Class invariants are conditions, the object state, that you know remain true. During a method call the object state will change, possibly to something that invalidates the object, but once a public method has completed, the object state must be left in a consistent state. There is no guarantee what order the user will call methods on a class, or even if they call methods at all, so an object must be usable whatever methods the user calls. The invariant aspects of an object applies on a method calls level: between method calls the object must be consistent and usable.
For example, imagine you have a class that represents a date: it holds a day number between 1 and 31, a month number between 1 and 12, and a year number. ...
