code: protected Set<String> getAttributeValueSet() {
          if (attributeValueSet == null) {
              synchronized (this) {
                  if (attributeValueSet == null) {
                      // because the load factor is 0.75f
                      attributeValueSet = new LinkedHashSet<>(2);
                  }
              }
          }
          return attributeValueSet;
      }
docs: NB:- this is only for getting values. Use addToAttributeValueSet method
  for adding
  
  @return the attributeValueSet
  @since 1.0.0
  @author WFF