in general, when a method is only defined for certain values, test for values on and over the border of what is acceptable. In other words, make sure your method does what it's supposed to do, but nothing more. This is important, because when you're going to fail, you want to fail early.
In inheritance hierarchies, make sure to test for LSP compliance.
Testing default getters and setters doesn't seem very useful to me, unless you're planning to do some validation later on.
