Access specific composite data in composite pattern

Question

Suppose I have an Item class which is the base class of a composite pattern.

I have two specific subclasses Movie and Music containing different data :

• Movie : director, producer, list of actors ...
• Music : chanteur, number of record sold ...

If I want to access Movie data (for example) in a client code, I need to downcast the Item corresponding to it because getProducer() is not a method of the Item class.

I am stuck between three different approach :

1. Put all the subclass methods in the Item interface
2. Downcast Item in client code
3. Use an object that would extract the data from the different Item (using a kind of visitor pattern)

The third proposition seems interesting because it hides Item subclasses to client, but I don't know how to let client update an Item (for example change the producer of a movie).

The Item subclasse types supported by the application will grow through releases, and I want to design it to facilitate those type additions.

Maybe I am totally going in the wrong way and there is a different and better solution.

Answer 1

Option 1 is a bad idea, simply because getProducer() does not belong to an item.

Option 3 would work if you'd also proved setters in addition to the getter methods you were planning to implement there.

Option 2 sounds the most reasonable to me. Casting is a common task in a polymorphic solutions and in your case it is adequate.