code: public static String getFieldType(Field field) {
          FieldType type = field.getType();
          if (type instanceof ScalarFieldType) {
              ScalarFieldType scalarFieldType = (ScalarFieldType) type;
              return ScalarFieldTypeUtil.getPrimitiveType(scalarFieldType);
          }
          if (type instanceof UserType) {
              UserType userType = (UserType) type;
              return UserTypeUtil.getCanonicalName(userType);
          }
          throw new IllegalArgumentException(field.toString());
      }
docs: Returns a java field type for proto field.