TheAlgorithms/Java · error · DuplicateKeyException

Duplicate key

Error message

Duplicate key: {key}

What it means

SplayTree does not allow duplicate keys. insertRec(Node, int) traverses the BST and, when it encounters a node whose key equals the key being inserted, throws DuplicateKeyException (a RuntimeException subclass) with the offending key in the message. This enforces set semantics for the tree.

Solutions

1. Call tree.search(key) first and skip insertion if it returns true.
2. Catch DuplicateKeyException at the call site if duplicates are acceptable and should be silently ignored.
3. Deduplicate your input collection before inserting into the tree.

Example fix

// before
tree.insert(10);
tree.insert(10); // throws DuplicateKeyException

// after
tree.insert(10);
if (!tree.search(10)) {
    tree.insert(10);
}

if (!tree.search(key)) {
    tree.insert(key);
}

try {
    tree.insert(key);
} catch (SplayTree.DuplicateKeyException e) {
    // key already present — treat as no-op or update
}

When it happens

Trigger: Calling tree.insert(key) when a node with that exact key already exists in the tree — including re-inserting after a prior successful insert of the same value.

Common situations: Ingesting data from a source with non-unique identifiers; retry/replay logic that re-inserts keys; concurrent-feeling code paths where two callers insert the same key.

Related errors

• Cannot delete from an empty tree
• Base must be greater than 01
• Base must be greater than 02
• Cannot enqueue null data
• Cannot enqueue null item.

AI-assisted analysis of TheAlgorithms/Java@fdfb9a395b (2026-08-13). Data as JSON: /api/errors/4e4e2e188001933c. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/com/thealgorithms/datastructures/trees/SplayTree.java:242

            } else if (root.right.key < key) {
                root.right.right = splay(root.right.right, key);
                root = rotateLeft(root);
            }
            return (root.right == null) ? root : rotateLeft(root);
        }
    }

    private Node insertRec(Node root, final int key) {
        if (root == null) {
            return new Node(key);
        }

        if (key < root.key) {
            root.left = insertRec(root.left, key);
        } else if (key > root.key) {
            root.right = insertRec(root.right, key);
        } else {
            throw new DuplicateKeyException("Duplicate key: " + key);
        }

        return root;
    }

    public static class EmptyTreeException extends RuntimeException {
        private static final long serialVersionUID = 1L;

        public EmptyTreeException(String message) {
            super(message);
        }
    }

    public static class DuplicateKeyException extends RuntimeException {
        private static final long serialVersionUID = 1L;

        public DuplicateKeyException(String message) {
            super(message);

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