Indeed, Design by Contract is not yet integrated into Java. The precondition is specified in PreCheck commands. The postcondition is specified in PostCheck commands. For JJ, yes it is. There are a few rules that make this DbC definition work for JJ, rules that may be a little too restrictive for full featured object oriented langauges, but rules that are more than reasonable for a beginner's language.
Functions are free of side effects.
Private methods cannot directly call public methods in the same class. For more on Design by Contract, please see the Eiffel programming language website.