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  • A Quantity class with units

    - by Ryan Ohs
    Goals Create a class that associates a numeric quantity with a unit of measurement. Provide support for simple arithmetic and comparison operations. Implementation An immutable class (Could have been struct but I may try inheritance later) Unit is stored in an enumeration Supported operations: Addition w/ like units Subtraction w/ like units Multiplication by scalar Division by scalar Modulus by scalar Equals() >, >=, <, <=, == IComparable ToString() Implicit cast to Decimal The Source The souce can be downloaded from Github. Notes This class does not support any arithmetic that would modify the unit. This class is not suitable for manipulating currencies. Future Ideas Have a CompositeQuantity class that would allow quantities with unlike units to be combined. Similar currency class with support for allocations/distributions. Provide conversion between units. (Actually I think this would be best placed in an external service. Many situations I deal with require some sort of dynamic conversion ratio.)

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  • Objective-C Lesson in Class Design

    - by Pota Onasys
    I have the following classes: Teacher Student Class (like a school class) They all extend from KObject that has the following code: - initWithKey - send - processKey Teacher, Student Class all use the functions processKey and initWithKey from KObject parent class. They implement their own version of send. The problem I have is that KObject should not be instantiated ever. It is more like an abstract class, but there is no abstract class concept in objective-c. It is only useful for allowing subclasses to have access to one property and two functions. What can I do so that KObject cannot be instantiated but still allow subclasses to have access to the functions and properties of KObject?

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  • How do I decide to which class a method should belong

    - by Eleeist
    I have TopicBusiness.class and PostBusiness.class. I have no problem with deciding into which class methods such as addPostToDatabase() or getAllPostsFromDatabase() should go. But what about getAllPostsFromTopic(TopicEntity topic) or getNumberOfPostsInTopic(TopicEntity topic)? Should the parameter be the deciding factor? So when the method takes TopicEntity as parameter it should belong to TopicBusiness.class? I am quite puzzled by this. EDIT: Some more info as requested. TopicBusiness.class and PostBusiness.class are classes holding all the business logic of the application concerning topics and posts respectively - that is fetching the data from database and/or performing some operations on them. TopicEntity is data (in this case representing single topic) fetched from database. getAllPostFromTopic(TopicEntity topic) gets all posts from database that belong to particular topic, while getNumberOfPostsInTopic(TopicEntity topic) performs database query and returns the number of posts that topic passed as parameter consists of.

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  • Architecting persistence (and other internal systems). Interfaces, composition, pure inheritance or centralization?

    - by Vandell
    Suppose that you need to implement persistence, I think that you're generally limited to four options (correct me if I'm wrong, please) Each persistant class: Should implement an interface (IPersistent) Contains a 'persist-me' object that is a specialized object (or class) that's made only to be used the class that contains it. Inherit from Persistent (a base class) Or you can create a gigantic class (or package) called Database and make your persistence logic there. What are the advantages and problems that can come from each of one? In a small (5kloc) and algorithmically (or organisationally) simple app what is probably the best option?

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  • What is the better design decision approach?

    - by palm snow
    I have two classes (MyFoo1 and MyFoo2) that share some common functionality. So far it does not seem like I need any polymorphic inheritence but at this point I am considering the following options: Have the common functionality in a utility class. Both of these classes call these methods from that utility class. Have an abstract class and implement common methods in that abstract class. Then derive MyFoo1 and MyFoo2 from that abstract class. Any suggestion on what would be a better design decision?

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  • Am I trying to Implement Multiple Inheritance. How can I do this.

    - by Shantanu Gupta
    I have created a class say A which has some functions defined as protected. Now Class B inherits A and class C inherits B. Class A has private default constructor and protected parameterized constructor. I want Class B to be able to access all the protected functions defined in Class A but class C can have access on some of the functions only not all the functions and class C is inheriting class B. How can I restrict access to some of the functions of Class A from Class C ? EDIT: namespace Db { public Class A { private A(){} protected A(string con){assign this value} protected DataTable getTable(){return Table;} protected Sqlparameters setParameters(){return parameter;} } } namespace Data { public Class B:A { protected B():base("constring"){} protected DataTable output(){return getTable();} protected sqlparameter values(param IDataParameter[] parameter){} } } namespace Bsns { public Class C:B { protected C():base(){} protected DataTable show() {return values(setparameter());} } } EDIT I think what I am trying to do here is Multiple inheritance. Please check. Class A { //suppose 10 functions are declared } Class B:A { //5 functions declared which are using A's function in internal body } Class C:B { //using all functions of B but require only 4 functions of A to be accessible by C. }

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  • jQuery works in FF but not in Safari

    - by Hristo
    I have some event handlers that work in FF and not in Safari. Simply put, I have a list of friends, some hard-coded, some pulled in from a database. Clicking on a buddy opens a chat window... this is much like the Facebook chat system. So in Firefox, everything works normally and as expected. In Safari, clicking on buddies that are hard-coded works fine, but clicking on buddies that are pulled in from the database doesn't pull up the chat window. <script type="text/javascript" src="js/jQuery.js"></script> <script type="text/javascript" src="js/chat.js"></script> <script type="text/javascript" src="js/ChatBar.js"></script> <script type="text/javascript" src="js/settings.js"></script> <script type="text/javascript"> var chat = new Chat(); var from = <?php echo "'" .$_SESSION['userid'] . "'"; ?>; chat.getUsers(<?php echo "'" .$_SESSION['userid'] . "'"; ?>); </script> So I load all my buddies with chat.getUsers. That function is: // get list of friends function getBuddyList(userName) { userNameID = userName; $.ajax({ type: "GET", url: "buddyList.php", data: { 'userName': userName, 'current': numOfUsers }, dataType: "json", cache: false, success: function(data) { if (numOfUsers != data.numOfUsers) { numOfUsers = data.numOfUsers; var list = "<li><span>Agents</span></li>"; for (var i = 0; i < data.friendlist.length; i++) { list += "<li><a class=\"buddy\" href=\"#\"><img alt=\"\" src=\"images/chat-thumb.gif\">"+ data.friendlist[i] +"</a></li>"; } $('#friend-list ul').append($(list)); } setTimeout('getBuddyList(userNameID)', 1000); } }); } buddyList.php just pulls in the Users from the database and returns an array with the user names. So the jQuery for clicking a buddy is: // click on buddy in #friends-panel $('#friends-panel a.buddy').click(function() { alert("Loaded"); // close #friends-panel $('.subpanel').hide(); $('#friends-panel a.chat').removeClass('active'); // if a chat window is already active, close it and deactivate $('#mainpanel li[class="active-buddy-tab"] div').not('#chat-box').removeAttr('id'); $('#mainpanel li[class="active-buddy-tab"]').removeClass('active-buddy-tab').addClass('buddy-tab'); // create active buddy chat window $('#mainpanel').append('<li class="active-buddy-tab"><a class="buddy-tab" href="#"></a><div id="chat-window"><h3><p id="to"></p></h3></div></li>'); // create name and close/minimize buttons $('.active-buddy-tab div h3 p#to').text($(this).text()); $('.active-buddy-tab div h3').append('<span class="close"> X </span><span class="minimize"> &ndash; </span>'); $('.active-buddy-tab').append('<span class="close"> X </span>'); // create chat area $('.active-buddy-tab div').append('<div id="chat-box"></div><form id="chat-message"><textarea id="message" maxlength="100"></textarea></form>'); // put curser in chat window $('.active-buddy-tab #message').focus(); // create a chat relationship return false; }); ... and the basic structure of the HTML is: <div id="footpanel"> <ul id="mainpanel"> <li id="friends-panel"> <a href="#" class="chat">Friends (<strong>18</strong>) </a> <div id="friend-list" class="subpanel"> <h3><span> &ndash; </span>Friends Online</h3> <ul> <li><span>Family Members</span></li> <!-- Hard coded buddies --> <li><a href="#" class="buddy"><img src="images/chat-thumb.gif" alt="" /> Your Friend 1</a></li> <li><a href="#" class="buddy"><img src="images/chat-thumb.gif" alt="" /> Your Friend </a></li> <!-- buddies will be added in dynamically here --> </ul> </div> </li> </ul> </div> I'm not too sure where to begin solving this issue. I thought it might be a rendering bug or something with the DOM but I've been staring at this code all day and I'm stuck. Any ideas on why it works in FF and not in Safari? btw... I'm testing on Snow Leopard. Thanks, Hristo

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  • Class<T> and static method Class.forName() drive me crazy.

    - by matt
    Hi, this code doesn't compile. I'm wondering what I am doing wrong: private static Importable getRightInstance(String s) throws Exception { Class<Importable> c = Class.forName(s); Importable i = c.newInstance(); return i; } where Importable is an interface and the string s is the name of an implementing class. The compiler says: ./Importer.java:33: incompatible types found : java.lang.Class<capture#964 of ?> required: java.lang.Class<Importable> Class<Importable> c = Class.forName(format(s)); thanks for any help! All the solutions Class<? extends Importable> c = Class.forName(s).asSubclass(Importable.class); and Class<? extends Importable> c = (Class<? extends Importable>) Class.forName(s); and Class<?> c = Class.forName(format(s)); Importable i = (Importable)c.newInstance(); give this error: Exception in thread "main" java.lang.IncompatibleClassChangeError: class C1 has interface Importable as super class where C1 is effectively a class implementing Importable, one of those i want to cast to Importable.

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  • Which of these design patterns is superior?

    - by durron597
    I find I tend to design class structures where several subclasses have nearly identical functionality, but one piece of it is different. So I write nearly all the code in the abstract class, and then create several subclasses to do the one different thing. Does this pattern have a name? Is this the best way for this sort of scenario? Option 1: public interface TaxCalc { String calcTaxes(); } public abstract class AbstractTaxCalc implements TaxCalc { // most constructors and fields are here public double calcTaxes(UserFinancials data) { // code double diffNumber = getNumber(data); // more code } abstract protected double getNumber(UserFinancials data); protected double initialTaxes(double grossIncome) { // code return initialNumber; } } public class SimpleTaxCalc extends AbstractCalc { protected double getNumber(UserFinancials data) { double temp = intialCalc(data.getGrossIncome()); // do other stuff return temp; } } public class FancyTaxCalc extends AbstractTaxCalc { protected double getNumber(UserFinancials data) { int temp = initialCalc(data.getGrossIncome()); // Do fancier math return temp; } } Option 2: This version is more like the Strategy pattern, and should be able to do essentially the same sorts of tasks. public class TaxCalcImpl implements TaxCalc { private final TaxMath worker; public DummyImpl(TaxMath worker) { this.worker = worker; } public double calcTaxes(UserFinancials data) { // code double analyzedDouble = initialNumber; int diffNumber = worker.getNumber(data, initialNumber); // more code } protected int initialTaxes(double grossIncome) { // code return initialNumber; } } public interface TaxMath { double getNumber(UserFinancials data, double initial); } Then I could do: TaxCalc dum = new TaxCalcImpl(new TaxMath() { @Override public double getNumber(UserFinancials data, double initial) { double temp = data.getGrossIncome(); // do math return temp; }); And I could make specific implementations of TaxMath for things I use a lot, or I could make a stateless singleton for certain kinds of workers I use a lot. So the question I'm asking is: Which of these patterns is superior, when, and why? Or, alternately, is there an even better third option?

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  • C#&ndash;Using a delegate to raise an event from one class to another

    - by Bill Osuch
    Even though this may be a relatively common task for many people, I’ve had to show it to enough new developers that I figured I’d immortalize it… MSDN says “Events enable a class or object to notify other classes or objects when something of interest occurs. The class that sends (or raises) the event is called the publisher and the classes that receive (or handle) the event are called subscribers.” Any time you add a button to a Windows Form or Web app, you can subscribe to the OnClick event, and you can also create your own event handlers to pass events between classes. Here I’ll show you how to raise an event from a separate class to a console application (or Windows Form). First, create a console app project (you could create a Windows Form, but this is easier for this demo). Add a class file called MyEvent.cs (it doesn’t really need to be a separate file, this is just for clarity) with the following code: public delegate void MyHandler1(object sender, MyEvent e); public class MyEvent : EventArgs {     public string message; } Your event can have whatever public properties you like; here we’re just got a single string. Next, add a class file called WorkerDLL.cs; this will simulate the class that would be doing all the work in the project. Add the following code: class WorkerDLL {     public event MyHandler1 Event1;     public WorkerDLL()     {     }     public void DoWork()     {         FireEvent("From Worker: Step 1");         FireEvent("From Worker: Step 5");         FireEvent("From Worker: Step 10");     }     private void FireEvent(string message)     {         MyEvent e1 = new MyEvent();         e1.message = message;         if (Event1 != null)         {             Event1(this, e1);         }         e1 = null;     } } Notice that the FireEvent method creates an instance of the MyEvent class and passes it to the Event1 handler (which we’ll create in just a second). Finally, add the following code to Program.cs: static void Main(string[] args) {     Program p = new Program(args); } public Program(string[] args) {     Console.WriteLine("From Console: Creating DLL");     WorkerDLL wd = new WorkerDLL();     Console.WriteLine("From Console: Wiring up event handler");     WireEventHandlers(wd);     Console.WriteLine("From Console: Doing the work");     wd.DoWork();     Console.WriteLine("From Console: Done - press any key to finish.");     Console.ReadLine(); } private void WireEventHandlers(WorkerDLL wd) {     MyHandler1 handler = new MyHandler1(OnHandler1);     wd.Event1 += handler; } public void OnHandler1(object sender, MyEvent e) {     Console.WriteLine(e.message); } The OnHandler1 method is called any time the event handler “hears” an event matching the specified signature – you could have it log to a file, write to a database, etc. Run the app in debug mode and you should see output like this: You can distinctly see which lines were written by the console application itself (Program.cs) and which were written by the worker class (WorkerDLL.cs). Technorati Tags: Csharp

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  • How can I implement an abstract singleton class in Java?

    - by Simon
    Here is my sample abstract singleton class: public abstract class A { protected static A instance; public static A getInstance() { return instance; } //...rest of my abstract methods... } And here is the concrete implementation: public class B extends A { private B() { } static { instance = new B(); } //...implementations of my abstract methods... } Unfortunately I can't get the static code in class B to execute, so the instance variable never gets set. I have tried this: Class c = B.class; A.getInstance() - returns null; and this ClassLoader.getSystemClassLoader().loadClass("B"); A.getInstance() - return null; Running both these in the eclipse debugger the static code never gets executed. The only way I could find to get the static code executed is to change the accessibility on B's constructor to public, and to call it. I'm using sun-java6-jre on Ubuntu 32bit to run these tests.

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  • How to get table cells evenly spaced?

    - by DaveDev
    I'm trying to create a page with a number of static html tables on them. What do I need to do to get them to display each column the same size as each other column in the table? The HTML is as follows: <span class="Emphasis">Interest rates</span><br /> <table cellpadding="0px" cellspacing="0px" class="PerformanceTable"> <tr><th class="TableHeader"></th><th class="TableHeader">Current rate as at 31 March 2010</th><th class="TableHeader">31 December 2009</th><th class="TableHeader">31 March 2009</th></tr> <tr class="TableRow"><td>Index1</td><td class="PerformanceCell">1.00%</td><td>1.00%</td><td>1.50%</td></tr> <tr class="TableRow"><td>index2</td><td class="PerformanceCell">0.50%</td><td>0.50%</td><td>0.50%</td></tr> <tr class="TableRow"><td>index3</td><td class="PerformanceCell">0.25%</td><td>0.25%</td><td>0.25%</td></tr> </table> <span>Source: Bt</span><br /><br /> <span class="Emphasis">Stock markets</span><br /> <table cellpadding="0px" cellspacing="0px" class="PerformanceTable"> <tr><th class="TableHeader"></th><th class="TableHeader">As at 31 March 2010</th><th class="TableHeader">1 month change</th><th class="TableHeader">QTD change</th><th class="TableHeader">12 months change</th></tr> <tr class="TableRow"><td>index1</td><td class="PerformanceCell">1169.43</td><td class="PerformanceCell">5.88%</td><td class="PerformanceCell">4.87%</td><td class="PerformanceCell">46.57%</td></tr> <tr class="TableRow"><td>index2</td><td class="PerformanceCell">1958.34</td><td class="PerformanceCell">7.68%</td><td class="PerformanceCell">5.27%</td><td class="PerformanceCell">58.31%</td></tr> <tr class="TableRow"><td>index3</td><td class="PerformanceCell">5679.64</td><td class="PerformanceCell">6.07%</td><td class="PerformanceCell">4.93%</td><td class="PerformanceCell">44.66%</td></tr> <tr class="TableRow"><td>index4</td><td class="PerformanceCell">2943.92</td><td class="PerformanceCell">8.30%</td><td class="PerformanceCell">-0.98%</td><td class="PerformanceCell">44.52%</td></tr> <tr class="TableRow"><td>index5</td><td class="PerformanceCell">978.81</td><td class="PerformanceCell">9.47%</td><td class="PerformanceCell">7.85%</td><td class="PerformanceCell">26.52%</td></tr> <tr class="TableRow"><td>index6</td><td class="PerformanceCell">3177.77</td><td class="PerformanceCell">10.58%</td><td class="PerformanceCell">6.82%</td><td class="PerformanceCell">44.84%</td></tr> </table> <span>Source: B</span><br /><br /> I'm also open to suggestion on how to tidy this up, if there are any? :-)

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  • How do i use findAll method with a class in Doctrine?

    - by terrani
    Hi, I am a little bit confused with Doctrine class. I created a class and its base class in Doctrine. The class's name is User. so..I created an object of class User. $oUser = new User(); when I try to use findAll method, it does not work. I found the following code on the doctrine documentation. Doctrine_Core::getTable('User')-findAll(); I don't understand why i need to call getTable to use findAll method when I have User class. Am I missing something?

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  • Passing an instantiated class to concrete class derived by Castle Windsor

    - by Tr1stan
    I have a system that I'm using to test some new architecture. I have the following setup (In MVC2 .Net - C Sharp): View < Controller < Service < Repository < DB I'm using Castle Windsor as my DI (IoC) controller, and this is working just fine in both the Service and Repo layers. However, I'm now at a point where I would like to pass an Entity Framework (DatabaseNameEntity) to the constructor to the Service, and then to the Repo, so that I have something similar to a Unit of Work pattern per request (This feels like what I'm trying to achieve anyway) - and I'm having trouble working out how this can be done using Castle Windsor. Am I going off on a silly tangent? Any pointers appreciated.

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  • ZipArchive php Class - Is it built-in to PHP?

    - by SoLoGHoST
    Ok, just wondering on the versions of PHP that this class is built into. And if they are built into all platforms (OS's). I'm wanting an approach to search through a zip file and place files using file_put_contents in different filepaths within the webroot. In any case, I'm familiar with how to do this with the ZipArchive class, but I'm wondering if using this class would be a good solution and support MOST, if not ALL servers?? I mean, I'd rather not use a method that requires the Server to have it installed. I'm looking for a solution to this that will support at least MOST servers without having to install the class... Thanks :) Also, I'd like to support opening tar.gz and/or .tgz files if possible, but I don't think the ZipArchive class supports this, but perhaps a different built-in php class does??

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  • ZipArchive php Class, would this be the best approach??

    - by SoLoGHoST
    Ok, just wondering on the versions of PHP that this class is built into. And if they are built into all platforms (OS's). I'm wanting an approach to search through a zip file and place files using file_put_contents in different filepaths within the webroot. In any case, I'm familiar with how to do this with the ZipArchive class, but I'm wondering if using this class would be a good solution and support MOST, if not ALL servers?? I mean, I'd rather not use a method that requires the Server to have it installed. I'm looking for a solution to this that will support at least MOST servers without having to install the class... Thanks :) Also, I'd like to support opening tar.gz and/or .tgz files if possible, but I don't think the ZipArchive class supports this, but perhaps a different built-in php class does??

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  • Java class Class<T> and static method Class.forName() drive me crazy.

    - by matt
    Hi, this code doesn't compile. i'm wandering what i am doing wrong: private static Importable getRightInstance(String s) throws Exception { Class<Importable> c = Class.forName(s); Importable i = c.newInstance(); return i; } where Importable is an interface and the string s is the name of an implementing class. The compiler says: ./Importer.java:33: incompatible types found : java.lang.Class<capture#964 of ?> required: java.lang.Class<Importable> Class<Importable> c = Class.forName(format(s)); thanks for any help!

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  • how to specify which child class this object belong to after retrieving from a hashmap?

    - by chandra wibowo
    hi everyone, i have a parent class called Course, and two child class PostgradCourse and UndergradCourse. i have a hashmap HashMap courses; i store all the postgradCourse and undergradCourse objects in the hashmap. i want to retrieve an undergradCourse object from the hashmap using the key. Course course = courses.get(courseCode); then i want to call a method in the UndergradCourse class, setUnits() method course.setUnits(); but the compiler say cannot find symbol- method setUnit() im pretty sure the problem is the compiler is looking for a method setUnit() in the Course class instead of UndergradCourse class i did this but its not working UndergradCourse course = courses.get(courseCode); results in incompatible type so how can i retrieve undergradCourse object from the hashmap as an undergradCourse object instead of course object? so then i can call a method inside the child class thanks in advance

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  • Why isn't this simple test class's method inherited in Ruby?

    - by Kevin Bannister
    Consider this very simple logging class: class MockLog def self.log_stub_method(*args) args.each do |a| define_method "#{a}" do |msg| t = Time.now.strftime("%Y-%m-%d %H:%M:%S") $stderr.puts "[#{a.upcase}] \u00bb [#{t}] #{msg}" end end end log_stub_method :fatal, :error, :warn, :info, :debug end Let's add logging to all our classes: class Module def has_logging() class_eval { @log = MockLog.new def log self.class.instance_variable_get :@log end } end end Now, why doesn't this work? class Foo has_logging end Foo.new.log.nil? # => false, as expected class Bar < Foo end Bar.new.log.nil? # => true?! Why wasn't the `log` method inherited?

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  • c# 3.5 class List<int> class initialisation

    - by josephj1989
    I can initialize a List like new List{1,2,3,4,5}; However List does not have a constructor which accepts a single parameter. So I tried to run this through the debugger and it seems to be calling the Add method. So how does the compiler know which method to invoke to add each individual element. This may be a silly question but I am a bit confused. Thanks

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  • I want to be able to derive from a class internally, but disallow class in other assemblies to derive from the class

    - by Rokke
    Hej I have the following setup: Assembly 1 public abstract class XX<T> : XX where T: YY { } public abstract class XX {} Assembly 2 public class ZZ : YY {} public class ZZFriend : XX<ZZ> {} I use this feature in reflection when in YY: public class YY { public Type FindFriend { return GetType().Assembly.GetTypes().FirstOrDefault( t => t.BaseType != null && t.BaseType.IsGenericType && typeof(XX).IsAssignableFrom(t) && t.BaseType.GetGenericArguments().FirstOrDefault() == GetType()); } } I would like do disallow inheritance of the non generic class XX like: public class ZZFriend: XX {} Alternatively, I need a method like (that can be used in the reflection in YY.FindFrind()): public Type F(Type t) { return GetTypeThatIsGeneric(XX, Type genericTypeParameter); } That can be used in YY as: Typeof(XX<ZZ) == F(typeof(GetType()) Hope that makes sense... Thanks in advance Søren Rokkedal

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  • any way to simplify this with a form of dynamic class instantiation?

    - by gnychis
    I have several child classes that extend a parent class, forced to have a uniform constructor. I have a queue which keeps a list of these classes, which must extend MergeHeuristic. The code that I currently have looks like the following: Class<? extends MergeHeuristic> heuristicRequest = _heuristicQueue.pop(); MergeHeuristic heuristic = null; if(heuristicRequest == AdjacentMACs.class) heuristic = new AdjacentMACs(_parent); if(heuristicRequest == SimilarInterfaceNames.class) heuristic = new SimilarInterfaceNames(_parent); if(heuristicRequest == SameMAC.class) heuristic = new SameMAC(_parent); Is there any way to simplify that to dynamically instantiate the class, something along the lines of: heuristic = new heuristicRequest.somethingSpecial(); That would flatten that block of if statements.

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  • A Reusable Builder Class for Ruby Testing

    - by Liam McLennan
    My last post was about a class for building test data objects in C#. This post describes the same tool, but implemented in Ruby. The C# version was written first but I originally came up with the solution in my head using Ruby, and then I translated it to C#. The Ruby version was easier to write and is easier to use thanks to Ruby’s dynamic nature making generics unnecessary.  Here are my example domain classes: class Person attr_accessor :name, :age def initialize(name, age) @name = name @age = age end end class Property attr_accessor :street, :manager def initialize(street, manager) @street = street @manager = manager end end and the test class showing what the builder does: class Test_Builder < Test::Unit::TestCase def setup @build = Builder.new @build.configure({ Property => lambda { Property.new '127 Creek St', @build.a(Person) }, Person => lambda { Person.new 'Liam', 26 } }) end def test_create assert_not_nil @build end def test_can_get_a_person @person = @build.a(Person) assert_not_nil @person assert_equal 'Liam', @person.name assert_equal 26, @person.age end def test_can_get_a_modified_person @person = @build.a Person do |person| person.age = 999 end assert_not_nil @person assert_equal 'Liam', @person.name assert_equal 999, @person.age end def test_can_get_a_different_type_that_depends_on_a_type_that_has_not_been_configured_yet @my_place = @build.a(Property) assert_not_nil @my_place assert_equal '127 Creek St', @my_place.street assert_equal @build.a(Person).name, @my_place.manager.name end end Finally, the implementation of Builder: class Builder # defaults is a hash of Class => creation lambda def configure defaults @defaults = defaults end def a(klass) temp = @defaults[klass].call() yield temp if block_given? temp end end

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  • Strategies for invoking subclass methods on generic objects

    - by Brad Patton
    I've run into this issue in a number of places and have solved it a bunch of different ways but looking for other solutions or opinions on how to address. The scenario is when you have a collection of objects all based off of the same superclass but you want to perform certain actions based only on instances of some of the subclasses. One contrived example of this might be an HTML document made up of elements. You could have a superclass named HTMLELement and subclasses of Headings, Paragraphs, Images, Comments, etc. To invoke a common action across all of the objects you declare a virtual method in the superclass and specific implementations in all of the subclasses. So to render the document you could loop all of the different objects in the document and call a common Render() method on each instance. It's the case where again using the same generic objects in the collection I want to perform different actions for instances of specific subclass (or set of subclasses). For example (an remember this is just an example) when iterating over the collection, elements with external links need to be downloaded (e.g. JS, CSS, images) and some might require additional parsing (JS, CSS). What's the best way to handle those special cases. Some of the strategies I've used or seen used include: Virtual methods in the base class. So in the base class you have a virtual LoadExternalContent() method that does nothing and then override it in the specific subclasses that need to implement it. The benefit being that in the calling code there is no object testing you send the same message to each object and let most of them ignore it. Two downsides that I can think of. First it can make the base class very cluttered with methods that have nothing to do with most of the hierarchy. Second it assumes all of the work can be done in the called method and doesn't handle the case where there might be additional context specific actions in the calling code (i.e. you want to do something in the UI and not the model). Have methods on the class to uniquely identify the objects. This could include methods like ClassName() which return a string with the class name or other return values like enums or booleans (IsImage()). The benefit is that the calling code can use if or switch statements to filter objects to perform class specific actions. The downside is that for every new class you need to implement these methods and can look cluttered. Also performance could be less than some of the other options. Use language features to identify objects. This includes reflection and language operators to identify the objects. For example in C# there is the is operator that returns true if the instance matches the specified class. The benefit is no additional code to implement in your object hierarchy. The only downside seems to be the lack of using something like a switch statement and the fact that your calling code is a little more cluttered. Are there other strategies I am missing? Thoughts on best approaches?

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