What is the difference between precondition, postcondition and invariant constraints?

Question

I am new to programming and this site and I am studying about some of the terminology for UML design and I am looking for a basic response. I looked up the definition of each of the terms but I am still trying to grasp the concept.

Answer 1

A pre condition is something that must be true before the use case is invoked. e.g. a precondition for a use case "Buy Book on WebSite" might be "The user has accessed the website and wants to buy a book".

A post condition is something that must be true after the use case is finished. e.g. The user has successfully purchased a book.

An invariant is something that must always be true throughout the use case. e.g. The user's ID matches an ID from the Users table in the database.