A Queue approach to Breadth-First searching of a binary tree

BinarySearchTree.java

/**
     * Searches through the current tree for a given key. Will return a match if
     * a node exists that has a value that starts with key or is equal to key.
     * All comparisons are in lowercase.
     *
     * @param key The key containing the value to search for
     * @return The node if it finds it
     * @throws EmptyTreeException    If the current tree is empty
     * @throws NodeNotFoundException If the node isn't found
     */
    public BinarySearchTreeNode<K> search(K key) throws EmptyTreeException, NodeNotFoundException {
        if (this.root == null) {
            throw new EmptyTreeException();
        }

        LinkedList<BinarySearchTreeNode<K>> node_queue = new LinkedList<>();
        BinarySearchTreeNode<K> current;
        node_queue.add(this.root);
        String key_string = key.toString().toLowerCase();

        while (!node_queue.isEmpty()) {
            current = node_queue.pop();
            String current_string = current.toString().toLowerCase();
            if (current_string.equals(key_string) || current_string.startsWith(key_string)) {
                return current;
            }
            if (current.left != null) {
                node_queue.add(current.left);
            }
            if (current.right != null) {
                node_queue.add(current.right);
            }
        }
        throw new NodeNotFoundException();
    }