Friday, October 6, 2006

layered OO design - testing, modular code reuse..

From experts to novice, everyone uses the "layer" jargon in the context of testing, code reuse, modular design,

For starters, i have a blog post on "DB entity object HAS-A DAO".
* import -- Ideally, a upper-layer class imports lower-layer classes. Shared-layer classes can be imported by all.
* compile-time dependency -- Ideally, low-layer classes should be compilable without upper-layer classes.
* reusable components -- therefore, lower-layer classes should be reusable in other projects in the absence of upper-layer classes.
* testing -- To test a upper-layer method (we didn't say "class"), you often need to instantiate (or mock) the lower-layer objects. Even if you use interfaces, you still need to instantiate these lower objects to run their methods.

(Suggestion: reduce cross-layer inter-dependencies.)

* use-a, has-a -- upper-layer classes could use-a or has-a lower-layer class, but not vice versa.
* separation of concern -- lower-layer objects should not need to know about upper-layer. If my class uses xstream (or a mailer, a scheduler, a serializer, a parser, a timer...), then xstream should be oblivious of my class.

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http://www.linkedin.com/in/tanbin