Java定义类的参数时的一个注意事项

public class BSTMap<K extends Comparable<K> , V> implements Map61B<K, V>{

}

private class Node<K,V>{
        private K key;
        private V value;
        private Node<K,V> left, right;
        public Node(K key, V value){
            this.key = key;
            this.value = value;
        }
}

private V get(Node node, K key){
        if (node == null) {
            return null;
        }
        if (key == null){
            throw new UnsupportedOperationException("Key cannot be null");
        }
        int cmp = key.compareTo(node.key);
        if (cmp == 0){
            return node.value;
        } else if (cmp < 0){
            return get(node.left, key);
        } else {
            return get(node.right, key);
        }

  }