private void registerFunction(String name, SqlOperator calciteFn, HiveToken hiveToken) {
      reverseOperatorMap.put(calciteFn, name);
      FunctionInfo hFn;
      try {
        hFn = FunctionRegistry.getFunctionInfo(name);
      } catch (SemanticException e) {
        LOG.warn("Failed to load udf " + name, e);
        hFn = null;
      }
      if (hFn != null) {
        String hFnName = getName(hFn.getGenericUDF());
        hiveToCalcite.put(hFnName, calciteFn);

        if (hiveToken != null) {
          calciteToHiveToken.put(calciteFn, hiveToken);
        }
      }
    }
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