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.

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");

View on GitHub (pinned to fdfb9a395b)

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.