code: public int ensureFeature(String featureName) {
          int feature = featureToIndex.getOrDefault(featureName, -1);
          if (feature == -1) {
              synchronized (featureToIndex) {
                  feature = featureToIndex.getOrDefault(featureName, -1);
                  if (feature == -1) {
                      feature = featureToIndex.size();
                      featureToIndex.put(featureName, feature);
                  }
              }
          }
          return feature;
      }
docs: An optimization, this lets clients inform the ConcatVectorNamespace of how many features to expect, so
  that we can avoid resizing ConcatVectors.
  @param featureName the feature to add to our index