private void validateConstraint(ObjectType constraint) {
    if (constraint == null) {
      throw new IllegalArgumentException(
          "constraint cannot be null");
    }
    // must be public
    if (!Modifier.isPublic(constraint.resolveClass().getModifiers())) {
      throw new IllegalArgumentException(
          "constraint class must be public");
    }
  }
no-arg constructor required for DataSerializable
geode-core/src/main/java/org/apache/geode/cache/query/internal/ResultsCollectionWrapper.java