invariants

问题 I often see the term Invariants in DDD. Here Dino Esposito talks about it. If I look at the .NET library, I see a ValidationAttribute class. Are Invariants and validation rules the same? For example, can I say 50% discount is available only if the order total is more than $250 is an Invariant? Or are they different where Invariants are to protect an object from becoming invalid and validation is to check the validity of the object even after it has changed it's state (it can be in a valid or

问题 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

问题 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

问题 I'm moving my first steps in Scala and I would like to make the following code works: trait Gene[+T] { val gene: Array[T] } The error that the compiler gives is: covariant type T occurs in invariant position in type => Array[T] of value gene I know I could do something like: trait Gene[+T] { def gene[U >: T]: Array[U] } but this doesn't solve the problem because I need a value: pratically what I'm trying to say is "I don't care of the inside type, I know that genes will have a gene field that

问题 As part of my learning i think the best answer(with meaning) for definition of abstraction that i found is from stackoverflow: What is abstraction? Besides that, As part of current online course cs61B Fall 2006, Berkeley, i learnt the similar below definition of ADT close to above definition but added an extra word 'invariant'. Shall i consider this word as corollary to the above definition? or Is this word part of the definition? An _Abstract_Data_Type_ (ADT) is a class that has a well

问题 As seen on Introduction to Algorithms (http://mitpress.mit.edu/algorithms), the exercise states the following: Input: Array A[1...n] Output: i, where A[i]=v or NIL when not found Write a pseudocode for LINEAR-SEARCH, which scans through the sequence, looking for v. Using a loop invariant, prove that your algorithm is correct. (Make sure that your loop invariant fulfills the three necessary properties – initialization, maintenance, termination.) I have no problem creating the algorithm, but

问题 I don't like the requirement on have at least one empty constructor and public setters on JPA entities. While I understand the issue on the EntityManager side, this invalidates class invariants. Does anyone have a solution for this (design pattern or idiom level) ? Thanks! Igor 回答1: With JPA, the default constructor is required, however, you are not required to use setters. You can choose a property access strategy(field or method) based on where you place the annotations. The following code

问题 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 . 回答1: 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)

问题 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? 回答1: 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

问题 I'm on the road to learn Scala and I'm having a hard time understanding contravariants, covariants, invariance, etc. From Why doesn't the example compile, aka how does (co-, contra-, and in-) variance work? I have learned how functions can be considered subtypes of another function. (Really useful to know!) The code below is what I believe are the important pieces to solving my puzzle. I have extracted parts that I think would add unneeded complexity to the problem. According to the example I