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Here's an answer that shows a questionable design on an API I use. A base class Entry is extended by File and Folder. I can ask for the entries in a folder that are files or folders or both.
Now I can call all the methods defined by Entry on the objects in this array. But what if I want to deal with Files and call some methods that only exist on File?
The getEntries method tells the compiler it returns Entry objects, so this fails:
Casting tells the compiler something it can't figure out by looking at the code. In the following I'm saying I know this object is really a File even though getEntries does not guarantee it in a way the compiler can understand.
What happens if I mess up and use the FOLDER parameter and then try to cast to File? When I try to cast a Folder to File I'll get an exception.
Finally, note that casting doesn't change the actual object at all. It only gives me a new reference to the object with a different type. With my new reference I can use methods that are defined on the subtype.
Did that help? Can you think of a couple examples you can code up and test? It would be a great experience to cause a cast exception on your own! [ March 22, 2006: Message edited by: Stan James ]
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi