public K nextKey(K key) {
        LinkEntry<K, V> entry = (LinkEntry<K, V>) getEntry(key);
        return (entry == null || entry.after == header ? null : entry.after.getKey());
    }
Gets the next key in sequence.

@param key the key to get after
@return the next key
jphp-runtime/src/php/runtime/common/collections/map/AbstractLinkedMap.java