immutability

问题 In our hibernate project, the entities are coded using the java beans pattern. There's quite a few spots in our code where someone has forgotten a to set a mutator and we get an exception due to a NOT NULL field. Is anyone using a builder to construct their entities or making them immutable? I'm trying to find an effective pattern that is not in the style of the java beans pattern. Thanks 回答1: If you make your beans immutable then you have to use field level access and this comes with its own

问题 For example I'm extracting a text String from a text file and I need those words to form an array. However, when I do all that some words end with comma (,) or a full stop (.) or even have brackets attached to them (which is all perfectly normal). What I want to do is to get rid of those characters. I've been trying to do that using those predefined String methods in Java but I just can't get around it. 回答1: Use: String str = "whatever"; str = str.replaceAll("[,.]", ""); replaceAll takes a

问题 I need to represent directed graphs in Clojure. I'd like to represent each node in the graph as an object (probably a record) that includes a field called :edges that is a collection of the nodes that are directly reachable from the current node. Hopefully it goes without saying, but I would like these graphs to be immutable. I can construct directed acyclic graphs with this approach as long as I do a topological sort and build each graph "from the leaves up". This approach doesn't work for

问题 So far most of "starter boilerplates" and some posts about react / redux I've seen encourage usage of immutable.js to address mutability. I personally rely on Object.assign or spread operators to handle this, hence don't really see advantage in immutable.js as it adds additional learning and shifts a bit from vanilla js techniques used for mutability. I was trying to find valid reasons for a switch, but wasn't able to hence I am asking here to see why it is so popular. 回答1: This is all about

问题 So I started learning React a week ago and I inevitably got to the problem of state and how components are supposed to communicate with the rest of the app. I searched around and Redux seems to be the flavor of the month. I read through all the documentation and I think it's actually a pretty revolutionary idea. Here are my thoughts on it: State is generally agreed to be pretty evil and a large source of bugs in programming. Instead of scattering it all throughout your app Redux says why not

问题 Kahn proposed an algorithm in 62 to topologically sort any DAG (directed acyclic graph), pseudo code copied from Wikipedia: L ← Empty list that will contain the sorted elements S ← Set of all nodes with no incoming edges while S is non-empty do remove a node n from S add n to tail of L for each node m with an edge e from n to m do remove edge e from the graph # This is a DESTRUCTIVE step! if m has no other incoming edges then insert m into S if graph has edges then return error (graph has at

问题 Currently taking a class that's using Scala which I've never used before, so the syntax and itself is new. I'm working on a simple division function but am running into some errors. First of all, am I using var sub=m right? In my code I simply wanted to do m = m-n but you can't change the variable, and I'm not sure what the best alternative is. Then my only other problem is the compiler barks at me for my print line.. <console>:14: error: reassignment to val m = m-n //////////////////////////

问题 An immutable object is initialized by its constuctor only, while a singleton is instantiated by a static method. How to make an immutable singleton in Java? 回答1: while a singleton is instantiated by a static method While this is the usual way of doing it, this is by no means the only way. In Java 1.5 a new version of Singleton is the enum singleton pattern: public enum Elvis{ INSTANCE // this is a singleton, no static methods involved } And since enums can have constructors, methods and

问题 I declared a variable outside the function like this: var s: Int = 0 passed it such as this: def function(s: Int): Boolean={ s += 1 return true } but the error lines wont go away under the "s +=" for the life of me. I tried everything. I am new to Scala btw. 回答1: First of all, I will repeat my words of caution: solution below is both obscure and inefficient , if it possible try to stick with val ues. implicit class MutableInt(var value: Int) { def inc() = { value+=1 } } def function(s:

问题 I declared a variable outside the function like this: var s: Int = 0 passed it such as this: def function(s: Int): Boolean={ s += 1 return true } but the error lines wont go away under the "s +=" for the life of me. I tried everything. I am new to Scala btw. 回答1: First of all, I will repeat my words of caution: solution below is both obscure and inefficient , if it possible try to stick with val ues. implicit class MutableInt(var value: Int) { def inc() = { value+=1 } } def function(s: