public K nextKey(K key) {
        int index = insertOrder.indexOf(key);
        if (index >= 0 && index < size() - 1) {
            return insertOrder.get(index + 1);
        }
        return null;
    }
Gets the next key to the one specified using insert order.
This method performs a list search to find the key and is O(n).

@param key the key to find previous for
@return the next key, null if no match or at start
jphp-runtime/src/php/runtime/common/collections/map/ListOrderedMap.java