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• How to implement the API/SPI Pattern in Java?

  - by Adam Tannon

  I am creating a framework that exposes an API for developers to use: public interface MyAPI { public void doSomeStuff(); public int getWidgets(boolean hasRun); } All the developers should have to do is code their projects against these API methods. I also want them to be able to place different "drivers"/"API bindings" on the runtime classpath (the same way JDBC or SLF4J work) and have the API method calls (doSomeStuff(), etc.) operate on different 3rd party resources (files, servers, whatever). Thus the same code and API calls will map to operations on different resources depending on what driver/binding the runtime classpath sees (i.e. myapi-ftp, myapi-ssh, myapi-teleportation). How do I write (and package) an SPI that allows for such runtime binding, and then maps MyAPI calls to the correct (concrete) implementation? In other words, if myapi-ftp allows you to getWidgets(boolean) from an FTP server, how would I could this up (to make use of both the API and SPI)? Bonus points for concrete, working Java code example! Thanks in advance!

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• My User control belonging to which Design Pattern??

  - by prashant

  Hello, I am creating a reusable component in C#.net. For that i have started a Control Library project and added a Control. Class MyControl : Control{} My user control just displays some images which will be used in many Windows Applications. Can you please tell me which design pattern i am using here. I am unable to decide which pattern they belongs. Thanks

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• Decorator Design Pattern Use With Service Objects (wSingleton)

  - by Dustin

  I'm working on a project where I need to add some functionality to a service object and using a decorator to add it in seems like a good fit. However, I've only ever used decorators with simple beans, never on a singleton like a service object. Has anyone ever done this before and what are the pros and cons? In this case I don't think creating a subclass will work so a decorator seems to be a good fit. What are your thoughts on doing this?

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• Alternative to singleton for unique resources

  - by user1320881

  I keep reading over and over again that one should avoid using singletons for various reasons. I'm wondering how to correctly handle a situation where a class represents a unique system resource. For example, a AudioOutput class using SDL. Since SDL_OpenAudio can only be open once at a time it makes no sense having more then one object of this type and it seems to me preventing accidentally making more then one object would actually be good. Just wondering what experienced programmers think about this, am i missing another option ?

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• What exactly is GRASP's Controller about?

  - by devoured elysium

  What is the idea behind Grasp's Controller pattern? My current interpretation is that sometimes you want to achieve something that needs to use a couple of classes but none of those classes could or has access to the information needed to do it, so you create a new class that does the job, having references to all the needed classes(this is, could be the information expert). Is this a correct view of what Grasp's Controller is about? Generally when googling or SO'ing controller, I just get results about MVC's (and whatnot) which are topics that I don't understand about, so I'd like answers that don't assume I know ASP.NET's MVC or something :( Thanks

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• What is the definition of a Service object ?

  - by Maskime

  I've been working a lot with PHP. But recently i was sent on a work wich use Java. In PHP i used to do a lot of Singleton object but this pattern has not the same signification in Java that it has in PHP. So i wanted to go for an utility class (a class with static method) but my chief doesn't like this kind of classes and ask me to go for services object. So my guess was that a service object is just a class with a constructor that implement some public methods... Am i right ?

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• Should the program logic reside inside the gui object class or be external to the class?

  - by hd112

  I have a question about how to structure code in relation to GUI objects. Suppose I have a dialog that has a list control that has a bunch of names obtained from a database. The user can edit the names. Does the logic reside inside that dialog class or should it be from the outside. To illustrate what I mean, here’s some pseudo code showing the structure of the code when the logic is handled outside the dialog class: NamesDialog : wxDialog { Private: ..stuff.. Public: ... SetNames(wxStringArray names); wxStringArray GetNames(); ..stuff.. } So the user of the class would do something like: wxStringArray names = DatabaseManager::Get()->GetNames(); names.Sort(); NamesDialogObject.SetNames(names); NamesDialogObject.ShowModal(); wxStringArray modified_names = NamesDialogObject.GetNames(); AddToDatabase(modified_names); //or something like this. On the other hand, the database logic can reside inside the NamesDialog class itself. In the show method I can query the database for the names and as the user interacts with the controls (list control in this case), the database can be updated from the event handlers. As a result the NamesDialog class only has the Show() method as there is no need to use SetNames or GetNames() etc. Which method is generally preferred? I don’t have much work experience so I’m not sure which is the proper way to handle it. Sometimes it's easier to handle everything in the class but getting access to objects it interacts with can be challenging. Generally can do it by having the relevant objects be singletons like the database manager in the above example.

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• Which design pattern fits - strategy makes sense ?

  - by user554833

  --Bump *One desperate try to get someone's attention I have a simple database table that stores list of users who have subscribed to folders either by email OR to show up on the site (only on the web UI). In the storage table this is controlled by a number(1 - show on site 2- by email). When I am showing in UI I need to show a checkbox next to each of folders for which the user has subscribed (both email & on site). There is a separate table which stores a set of default subscriptions which would apply to each user if user has not expressed his subscription. This is basically a folder ID and a virtual group name. But, Email subscriptions do not count for applying these default groups. So if no "on site" subscription apply default group. Thats the rule. How about a strategy pattern here (Pseudo code) Interface ISubscription public ArrayList GetSubscriptionData(Pass query object) Public class SubscriptionWithDefaultGroup Implement ArrayList GetSubscriptionData(Pass query object) Public class SubscriptionWithoutDefaultGroup Implement ArrayList GetSubscriptionData(Pass query object) Public class SubscriptionOnlyDefaultGroup Implement ArrayList GetSubscriptionData(Pass query object) does this even make sense? I would be more than glad for receive any criticism / help / notes. I am learning. Cheers

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• Writing a simple incrementer counter in rails

  - by Trip

  For every Card, I would like to attach a special number to them that increments by one. I assume I can do this all in the controller. def create @card = Card.new(params[:card]) @card.SpecNum = @card.SpecNum ++ ... end Or. I can be blatantly retarded. And maybe the best bet is to add an auto-incremement table to mysql. The problem is the number has to start at a specific number, 1020. Any ideas?

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• How to handle request/response propagation up and down a widget hierarchy in a GUI app?

  - by fig-gnuton

  Given a GUI application where widgets can be composed of other widgets: If the user triggers an event resulting in a lower level widget needing data from a model, what's the cleanest way to be able to send that request to a controller (or the datastore itself)? And subsequently get the response back to that widget? Presumably one wouldn't want the controller or datastore to be a singleton directly available to all levels of widgets, or is this an acceptable use of singleton? Or should a top level controller be injected as a dependency through a widget hierarchy, as far down as the lowest level widget that might need that controller? Or a different approach entirely?

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• How should nested components interact with model in a GUI application?

  - by fig-gnuton

  Broad design/architecture question. If you have nested components in a GUI, what's the most common way for those components to interact with data? For example, let's say a component receives a click on one of its buttons to save data. Should the save request be delegated up that component's ancestors, with the uppermost ancestor ultimately passing the request to a controller? Or are models/datastores in a GUI application typically singletons, so that a component at any level of a hierarchy can directly get/set data? Or is a controller injected as a dependency down the hierarchy of components, so that any given component is only one intermediary away from the datastore/model?

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• c# class design - what can I use instead of "static abstract"?

  - by Ryan

  I want to do the following public abstract class MyAbstractClass { public static abstract int MagicId { get; } public static void DoSomeMagic() { // Need to get the MagicId value defined in the concrete implementation } } public class MyConcreteClass : MyAbstractClass { public static override int MagicId { get { return 123; } } } However I can't because you can't have static abstract members. I understand why I can't do this - any recommendations for a design that will achieve much the same result? (For clarity - what I am trying to do is provide a library with an abstract base class but the concrete versions MUST implement a few properties/methods themselves and yes, there are good reasons for keeping it static.)

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• writing structs and classes to disk

  - by Phenom

  The following function writes a struct to a file. int btwrite(short rrn, BTPAGE *page_ptr) { long addr; addr = (long) rrn * (long) PAGESIZE + HEADERSIZE; lseek(btfd, addr, 0); return (write(btfd, page_ptr, PAGESIZE)); } The following is the struct. typedef struct { short keycount; /* number of keys in page */ int key[MAXKEYS]; /* the actual keys */ int value[MAXKEYS]; /* the actual values */ short child[MAXKEYS+1]; /* ptrs to rrns of descendants */ } BTPAGE; What would happen if I changed the struct to a class, would it still work the same? If I added class functions, would the size it takes up on disk increase?

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• Writing an AI for a turn-based board game

  - by Cyril

  Hi, i'm currently programming a board game (8x8) in which I need to develop an AI. I have read a lot of articles about AI in board games, minmax with or without alphabeta pruning, but I don't really know how to implements this, I don't know where to start... About my game, this is a turn-based game, each player has pieces on the board, they have to pick one and choose in moving this piece (1 or 2 cells max) or clone the piece (1 cell max). At the moment, I have a really stupid AI which choose a random piece then choose a random move to play... Could you please give me some clues, on how to implement this functionality ? Best regards

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• Many-to-many relationship in oop

  - by Manu

  what is best way to model many-to-many relationship? lets say we have a two classes , Team and Player any given Player can be in multiple Team s any Team can have as many Player s as they like I like to call methods like playerX.getTeamList() to get the list of all the Team s he/she is in teamY.getPlayerList() to get the list of all the Player s in the team (or have some other way to do this effectively) I can think of two ways of doing this , but they just don't feels like good oop pattens. can you think of any good ways , perhaps a design patten ?

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• Session vs singleton pattern

  - by chobo

  Hi, I have a web application where I would like to pull user settings from a database and store them for Global access. Would it make more sense to store the data in a Singleton, or a Session object? What's the difference between the two? Is it better to store the data as an object reference or break it up into value type objects (ints and strings)? Thanks!

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• How to implement the disposable pattern in a class that inherits from another disposable class?

  - by TheRHCP

  Hi, I often used the disposable pattern in simple classes that referenced small amount of resources, but I never had to implement this pattern on a class that inherits from another disposable class and I am starting to be a bit confused in how to free the whole resources. I start with a little sample code: public class Tracer : IDisposable { bool disposed; FileStream fileStream; public Tracer() { //Some fileStream initialization } public void Dispose() { this.Dispose(true); GC.SuppressFinalize(this); } protected virtual void Dispose(bool disposing) { if (!disposed) { if (disposing) { if (fileStream != null) { fileStream.Dispose(); } } disposed = true; } } } public class ServiceWrapper : Tracer { bool disposed; ServiceHost serviceHost; //Some properties public ServiceWrapper () { //Some serviceHost initialization } //protected override void Dispose(bool disposing) //{ // if (!disposed) // { // if (disposing) // { // if (serviceHost != null) // { // serviceHost.Close(); // } // } // disposed = true; // } //} } My real question is: how to implement the disposable pattern inside my ServiceWrapper class to be sure that when I will dispose an instance of it, it will dispose resources in both inherited and base class? Thanks.

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• Writing a script with the cmd prompt.

  - by Josh

  I'm trying to make a script that will place a list (in a .csv file) of processes that are running that take up more than 10 mb of RAM and shows the time + date the script was run. My teacher did this during his lecture but I can't remember how he did it. Just trying to figure out how to be better at IT. So my question is, can anyone help me with this? I don't even know where to start.

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• Writing a script for reading many .csv files with similar filenames

  - by wahalulu

  I have several .csv files with similar filenames except a numeric month (i.e. 03_data.csv, 04_data.csv, 05_data.csv, etc.) that I'd like to read into R. I have two questions: Is there a function in R similar to MATLAB's varname and assignin that will let me create/declare a variable name within a function or loop that will allow me to read the respective .csv file - i.e. 03_data.csv into 03_data data.frame, etc.? I want to write a quick loop to do this because the filenames are similar. As an alternative, is it better to create one dataframe with the first file and then append the rest using a for loop? How would I do that?

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• Why use sinlgeton instead of static class?

  - by Jouke van der Maas

  When would a singleton actually be easier or better than a static class? It seems to me creating a singleton is just extra effort that's not actually needed, but I'm sure there is a good reason. Otherwise, they wouldn't be used, obviously.

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• benefit of having a factory for object creation?

  - by ajsie

  I'm trying to understand the factory design pattern. I don't understand why it's good to have a middleman between the client and the product (object that the client wants). example with no factory: $mac = new Mac(); example with a factory: $appleStore = new AppleStore(); $mac = $appleStore->getProduct('mac'); How does the factory pattern decouple the client from the product? Could someone give an example of a future code change that will impact on example 1 negative, but positive in example 2 so I understand the importance of decoupling? Thanks.

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• Help needed in writing regular expression using TCL

  - by user330727

  Hello Everyone, Just seeking a favour to write a regular expression to match the following set of strings. I want to write an expression which matches all the following strings TCL ( XYZ XZZ XVZ XWZ ) Clue : Starting string is X and Z ending string is same for all the pairs. Only the middle string is differs Y Z V W. My trial: [regexp {^X([Y|Z|V|W]*)Z$}

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• Help needed in writing regular expression -- TCL

  - by user330727

  Hello Everyone, Just seeking a favour to write a regular expression to match the following set of strings. I want to write an expression which matches all the following strings TCL i) ( XYZ XZZ XVZ XWZ ) Clue : Starting string is X and Z ending string is same for all the pairs. Only the middle string is differs Y Z V W. My trial: [regexp {^X([Y|Z|V|W]*)Z$}] I want to write an another regexp which catches/matches only the following string wherever comes ii) (XYZ) My trial: [regexp {^X([Y]*)Z$}]

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• correct technical term for this pattern

  - by Oliver A.

  sometimes I use a pattern which is very similar to the singleton pattern: There is one default instance which and a static get method to aces it. But you may create other instances and pass it as optional parameter and if you want to and you can even replace the default instance with a instance from a child class. So it is NO SINGLETON at all but it is used like one singleton in most cases. Anyone got an idea who to call something like this ? Maybe half*** singleton? domiton?

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• Avoiding sub-type selection in view code

  - by John Donoghue

  Hi, I have some code where the model contains some classes like (vb.net pseudocode, but could be any OO language): Enum AttributeType Boolean Date String End Enum MustInherit Class Attibute Must Override Function Type As AttributeType End Class Class BooleanAttribute: Attribute Function Type As AttributeType Return AttributeType.Boolean End Function End Class And the view contains some code like: Select Case AttributeType Case Boolean //Display checkbox control Case Date //Display date picker control Case String //Display textbox control End Select I don't really like the code in the view, for the hopefully obvious reasons (what happens when I get a new attribute type etc). My question is, how should I replace it? I could easily add a method to the concrete classes, but that pollutes the model with UI stuff so that's a horrible idea. I could move the select into a factory, but that seems to be just hiding the problem. Can anybody advise a better approach?

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