private void registerPlugin(HiveUDFPlugin plugin) {
      for (UDFDescriptor udfDesc : plugin.getDescriptors()) {
        Optional<SqlFunction> calciteFunction = udfDesc.getCalciteFunction();
        if (calciteFunction.isPresent()) {
          registerDuplicateFunction(udfDesc.getFunctionName(), calciteFunction.get(),
              hToken(HiveParser.Identifier, udfDesc.getFunctionName()));
        }
      }

    }
This class is used to build immutable hashmaps in the static block above.
ql/src/java/org/apache/hadoop/hive/ql/optimizer/calcite/translator/SqlFunctionConverter.java