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  • designing solution to dynamically load class

    - by dot
    Background Information I have a web app that allows end users to connect to ssh-enabled devices and manipulate them. Right now, i only support one version of firmware. The logic is something like this: user clicks on a button to run some command on device. web application looks up the class name containing the correct ssh interface for the device, using the device's model name. (because the number of hardware models is so small, i have a list that's hardcoded in my web app) web app creates a new ssh object using the class loaded in step 2. ssh command is run and session closed. command results displayed on web page. This all works fine. Now the end user wants me to be able to support multiple versions of firmware. But the catch is, they don't want to have to document the firmware version anywhere becuase the amount of overhead this will create in maintaining the system database. In other words, I can't look up the firmware version based on the device. The good news is that it sounds like at most, I'll have to support two different versions of firmware per device. One option is to name the the classes like this: deviceX.1.php deviceX.2.php deviceY.1.php deviceY.2.php where "X" and "Y" represent the model names, and 1 and 2 represent the firmware versions. When a user runs a command, I will first try it with one of the class files, if it fails, i can try with the second. I think always try the newer version of firmware first... so let's say in the above example, I would load deviceX.2.php before deviceX.1.php. This will work, but it's not very efficient. But I can't think of another way around this. Any suggestions?

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  • VB.NET looping through XML to store in singleton

    - by rockinthesixstring
    I'm having a problem with looping through an XML file and storing the value in a singleton My XML looks like this <values> <value></value> <value>$1</value> <value>$5,000</value> <value>$10,000</value> <value>$15,000</value> <value>$25,000</value> <value>$50,000</value> <value>$75,000</value> <value>$100,000</value> <value>$250,000</value> <value>$500,000</value> <value>$750,000</value> <value>$1,000,000</value> <value>$1,250,000</value> <value>$1,500,000</value> <value>$1,750,000</value> <value>$2,000,000</value> <value>$2,500,000</value> <value>$3,000,000</value> <value>$4,000,000</value> <value>$5,000,000</value> <value>$7,500,000</value> <value>$10,000,000</value> <value>$15,000,000</value> <value>$25,000,000</value> <value>$50,000,000</value> <value>$100,000,000</value> <value>$100,000,000+</value> </values> And my function looks like this Public Class LoadValues Private Shared SearchValuesInstance As List(Of SearchValues) = Nothing Public Shared ReadOnly Property LoadSearchValues As List(Of SearchValues) Get Dim sv As New List(Of SearchValues) If SearchValuesInstance Is Nothing Then Dim objDoc As XmlDocument = New XmlDataDocument Dim objRdr As XmlTextReader = New XmlTextReader(HttpContext.Current.Server.MapPath("~/App_Data/Search-Values.xml")) objRdr.Read() objDoc.Load(objRdr) Dim root As XmlElement = objDoc.DocumentElement Dim itemNodes As XmlNodeList = root.SelectNodes("/values") For Each n As XmlNode In itemNodes sv.Add(New SearchValues(n("@value").InnerText, n("@value").InnerText)) Next SearchValuesInstance = sv Else : sv = SearchValuesInstance End If Return sv End Get End Property End Class My problem is that I'm getting an object not set to an instance of an object on the sv.Add(New SearchValues(n("@value").InnerText, n("@value").InnerText)) line.

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  • How to find rows between other rows w/ID then add class

    - by Ravex
    Hi guys. i'm stuck with my table. need create toggle rows function. but i no idea how to find sub rows in table. Some one can help me? I have table with many rows 500 All Rows have class="row-1,row-2.....row-600 etc" And all main rows also have class="parent" between each "parent" rows i have 6 rows So i need for toggle/collapse purposes find all (sub)rows betwen parent rows. and add class with id like in prevous parent row. For example: parent have class="row-1 parent" all sub must have - class="child-row-1" default table <table id="table"> <tr class="row-1 odd parent"> <th class="column-1">st. 3 - 5</th> <th class="column-2">Profile</th> <th class="column-3">Purpose</th> </tr> <tr class="row-2 even"> <td class="column-1">Metal Stamp</td> <td class="column-2">Width</td> <td class="column-3">Price</td> </tr> <tr class="row-3 odd"> <td class="column-1">Circle 3 - 5</td> <td class="column-2">28-110</td> <td class="column-3">21500</td> </tr> <tr class="row-4 even"> <td class="column-1">Circle 3 - 5</td> <td class="column-2">115-180</td> <td class="column-3">20700</td> </tr> <tr class="row-5 odd"> <td class="column-1">Cube 3 - 5</td> <td class="column-2">63-80</td> <td class="column-3">21500</td> </tr> <tr class="row-6 even"> <td class="column-1">Cube 3 - 5</td> <td class="column-2">100-220</td> <td class="column-3">20700</td> </tr> <tr class="row-7 odd"> <td class="column-1">Line 3 - 5</td> <td class="column-2">10-50 ? 40-200</td> <td class="column-3">27000</td> </tr> </table> in the end it should look like this: <table id="table"> <tr class="row-1 odd parent"> <th class="column-1">st. 3 - 5</th> <th class="column-2">Profile</th> <th class="column-3">Purpose</th> </tr> <tr class="row-2 even child-row-1"> <td class="column-1">Metal Stamp</td> <td class="column-2">Width</td> <td class="column-3">Price</td> </tr> <tr class="row-3 odd child-row-1"> <td class="column-1">Circle 3 - 5</td> <td class="column-2">28-110</td> <td class="column-3">21500</td> </tr> <tr class="row-4 even child-row-1"> <td class="column-1">Circle 3 - 5</td> <td class="column-2">115-180</td> <td class="column-3">20700</td> </tr> <tr class="row-5 odd child-row-1"> <td class="column-1">Cube 3 - 5</td> <td class="column-2">63-80</td> <td class="column-3">21500</td> </tr> <tr class="row-6 even child-row-1"> <td class="column-1">Cube 3 - 5</td> <td class="column-2">100-220</td> <td class="column-3">20700</td> </tr> <tr class="row-7 odd child-row-1"> <td class="column-1">Line 3 - 5</td> <td class="column-2">10-50 ? 40-200</td> <td class="column-3">27000</td> </tr> </table>

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  • Should I create protected constructor for my singleton classes?

    - by Vijay Shanker
    By design, in Singleton pattern the constructor should be marked private and provide a creational method retuning the private static member of the same type instance. I have created my singleton classes like this only. public class SingletonPattern {// singleton class private static SingletonPattern pattern = new SingletonPattern(); private SingletonPattern() { } public static SingletonPattern getInstance() { return pattern; } } Now, I have got to extend a singleton class to add new behaviors. But the private constructor is not letting be define the child class. I was thinking to change the default constructor to protected constructor for the singleton base class. What can be problems, if I define my constructors to be protected? Looking for expert views....

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  • Problem with SIngleton Class

    - by zp26
    Hi, I have a prblem with my code. I have a viewController and a singleton class. When i call the method readXml and run a for my program update the UITextView. When i call the clearTextView method the program exit with EXC_BAD_ACCESS. The prblem it's the name of the variable position. This is invalid but i don't change anything between the two methods. You have an idea? My code: #import "PositionIdentifierViewController.h" #import "WriterXML.h" #import "ParserXML.h" #define timeToScan 0.1 @implementation PositionIdentifierViewController @synthesize accelerometer; @synthesize actualPosition; @synthesize actualX; @synthesize actualY; @synthesize actualZ; -(void)updateTextView:(NSString*)nomePosizione { NSString *string = [NSString stringWithFormat:@"%@",nomePosizione]; textEvent.text = [textEvent.text stringByAppendingString:@"\n"]; textEvent.text = [textEvent.text stringByAppendingString:string]; } -(IBAction)clearTextEvent{ textEvent.text = @""; //with this for my program exit for(int i=0; i<[[sharedController arrayPosition]count]; i++){ NSLog(@"sononelfor"); Position *tempPosition = [[Position alloc]init]; tempPosition = [[sharedController arrayPosition]objectAtIndex:i]; [self updateTextView:(NSString*)[tempPosition name]]; } } -(void)readXml{ if([sharedController readXml]){ UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Caricamento Posizioni" message:@"Caricamento effettuato con successo" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:nil]; [alert show]; [alert release]; NSString *string = [NSString stringWithFormat:@"%d", [[sharedController arrayPosition]count]]; [self updateTextView:(NSString*)string]; //with only this the program is ok for(int i=0; i<[[sharedController arrayPosition]count]; i++){ NSLog(@"sononelfor"); Position *tempPosition = [[Position alloc]init]; tempPosition = [[sharedController arrayPosition]objectAtIndex:i]; [self updateTextView:(NSString*)[tempPosition name]]; } } else{ UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Caricamento Posizioni" message:@"Caricamento non riuscito" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:nil]; [alert show]; [alert release]; } } // Implement viewDidLoad to do additional setup after loading the view, typically from a nib. - (void)viewDidLoad { [super viewDidLoad]; sharedController = [SingletonController sharedSingletonController]; actualPosition = [[Position alloc]init]; self.accelerometer = [UIAccelerometer sharedAccelerometer]; self.accelerometer.updateInterval = timeToScan; self.accelerometer.delegate = self; actualX=0; actualY=0; actualZ=0; [self readXml]; } - (void)dealloc { [super dealloc]; [actualPosition dealloc]; [super dealloc]; } @end #import "SingletonController.h" #import "Position.h" #import "WriterXML.h" #import "ParserXML.h" #define standardSensibility 2 #define timeToScan .1 @implementation SingletonController @synthesize arrayPosition; @synthesize arrayMovement; @synthesize actualPosition; @synthesize actualMove; @synthesize stopThread; +(SingletonController*)sharedSingletonController{ static SingletonController *sharedSingletonController; @synchronized(self) { if(!sharedSingletonController){ sharedSingletonController = [[SingletonController alloc]init]; } } return sharedSingletonController; } -(BOOL)readXml{ ParserXML *newParser = [[ParserXML alloc]init]; if([newParser startParsing:(NSString*)@"filePosizioni.xml"]){ [arrayPosition addObjectsFromArray:[newParser arrayPosition]]; return TRUE; } else return FALSE; } -(id)init{ self = [super init]; if (self != nil) { arrayPosition = [[NSMutableArray alloc]init]; arrayMovement = [[NSMutableArray alloc]init]; actualPosition = [[Position alloc]init]; actualMove = [[Movement alloc]init]; stopThread = FALSE; } return self; } -(void) dealloc { [super dealloc]; } @end

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  • Is it appropriate for a class to only be a collection of information with no logic?

    - by qegal
    Say I have a class Person that has instance variables age, weight, and height, and another class Fruit that has instance variables sugarContent and texture. The Person class has no methods save setters and getters, while the Fruit class has both setters and getters and logic methods like calculateSweetness. Is the Fruit class the type of class that is better practice than the Person class. What I mean by this is that the Person class seems like it doesn't have much purpose; it exists solely to organize data, while the Fruit class organizes data and actually contains methods for logic.

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  • Verification of UML Class Diagram

    - by Jean Carlos Suárez Marranzini
    This is my UML Class Diagram made in Astah Community, for a tennis scoreboard game. Here's a link to the image (I don't have enough rep to post images): http://i47.tinypic.com/2lsxx90.png Points are calculated based on moves. Moves can be either points (for the player's advantage) or errors (for the opponent's advantage). The Time Machine allows you to travel to previous game states (expressed as scoreboards). The storage component should be able to store matches independently of the serialization format. The serializers and deserializers should be able to do their job regardless of where the storage lies. The GameEngine should be able to apply the rules of the game regardless of the particularities of the game (hence, dependency injection through the Settings class). The outcomes of games, sets and matches should be deducible based on the points and the rules to apply (the logic implementations are there to provide the rules). Could you please verify my design and tell me if there's anything wrong with it? Thanks in advance.

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  • Class as first-class object

    - by mrpyo
    Could a class be a first-class object? If yes, how would the implementation look? I mean, how could syntax for dynamically creating new classes look like? EDIT: I mean what example syntax could look like (I'm sorry, English is not my native language), but still I believe this question makes sense - how you give this functionality while keeping language consistent. For example how you create reference for new type. Do you make reference first-class object too and then use something like this: Reference<newType> r = new Reference<newType>(); r.set(value); Well this could get messy so you may just force user to use Object type references for dynamically created classes, but then you loose type-checking. I think creating concise syntax for this is interesting problem which solving could lead to better language design, maybe language which is metalanguage for itself (I wonder if this is possible).

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  • Is this a correct implementation of singleton C++?

    - by Kamal
    class A{ static boost::shared_ptr<A> getInstance(){ if(pA==NULL){ pA = new A(); } return boost::shared_ptr(pA); } //destructor ~A(){ delete pA; pA=NULL; } private: A(){ //some initialization code } //private assigment and copy constructors A(A const& copy); // Not Implemented A& operator=(A const& copy); // Not Implemented static A* pA; }; A* A::pA = NULL;

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  • C++ class initialisation containing class variable initialization

    - by Phil Hannent
    I noticed some code of a colleague today that initialized class variables in the initialization. However it was causing a warning, he says because of the order they are in. My question is why is it better to do variable initialization where it currently is and not within the curly brackets? DiagramScene::DiagramScene( int slideNo, QRectF screenRect, MainWindow* parent ) : QGraphicsScene( screenRect, parent ), myParent( parent ), slideUndoImageCurrentIndex(-1), nextGroupID(0), m_undoInProgress(false), m_deleteItemOnNextUndo(0) line(0), path(0) { /* Setup default brush for background */ scDetail->bgBrush.setStyle(Qt::SolidPattern); scDetail->bgBrush.setColor(Qt::white); setBackgroundBrush(scDetail->bgBrush); }

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  • Check if a class is subclass of another class in Java

    - by craesh
    Hi! I'm playing around with Java's reflection API and trying to handle some fields. Now I'm stuck with identifying the type of my fields. Strings are easy, just do myField.getType().equals(String.class). The same applies for other non-derived classes. But how do I check derived classes? E.g. LinkedList as subclass of List. I can't find any isSubclassOf(...) or extends(...) method. Do I need to walk through all getSuperClass() and find my supeclass by my own? Thanks! craesh

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  • Static member class - declare class private and class member package-private?

    - by Helper Method
    Consider you have the following class public class OuterClass { ... private static class InnerClass { int foo; int bar; } } I think I've read somewhere (but not the official Java Tutorial) that if I would declare the static member classes attributes private, the compiler had to generate some sort of accessor methods so that the outer class can actually access the static member class's (which is effectively a package-private top level class) attributes. Any ideas on that?

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  • VS2012 - How to manually convert .NET Class Library to a Portable Class Library

    - by Igor Milovanovic
    The portable libraries are the  response to the growing profile fragmentation in .NET frameworks. With help of portable libraries you can share code between different runtimes without dreadful #ifdef PLATFORM statements or even worse “Add as Link” source file sharing practices. If you have an existing .net class library which you would like to reference from a different runtime (e.g. you have a .NET Framework 4.5 library which you would like to reference from a Windows Store project), you can either create a new portable class library and move the classes there or edit the existing .csproj file and change the XML directly. The following example shows how to convert a .NET Framework 4.5 library to a Portable Class Library. First Unload the Project and change the following settings in the .csproj file: <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" /> to: <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable \$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" /> and add the following keys to the first property group in order to get visual studio to show the framework picker dialog: <ProjectTypeGuids>{786C830F-07A1-408B-BD7F-6EE04809D6DB}; {FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>   After that you can select the frameworks in the Library Tab of the Portable Library:   As last step, delete any framework references from the library as you have them already referenced via the .NET Portable Subset.     [1] Cross-Platform Development with the .NET Framework - http://msdn.microsoft.com/en-us/library/gg597391.aspx [2] Framework Profiles in .NET: http://nitoprograms.blogspot.de/2012/05/framework-profiles-in-net.html

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  • jQuery how to add class to a parent div which has a 3rd level child div with a specific class name.

    - by Vikram
    Hello friends I have an issue adding a special class to a couple of my divs. My layout is like this. <div class="container"> <div class="grid-6 push-3 equal" style="height: 999px;"> <div class="block"> <div id="mainbody"> <!-- Body content here --> </div> </div> </div> <div class="grid-2 equal" style="height: 999px;"> <div class="block"> <div id="sidebar-a"> <!-- Sidebar-a content here --> </div> </div> </div> <div class="grid-2 equal" style="height: 999px;"> <div class="block"> <div id="sidebar-b"> <!-- Sidebar-b content here --> </div> </div> </div> <div class="grid-2 equal" style="height: 999px;"> <div class="block"> <div id="sidebar-b"> <!-- Sidebar-c content here --> </div> </div> </div> </div> I want to add a different background color to each of my sidebars via CSS and when I code like: #mainbody { background : #fff; } #sidebar-a { background : #eee; } #sidebar-b { background : #ddd; } #sidebar-c { background : #ccc; } It is applying the background only to that specific class but that specific class is not of equal height. I actually need to apply to this <div class="grid-2 equal" style="height: 999px;"> div. Now the issue is that in this <div class="grid-6 push-3 equal" style="height: 999px;"> and class="grid-2 equal" style="height: 999px;"> the class names grid-6 and grid-2 are generated dynamically by my PHP of 960 Grid System and also the style="height: 999px; is generated by a jQuery script for Equal-Columns. What I want is to add a unique class name like this...... Look for a div with a class of .equal which has a child div with a class of .block and which further has a child div with an ID of sidebar-a. IF TRUE then add a class of .sidebar-a to the maindiv which has a class of .equal So that the result looks like this: <div class="grid-6 equal push-3 mainbody" style="height: 999px;"> <div class="grid-2 equal sidebar-a" style="height: 999px;"> <div class="grid-2 equal sidebar-b" style="height: 999px;"> <div class="grid-2 equal sidebar-c" style="height: 999px;"> Then I'll be able to style it like this: .mainbody { background : #fff; } .sidebar-a { background : #eee; } .sidebar-b { background : #ddd; } .sidebar-c { background : #ccc; } Hence I thought since I am anyway using jQuery in my Template, why not use it to deal with this issue. Please feel free to suggest a better way if you have something else in mind.

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  • What is design principle behind Servlets being Singleton

    - by Sandeep Jindal
    A servlet container "generally" create one instance of a servlet and different threads of the same instance to serve multiple requests. (I know this can be changed using deprecated SingleThreadModel and other features, but this is the usual way). I thought, the simple reason behind this is performance gain, as creating threads is better than creating instances. But it seems this is not the reason. On the other hand, creating instances have little advantage that developers never have to worry about thread safety. I am trying to understand the reason for this decision over the trade-off of thread-safety.

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  • Why is an anonymous inner class containing nothing generated from this code?

    - by Andrew Westberg
    When run through javac on the cmd line Sun JVM 1.6.0_20, this code produces 6 .class files OuterClass.class OuterClass$1.class OuterClass$InnerClass.class OuterClass$InnerClass2.class OuterClass$InnerClass$InnerInnerClass.class OuterClass$PrivateInnerClass.class When run through JDT in eclipse, it produces only 5 classes. OuterClass.class OuterClass$1.class OuterClass$InnerClass.class OuterClass$InnerClass2.class OuterClass$InnerClass$InnerInnerClass.class OuterClass$PrivateInnerClass.class When decompiled, OuterClass$1.class contains nothing. Where is this extra class coming from and why is it created? package com.test; public class OuterClass { public class InnerClass { public class InnerInnerClass { } } public class InnerClass2 { } //this class should not exist in OuterClass after dummifying private class PrivateInnerClass { private String getString() { return "hello PrivateInnerClass"; } } public String getStringFromPrivateInner() { return new PrivateInnerClass().getString(); } }

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  • Cocoa Singleton conventions

    - by MikeyWard
    Cocoa is full of singletons. Is there a logical/conventional difference between when the Cocoa APIs use NSSingletonObject *so = [NSSingletonObject defaultSingleton]; versus NSSingletonObject *so = [NSSingletonObject sharedSingleton]; ? Not a huge thing, but I don't really see why sometimes one is used versus the other.

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  • Class Loading Deadlocks

    - by tomas.nilsson
    Mattis follows up on his previous post with one more expose on Class Loading Deadlocks As I wrote in a previous post, the class loading mechanism in Java is very powerful. There are many advanced techniques you can use, and when used wrongly you can get into all sorts of trouble. But one of the sneakiest deadlocks you can run into when it comes to class loading doesn't require any home made class loaders or anything. All you need is classes depending on each other, and some bad luck. First of all, here are some basic facts about class loading: 1) If a thread needs to use a class that is not yet loaded, it will try to load that class 2) If another thread is already loading the class, the first thread will wait for the other thread to finish the loading 3) During the loading of a class, one thing that happens is that the <clinit method of a class is being run 4) The <clinit method initializes all static fields, and runs any static blocks in the class. Take the following class for example: class Foo { static Bar bar = new Bar(); static { System.out.println("Loading Foo"); } } The first time a thread needs to use the Foo class, the class will be initialized. The <clinit method will run, creating a new Bar object and printing "Loading Foo" But what happens if the Bar object has never been used before either? Well, then we will need to load that class as well, calling the Bar <clinit method as we go. Can you start to see the potential problem here? A hint is in fact #2 above. What if another thread is currently loading class Bar? The thread loading class Foo will have to wait for that thread to finish loading. But what happens if the <clinit method of class Bar tries to initialize a Foo object? That thread will have to wait for the first thread, and there we have the deadlock. Thread one is waiting for thread two to initialize class Bar, thread two is waiting for thread one to initialize class Foo. All that is needed for a class loading deadlock is static cross dependencies between two classes (and a multi threaded environment): class Foo { static Bar b = new Bar(); } class Bar { static Foo f = new Foo(); } If two threads cause these classes to be loaded at exactly the same time, we will have a deadlock. So, how do you avoid this? Well, one way is of course to not have these circular (static) dependencies. On the other hand, it can be very hard to detect these, and sometimes your design may depend on it. What you can do in that case is to make sure that the classes are first loaded single threadedly, for example during an initialization phase of your application. The following program shows this kind of deadlock. To help bad luck on the way, I added a one second sleep in the static block of the classes to trigger the unlucky timing. Notice that if you uncomment the "//Foo f = new Foo();" line in the main method, the class will be loaded single threadedly, and the program will terminate as it should. public class ClassLoadingDeadlock { // Start two threads. The first will instansiate a Foo object, // the second one will instansiate a Bar object. public static void main(String[] arg) { // Uncomment next line to stop the deadlock // Foo f = new Foo(); new Thread(new FooUser()).start(); new Thread(new BarUser()).start(); } } class FooUser implements Runnable { public void run() { System.out.println("FooUser causing class Foo to be loaded"); Foo f = new Foo(); System.out.println("FooUser done"); } } class BarUser implements Runnable { public void run() { System.out.println("BarUser causing class Bar to be loaded"); Bar b = new Bar(); System.out.println("BarUser done"); } } class Foo { static { // We are deadlock prone even without this sleep... // The sleep just makes us more deterministic try { Thread.sleep(1000); } catch(InterruptedException e) {} } static Bar b = new Bar(); } class Bar { static { try { Thread.sleep(1000); } catch(InterruptedException e) {} } static Foo f = new Foo(); }

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  • should I extend or create instance of the class

    - by meWantToLearn
    I have two classes Class A and Class B in Class A, i have three methods that perform the save, delete and select operation based upon the object I pass them. in Class B I perform the logic operations, such as modification to the property of the object before being passed to the methods of Class A, My problem is in Class B, should it extend Class A, and call the methods of class A , by parent::methodName or create instance of class A and then call Class A does not includes any property just methods. class A{ public function save($obj){ //code here } public function delete($obj){ //code here } public function select($obj){ //code here } } //Should I extend class A, and call the method by parent::methodName($obj) or create an instance of class A, call the method $instanceOfA-methodName($obj); class B extends A{ public function checkIfHasSaved($obj){ if($obj->saved == 'Yes'){ parent::save($obj); //**should I call the method like this** $instanceOFA = new A(); //**or create instance of class A and call without extending class A** instanceOFA->save($obj); } //other logic operations here } }

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  • C#: System.Lazy&lt;T&gt; and the Singleton Design Pattern

    - by James Michael Hare
    So we've all coded a Singleton at one time or another.  It's a really simple pattern and can be a slightly more elegant alternative to global variables.  Make no mistake, Singletons can be abused and are often over-used -- but occasionally you find a Singleton is the most elegant solution. For those of you not familiar with a Singleton, the basic Design Pattern is that a Singleton class is one where there is only ever one instance of the class created.  This means that constructors must be private to avoid users creating their own instances, and a static property (or method in languages without properties) is defined that returns a single static instance. 1: public class Singleton 2: { 3: // the single instance is defined in a static field 4: private static readonly Singleton _instance = new Singleton(); 5:  6: // constructor private so users can't instantiate on their own 7: private Singleton() 8: { 9: } 10:  11: // read-only property that returns the static field 12: public static Singleton Instance 13: { 14: get 15: { 16: return _instance; 17: } 18: } 19: } This is the most basic singleton, notice the key features: Static readonly field that contains the one and only instance. Constructor is private so it can only be called by the class itself. Static property that returns the single instance. Looks like it satisfies, right?  There's just one (potential) problem.  C# gives you no guarantee of when the static field _instance will be created.  This is because the C# standard simply states that classes (which are marked in the IL as BeforeFieldInit) can have their static fields initialized any time before the field is accessed.  This means that they may be initialized on first use, they may be initialized at some other time before, you can't be sure when. So what if you want to guarantee your instance is truly lazy.  That is, that it is only created on first call to Instance?  Well, there's a few ways to do this.  First we'll show the old ways, and then talk about how .Net 4.0's new System.Lazy<T> type can help make the lazy-Singleton cleaner. Obviously, we could take on the lazy construction ourselves, but being that our Singleton may be accessed by many different threads, we'd need to lock it down. 1: public class LazySingleton1 2: { 3: // lock for thread-safety laziness 4: private static readonly object _mutex = new object(); 5:  6: // static field to hold single instance 7: private static LazySingleton1 _instance = null; 8:  9: // property that does some locking and then creates on first call 10: public static LazySingleton1 Instance 11: { 12: get 13: { 14: if (_instance == null) 15: { 16: lock (_mutex) 17: { 18: if (_instance == null) 19: { 20: _instance = new LazySingleton1(); 21: } 22: } 23: } 24:  25: return _instance; 26: } 27: } 28:  29: private LazySingleton1() 30: { 31: } 32: } This is a standard double-check algorithm so that you don't lock if the instance has already been created.  However, because it's possible two threads can go through the first if at the same time the first time back in, you need to check again after the lock is acquired to avoid creating two instances. Pretty straightforward, but ugly as all heck.  Well, you could also take advantage of the C# standard's BeforeFieldInit and define your class with a static constructor.  It need not have a body, just the presence of the static constructor will remove the BeforeFieldInit attribute on the class and guarantee that no fields are initialized until the first static field, property, or method is called.   1: public class LazySingleton2 2: { 3: // because of the static constructor, this won't get created until first use 4: private static readonly LazySingleton2 _instance = new LazySingleton2(); 5:  6: // Returns the singleton instance using lazy-instantiation 7: public static LazySingleton2 Instance 8: { 9: get { return _instance; } 10: } 11:  12: // private to prevent direct instantiation 13: private LazySingleton2() 14: { 15: } 16:  17: // removes BeforeFieldInit on class so static fields not 18: // initialized before they are used 19: static LazySingleton2() 20: { 21: } 22: } Now, while this works perfectly, I hate it.  Why?  Because it's relying on a non-obvious trick of the IL to guarantee laziness.  Just looking at this code, you'd have no idea that it's doing what it's doing.  Worse yet, you may decide that the empty static constructor serves no purpose and delete it (which removes your lazy guarantee).  Worse-worse yet, they may alter the rules around BeforeFieldInit in the future which could change this. So, what do I propose instead?  .Net 4.0 adds the System.Lazy type which guarantees thread-safe lazy-construction.  Using System.Lazy<T>, we get: 1: public class LazySingleton3 2: { 3: // static holder for instance, need to use lambda to construct since constructor private 4: private static readonly Lazy<LazySingleton3> _instance 5: = new Lazy<LazySingleton3>(() => new LazySingleton3()); 6:  7: // private to prevent direct instantiation. 8: private LazySingleton3() 9: { 10: } 11:  12: // accessor for instance 13: public static LazySingleton3 Instance 14: { 15: get 16: { 17: return _instance.Value; 18: } 19: } 20: } Note, you need your lambda to call the private constructor as Lazy's default constructor can only call public constructors of the type passed in (which we can't have by definition of a Singleton).  But, because the lambda is defined inside our type, it has access to the private members so it's perfect. Note how the Lazy<T> makes it obvious what you're doing (lazy construction), instead of relying on an IL generation side-effect.  This way, it's more maintainable.  Lazy<T> has many other uses as well, obviously, but I really love how elegant and readable it makes the lazy Singleton.

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  • Changing multiple objects with a new class name using Jquery

    - by liquilife
    I'd like to click on a trigger and show a specific image. There are multiple triggers which would show a specific image related to it within a set. There are 4 sets The challenge for me is toggling the other images to hide only in this 'set' when one of these triggers are clicked, as there can only be one image showing at a time in each set. Here is the HTML I've put together thus far: <!-- Thumbnails which can be clicked on to toggle the larger preview image --> <div class="materials"> <a href="javascript:;" id="shirtgrey"><img src="/grey_shirt.png" height="122" width="122" /></a> <a href="javascript:;" id="shirtred"><img src="red_shirt.png" height="122" width="122" /></a> <a href="javascript:;" id="shirtblue"><img src="hblue_shirt.png" height="122" width="122" /></a> <a href="javascript:;" id="shirtgreen"><img src="green_shirt.png" height="122" width="122" /></a> </div> <div class="collars"> <a href="javascript:;" id="collargrey"><img src="grey_collar.png" height="122" width="122" /></a> <a href="javascript:;" id="collarred"><img src="red_collar.png" height="122" width="122" /></a> <a href="javascript:;" id="collarblue"><img src="blue_collar.png" height="122" width="122" /></a> <a href="javascript:;" id="collargreen"><img src="green_collar.png" height="122" width="122" /></a> </div> <div class="cuffs"> <a href="javascript:;" id="cuffgrey"><img src="grey_cuff.png" height="122" width="122" /></a> <a href="javascript:;" id="cuffred"><img src="red_cuff.png" height="122" width="122" /></a> <a href="javascript:;" id="cuffblue"><img src="blue_cuff.png" height="122" width="122" /></a> <a href="javascript:;" id="cuffgreen"><img src="/green_cuff.png" height="122" width="122" /></a> </div> <div class="pockets"> <a href="javascript:;" id="pocketgrey"><img src="grey_pocket.png" height="122" width="122" /></a> <a href="javascript:;" id="pocketred"><img src=".png" height="122" width="122" /></a> <a href="javascript:;" id="pocketblue"><img src="blue_pocket.png" height="122" width="122" /></a> <a href="javascript:;" id="pocketgreen"><img src="green_pocket.png" height="122" width="122" /></a> </div> <!-- The larger images where one from each set should be viewable at one time, triggered by the thumb clicked above --> <div class="selectionimg"> <div class="selectShirt"> <img src="grey_shirt.png" height="250" width="250" class="selectShirtGrey show" /> <img src="red_shirt.png" height="250" width="250" class="selectShirtRed hide" /> <img src="blue_shirt.png" height="250" width="250" class="selectShirtBlue hide" /> <img src="green_shirt.png" height="250" width="250" class="selectShirtGreen hide" /> </div> <div class="selectCollar"> <img src="grey_collar.png" height="250" width="250" class="selectCollarGrey show" /> <img src="red_collar.png" height="250" width="250" class="selectCollarRed hide" /> <img src="blue_collar.png" height="250" width="250" class="selectCollarBlue hide" /> <img src="green_collar.png" height="250" width="250" class="selectCollarGreen hide" /> </div> <div class="selectCuff"> <img src="grey_cuff.png" height="250" width="250" class="selectCuffGrey show" /> <img src="red_cuff.png" height="250" width="250" class="selectCuffRed hide" /> <img src="blue_cuff.png" height="250" width="250" class="selectCuffBlue hide" /> <img src="green_cuff.png" height="250" width="250" class="selectCuffGreen hide" /> </div> <div class="selectPocket"> <img src="grey_pocket.png" height="250" width="250" class="selectPocketGrey show" /> <img src="hred_pocket.png" height="250" width="250" class="selectPocketRed hide" /> <img src="blue_pocket.png" height="250" width="250" class="selectPocketBlue hide" /> <img src="green_pocket.png" height="250" width="250" class="selectPocketGreen hide" /> </div> </div> How can jQuery be used to change a class of an image to "show" and ensure that all other images in that same div are set to a class of "hide"? First time posting here. I'm very efficient with HTML and CSS and have a basic understanding of jQuery. I'm learning and this just seems a little bit beyond my abilities at the moment. I hope this all makes sense. Thanks for any help.

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