code: public static String getMapFieldValueType(Field field) {
          FieldType type = field.getType();
          if (!(type instanceof Message)) {
              throw new IllegalArgumentException(field.toString());
          }
          Message entryType = (Message) type;
          Type valueType = entryType.getField(MAP_ENTRY_VALUE).getType();
          String v;
          if (valueType instanceof ScalarFieldType) {
              ScalarFieldType scalarValueType = (ScalarFieldType) valueType;
              v = ScalarFieldTypeUtil.getWrapperType(scalarValueType);
          } else {
              UserType userType = (UserType) valueType;
              v = UserTypeUtil.getCanonicalName(userType);
          }
          return v;
      }
docs: Returns map field value type name.