Reuse Code, Not Objects

We all know of the importance of code reuse and DRY code. It is important to remember that this applies to code, and not to objects!
A common anti-pattern I see in stateful classes is something along the following lines:

class SomeStatefulClass {
  private ResponseType responseType;
  private final ErrMessages errMessages;
  
  public SomeStatefulClass() {
    this.ResponseType = null;
    this.errMessages = new ErrMessages();
  }
  
  public void init() {
    errMessages.clear();
  }
  
  public Response process(Request request) {
    try {
      // Process the request.
      // Update stats.
      // Update otherState.
      Response response = computeResponse();
      responseType = computeResponseType();
      return response;
    } catch (Exception e) {
      this.errMessages.append(e);
    }
    return null;
  }
  
  public LogMessages getErrMessages() {
    return this.errMessages;
  }
  
  public ResponseType getResponseType() {
    return responseType;
  }
}

This design pattern is a major code smell. Ironically, such classes are prone to be misused through reuse. A common example of this is reusing the object within a loop:

public void process(List<Request> requests) {
  final SomeStatefulClass statefulObject
      = new SomeStatefulClass();
  Response response;
  for (Request request: requests) {
    statefulObject.init();
    response = statefulObject.process(request);
    appendResponse(response, statefulObject.getResponseType());
    
  }
}

The issue here is subtle, but dangerous. Consider what happens if one of the requests in the list of requests passed to process() causes statefulObjectto throw an exception inside computeResponse(). Dutifully, this exception is caught by process() and it returns null. However, note that the value of responseType in statefulObject was never modified by processing of this request, and therefore, it still hold the ResponseType of the previous request! Therefore, the line appendResponse(response, statefulObject.getResponseType()); is now passing in a null response and the response type of the previous request!
These types of bugs are subtle and a pain to track down.
And this happened because we chose to reuse the statefulObject. If we were to use a new instance each time, this would not really be an issue.
Moral: If feasible, do not reuse objects; create new instances and throw them away when done!

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