TheAlgorithms/Java · error · IllegalStateException

Stack is empty, cannot pop element

Error message

Stack is empty, cannot pop element

What it means

Thrown by StackArray.pop() when the stack holds no elements. pop() reads stackArray[top--], so on an empty stack top is -1 and the index would be invalid. The IllegalStateException guards a precondition violation: removing an element that does not exist.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/stacks/StackArray.java:69

    @Override
    public void push(T value) {
        if (isFull()) {
            resize(maxSize * 2);
        }
        stackArray[++top] = value;
    }

    /**
     * Removes and returns the element from the top of the stack. Shrinks the stack if
     * its size is below a quarter of its capacity, but not below the default capacity.
     *
     * @return the element removed from the top of the stack
     * @throws IllegalStateException if the stack is empty
     */
    @Override
    public T pop() {
        if (isEmpty()) {
            throw new IllegalStateException("Stack is empty, cannot pop element");
        }
        T value = stackArray[top--];
        if (top + 1 < maxSize / 4 && maxSize > DEFAULT_CAPACITY) {
            resize(maxSize / 2);
        }
        return value;
    }

    /**
     * Returns the element at the top of the stack without removing it.
     *
     * @return the top element of the stack
     * @throws IllegalStateException if the stack is empty
     */
    @Override
    public T peek() {
        if (isEmpty()) {
            throw new IllegalStateException("Stack is empty, cannot peek element");

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Check isEmpty() before every pop().
2. Drive pop loops with while (!isEmpty()) instead of a fixed iteration count.
3. If an empty pop is an expected branch, catch IllegalStateException and break.
4. Audit caller logic for unbalanced push/pop counts.

Example fix

// before
for (int i = 0; i < n; i++) {
    T v = stack.pop();
}
// after
while (!stack.isEmpty()) {
    T v = stack.pop();
}

while (!stack.isEmpty()) {
    T v = stack.pop();
}

When it happens

Trigger: Calling pop() on a freshly constructed StackArray (top==-1). Calling pop() more times than push(). Popping inside a loop controlled by an external count rather than isEmpty().

Common situations: Unbalanced push/pop in evaluation loops. Reusing a stack across operations without verifying state. Off-by-one loop termination causing an extra pop after drain.

Related errors

• Cannot pop from an empty stack.
• Cannot peek from an empty stack.
• Stack is empty, cannot peek element
• Empty stack. Nothing to pop
• Empty stack. Nothing to peek

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/777902ab33c0278b. Report an issue: GitHub.