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Technical Life - cellophane — LiveJournal
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renniekins
renniekins
Technical Life
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specialagentm From: specialagentm Date: April 1st, 2008 01:57 pm (UTC) (Link)
One definition of subtypes in OO [1] is that you should be able to freely substitute a subtype for any instance of its supertype, and the caller should not know the difference. Different behaviors may result, but the message-passing, flow of control, and correctness is identical.

All that factories or dependency injection does is pull that up one more level and let you make a decision to, application-wide, swap in and out different implementations without having to change every single call to new(). Some very dynamic languages like Lisp or Ruby let you do this directly (I can, at runtime, redefine class Foo to be some other piece of code entirely). Less smart languages like Java need a little help, but you can write a factory class or static factory method pretty quickly, and a global search and replace to change:

Foo bar = new Foo();

to this:

Foo bar = FooFactory.fetchMeAFoo();

is pretty easy too.

So, yes... upfront work is needed. But this doesn't just drive TDD, it drives a lot of other pieces of flexibility that are often useful. For us, use of factories means we can write core code that doesn't care where it's running at, and how it gets to the data it needs -- in some cases, it has local database access. In other cases, it has to fetch the data over a Web Service to someone else who has DB access. In the TDD case, it's using fake in-memory data.

I'm not trying to sell anything, just trying to show the full picture of why these practices are followed. I'll admit that the way you implement these concepts could be more elegant, but the dirty bits are usually all quite well hidden once you've got it all setup.

[1] http://en.wikipedia.org/wiki/Liskov_substitution_principle
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