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• singledispatch in class, how to dispatch self type

  - by yanxinyou

  Using python3.4. Here I want use singledispatch to dispatch different type in __mul__ method . The code like this : class Vector(object): ## some code not paste @functools.singledispatch def __mul__(self, other): raise NotImplementedError("can't mul these type") @__mul__.register(int) @__mul__.register(object) # Becasue can't use Vector , I have to use object def _(self, other): result = Vector(len(self)) # start with vector of zeros for j in range(len(self)): result[j] = self[j]*other return result @__mul__.register(Vector) # how can I use the self't type @__mul__.register(object) # def _(self, other): pass # need impl As you can see the code , I want support Vector*Vertor , This has Name error Traceback (most recent call last): File "p_algorithms\vector.py", line 6, in <module> class Vector(object): File "p_algorithms\vector.py", line 84, in Vector @__mul__.register(Vector) # how can I use the self't type NameError: name 'Vector' is not defined The question may be How Can I use class Name a Type in the class's method ? I know c++ have font class statement . How python solve my problem ? And it is strange to see result = Vector(len(self)) where the Vector can be used in method body .

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• Class templating std::set key types

  - by TomFLuff

  I have a class to evaluate set algebra but wish to template it. At the minute it looks a bit like this set.h: template<typename T> class SetEvaluation { public: SetEvaluation<T>(); std::set<T> evaluate(std::string in_expression); } set.cpp template<typename T> std::set<T> SetEvaluation<T>::evaluate(std::string expression) { std::set<T> result; etc etc... } But i'm getting undefined reference errors when compiling. Is it possible to declare the return type as std::set<T> and then pass std::string as the class template param. There are no errors in the class but only when I try to instantiate SetEvaluation<std::string> Can anyone shed light on this problem? thanks

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• AS3 - Can I know if a class implements an interface (or is a subclass of another class) ?

  - by lk

  With this code function someFunction(classParam:Class):Boolean { // how to know if classParam implements some interface? } i.e. Comparing classParam with IEventDispatcher interface: someFunction(EventDispatcher) // returns true someFunction(Object) // returns false I know it can't be done with is operator. But, is there a way to do it? Is there a way to know if a class implements some interface? (or is a subclass of another class?)

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• Access property from include inside a class method in PHP

  - by Jojo

  How do you make a class property available to the other included file inside the same class' method? // file A.php class A { private $var = 1; public function go() { include('another.php'); } } in another file: // this is another.php file // how can I access class A->var? echo $var; // this can't be right Is this possible given the scope. If var is an array then we can use extract but if var is not, we can wrap it in an array. Is there a better way? Thanks!

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• Rails: i have a class method and i want to modify something of the instance

  - by Totty

  Rails: i have a class method and i want to modify something of the instance something like this: class Test < Main template :box def test # here I want to access the template name, that is box end end class Main def initialize end def self.template(name) # here I have to save somehow the template name # remember is not an instance. end end that is similar to the model classes: # in the model has_many :projects How do I do it?

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• Dynamic XAP loading in Task-It - Part 1

  Download Source Code NOTE 1: The source code provided is running against the RC versions of Silverlight 4 and VisualStudio 2010, so you will need to update to those bits to run it. NOTE 2: After downloading the source, be sure to set the .Web project as the StartUp Project, and Default.aspx as the Start Page In my MEF into post, MEF to the rescue in Task-It, I outlined a couple of issues I was facing and explained why I chose MEF (the Managed Extensibility Framework) to solve these issues. Other posts to check out There are a few other resources out there around dynamic XAP loading that you may want to review (by the way, Glenn Block is the main dude when it comes to MEF): Glenn Blocks 3-part series on a dynamically loaded dashboard Glenn and John Papas Silverlight TV video on dynamic xap loading These provide some great info, but didnt exactly cover the scenario I wanted to achieve in Task-Itand that is dynamically loading each of the apps pages the first time the user enters a page. The code In the code I provided for download above, I created a simple solution that shows the technique I used for dynamic XAP loading in Task-It, but without all of the other code that surrounds it. Taking all that other stuff away should make it easier to grasp. Having said that, there is still a fair amount of code involved. I am always looking for ways to make things simpler, and to achieve the desired result with as little code as possible, so if I find a better/simpler way I will blog about it, but for now this technique works for me. When I created this solution I started by creating a new Silverlight Navigation Application called DynamicXAP Loading. I then added the following line to my UriMappings in MainPage.xaml: <uriMapper:UriMapping Uri="/{assemblyName};component/{path}" MappedUri="/{assemblyName};component/{path}"/> In the section of MainPage.xaml that produces the page links in the upper right, I kept the Home link, but added a couple of new ones (page1 and page 2). These are the pages that will be dynamically (lazy) loaded: <StackPanel x:Name="LinksStackPanel" Style="{StaticResource LinksStackPanelStyle}">      <HyperlinkButton Style="{StaticResource LinkStyle}" NavigateUri="/Home" TargetName="ContentFrame" Content="home"/>      <Rectangle Style="{StaticResource DividerStyle}"/>      <HyperlinkButton Style="{StaticResource LinkStyle}" Content="page 1" Command="{Binding NavigateCommand}" CommandParameter="{Binding ModulePage1}"/>      <Rectangle Style="{StaticResource DividerStyle}"/>      <HyperlinkButton Style="{StaticResource LinkStyle}" Content="page 2" Command="{Binding NavigateCommand}" CommandParameter="{Binding ModulePage2}"/>  </StackPanel> In App.xaml.cs I added a bit of MEF code. In Application_Startup I call a method called InitializeContainer, which creates a PackageCatalog (a MEF thing), then I create a CompositionContainer and pass it to the CompositionHost.Initialize method. This is boiler-plate MEF stuff that allows you to do 'composition' and import 'packages'. You're welcome to do a bit more MEF research on what is happening here if you'd like, but for the purpose of this example you can just trust that it works. :-) private void Application_Startup(object sender, StartupEventArgs e) {     InitializeContainer();     this.RootVisual = new MainPage(); }   private static void InitializeContainer() {     var catalog = new PackageCatalog();     catalog.AddPackage(Package.Current);     var container = new CompositionContainer(catalog);     container.ComposeExportedValue(catalog);     CompositionHost.Initialize(container); } Infrastructure In the sample code you'll notice that there is a project in the solution called DynamicXAPLoading.Infrastructure. This is simply a Silverlight Class Library project that I created just to move stuff I considered application 'infrastructure' code into a separate place, rather than cluttering the main Silverlight project (DynamicXapLoading). I did this same thing in Task-It, as the amount of this type of code was starting to clutter up the Silverlight project, and it just seemed to make sense to move things like Enums, Constants and the like off to a separate place. In the DynamicXapLoading.Infrastructure project you'll see 3 classes: Enums - There is only one enum in here called ModuleEnum. We'll use these later. PageMetadata - We will use this class later to add metadata to a new dynamically loaded project. ViewModelBase - This is simply a base class for view models that we will use in this, as well as future samples. As mentioned in my MVVM post, I will be using the MVVM pattern throughout my code for reasons detailed in the post. By the way, the ViewModelExtension class in there allows me to do strongly-typed property changed notification, so rather than OnPropertyChanged("MyProperty"), I can do this.OnPropertyChanged(p => p.MyProperty). It's just a less error-prown approach, because if you don't spell "MyProperty" correctly using the first method, nothing will break, it just won't work. Adding a new page We currently have a couple of pages that are being dynamically (lazy) loaded, but now let's add a third page. 1. First, create a new Silverlight Application project: In this example I call it Page3. In the future you may prefer to use a different name, like DynamicXAPLoading.Page3, or even DynamicXAPLoading.Modules.Page3. It can be whatever you want. In my Task-It application I used the latter approach (with 'Modules' in the name). I do think of these application as 'modules', but Prism uses the same term, so some folks may not like that. Use whichever naming convention you feel is appropriate, but for now Page3 will do. When you change the name to Page3 and click OK, you will be presented with the Add New Project dialog: It is important that you leave the 'Host the Silverlight application in a new or existing Web site in the solution' checked, and the .Web project will be selected in the dropdown below. This will create the .xap file for this project under ClientBin in the .Web project, which is where we want it. 2. Uncheck the 'Add a test page that references the application' checkbox, and leave everything else as is. 3. Once the project is created, you can delete App.xaml and MainPage.xaml. 4. You will need to add references your new project to the following: DynamicXAPLoading.Infrastructure.dll (this is a Project reference) DynamicNavigation.dll (this is in the Libs directory under the DynamicXAPLoading project) System.ComponentModel.Composition.dll System.ComponentModel.Composition.Initialization.dll System.Windows.Controls.Navigation.dll If you have installed the latest RC bits you will find the last 3 dll's under the .NET tab in the Add Referenced dialog. They live in the following location, or if you are on a 64-bit machine like me, it will be Program Files (x86).       C:\Program Files\Microsoft SDKs\Silverlight\v4.0\Libraries\Client Now let's create some UI for our new project. 5. First, create a new Silverlight User Control called Page3.dyn.xaml 6. Paste the following code into the xaml: <dyn:DynamicPageShim xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:dyn="clr-namespace:DynamicNavigation;assembly=DynamicNavigation"     xmlns:my="clr-namespace:Page3;assembly=Page3">     <my:Page3Host /> </dyn:DynamicPageShim> This is just a 'shim', part of David Poll's technique for dynamic loading. 7. Expand the icon next to Page3.dyn.xaml and delete the code-behind file (Page3.dyn.xaml.cs). 8. Next we will create a control that will 'host' our page. Create another Silverlight User Control called Page3Host.xaml and paste in the following XAML: <dyn:DynamicPage x:Class="Page3.Page3Host"     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"     xmlns:dyn="clr-namespace:DynamicNavigation;assembly=DynamicNavigation"     xmlns:Views="clr-namespace:Page3.Views"      mc:Ignorable="d"     d:DesignHeight="300" d:DesignWidth="400"     Title="Page 3">       <Views:Page3/>   </dyn:DynamicPage> 9. Now paste the following code into the code-behind for this control: using DynamicXAPLoading.Infrastructure;   namespace Page3 {     [PageMetadata(NavigateUri = "/Page3;component/Page3.dyn.xaml", Module = Enums.Page3)]     public partial class Page3Host     {         public Page3Host()         {             InitializeComponent();         }     } } Notice that we are now using that PageMetadata custom attribute class that we created in the Infrastructure project, and setting its two properties. NavigateUri - This tells it that the assembly is called Page3 (with a slash beforehand), and the page we want to load is Page3.dyn.xaml...our 'shim'. That line we added to the UriMapper in MainPage.xaml will use this information to load the page. Module - This goes back to that ModuleEnum class in our Infrastructure project. However, setting the Module to ModuleEnum.Page3 will cause a compilation error, so... 10. Go back to that Enums.cs under the Infrastructure project and add a 3rd entry for Page3: public enum ModuleEnum {     Page1,     Page2,     Page3 } 11. Now right-click on the Page3 project and add a folder called Views. 12. Right-click on the Views folder and create a new Silverlight User Control called Page3.xaml. We won't bother creating a view model for this User Control as I did in the Page 1 and Page 2 projects, just for the sake of simplicity. Feel free to add one if you'd like though, and copy the code from one of those other projects. Right now those view models aren't really doing anything anyway...though they will in my next post. :-) 13. Now let's replace the xaml for Page3.xaml with the following: <dyn:DynamicPage x:Class="Page3.Views.Page3"     xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"     xmlns:dyn="clr-namespace:DynamicNavigation;assembly=DynamicNavigation"     mc:Ignorable="d"     d:DesignHeight="300" d:DesignWidth="400"     Style="{StaticResource PageStyle}">       <Grid x:Name="LayoutRoot">         <ScrollViewer x:Name="PageScrollViewer" Style="{StaticResource PageScrollViewerStyle}">             <StackPanel x:Name="ContentStackPanel">                 <TextBlock x:Name="HeaderText" Style="{StaticResource HeaderTextStyle}" Text="Page 3"/>                 <TextBlock x:Name="ContentText" Style="{StaticResource ContentTextStyle}" Text="Page 3 content"/>             </StackPanel>         </ScrollViewer>     </Grid>   </dyn:DynamicPage> 14. And in the code-behind remove the inheritance from UserControl, so it should look like this: namespace Page3.Views {     public partial class Page3     {         public Page3()         {             InitializeComponent();         }     } } One thing you may have noticed is that the base class for the last two User Controls we created is DynamicPage. Once again, we are using the infrastructure that David Poll created. 15. OK, a few last things. We need a link on our main page so that we can access our new page. In MainPage.xaml let's update our links to look like this: <StackPanel x:Name="LinksStackPanel" Style="{StaticResource LinksStackPanelStyle}">     <HyperlinkButton Style="{StaticResource LinkStyle}" NavigateUri="/Home" TargetName="ContentFrame" Content="home"/>     <Rectangle Style="{StaticResource DividerStyle}"/>     <HyperlinkButton Style="{StaticResource LinkStyle}" Content="page 1" Command="{Binding NavigateCommand}" CommandParameter="{Binding ModulePage1}"/>     <Rectangle Style="{StaticResource DividerStyle}"/>     <HyperlinkButton Style="{StaticResource LinkStyle}" Content="page 2" Command="{Binding NavigateCommand}" CommandParameter="{Binding ModulePage2}"/>     <Rectangle Style="{StaticResource DividerStyle}"/>     <HyperlinkButton Style="{StaticResource LinkStyle}" Content="page 3" Command="{Binding NavigateCommand}" CommandParameter="{Binding ModulePage3}"/> </StackPanel> 16. Next, we need to add the following at the bottom of MainPageViewModel in the ViewModels directory of our DynamicXAPLoading project: public ModuleEnum ModulePage3 {     get { return ModuleEnum.Page3; } } 17. And at last, we need to add a case for our new page to the switch statement in MainPageViewModel: switch (module) {     case ModuleEnum.Page1:         DownloadPackage("Page1.xap");         break;     case ModuleEnum.Page2:         DownloadPackage("Page2.xap");         break;     case ModuleEnum.Page3:         DownloadPackage("Page3.xap");         break;     default:         break; } Now fire up the application and click the page 1, page 2 and page 3 links. What you'll notice is that there is a 2-second delay the first time you hit each page. That is because I added the following line to the Navigate method in MainPageViewModel: Thread.Sleep(2000); // Simulate a 2 second initial loading delay The reason I put this in there is that I wanted to simulate a delay the first time the page loads (as the .xap is being downloaded from the server). You'll notice that after the first hit to the page though that there is no delay...that's because the .xap has already been downloaded. Feel free to comment out this 2-second delay, or remove it if you'd like. I just wanted to show how subsequent hits to the page would be quicker than the initial one. By the way, you may want to display some sort of BusyIndicator while the .xap is loading. I have that in my Task-It appplication, but for the sake of simplicity I did not include it here. In the future I'll blog about how I show and hide the BusyIndicator using events (I'm currently using the eventing framework in Prism for that, but may move to the one in the MVVM Light Toolkit some time soon). Whew, that felt like a lot of steps, but it does work quite nicely. As I mentioned earlier, I'll try to find ways to simplify the code (I'd like to get away from having things like hard-coded .xap file names) and will blog about it in the future if I find a better way. In my next post, I'll talk more about what is actually happening with the code that makes this all work.Did you know that DotNetSlackers also publishes .net articles written by top known .net Authors? We already have over 80 articles in several categories including Silverlight. Take a look: here.

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• Can You Have "Empty" Abstract/Classes?

  - by ShrimpCrackers

  Of course you can, I'm just wondering if it's rational to design in such a way. I'm making a breakout clone and was doing some class design. I wanted to use inheritance, even though I don't have to, to apply what I've learned in C++. I was thinking about class design and came up with something like this: GameObject - base class (consists of data members like x and y offsets, and a vector of SDL_Surface* MovableObject : GameObject - abstract class + derived class of GameObject (one method void move() = 0; ) NonMovableObject : GameObject - empty class...no methods or data members other than constructor and destructor(at least for now?). Later I was planning to derive a class from NonMovableObject, like Tileset : NonMovableObject. I was just wondering if "empty" abstract classes or just empty classes are often used...I notice that the way I'm doing this, I'm just creating the class NonMovableObject just for sake of categorization. I know I'm overthinking things just to make a breakout clone, but my focus is less on the game and more on using inheritance and designing some sort of game framework.

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• How can I access bitmaps created in another activity?

  - by user22241

  I am currently loading my game bitmaps when the user presses 'start' in my animated splash screen activity (the first / launch activity) and the app progresses from my this activity to the main game activity, This is causing choppy animation in the splashscreen while it loads/creates the bitmaps for the new activity. I've been told that I should load all my bitmaps in one go at the very beginning. However, I can't work out how to do this - could anyone please point me in the right direction? I have 2 activities, a splash screen and the main game. Each consist of a class that extends activity and a class that extends SurfaceView (with an inner class for the rendering / logic updating). So, for example at the moment I am creating my bitmaps in the constructor of my SurfaceView class like so: public class OptionsScreen extends SurfaceView implements SurfaceHolder.Callback { //Create variables here public OptionsScreen(Context context) { Create bitmaps here } public void intialise(){ //This method is called from onCreate() of corresponding application context // Create scaled bitmaps here (from bitmaps previously created) }

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• Is there a name for a mini class that is not a struct?

  - by user1555300

  I have a couple of mini-classes that are not nested classes. They need to be passed around between different larger classes to use. In a way, they act like Tuples, storing fields in them. For example, [Serializable] public class TransformObject { public GameObject GameObj; public tk2dCameraAnchor Anchor; public ManagerTransform MTransform; } I have a few of them for my game I have been developing. They have to be classes, not struct because the Unity3d editor will not show in the inspector if so. I was just wondering if there is a official name for these kind of mini classes.

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• Create inherited class from base class

  - by Raj

  public class Car { private string make; private string model; public Car(string make, string model) { this.make = make; this.model = model; } public virtual void Display() { Console.WriteLine("Make: {0}", make); Console.WriteLine("Model: {0}", model); } public string Make { get{return make;} set{make = value;} } public string Model { get{return model;} set{model = value;} } } public class SuperCar:Car { private Car car; private int horsePower; public SuperCar(Car car) { this.car = car; } public int HorsePower { get{return horsePower;} set{horsepower = value;} } public override void Display() { base.Display(); Console.WriteLine("I am a super car"); } When I do something like Car myCar = new Car("Porsche", "911"); SuperCar mySupcar = new SuperCar(myCar); mySupcar.Display(); I only get "I am a supercar" but not the properties of my base class. Should I explicitly assign the properties of my base class in the SuperCar constructor? In fact I'm trying Decorator pattern where I want a class to add behaviour to a base class.

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• Python: Class factory using user input as class names

  - by Sano98

  Hi everyone, I want to add class atttributes to a superclass dynamically. Furthermore, I want to create classes that inherit from this superclass dynamically, and the name of those subclasses should depend on user input. There is a superclass "Unit", to which I can add attributes at runtime. This already works. def add_attr (cls, name, value): setattr(cls, name, value) class Unit(object): pass class Archer(Unit): pass myArcher = Archer() add_attr(Unit, 'strength', 5) print "Strenght ofmyarcher: " + str(myArcher.strength) Archer.strength = 2 print "Strenght ofmyarcher: " + str(myArcher.strength) This leads to the desired output: Strenght ofmyarcher: 5 Strenght ofmyarcher: 2 But now I don't want to predefine the subclass Archer, but I'd rather let the user decide how to call this subclass. I've tried something like this: class Meta(type, subclassname): def __new__(cls, subclassname, bases, dct): return type.__new__(cls, subclassname, Unit, dct) factory = Meta() factory.__new__("Soldier") but no luck. I guess I haven't really understood what new does here. What I want as a result here is class Soldier(Unit): pass being created by the factory. And if I call the factory with the argument "Knight", I'd like a class Knight, subclass of Unit, to be created. Any ideas? Many thanks in advance! Bye -Sano

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• A dynamic array of class "landmark", inside another single class "landmarks"

  - by pinnacler

  I'm working on a robot localization simulator and I created a class called "landmark". The end result is going to be a robot that is always centered and always faces the top of the screen. As it turns, the birds eye view map will rotate around the robot. To accomplish this, I'm assuming I can rotate one class and have all elements inside rotate as well. So, the landmark class has properties x,y, label, and radius. This is suppose to simulate a tree location in a forest. To test everything, I need "forest data," and I wrote a script to generate 100 trees in a 100m x 100m area. The script automatically generates values within an acceptable range for x,y, radius. The generated data is stored in an object called tempForest and is 100x3. Ideally, I want to create a class called "landmarks" (plural) that has 100 landmark instances inside. How would I instantiate 100 instances of landmark in one instance of landmarks using that randomly generated data? Ideally, I'd just type treeBeacons = landmarks(); and it would randomly populate 100 (user definable, set in config file) instances with x, y, radius data. I'm not sure how to deal with a dynamic array of class "Landmark", inside another single class "landmarks." Any ideas?

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• Why does a sub-class class of a class have to be static in order to initialize the sub-class in the

  - by Alex

  So, the question is more or less as I wrote. I understand that it's probably not clear at all so I'll give an example. I have class Tree and in it there is the class Node, and the empty constructor of Tree is written: public class RBTree { private RBNode head; public RBTree(RBNode head,RBTree leftT,RBTree rightT){ this.head=head; this.head.leftT.head.father = head; this.head.rightT.head.father = head; } public RBTree(RBNode head){ this(head,new RBTree(),new RBTree()); } public RBTree(){ this(new RBNode(),null,null); } public class RBNode{ private int value; private boolean isBlack; private RBNode father; private RBTree leftT; private RBTree rightT; } } Eclipse gives me the error: "No enclosing instance of type RBTree is available due to some intermediate constructor invocation" for the "new RBTree()" in the empty constructor. However, if I change the RBNode to be a static class, there is no problem. So why is it working when the class is static. BTW, I found an easy solution for the cunstructor: public RBTree(){ this.head = new RBNode(); } So, I have no idea what is the problem in the first piece of code.

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• C# class can not disguise to be another class because GetType method cannot be override

  - by zinking

  there is a statement in the CLR via C# saying in C#, one class cannot disguise to be another, because GetType is virutal and thus it cannot be override but I think in C# we can still hide the parent implementation of GetType. I must missed something if I hide the base GetType implementation then I can disguise my class to be another class, is that correct? The key here is not whether GetType is virutal or not, the question is can we disguise one class to be another in C# Following is the NO.4 answer from the possible duplicate, so My question is more on this. is this kind of disguise possible, if so, how can we say that we can prevent class type disguise in C# ? regardless of the GetType is virtual or not While its true that you cannot override the object.GetType() method, you can use "new" to overload it completely, thereby spoofing another known type. This is interesting, however, I haven't figured out how to create an instance of the "Type" object from scratch, so the example below pretends to be another type. public class NotAString { private string m_RealString = string.Empty; public new Type GetType() { return m_RealString.GetType(); } } After creating an instance of this, (new NotAString()).GetType(), will indeed return the type for a string. share|edit|flag answered Mar 15 at 18:39 Dr Snooze 213 By almost anything that looks at GetType has an instance of object, or at the very least some base type that they control or can reason about. If you already have an instance of the most derived type then there is no need to call GetType on it. The point is as long as someone uses GetType on an object they can be sure it's the system's implementation, not any other custom definition. – Servy Mar 15 at 18:54 add comment

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• Java: Comparing a class with another within that class using a my own .equals

  - by user1670252

  I am making a method .equals replacing the equals method used. It accepts a object. I want it to check if that object equals the class that runs the .equals class. I know I want to compare all the private methods I have to that object. Is there a way to do this without making another private class to get the private variables from the object? How do I do this to compare equality not identity? I am stuck on this. Do i have to use == to compare? Also looking online i see others use recursion. If this is the way i have to do it can you show and explain it to me? so an example i have public boolean equals(Object o) { this is in a class we will call bobtheBuilder (first thing to pop in my head) I want to check if the object o is equal to the class he has private object array and a private int. I assume I want to check if the array and int of this class equal the array and int of the object.

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• static member specialization of templated child class and templated base class

  - by b3nj1

  I'm trying to have a templated class (here C) that inherits from another templated class (here A) and perform static member specialization (of int var here), but I cant get the right syntax to do so (if it's possible #include <iostream> template<typename derived> class A { public: static int var; }; //This one works fine class B :public A<B> { public: B() { std::cout << var << std::endl; } }; template<> int A<B>::var = 9; //This one doesn't works template<typename type> class C :public A<C<type> > { public: C() { std::cout << var << std::endl; } }; //template<> template<typename type> int A<C<type> >::a = 10; int main() { B b; C<int> c; return 0; } I put an example that works with a non templated class (here B) and i can get the static member specialization of var, but for C that just doesn't work. Here is what gcc tells me : test.cpp: In constructor ‘C<type>::C()’: test.cpp:29:26: error: ‘var’ was not declared in this scope test.cpp: At global scope: test.cpp:34:18: error: template definition of non-template ‘int A<C<type> >::a’ I'm using gcc version 4.6.3, thanks for any help

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• Outer class jframe is not hiding when calling the inner class jframe

  - by user2909960

  When i am calling the inner jframe, it is called, but outer jframe is not hiding. instead it gets overlapped. so what will be the solution for this. Is there any way to get out of this. As i tried when i am calling the inner class frame, the outer class frame is also called, and it is not hidden. package com.exp.example; import java.awt.Color; import java.awt.Container; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JLabel; import javax.swing.JTextField; @SuppressWarnings("serial") public class A extends JFrame implements ActionListener { JFrame rframe = new JFrame(); JLabel CFirstName; JTextField Cfname; JButton jbsubmit; Container cp; public A() { rframe.setSize(500, 200); rframe.setLocationRelativeTo(null); cp = getContentPane(); cp.setLayout(null); setSize(550, 300); rframe.setTitle("Outer Frame"); cp.setBackground(new Color(140, 180, 180)); CFirstName = new JLabel("First Name"); Cfname = new JTextField(10); jbsubmit = new JButton("PREVIEW"); CFirstName.setBounds(10, 20, 100, 35); Cfname.setBounds(150, 20, 150, 25); jbsubmit.setBounds(190, 110, 92, 25); cp.add(CFirstName); cp.add(Cfname); cp.add(jbsubmit); jbsubmit.addActionListener(this); rframe.add(cp); rframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); rframe.setVisible(true); } public void actionPerformed(ActionEvent ae) { String action = ae.getActionCommand(); if (action == "PREVIEW") { /* Write the code here * When we click on preview button the frame of outer class(class A) gets * deactivated(closed) and inner frame, frame of inner class(class B) gets visible. * it should not be overlapped. */ /* My Code */ new B(); rframe.setVisible(false); } } public class B { JFrame frm = new JFrame(); Container cp; public B() { frm.setSize(500, 200); frm.setLocationRelativeTo(null); cp = getContentPane(); cp.setLayout(null); setSize(550, 300); frm.setTitle("Inner Frame"); cp.setBackground(new Color(140, 180, 180)); JLabel cpn = new JLabel("hello"); cpn.setBounds(10, 20, 100, 35); cp.add(cpn); frm.add(cp); frm.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frm.setVisible(true); } } public static void main(String[] args) { new A(); } }

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• Beginning Java (Working with Arrays; Class Assignment)

  - by Jason

  I am to the point where I feel as if I correctly wrote the code for this homework assignment. We were given a skeleton and 2 classes that we had to import (FileIOHelper and Student). /* * Created: *** put the date here *** * * Author: *** put your name here *** * * The program will read information about students and their * scores from a file, and output the name of each student with * all his/her scores and the total score, plus the average score * of the class, and the name and total score of the students with * the highest and lowest total score. */ // import java.util.Scanner; import java.io.*; // C:\Users\Adam\info.txt public class Lab6 { public static void main(String[] args) throws IOException { // Fill in the body according to the following comments Scanner key boardFile = new Scanner(System.in); // Input file name String filename = getFileName(keyboardFile); //Open the file // Input number of students int numStudents = FileIOHelper.getNumberOfStudents(filename); Student students[] = new Student[numStudents]; // Input all student records and create Student array and // integer array for total scores int totalScore[] = new int[students.length]; for (int i = 0; i < students.length; i++){ for(int j = 1; j < 4; j++){ totalScore[i] = totalScore[i] + students[i].getScore(j); } } // Compute total scores and find students with lowest and // highest total score int maxScore = 0; int minScore = 0; for(int i = 0; i < students.length; i++){ if(totalScore[i] >= totalScore[maxScore]){ maxScore = i; } else if(totalScore[i] <= totalScore[minScore]){ minScore = i; } } // Compute average total score int allScores = 0; int average = 0; for (int i = 0; i < totalScore.length; i++){ allScores = allScores + totalScore[i]; } average = allScores / totalScore.length; // Output results outputResults(students, totalScore, maxScore, minScore, average); } // Given a Scanner in, this method prompts the user to enter // a file name, inputs it, and returns it. private static String getFileName(Scanner in) { // Fill in the body System.out.print("Enter the name of a file: "); String filename = in.next(); return filename; // Do not declare the Scanner variable in this method. // You must use the value this method receives in the // argument (in). } // Given the number of students records n to input, this // method creates an array of Student of the appropriate size, // reads n student records using the FileIOHelper, and stores // them in the array, and finally returns the Student array. private static Student[] getStudents(int n) { Student[] myStudents = new Student[n]; for(int i = 0; i <= n; i++){ myStudents[i] = FileIOHelper.getNextStudent(); } return myStudents; } // Given an array of Student records, an array with the total scores, // the indices in the arrays of the students with the highest and // lowest total scores, and the average total score for the class, // this method outputs a table of all the students appropriately // formatted, plus the total number of students, the average score // of the class, and the name and total score of the students with // the highest and lowest total score. private static void outputResults( Student[] students, int[] totalScores, int maxIndex, int minIndex, int average ) { // Fill in the body System.out.println("\nName \t\tScore1 \tScore2 \tScore3 \tTotal"); System.out.println("--------------------------------------------------------"); for(int i = 0; i < students.length; i++){ outputStudent(students[i], totalScores[i], average); System.out.println(); } System.out.println("--------------------------------------------------------"); outputNumberOfStudents(students.length); outputAverage(average); outputMaxStudent(students[maxIndex], totalScores[maxIndex]); outputMinStudent(students[minIndex], totalScores[minIndex]); System.out.println("--------------------------------------------------------"); } // Given a Student record, the total score for the student, // and the average total score for all the students, this method // outputs one line in the result table appropriately formatted. private static void outputStudent(Student s, int total, int avg) { System.out.print(s.getName() + "\t"); for(int i = 1; i < 4; i++){ System.out.print(s.getScore(i) + "\t"); } System.out.print(total + "\t"); if(total < avg){ System.out.print("-"); }else if(total > avg){ System.out.print("+"); }else{ System.out.print("="); } } // Given the number of students, this method outputs a message // stating what the total number of students in the class is. private static void outputNumberOfStudents(int n) { System.out.println("The total number of students in this class is: \t" + n); } // Given the average total score of all students, this method // outputs a message stating what the average total score of // the class is. private static void outputAverage(int average) { System.out.println("The average total score of the class is: \t" + average); } // Given the Student with highest total score and the student's // total score, this method outputs a message stating the name // of the student and the highest score. private static void outputMaxStudent( Student student, int score ) { System.out.println(student.getName() + " got the maximum total score of: \t" + score); } // Given the Student with lowest total score and the student's // total score, this method outputs a message stating the name // of the student and the lowest score. private static void outputMinStudent( Student student, int score ) { System.out.println(student.getName() + " got the minimum total score of: \t" + score); } } But now I get an error at the line totalScore[i] = totalScore[i] + students[i].getScore(j); Exception in thread "main" java.lang.NullPointerException at Lab6.main(Lab6.java:42)

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• Interfaces on an abstract class

  - by insta

  My coworker and I have different opinions on the relationship between base classes and interfaces. I'm of the belief that a class should not implement an interface unless that class can be used when an implementation of the interface is required. In other words, I like to see code like this: interface IFooWorker { void Work(); } abstract class BaseWorker { ... base class behaviors ... public abstract void Work() { } protected string CleanData(string data) { ... } } class DbWorker : BaseWorker, IFooWorker { public void Work() { Repository.AddCleanData(base.CleanData(UI.GetDirtyData())); } } The DbWorker is what gets the IFooWorker interface, because it is an instantiatable implementation of the interface. It completely fulfills the contract. My coworker prefers the nearly identical: interface IFooWorker { void Work(); } abstract class BaseWorker : IFooWorker { ... base class behaviors ... public abstract void Work() { } protected string CleanData(string data) { ... } } class DbWorker : BaseWorker { public void Work() { Repository.AddCleanData(base.CleanData(UI.GetDirtyData())); } } Where the base class gets the interface, and by virtue of this all inheritors of the base class are of that interface as well. This bugs me but I can't come up with concrete reasons why, outside of "the base class cannot stand on its own as an implementation of the interface". What are the pros & cons of his method vs. mine, and why should one be used over another?

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• Java multiple class compositing and boiler plate reduction

  - by h2g2java

  We all know why Java does/should not have multiple inheritance. So this is not questioning about what has already been debated till-cows-come-home. This discusses what we would do when we wish to create a class that has the characteristics of two or more other classes. Probably, most of us would do this to "inherit" from three classes. For simplicity, I left out the constructor.: class Car extends Vehicle { final public Transport transport; final public Machine machine; } So that, Car class directly inherits methods and objects of Vehicle class, but would have to refer to transport and machine explicitly to refer to objects instantiated in Transport and Machine. Car car = new Car(); car.drive(); // from Vehicle car.transport.isAmphibious(); // from Transport car.machine.getCO2Footprint(); // from Machine I thought this was a good idea until when I encounter frameworks that require setter and getter methods. For example, the XML <Car amphibious='false' footPrint='1000' model='Fordstatic999'/> would look for the methods setAmphibious(..), setFootPrint(..) and setModel(..). Therefore, I have to project the methods from Transport and Machine classes class Car extends Vehicle { final public Transport transport; final public Machine machine; public void setAmphibious(boolean b){ this.transport.setAmphibious(b); } public void setFootPrint(String fp){ this.machine.setFootPrint(fp); } } This is OK, if there were just a few characteristics. Right now, I am trying to adapt all of SmartGWT into GWT UIBinder, especially those classes that are not a GWT widget. There are lots of characteristics to project. Wouldn't it be nice if there exists some form of annotation framework that is like this: class Car extends Vehicle @projects {Transport @projects{Machine @projects Guzzler}} { /* No need to explicitly instantiate Transport, Machine or Guzzler */ .... } Where, in case of common names of characteristics exist, the characteristics of Machine would take precedence Guzzler's, and Transport's would have precedence over Machine's, and Vehicle's would have precedence over Transport's. The annotation framework would then instantiate Transport, Machine and Guzzler as hidden members of Car and expand to break-out the protected/public characteristics, in the precedence dictated by the @project annotation sequence, into actual source code or into byte-code. Preferably into byte-code. So that the setFootPrint method is found in both Machine and Guzzler, only that of Machine's would be projected. Questions: Don't you think this is a good idea to have such a framework? Does such a framework already exist? Tell me where/what. Is there an eclipse plugin that does it? Is there a proposal or plan anywhere that you know about such an annotation framework? It would be wonderful too, if the annotation/plugin framework lets me specify that boolean, int, or whatever else needs to be converted from String and does the conversion/parsing for me too. Please advise, somebody. I hope wording of my question was clear enough. Thx. Edited: To avoid OO enthusiasts jumping to conclusion, I have renamed the title of this question.

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• Java: how do I get a class literal from a generic type?

  - by Tom

  Typically, I've seen people use the class literal like this: Class<Foo> cls = Foo.class; But what if the type is generic, e.g. List? This works fine, but has a warning since List should be parameterized: Class<List> cls = List.class So why not add a <?>? Well, this causes a type mismatch error: Class<List<?>> cls = List.class I figured something like this would work, but this is just a plain ol' a syntax error: Class<List<Foo>> cls = List<Foo>.class How can I get a Class<List<Foo>> statically, e.g. using the class literal? I could use @SuppressWarnings("unchecked") to get rid of the warnings caused by the non-parameterized use of List in the first example, Class<List> cls = List.class, but I'd rather not. Any suggestions? Thanks!

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• SQLAuthority News – SQL Server Technical Article – The Data Loading Performance Guide

  - by pinaldave

  The white paper describes load strategies for achieving high-speed data modifications of a Microsoft SQL Server database. “Bulk Load Methods” and “Other Minimally Logged and Metadata Operations” provide an overview of two key and interrelated concepts for high-speed data loading: bulk loading and metadata operations. After this background knowledge, white paper describe how these methods can be [...]

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• How to refactor a Python “god class”?

  - by Zearin

  Problem I’m working on a Python project whose main class is a bit “God Object”. There are so friggin’ many attributes and methods! I want to refactor the class. So Far… For the first step, I want to do something relatively simple; but when I tried the most straightforward approach, it broke some tests and existing examples. Basically, the class has a loooong list of attributes—but I can clearly look over them and think, “These 5 attributes are related…These 8 are also related…and then there’s the rest.” getattr I basically just wanted to group the related attributes into a dict-like helper class. I had a feeling __getattr__ would be ideal for the job. So I moved the attributes to a separate class, and, sure enough, __getattr__ worked its magic perfectly well… At first. But then I tried running one of the examples. The example subclass tries to set one of these attributes directly (at the class level). But since the attribute was no longer “physically located” in the parent class, I got an error saying that the attribute did not exist. @property I then read up about the @property decorator. But then I also read that it creates problems for subclasses that want to do self.x = blah when x is a property of the parent class. Desired Have all client code continue to work using self.whatever, even if the parent’s whatever property is not “physically located” in the class (or instance) itself. Group related attributes into dict-like containers. Reduce the extreme noisiness of the code in the main class. For example, I don’t simply want to change this: larry = 2 curly = 'abcd' moe = self.doh() Into this: larry = something_else('larry') curly = something_else('curly') moe = yet_another_thing.moe() …because that’s still noisy. Although that successfully makes a simply attribute into something that can manage the data, the original had 3 variables and the tweaked version still has 3 variables. However, I would be fine with something like this: stooges = Stooges() And if a lookup for self.larry fails, something would check stooges and see if larry is there. (But it must also work if a subclass tries to do larry = 'blah' at the class level.) Summary Want to replace related groups of attributes in a parent class with a single attribute that stores all the data elsewhere Want to work with existing client code that uses (e.g.) larry = 'blah' at the class level Want to continue to allow subclasses to extend, override, and modify these refactored attributes without knowing anything has changed Is this possible? Or am I barking up the wrong tree?

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• Determining cause of random latency and loading issues

  - by Sherwin Flight

  I'm not sure exactly what details to post in regards to my issue, because I'm not sure what is relevant. Prior to the end of September my websites all loaded quickly, in almost all cases. Loading time wasn't usually more than a few seconds. However, since the end of September I noticed a big increase in page loading times. In some cases pages were taking 30 seconds or more to load. I do have a remote monitoring service monitoring some of the sites as well, and the image below shows the response times over the past month. The response times shown at the beginning of this graph were what the usual response times were prior to this issue occurring. You can see that there has been a significant increase in response times from the beginning to the end of this graph. The thing is, the problem is not happening 100% of the time. If I click through the site, or even just keep refreshing the page, about 25% of the time the pages load quickly, the remaining 75% of the time they load slowly. Sometimes the pages take so long to load that they time out, and don't load at all. I have contacted my hosting provider, and they said things at their end was fine. I don't believe the problem is my home internet provider, because all other websites load without a problem. The server is located in Texas, USA. This also raises another interesting point. My remote monitor checks my site from two locations, California, USA, and London, England. As you can see in the chart below the response time is actually shorter when checked from London, which doesn't seem to make sense, since the server is physically closer to the California monitoring location. I would have expected the London monitoring location to have higher response times since they are physically farther away. I should also point out that in some traceroute test I've done it seem like the first connection to the server seems to take the longest, then after that the rest of the page loads quickly. Below is a little chart showing the times for the first connection to the server. So, what could be causing this problem, and what steps can I take to resolve it or at least narrow down the problem? Sending the request to the server was very quick, and receiving the reply back seems pretty quick, but the WAIT time is really long. So it connects, sends the request, but then waits close to 30 seconds before it starts receiving data back. I am also aware that there are things I can do to speed up page loading times, like reducing the number of CSS and JS files used on a page, compressing images, etc. This is not really what the source of the problem is though, because nothing has really changed on the site since before the problem started, and other sites on the same server are loading slowly as well.

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• Determining cause of random latency/loading issues

  - by Sherwin Flight

  I'm not sure exactly what details to post in regards to my issue, because I'm not sure what is relevant. Prior to the end of September my websites all loaded quickly, in almost all cases. Loading time wasn't usually more than a few seconds. However, since the end of September I noticed a big increase in page loading times. In some cases pages were taking 30 seconds or more to load. I do have a remote monitoring service monitoring some of the sites as well, and the image below shows the response times over the past month. The response times shown at the beginning of this graph were what the usual response times were prior to this issue occurring. You can see that there has been a significant increase in response times from the beginning to the end of this graph. The thing is, the problem is not happening 100% of the time. If I click through the site, or even just keep refreshing the page, about 25% of the time the pages load quickly, the remaining 75% of the time they load slowly. Sometimes the pages take so long to load that they time out, and don't load at all. I have contacted my hosting provider, and they said things at their end was fine. I don't believe the problem is my home internet provider, because all other websites load without a problem. The server is located in Texas, USA. This also raises another interesting point. My remote monitor checks my site from two locations, California, USA, and London, England. As you can see in the chart below the response time is actually shorter when checked from London, which doesn't seem to make sense, since the server is physically closer to the California monitoring location. I would have expected the London monitoring location to have higher response times since they are physically farther away. I should also point out that in some traceroute test I've done it seem like the first connection to the server seems to take the longest, then after that the rest of the page loads quickly. Below is a little chart showing the times for the first connection to the server. So, what could be causing this problem, and what steps can I take to resolve it or at least narrow down the problem? Sending the request to the server was very quick, and receiving the reply back seems pretty quick, but the WAIT time is really long. So it connects, sends the request, but then waits close to 30 seconds before it starts receiving data back. I am also aware that there are things I can do to speed up page loading times, like reducing the number of css/js files used on a page, compressing images, etc. This is not really what the source of the problem is though, because nothing has really changed on the site since before the problem started, and other sites on the same server are loading slowly as well. Any help or advice is much appreciated.

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