How does one usually make a Factory choose between multiple Impl classes?

I need to make a Factory choose between multiple Impl classes and I would like to know what the best practice is for this? How does one typically pass the data from a class to the ImplFactory in order to choose the appropriate Impl for that class while still maintaining good design principles?

I would prefer a thoughtful conceptual answer. No quick posting or links please.
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The best practice would probably be

a. use a configuration file
b. put the name of the impl class in it
c. make the factory load it

That way, no recompilation is necessary in order to change impl

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If that seems like overkill (could be for just one class) then the following is good
java YourApp

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EindoofusAuthor Commented:
Could you please elaborate a bit more on "b" and "c"? I'm having a hard time understanding them. If you have any code as an example I would greatly appreciate it.
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I'm going to abandon my first suggestion if favour of my second - unless you have several classes that have to be selected
EindoofusAuthor Commented:
java YourApp

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I'm not sure if we are on the same page. Can you please take a look at my other question to see what I am doing:

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