TheAlgorithms/Java · error · NoSuchElementException

Empty stack. Nothing to peek

Error message

Empty stack. Nothing to peek

What it means

Thrown by StackOfLinkedList.peek() when the stack holds no elements (size==0). peek() reads head.data, which would NPE on an empty stack. The NoSuchElementException enforces that a top element must exist before it is inspected.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/stacks/StackOfLinkedList.java:94

            throw new NoSuchElementException("Empty stack. Nothing to pop");
        }
        Node destroy = head;
        head = head.next;
        int retValue = destroy.data;
        destroy = null; // Help garbage collection
        size--;
        return retValue;
    }

    /**
     * Returns the top element of the stack without removing it.
     *
     * @return the element at the top of the stack
     * @throws NoSuchElementException if the stack is empty
     */
    public int peek() {
        if (size == 0) {
            throw new NoSuchElementException("Empty stack. Nothing to peek");
        }
        return head.data;
    }

    @Override
    public String toString() {
        Node cur = head;
        StringBuilder builder = new StringBuilder();
        while (cur != null) {
            builder.append(cur.data).append("->");
            cur = cur.next;
        }
        return builder.replace(builder.length() - 2, builder.length(), "").toString(); // Remove the last "->"
    }

    /**
     * Checks if the stack is empty.
     *

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Check size==0 before calling peek().
2. Seed the stack with a sentinel so it is never empty during the peek window.
3. Terminate loops on the empty condition rather than an external counter.
4. Catch NoSuchElementException if empty-peek is a legitimate branch.

Example fix

// before
int top = stack.peek();
// after
int top = (stack.size() == 0) ? defaultValue : stack.peek();

int top = (stack.size() == 0) ? defaultValue : stack.peek();

When it happens

Trigger: Calling peek() on a fresh StackOfLinkedList. Calling peek() after draining the stack. Inspecting the top in an algorithm before any push has occurred.

Common situations: Parsing or matching algorithms that peek before seeding a sentinel. Stateful processors with an uninitialized first iteration. Off-by-one loop termination.

Related errors

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

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/17c6bc777015dc3f. Report an issue: GitHub.