Thursday, April 23, 2009

pure VO wrapped in a domain object

why do Viktor and many java data modelers prefer to break a domain object into a pure VO and a wrapper object hosting the behavior or api methods?

Here's a pratical reason. There's negligible downside but increasing benefit as the object gets bigger. The one-class design can present hundreds (for a large object) of fields and methods, some belong to the VO, some obviously not. Hard to read. At some point you end up splitting the class anyway, for maintainability and readability. Why not adopt a coding standard?

A purist design is to have the VO as a public inner class of the wrapper. Argument -- the VO should never exist outside.

A bad design is to make the wrapper a factory. The wrapper should still be a domain model. One instance represents one real-world instance.

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