from the front of the queue.
     *
     * @return The element removed from the front of the queue.
     * @throws IllegalStateException if the queue is empty.
     */
    @SuppressWarnings("unchecked")
    public T remove() {
        if (isEmpty()) {
            throw new IllegalStateException("Queue is empty, cannot remove element");
        }
        T removedElement = (T) queueArray[front];
        queueArray[front] = null; // Optional: Clear the reference for garbage collection
        front = (front + 1) % maxSize;
        nItems--;
        return removedElement;
    }

    /**
     * Checks the element at the front of the queue without removing it.
     *
     * @return Element at the front of the queue.
     * @throws IllegalStateException if the queue is empty.
     */
    @SuppressWarnings("unchecked")
    public T peekFront() {
        if (isEmpty()) {
            throw new IllegalStateException("Queue is empty, cannot peek front");

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