docs: public static String getMappedTypeDefined(int order, FieldType type, String express, boolean isList) {
  
          String fieldName = getFieldName(order);
          if ((type == FieldType.STRING || type == FieldType.BYTES) && !isList) {
              // add null check
              String code = "com.google.protobuf.ByteString " + fieldName + "=null" + ClassCode.JAVA_LINE_BREAK;
              code += "if (!CodedConstant.isNull(" + express + ")) {\n";
  
              String method = "copyFromUtf8";
              if (type == FieldType.BYTES) {
                  method = "copyFrom";
              }
  
              code += fieldName + " = com.google.protobuf.ByteString." + method + "(" + express + ")"
                      + ClassCode.JAVA_LINE_BREAK;
              code += "}";
              return code;
          }
          // add null check
          String defineType = type.getJavaType();
          if (isList) {
              defineType = "List";
          }
  
          String code = defineType + " " + fieldName + "=null" + ClassCode.JAVA_LINE_BREAK;
          code += "if (!CodedConstant.isNull(" + express + ")) {\n";
  
          code += fieldName + "=" + express + ClassCode.JAVA_LINE_BREAK;
          code += "}";
          return code;
      }