@org.apache.thrift.annotation.Nullable
    public static _Fields findByThriftId(int fieldId) {
      switch(fieldId) {
        case 1: // INT_VALUE
          return INT_VALUE;
        case 2: // LONG_VALUE
          return LONG_VALUE;
        case 3: // STRING_VALUE
          return STRING_VALUE;
        case 4: // DOUBLE_VALUE
          return DOUBLE_VALUE;
        case 5: // FLAG
          return FLAG;
        case 6: // L_STRING
          return L_STRING;
        case 7: // UNION_MSTRING_STRING
          return UNION_MSTRING_STRING;
        default:
          return null;
      }
    }
Find the _Fields constant that matches fieldId, or null if its not found.
serde/src/gen/thrift/gen-javabean/org/apache/hadoop/hive/serde2/thrift/test/PropValueUnion.java