I am planning on using XStream with Java to convert between objects and XML requests and XML responses and objects, where the XML is flowing over HTTP/HTTPS. On the response side, I can get a "successful" response, which seems like it would map to one Java class, or a "failure" response, which seems like it would map to another Java class.

For example, for a "file list" request, I could get an affirmative response e.g.,

<?xml version="1.0" encoding="UTF-8"?>
<response>
  <success>true</success>
  <files>
    <file>[...]</file>
    <file>[...]</file>
    <file>[...]</file>
  </files>
</response>

or I could get a negative response e.g.,

<?xml version="1.0" encoding="UTF-8"?>
<response>
  <success>false</success>
  <error>
    <errorCode>-502</errorCode>
    <systemMessage>[...]AuthenticationException</systemMessage>
    <userMessage>Not authenticated</userMessage>
  </error>
</response>

To handle this, should I include fields in one class for both cases or should I somehow use XStream to "conditionally" create one of the two potential classes?

The case with fields from both response cases in the same object would look something like this:

Class Response {
  boolean success;
  ArrayList<File> files;
  ResponseError error;
  [...]
}

Class File {
  String name;
  long size;
  [...]
}

Class ResponseError {
  int errorCode;
  String systemMessage;
  String userMessage;
  [...]
}

I don't know what the "use XStream and create different objects in case of success or error" looks like. Is it possible to do that somehow? Is it better or worse way to go?

Anyway, any advice on how to handle using XStream to deal with this success vs. failure response case would be appreciated. Thanks in advance!