Java“扩展”对象

Question

I have only recently gotten back into Java programming and am currently stuck with a certain problem.

I have an object of a certain type:

StandardClass stdClass = (StandardClass)request.getAttribute(ATTR);  

But now I need to make the Object stdClass an Observer of some Observable.
Since StandardClass doesn't implement the Observer interface I can't add it as an observer directly. So I tried:

ExtendedClass extends StandardClass implements Observer { ... }

But I haven't been able to find a way to cast the original object into the new class.

Possibly this whole approach is totally wrong and I currently can't see the forest for the trees, so I'd appreciate any input on how to solve this kind of problem.

Thank you very much in advance for your time and help, I really appreciate it!!

Cheers,
Thomas

Answer 1

You won't be able to cast the original object into an ExtendedClass , because it is not an instance of that class. The existence of a subclass does not mean that superclass instances can be somehow coerced into these subclasses. In this case, your stdClass object is an instance of StandardClass only, so it cannot be downcast successfully.

EDIT to clarify: Of course, some objects stored in StandardClass variables might well actually be ExtendedClass objects. My understanding was that you haven't modified the actual object stored in the request, merely created a subclass. When you said you couldn't perform the cast, I presumed that you mean the cast failed at runtime with a ClassCastException (some casts will be rejected by the compiler because they cannot succeed, eg an Integer can never be cast to a String , but this shouldn't be one of those cases). If you created the subclass and modified your application such that the object stored in the request was created as an ExtendedClass object, then casting should succeed.

To achieve what you want to do, a cleaner approach would be some sort of wrapper class , which contains a StandardClass instance and implements the Observer methods itself. It can then call appropriate methods on StandardClass in response to the observed events.

Answer 2

If the object at request.getAttribute(ATTR) is truly an Observable , then you can just cast it as so:

Observable obs = (Observable) request.getAttribute(ATTR);

The compiler will not allow you to cast from one type to an incompatible type, if it knows that StandardClass is not an Observable . Or, you could simply declare the reference as ExtendedClass .

Answer 3

With your approach:

request.getAttribute(ATTR);

Should be returning an object type of ExtendedClass in order to cast it to an Observer for example:

Observer observer = (Observer) request.getAttribute(ATTR);

Answer 4

If you need a visual, take a look at the at the Observer Pattern Diagram

http://upload.wikimedia.org/wikipedia/commons/8/8d/Observer.svg