And Effective Java Programming Language Guide, rule 23.
Programming With Assertions
Do not use assertions for argument checking in public methods.
Argument checking is typically part of the published specifications (or contract) of a method, and these specifications must be obeyed whether assertions are enabled or disabled. Another problem with using assertions for argument checking is that erroneous arguments should result in an appropriate runtime exception (such as IllegalArgumentException, IndexOutOfBoundsException, or NullPointerException). An assertion failure will not throw an appropriate exception.
Do not use assertions to do any work that your application requires for correct operation.
Because assertions may be disabled, programs must not assume that the boolean expression contained in an assertion will be evaluated.
Effective Java Programming Language Guide, rule 23
For an unexported method, you as the package author control the circumstances under which the method is called, so you can and should ensure that only valid parameter values are ever passed in. Therefore nonpublic methods should generally check their parameters using assertions rather than normal checks. If you are using a release of the platform that supports assertions (1.4 or later), you should use the assert construct; otherwise you should use a makeshift assertion mechanism.