@Override
  public synchronized boolean add(Object obj) {
    throw new UnsupportedOperationException(
        "add(Object) not supported, try add(Object key, Object obj) instead");
  }
Unsupported, we do not use HashIndexSet as a general all purpose set
geode-core/src/main/java/org/apache/geode/cache/query/internal/index/HashIndexSet.java