Determine type of serialized object

Question

I have a need to send messages (requests from the User to the Engine, and responses from the Engine to the User) over a socket. So the flow is essentially

       +--------+ serialized  request +--------+      
       | Server | <==== network ====> | Client | 
       +--------+ serialized response +--------+      
           ^                              ^
           | request/response             | mouse/keyclicks
           |    object                    |
           v                              v
       +--------+                     +--------+
       | Engine |                     |  User  |
       +--------+                     +--------+

Now, it only makes sense to not reinvent the wheel here. I'm dealing with Java on both sides, so I plan use Objects like so:

/**
 * A Client makes a Request.
 */
abstract class UserRequest implements Serializable {
    /**
     * The engine will call request.engineCallback(this);
     * This will tell the Engine what the request is, at which point
     * the Engine will determine its response.
     */
    abstract EngineResponse engineCallback(Engine engine);
}

/**
 * The Engine has calculated a Response.
 */
abstract class EngineResponse implements Serializable {
    /**
     * The User will call response.userCallback(this);
     * This tells the User what the Engine thought of its request,
     * and what happened as a result.
     */
     abstract void userCallback(User user);
}

What I'm not following is, in my Server and Client sockets, how will I know what subclass of Request and Response is coming in? I see a situation like

Object request = in.readObject();

// now what? How do I know what to cast it to?

// Can I just cast it like
UserRequest request = (UserRequest)(in.readObject());
engine.addToRequestQueue(request); // let it take care of implementation details?

My first thought was to just pass everything through over Strings, but that seemed a bit silly when Java provides Serialization. But how do I make sure I know what class came across the wire? For that matter, do I need to know, as long as I only send decedents of UserRequest to the server and EngineResponse to the client?

Answer 1

Just use the instanceof keyword provided:

Object o = in.readObject();

if (o instanceof SomeUserRequest)
{
  SomeUserRequest sur = (SomeUserRequest)o;
  ..
}
else if (o instanceof OtherUserRequest)
{
  ..
}