OO design patterns to use for validation

Question

I am in the process of writing some validation code based on these assumptions:

• Validation code must be in an external class
  • i.e. no data class contains

Answer 1

If you're doing any sort of of GUI work, you should take a look at JGoodies Validation: http://www.jgoodies.com/downloads/libraries.html (also some articles here: www.jgoodies.com/articles/).

I would create a validator for any class that needs validation. You can actually create more than one validator if you need different ways of validating, e.g. strict or not. You can group common functionality and methods into classes like AbstractValidator and ValidationResult (which may have a list of errors, severity, etc.).

Be wary of over-design. Try starting with something simple like:

new UserValidator().validate(user)

or to validate a view:

new UserPanelValidator().validate(userPanel)

It does depend on your architecture though. For example, if you automatically propagate input from the view to the domain, then you don't need to do as much validation at the view level.