invariants

I am reading the book Java Concurrency in Practice and getting a little bit confused with these terms: Pre condition Post condition Invariants Can someone please explain me them (with an example, if it's possible)? You'll have a lot of problems writing Java, especially multi-threaded code, if you can't understand these simple ideas: 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". Post-conditions are the things that must be true after the method is complete. The method tells clients "this is what I promise to

I'm interested in hearing what technique(s) you're using to validate the internal state of an object during an operation that, from it's own point of view, only can fail because of bad internal state or invariant breach. My primary focus is on C++, since in C# the official and prevalent way is to throw an exception, and in C++ there's not just one single way to do this (ok, not really in C# either, I know that). Note that I'm not talking about function parameter validation, but more like class invariant integrity checks. For instance, let's say we want a Printer object to Queue a print job

When using formal aspects to create some code is there a generic method of determining a loop invariant or will it be completely different depending on the problem? It has already been pointed out that one same loop can have several invariants, and that Calculability is against you. It doesn't mean that you cannot try. You are, in fact, looking for an inductive invariant : the word invariant may also be used for a property that is true at each iteration but for which is it not enough to know that it hold at one iteration to deduce that it holds at the next. If I is an inductive invariant, then

Could someone provide me simple C# examples of convariance, contravariance, invariance and contra-invariance (if such thing exists). All samples I've seen so far was just casting some object into System.Object . Could someone provide me simple C# examples of convariance, contravariance, invariance and contra-invariance (if such thing exists). I have no idea what "contra-invariance" means. The rest are easy. Here's an example of covariance: void FeedTheAnimals(IEnumerable<Animal> animals) { foreach(Animal animal in animals) animal.Feed(); } ... List<Giraffe> giraffes = ...; FeedTheAnimals

I'm currently programming a scientific imaging application using OpenGL. I would like to know if OpenGL rendering (in term of retrieved pixel from FBO) is supposed to be fully deterministic when my code (C++ / OpenGL and simple GLSL) is executed on different hardware (ATI vs NVidia, various NVidia generations and various OS)? More precisely, I'd need the exact same pixels buffer everytime I run my code on any hardware (that can runs basic GLSL and OpenGL 3.0)... Is that possible? Is there some advice I should consider? If it's not possible, is there a specific brand of video card (perhaps

The word seems to get used in a number of contexts. The best I can figure is that they mean a variable that can't change. Isn't that what constants/finals (darn you Java!) are for? An invariant is more "conceptual" than a variable. In general, it's a property of the program state that is always true. A function or method that ensures that the invariant holds is said to maintain the invariant. For instance, a binary search tree might have the invariant that for every node, the key of the node's left child is less than the node's own key. A correctly written insertion function for this tree will