@Override
        public void putAll(Map<? extends K, ? extends V> m) {
            for (Map.Entry<?, ?> e : m.entrySet()) {
                map.put(unwrap(e.getKey()), unwrap(e.getValue()));
            }
        }
Map of field values backed by a normal map of Java objects
@author vegardh
document/src/main/java/com/yahoo/document/datatypes/MapFieldValue.java