Java: specify guidelines for argument validation

[damithc]

Specify when to (and when not to ) validate method arguments

[damithc]

It's quite hard to find established guidelines on this aspect. The best I could find is this extract from Effective Java.

Previous version of the guidelines (discussed halfway, but not merged) can be found here (refer to the deleted lines). Those guidelines are a bit hard to follow/enforce.

Perhaps a simpler (to follow/enforce) approach is to mandate argument validation for all public methods? i.e. for all public methods,

1. Try to minimize restrictions on arguments. Instead of restricting a specific type of argument (e.g. null), implement a reasonable behavior for that argument.
2. At the start of the method, validate arguments against all restrictions in force. Use suitable type of exceptions (preferably, built-in exceptions) to indicate violations.
3. Document all restrictions and the corresponding exceptions in the header comment.

That still leaves the question of validating arguments in non-public methods.