@Override
        public Set<K> keySet() {
            Set<K> ret = new HashSet<K>();
            for (Map.Entry e : map.entrySet()) {
                ret.add(wrapKey(e.getKey()));
            }
            return ret;
        }
Map of field values backed by a normal map of Java objects
@author vegardh
document/src/main/java/com/yahoo/document/datatypes/MapFieldValue.java