TheAlgorithms/Java · error · NullPointerException

Element cannot be null

Error message

Element cannot be null

What it means

Thrown by CursorLinkedList.indexOf(T) when element is null. The method iterates logical nodes calling iterator.element.equals(element), so a null element would NPE on the equals call. The library rejects null upfront with NullPointerException.

Source

Thrown at src/main/java/com/thealgorithms/datastructures/lists/CursorLinkedList.java:77

            int start = head;
            while (start != -1) {
                T element = cursorSpace[start].element;
                System.out.println(element.toString());
                start = cursorSpace[start].next;
            }
        }
    }

    /**
     * Finds the logical index of a specified element in the list.
     *
     * @param element the element to search for in the list
     * @return the logical index of the element, or -1 if not found
     * @throws NullPointerException if element is null
     */
    public int indexOf(T element) {
        if (element == null) {
            throw new NullPointerException("Element cannot be null");
        }
        try {
            Objects.requireNonNull(element);
            Node<T> iterator = cursorSpace[head];
            for (int i = 0; i < count; i++) {
                if (iterator.element.equals(element)) {
                    return i;
                }
                iterator = cursorSpace[iterator.next];
            }
        } catch (Exception e) {
            return -1;
        }
        return -1;
    }

    /**
     * Retrieves an element at a specified logical index in the list.

View on GitHub (pinned to fdfb9a395b)

Solutions

1. Null-check before calling indexOf.
2. Wrap the element in Optional and skip the search when empty.
3. Filter null values out of the data feeding the list and its queries.
4. Return -1 yourself when the input is null rather than calling indexOf.

Example fix

// before
int idx = list.indexOf(map.get(key));

// after
T e = map.get(key);
int idx = (e == null) ? -1 : list.indexOf(e);

if (element != null) {
    return list.indexOf(element);
}
return -1;

try {
    return list.indexOf(element);
} catch (NullPointerException e) {
    return -1;
}

When it happens

Trigger: Calling indexOf(null). Searching for an element obtained from a nullable source. Passing the result of Map.get on a missing key.

Common situations: User input that was not null-validated. Optional.orElse(null) fed to indexOf. Lookup tables that return null for absent entries.

Related errors

• Cannot add null element to the list
• Cannot add null element to the list
• Input lists must not be null.
• Cannot insert null element
• Input lists and result collection must not be null.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/27c135406993087b. Report an issue: GitHub.