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I am reading the book Java Concurrency in Practice and getting a little bit confused with these terms:

  1. Pre condition
  2. Post condition
  3. Invariants

Can someone please explain me them (with an example, if it's possible)?

MarianD
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Inquisitive
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  • Also Assume when Unit Test and Given in BDD are examples of real world use of pre-conditions, and Assert & BDD Then clauses are examples of post-conditions. – Martin Spamer Mar 25 '19 at 19:34

1 Answers1

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You'll have a lot of problems writing Java, especially multi-threaded code, if you can't understand these simple ideas:

  1. Pre-conditions are the things that must be true before a method is called. The method tells clients "this is what I expect from you".
  2. Post-conditions are the things that must be true after the method is complete. The method tells clients "this is what I promise to do for you".
  3. Invariants are the things that are always true and won't change. The method tells clients "if this was true before you called me, I promise it'll still be true when I'm done".

They're all part of an idea called "programming by contract". It was invented by a guy named C.A.R. Hoare. Bertrand Meyer built an object oriented language called Eiffel around it. No one uses it much, but he had a day in the sun because of it.

Eiffel isn't very popular. There are over four million questions on SO as I write this, but only 32 of them are tagged "eiffel".

Update: There were 11,966,392 question on SO on 29-Jun-2016. Only 92 of them were tagged "eiffel". The percentage of Eiffel questions is staying roughly constant at ~0.00077%.

j.developer
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duffymo
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    http://en.wikipedia.org/wiki/Hoare_logic – lexicalscope Dec 04 '12 at 11:43
  • So, when I'm writing a method with certain pre-conditions, would I check to see if those pre-conditions are satisfied, or am I to assume that the client is smart enough to not violate the pre-condition? – Ungeheuer Apr 06 '17 at 05:46
  • Your choice, but the whole idea is to validate and prevent errors. Ask yourself how consumers will know your contract. – duffymo Apr 06 '17 at 07:26
  • I got here by reading https://stackoverflow.com/q/77127/845117 regarding 'When to throw an Exception'. The 3 Terms of Pre-, Postconditions and Invariants do sound clearly logic to me. And I would throw an Exception if any of these cannot be met by 'my' executing function. For me then, an Exception is a marker telling me execution did not terminate as expected/as declared - watch out! – Dirk Schumacher Aug 19 '21 at 05:42