docs: public static String getMappedTypeDefined(int order, FieldType type, String express, boolean isList,
              boolean isMap) {
          StringBuilder code = new StringBuilder();
          String fieldName = getFieldName(order);
          if ((type == FieldType.STRING || type == FieldType.BYTES) && !isList) {
              // add null check
              code.append("com.google.protobuf.ByteString ").append(fieldName).append(" = null")
                      .append(CodeGenerator.JAVA_LINE_BREAK);
              code.append("if (!CodedConstant.isNull(").append(express).append(")) {").append(CodeGenerator.LINE_BREAK);
  
              String method = "copyFromUtf8";
              if (type == FieldType.BYTES) {
                  method = "copyFrom";
              }
              code.append(fieldName).append(" = com.google.protobuf.ByteString.").append(method).append("(")
                      .append(express).append(")").append(CodeGenerator.JAVA_LINE_BREAK);
              code.append("}").append(CodeGenerator.LINE_BREAK);
              return code.toString();
          }
          // add null check
          String defineType = type.getJavaType();
          if (isList) {
              defineType = "List";
          } else if (isMap) {
              defineType = "Map";
          }
          code.setLength(0);
          code.append(defineType).append(" ").append(fieldName).append(" = null").append(CodeGenerator.JAVA_LINE_BREAK);
          code.append("if (!CodedConstant.isNull(").append(express).append(")) {").append(CodeGenerator.LINE_BREAK);
          code.append(fieldName).append(" = ").append(express).append(CodeGenerator.JAVA_LINE_BREAK);
          code.append("}").append(CodeGenerator.LINE_BREAK);
          return code.toString();
      }