TheAlgorithms/Java · error · NoSuchElementException

Queue is empty

Error message

Queue is empty

What it means

Thrown by QueueByTwoStacks.get() as a NoSuchElementException when both internal stacks are empty. get() first transfers any pending elements from enqueueStk to dequeueStk; only if dequeueStk is still empty afterward does it throw. This means the queue is genuinely empty (no put() since the last full drain).

Solutions

1. Track emptiness via size() (enqueueStk + dequeueStk sizes) and guard get() with size() > 0.
2. Bound get() loops by the number of put() calls actually made.
3. Catch NoSuchElementException when an empty queue is an expected outcome.

Example fix

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

// after
T v = null;
if (queue.size() > 0) {
    v = queue.get();
}

T v = null;
if (queue.size() > 0) {
    v = queue.get();
}

boolean nonEmpty = queue.size() > 0;

null

When it happens

Trigger: Calling get() on a freshly constructed QueueByTwoStacks, or calling get() more times than put(). The transfer step means a get() right after put() calls succeeds; the throw only happens when there is nothing on either stack.

Common situations: Consumer thread calling get() before the producer has put() anything; drain loop that calls get() one time too many; misuse where get() is the only emptiness signal; assuming get() blocks (it does not — this is non-blocking).

Related errors

• Queue is empty
• Queue is empty
• Queue is empty, cannot peek front
• Queue is empty, cannot peek rear
• Queue is empty, cannot remove element

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/fad81098cdded73a. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/datastructures/queues/QueueByTwoStacks.java:56

    }

    /**
     * Removes and returns the element at the front of the queue.
     * If `dequeueStk` is empty, it transfers all elements from
     * `enqueueStk` to `dequeueStk` to maintain the correct FIFO
     * (First-In-First-Out) order before popping.
     *
     * @return The element at the front of the queue.
     * @throws NoSuchElementException If the queue is empty.
     */
    public T get() {
        if (dequeueStk.isEmpty()) {
            while (!enqueueStk.isEmpty()) {
                dequeueStk.push(enqueueStk.pop());
            }
        }
        if (dequeueStk.isEmpty()) {
            throw new NoSuchElementException("Queue is empty");
        }
        return dequeueStk.pop();
    }

    /**
     * Returns the total number of elements currently in the queue.
     * This is the sum of the sizes of both stacks.
     *
     * @return The number of elements in the queue.
     */
    public int size() {
        return enqueueStk.size() + dequeueStk.size();
    }

    /**
     * Returns a string representation of the queue, showing the elements
     * in the correct order (from front to back).
     * The `dequeueStk` is first cloned, and then all elements from the

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