code: public static String getRetRequiredCheck(String express, Field field) {
          String code = "if (CodedConstant.isNull(" + express + ")) {\n";
          code += "throw new UninitializedMessageException(CodedConstant.asList(\"" + field.getName() + "\"));\n";
          code += "}\n";
  
          return code;
      }
docs: get return required field check java expression.
  
  @param express java expression
  @param field java field
  @return full java expression