@Override
  public Object put(Object key, Object value) {
    if (containsKey(key)) {
      return m.put(key, value);
    } else {
      throw new UnsupportedOperationException("can not add the key \"" + key + "\"");
    }
  }
Only called by internal code to bypass exportValue() method

@return internal map
geode-core/src/main/java/org/apache/geode/cache/operations/internal/UpdateOnlyMap.java