Question

I’m reading a HTTP POST and the body of the HTTP request can be either JSON or XML. Now I’ve delegated the reading to a special utility class.

interface HttpUtils {     BodyWrapper parseBody( HttpServletRequest req ); }  interface BodyWrapper {     boolean isXML();  // 1     boolean isJSON(); // 2     String body();    // 3 } 

• I hate the fact that BodyWrapper has methods (1 & 2) to identify its type. Perhaps I should use inheritance. If I do that, I will need to do an instanceof to find out what is being returned by HttpUtils.parseBody(..)
• Ideally I would also want the body() method to return either a JSONObject or an DOM node. How would I do that?

Answer 1

Don’t ask your objects for information, and then make decisions on what they tell you. Make your objects do the work for you. That is, don’t do this:

if (body.isXML()) {   // do XML stuff } else if (body.isJSON()) {   // do JSON stuff } 

It’s a maintenance headache. Do something like this instead (the BodyWrapper implementations would created using an abstract factory method or similar)

public interface BodyWrapper {    Object doStuff(); }  public class DOMBodyWrapper implements BodyWrapper {    public Object doStuff() {    } }  public class JSONBodyWrapper implements BodyWrapper {    public Object doStuff() {       // do something and return a success/failure result. I've       // deliberately not defined what this object is....    } } 

and then:

// get the body via a factory or similar body.doStuff(); 

That way, something creates the appropriate BodyWrapper implementation, and then instead of asking it what type it is, you just use it. Note that the BodyWrapper isn’t returning different types of internal structures, because it (perhaps an abstract base class) is doing the work for you.