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  • Why the concept of "Covariance" and "Contravariance" are applicable while implementing the methods o

    - by Amy
    The use case is some what like this: public class SomeClass: IClonable { // Some Code //Implementing interface method Public object Clone() { //Some Clonning Code } } Now my question is "Why is it not possible to use "SomeClass(As it is derivd from objec)" as a return type of Clone() method if we consider the Funda's of Covariance and Contravariance. Can somebody explain me the reason behind this implemementaion of Microsoft ????

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  • Concept of GUI's - Centralized or decentralized

    - by wvd
    Hello all, Since a few months I've been learning Erlang, and now it was time to do some basic GUI. After some quick research I saw there was an interesting library called 'wxi' (based on Fudgets of Haskell) which uses a different approach on GUI's. No central loop, every widget processes it's own events and handles it's own data. What do you guys think about this? It looks like it kind of can be efficient in languages such as Erlang, and it's an interesting approach. William van Doorn

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  • function concept

    - by anurag18294
    void execute(int &x,int y=100) { x=x+y; cout< will the following program work in spite of not assigning a default value to the x(formal parameters in the fuction prototype).

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  • ASP.NET MVC 2 AJAX dilemma: Lose Models concept or create unmanageable JavaScript

    - by Slightly Frustrated
    Hi, Ok, let's assume we are working with ASP.NET MVC 2 (latest and greatest preview) and we want to create AJAX user interface with jQuery. So what are our real options here? Option 1 - Pass Json from the Controller to the view, and then the view submits Json back to the controller. This means (in the order given): User opens some View (let's say - /Invoices/January) which has to visualize a list of data (e.g. <IEnumerable<X.Y.Z.Models.Invoice>>) Controller retrieves the Model from the repository (assuming we are using repository pattern). Controller creates a new instance of a class which we will serialize to Json. The reasaon we do this, is because the model may not be serializable (circular reference ftl) Controller populates the soon-to-be-serialized class with data Controller serializes the class to Json and passes it the view. User does some change and submits the 'form' The View submits back Json to the controller The Controller now must 'manually' validate the input, because the Json passed does not bind to a Model See, if our View is communicating to the controller via Json, we lose the Model validation, which IMHO is incredible disadvantage. In this case, forget about data annotations and stuff. Option 2 - Ok, the alternative of the first approach is to pass the Models to the Views, which is the default behavior in the template when you start a new project. We pass a strong typed model to the view The view renders the appropriate html and javascript, sticking to the model property names. This is important! The user submits the form. If we stick to the model names, when we .serialize() the form and submit it to the controller it will map to a model. There is no Json mapping. The submitted form directly binds to a strongly typed model, hence, we can use the model validation. E.g. we keep the business logic where it should be. Problem with this approach is, if we refactor some of the Models (change property names, types, etc), the javascript we wrote would become invalid. We will have to manually refactor the scripting and hope we don't miss something. There is no way you can test it either. Ok, the question is - how to write an AJAX front end, which keeps the business logic validation in the model (e.g. controller passes and receives a Model type), but in the same time doesn't screw up the javascript and html when we refactor the model?

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  • DATE concept in VB.NET

    - by megala
    i have created VB.net project.In that i have two textbox,and two buttons button1--submit buttom2-Duedate textbox1 contain the current date My constraints is if i click button2(Duedate) than add 30 days to textbox1 date and assign that value into textbox2. How to achieve this? I want the result like as folloes If I give textbox1 = 12/12/2009 than I click Duedatetextbox2.text =11/1/2010 Is it possible. Thanks in advance.

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  • MUD (game) design concept question about timed events.

    - by mudder
    I'm trying my hand at building a MUD (multiplayer interactive-fiction game) I'm in the design/conceptualizing phase and I've run into a problem that I can't come up with a solution for. I'm hoping some more experienced programmers will have some advice. Here's the problem as best I can explain it. When the player decides to perform an action he sends a command to the server. the server then processes the command, determines whether or not the action can be performed, and either does it or responds with a reason as to why it could not be done. One reason that an action might fail is that the player is busy doing something else. For instance, if a player is mid-fight and has just swung a massive broadsword, it might take 3 seconds before he can repeat this action. If the player attempts to swing again to soon, the game will respond indicating that he must wait x seconds before doing that. Now, this I can probably design without much trouble. The problem I'm having is how I can replicate this behavior from AI creatures. All of the events that are being performed by the server ON ITS OWN, aka not as an immediate reaction to something a player has done, will have to be time sensitive. Some evil monster has cast a spell on you but must wait 30 seconds before doing it again... I think I'll probably be adding all these events to some kind of event queue, but how can I make that event queue time sensitive?

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  • help understanding the concept of javascript callbacks with node.js, especially in loops

    - by Mr JSON
    hi I am just starting with node.js. I have done a little ajax stuff but nothing too complicated so callbacks are still kind of over my head. I looked at async but all I need is to run a few functions sequentially. I basically have something that pulls some json from an api, creates a new one and then does something with that. obviously i can't just run it because it runs everything at once and has an empty json. mostly they have to go sequentially but if while pulling a json from the api it can pull other json while it's waiting that is fine. I just got confused when putting the callback in a loop. what do I do with the index? i think i have seen some places that use callback inside the loop as kind of a recusive function and don't use for loops at all. simple examples would help alot thanks!

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  • clarification on the concept of "web service"

    - by udit
    Im a little confused on the varying definitions and implementations of web services available as implementations. Need some clarification please. Ones I have used till now: If a vendor gives me a specific format of XML that I can send populated with data to request and I make a simple HTTP POST over the internet passing in the XML String as the payload, is this a web service call ? If so, is there a specific name to it, this kind of web service ? Because obviously, it does not use anything like Axis, WSDL or SOAP to establish this connection. A variant of this is If the vendor gives me an XSD, I use JAXB to make a java class out of it and pass in the serialized version of the object, which eventually works out to be the same as option 1. RESTful web service: Vendor gives me a URL like http://restfulservice/products and I can make HTTP Requests to the URL and depending on what HTTP verb I use, the appropropriate action is called and the response sent over the wire. Ones I have only read about\ have a vague idea about SOAP. How does this work?.. Ive read the W3Schools tutorial and I undertsand that there is a very specific form of XML that is standardized according to W3C standards that we use to pass the same kind of messages as we did in option 1. But how does this work in real life? Vendor sends me what? Do I generate classes? Do I serialize some objects and http post them over to an address? Or do the generated objects themselves have connection methods that will do them for me? What about WSDL? When does a vendor send me WSDL and what do I do with it ? I guess I can generate classes from it. If yes, then what do I do with the generated classes ? When do I need that axis jar to generate classes from something that the vendor sends ? As you can see, I have some clear and other mostly vague ideas about the different kinds of web services available. would help if someone ould clarify and\or point to more real-world resources. I've looked a little bit into Java Web Services on the internet and the numerous four letter acronyms that get thrown at me make me dizzy. Thanks

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  • Concept: Mapping irregular shapes (cartoons, sprites) to triangles in OpenGL ES

    - by Moshe
    I understand how mapping a triangle texture to a triangle works, but how do you map other things? I can't see myself mapping a circle onto a triangle. If it were a quad (square), I could see it happening, but why would a graphic not get warped on a triangle? EDIT: Bonus question: What are some good OpenGL ES tutorials online? Videos and articles count. (I've seen the Stanford University stuff on iTunes U and think it's excellent, but I want more.)

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  • Concept of creating static Map

    - by chetan
    I want to create a Map object that can lose value only if we explicitly delete it from map. I have created Map object like public static Map<Long,String> listUser =new HasMap<Long,String>(); but every time class object is created new instance of Map is also created that contain null value. I want to create a Map instance that contain value we stored before.

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  • Statement.RETURN_GENERATED_KEYS concept

    - by rits
    import java.sql.* ; import java.util.* ; import java.io.* ; class DataBaseFactory{ public static Connection getConnection(){ ... ... ... ... } } class Demo{ public static void main(String []args) throws SQLException{ Connection con = DataBaseFactory.getConnection() ; //This is throwing exception //PreparedStatement ps = con.prepareStatement("insert into user values(?,?)", Statement.RETURN_GENERATED_KEYS) ; //But this is working fine PreparedStatement ps = con.prepareStatement("insert into user values(?,?)") ; } } thnx in advance to all the java people here....

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  • Understanding Async Concept in WebServices

    - by 8EM
    I've had the thrill recently of developing web service applications. Most of my experience is with GWT and mainly doing most things on the client side then doing an async call back for any additional data needed. However at the moment, I want a process that will be triggered on the client side, then on the server side, a loop will occur, where if a certain condition is met, it will 'push' back to the client. This will hopefully remove the processor usage on the client side and also saves bandwidth. What is this called? I understand 'polling' is where the client side continuously hits a server, however what I want is the opposite. Is this possible? Am I misunderstanding what happened when I trigger an AsyncService in GWT? Please advise. EDIT: Just for further clarification: Having some kind of weather data service. Where, you trigger 'go' on the client side, then on the server side, it checks to see the degrees, if it has moved since last time, it will spit back the degrees to the client, if it hasn't, it will keep looping.

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  • Help with concept - filters and number of items

    - by dreamer
    Please check http://www.alibaba.com/catalogs/cid/702/Laptops.html they have nice filter here with number of items for each. Note one detail - they have locations here. Same thing on olx.com - location and number of items for each category. Now imagine I have tables: [products] (Id, Name, CategoryId, LocationId) [Categories] (Id,Name) [Location] (Id, Name) My question how can I do the same, cause count things even with caching looks expensive? And they give results pretty fast... Please advice with possible ways to do that in ASP.NET, C#, MVC, MS SQL, but avoice simple answers like "count and change" Thank you in advance.

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  • Multiset of shared_ptrs as a dynamic priority queue: Concept and practice

    - by Sarah
    I was using a vector-based priority queue typedef std::priority_queue< Event, vector< Event >, std::greater< Event > > EventPQ; to manage my Event objects. Now my simulation has to be able to find and delete certain Event objects not at the top of the queue. I'd like to know if my planned work-around can do what I need it to, and if I have the syntax right. I'd also like to know if dramatically better solutions exist. My plan is to make EventPQ a multiset of smart pointers to Event objects: typedef std::multi_set< boost::shared_ptr< Event > > EventPQ; I'm borrowing functions of the Event class from a related post on a multimap priority queue. // Event.h #include <cstdlib> using namespace std; #include <set> #include <boost/shared_ptr.hpp> class Event; typedef std::multi_set< boost::shared_ptr< Event > > EventPQ; class Event { public: Event( double t, int eid, int hid ); ~Event(); void add( EventPQ& q ); void remove(); bool operator < ( const Event & rhs ) const { return ( time < rhs.time ); } bool operator > ( const Event & rhs ) const { return ( time > rhs.time ); } double time; int eventID; int hostID; EventPQ* mq; EventPQ::iterator mIt; }; // Event.cpp Event::Event( double t, int eid, int hid ) { time = t; eventID = eid; hostID = hid; } Event::~Event() {} void Event::add( EventPQ& q ) { mq = &q; mIt = q.insert( boost::shared_ptr<Event>(this) ); } void Event::remove() { mq.erase( mIt ); mq = 0; mIt = EventPQ::iterator(); } I was hoping that by making EventPQ a container of pointers, I could avoid wasting time copying Events into the container and avoid accidentally editing the wrong copy. Would it be dramatically easier to store the Events themselves in EventPQ instead? Does it make more sense to remove the time keys from Event objects and use them instead as keys in a multimap? Assuming the current implementation seems okay, my questions are: Do I need to specify how to sort on the pointers, rather than the objects, or does the multiset automatically know to sort on the objects pointed to? If I have a shared_ptr ptr1 to an Event that also has a pointer in the EventPQ container, how do I find and delete the corresponding pointer in EventPQ? Is it enough to .find( ptr1 ), or do I instead have to find by the key (time)? Is the Event::remove() sufficient for removing the pointer in the EventPQ container? There's a small chance multiple events could be created with the same time (obviously implied in the use of multiset). If the find() works on event times, to avoid accidentally deleting the wrong event, I was planning to throw in a further check on eventID and hostID. Does this seem reasonable? (Dumb syntax question) In Event.h, is the declaration of dummy class Event;, then the EventPQ typedef, and then the real class Event declaration appropriate? I'm obviously an inexperienced programmer with very spotty background--this isn't for homework. Would love suggestions and explanations. Please let me know if any part of this is confusing. Thanks.

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  • Does python have a session variable concept???

    - by gizgok
    I have a datetime.date variable in python.I need to pass it to a function do operations according to the date given and then increment the date for the next set of operations.The problem is I have to do the operations in diff pages and hence I need the date as a variable which can go from page to page. Can we do this in python.......

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  • Freelance site - Escrow Concept

    - by jose
    How does escrow feature work in freelancing sites? Are they using any 3rd party escrow providers? OR Is it possible to develop the same feature using PHP? I know, it is possible. But I dont know how to develop. Please advise

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  • Understanding the concept of Inodes

    - by darkie15
    Hi All, I am referring to the link: http://www.tux4u.nl/freedocs/unix/draw/inode.pdf I am confused on parts: 1 12 direct block pointers 2 1 single indirect block pointer 3 1 double indirect block pointer 4 1 triple indirect block pointer Now the diagram says that each pointer is 32/64 bits. [Query]: Why and how are these values inferred? I mean why specifically have only 32 or 64 bit pointers? The diagram says, One data block{8 KB} for each pointer {4 bytes/8 bytes} [Query]: How does this actually work out? i.e. 8*1024 bytes / 8 bytes = 1024 bytes? What is the logic behind having a 8 bytes pointer for 8KB block? Regards, darkie.

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  • CS Majors: Hardest concept(s) you learned in school?

    - by Mark Lubin
    For the CS majors out there what were the hardest CS classes or concepts that you learned in your undergraduate schooling? Did you find once you learned the basics,(data structs, OOP fundamentals, discrete math, pointers, recursion, etc) the rest followed naturally or did you hit a wall at any point in your higher classes like OS'es and Compilers? Thanks for the input!

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  • Multi-threading concept and lock in c#

    - by Neeraj
    I read about lock, though not understood nothing at all. My question is why do we use a un-used object and lock that and how this makes something thread-safe or how this helps in multi-threading ? Isn't there other way to make thread-safe code. public class test { private object Lock { get; set; } ... lock (this.Lock) { ... } ... } Sorry is my question is very stupid, but i don't understand, although i've used it many times.

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  • How the websocket bi-directional concept work?

    - by GMsoF
    I think the main difference between websocket and http streaming (I am not refering to polling and long polling) is websocket allows bi-directional communication which is similar to usual raw socket programming. (above is my understanding, could be wrong, feel free to correct me.) My question is how the web client (browser) continue to send another request in the already-opened websocket? Usual http request will treat another request as new socket connection, but websocket does not, that is why I am confused, how it achieve that? It should be handled in Server side or Client (browser) side?

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  • backbonejs remove anomaly concept

    - by user1537158
    I was building a small app for adding and deleting li from ul using Backbonejs.One of the SO members cymen helped me code it, using that i tailored the code a little.currently if i add one element and delete , it works , but the second time i add an element (to ul) and go to delete it , i get Uncaught TypeError: Cannot call method 'remove' of undefined Pasting my code here , HTML : <input type="text" id="name"> <button id="add">Add</button> <ul id="mylist"></ul> JS: $(function(){ var myCollection = Backbone.Collection.extend(); var myView = Backbone.View.extend({ el:$('body'), tagName:'li', initialize : function(e){ this.collection.bind("add",this.render,this); this.collection.bind("remove",this.render,this); }, events:{ 'click #add' : 'addfoo' }, addfoo : function(){ var myname= $('#name').val(); $('#name').val(''); this.collection.add({name:myname}); }, render : function(){ $('#mylist').empty(); this.collection.each(function(model){ console.log("myView"); var remove = new myRemoveView({model:model}); remove.render(); }); } }); var myRemoveView = Backbone.View.extend({ el:$('body'), events:{ 'click .button':'removeFoo' }, removeFoo : function(){ console.log("here"); this.model.collection.remove(this.model); }, render : function(){ console.log("second view"); $('#mylist').append('<li>'+this.model.get('name') + "<button class='button'>"+"delete"+"</button></li>"); return; } }); var view = new myView({collection: new myCollection()}); }); Two things i did not understand : i) in the removeFoo function , we write this.model.collection.remove(this.model) shouldnt this have been this.collection.model.remove , something of that sort ? ii) i add a li to ul , then i delete it , when i add another li (appending to ul works perfect) but this time when i go to delete it throws me the above error : Uncaught TypeError :cannot call method 'remove' of undefined can you please help me figure out these 2 doubts in my code , btw SO member cymen's code works like a charm only my tailored code (above) is giving me errors. SO member cymen's code : JS Fiddle for his code Thank you

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  • Dont understand the concept of extends in URL.openConnection() in JAVA

    - by user1722361
    Hi I am trying to learn JAVA deeply and so I am digging into the JDK source code in the following lines: URL url = new URL("http://www.google.com"); URLConnection tmpConn = url.openConnection(); I attached the source code and set the breakpoint at the second line and stepped into the code. I can see the code flow is: URL.openConnection() - sun.net.www.protocol.http.Handler.openConnection() I have two questions about this First In URL.openConnection() the code is: public URLConnection openConnection() throws java.io.IOException { return handler.openConnection(this); } handler is an object of URLStreamHandler, define as blow transient URLStreamHandler handler; But URLStreamHandler is a abstract class and method openConnection() is not implement in it so when handler calls this method, it should go to find a subclass who implement this method, right? But there are a lot classes who implement this methods in sun.net.www.protocol (like http.Hanlder, ftp.Handler ) How should the code know which "openConnection" method it should call? In this example, this handler.openConnection() will go into http.Handler and it is correct. (if I set the url as ftp://www.google.com, it will go into ftp.Handler) I cannot understand the mechanism. second. I have attached the source code so I can step into the JDK and see the variables but for many classes like sun.net.www.protocol.http.Handler, there are not source code in src.zip. I googled this class and there is source code online I can get but why they did not put it (and many other classes) in the src.zip? Where can I find a comprehensive version of source code? Thanks!

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