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  • Why Java interfaces can't have constructors?

    - by AndrejaKo
    This question showed up on my mid-term exams and I've been searching for correct answer for some time. I know that Java interfaces can't be directly instantiated so they don't need constructors and that they can have only public static final attributes so they don't need constructors to set them up but that's not the expected answer.

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  • Choosing a design pattern for a class that might change it's internal attributes

    - by the_drow
    I have a class that holds arbitrary state and it's defined like this: class AbstractFoo { }; template <class StatePolicy> class Foo : public StatePolicy, public AbstractFoo { }; The state policy contains only protected attributes that represent the state. The state might be the same for multiple behaviors and they can be replaced at runtime. All Foo objects have the same interface to abstract the state itself and to enable storing Foo objects in containers. I would like to find the least verbose and the most maintainable way to express this.

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  • ASP.Net Architecture Specific to Shared/Static functions

    - by Maxim Gershkovich
    Hello All, Could someone please advise in the context of a ASP.Net application is a shared/static function common to all users? If for example you have a function Public shared function GetStockByID(StockID as Guid) as Stock Is that function common to all current users of your application? Or is the shared function only specific to the current user and shared in the context of ONLY that current user? So more specifically my question is this, besides database concurrency issues such as table locking do I need to concern myself with threading issues in shared functions in an ASP.Net application? In my head; let’s say my application namespace is MyTestApplicationNamespace. Everytime a new user connects to my site a new instance of the MyTestApplicationNamespace is created and therefore all shared functions are common to that instance and user but NOT common across multiple users. Is this correct?

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  • Dynamically create and cast objects at runtime

    - by vaibhav bindroo
    Let's say we have 2 classes A and B public class A{ private int member1; A() { member1 = 10; } public getMember(){ return member1; } } Class B is also on the same lines except that its member variable is named member2 and gets intitialized to say 20 inside the constructor. My Requirement : At runtime , I get a string which contains a className ( could be A or B). I want to dynamically create an object of this class along with invoking the constructor. How can I achieve this . I don't want to use interfaces for common functionality of above classes Morever, later on I set the properties of this raw object using Propery Builder Bean Util class based on a list of columns . Class clazz = Class.forName("className"); Obj obj = clazz.newInstance(); How I can dynamically convert that obj to className object.

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  • Google App Engine - Dealing with concurrency issues of storing an object

    - by Spines
    My User object that I want to create and store in the datastore has an email, and a username. How do I make sure when creating my User object that another User object doesn't also have either the same email or the same username? If I just do a query to see if any other users have already used the username or the email, then there could be a race condition. UPDATE: The solution I'm currently considering is to use the MemCache to implement a locking mechanism. I would acquire 2 locks before trying to store the User object in the datastore. First a lock that locks based on email, then another that locks based on username. Since creating new User objects only happens at user registration time, and it's even rarer that two people try to use either the same username or the same email, I think it's okay to take the performance hit of locking. I'm thinking of using the MemCache locking code that is here: http://appengine-cookbook.appspot.com/recipe/mutex-using-memcache-api/ What do you guys think?

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  • populate object graph from database

    - by Rama
    Hi, I would like to know the best way to populate an object that has a collection of child objects and each child object may inturn have a collection of objects, from database without making multiple calls to the database to get child objects for each object. basically in hierarchical format something like for example a customer has orders and each order has order items. is it best to retrieve the data in xml format (SQL server 2005), or retrieve a dataset by joining the related tables together and then map that the data to the object? thanks in advance for your help.

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  • vs2003 : assign datarow object

    - by dotnet-practitioner
    currently I am doing this... object s = new object(); ... s = mydatarow["mycolumn"]; What I would like to do is... DataRow r = null; ... r = mydatarow["mycolumn"]; and I get an error saying that can not covert from object to DataRow.. is there a better way of doing this?

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  • c# - Object reference not set to an instance of an object.

    - by tom
    Line 30: sUsername.Trim(); Line 31: sPassword.Trim(); Line 32: string ConnectionString = WebConfigurationManager.ConnectionStrings["dbnameConnectionString"].ConnectionString; Line 33: SqlConnection myConnection = new SqlConnection(ConnectionString); Line 34: try Object reference not set to an instance of an object. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: System.NullReferenceException: Object reference not set to an instance of an object. Any ideas? I don't understand the error.

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  • How should I declare default values for instance variables in Python?

    - by int3
    Should I give my class members default values like this: class Foo: num = 1 or like this? class Foo: def __init__(self): self.num = 1 In this question I discovered that in both cases, bar = Foo() bar.num += 1 is a well-defined operation. I understand that the first method will give me a class variable while the second one will not. However, if I do not require a class variable, but only need to set a default value for my instance variables, are both methods equally good? Or one of them more 'pythonic' than the other? One thing I've noticed is that in the Django tutorial, they use the second method to declare Models. Personally I think the second method is more elegant, but I'd like to know what the 'standard' way is.

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  • How can I define an empty array in a Perl construtor?

    - by Laimoncijus
    I am just beginner with Perl, so if it sounds stupid - sorry for that :) My problem is - I am trying to write a class, which has an empty array, defined in constructor of a class. So I am doing this like this: package MyClass; use strict; sub new { my ($C) = @_; my $self = { items => () }; bless $self, ref $C || $C; } sub get { return $_[0]->{items}; } 1; Later I am testing my class with simple script: use strict; use Data::Dumper; use MyClass; my $o = MyClass->new(); my @items = $o->get(); print "length = ", scalar(@items), "\n", Dumper(@items); And while running the script I get following: $ perl my_test.pl length = 1 $VAR1 = undef; Why am I doing wrong what causes that I get my items array filled with undef? Maybe someone could show me example how the class would need to be defined so I would not get any default values in my array?

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  • Observer pattern and violation of Single Responsibility Principle

    - by Devil Jin
    I have an applet which repaints itself once the text has changed Design 1: //MyApplet.java public class MyApplet extends Applet implements Listener{ private DynamicText text = null; public void init(){ text = new DynamicText("Welcome"); } public void paint(Graphics g){ g.drawString(text.getText(), 50, 30); } //implement Listener update() method public void update(){ repaint(); } } //DynamicText.java public class DynamicText implements Publisher{ // implements Publisher interface methods //notify listeners whenever text changes } Isn't this a violation of Single Responsibility Principle where my Applet not only acts as Applet but also has to do Listener job. Same way DynamicText class not only generates the dynamic text but updates the registered listeners. Design 2: //MyApplet.java public class MyApplet extends Applet{ private AppletListener appLstnr = null; public void init(){ appLstnr = new AppletListener(this); // applet stuff } } // AppletListener.java public class AppletListener implements Listener{ private Applet applet = null; public AppletListener(Applet applet){ this.applet = applet; } public void update(){ this.applet.repaint(); } } // DynamicText public class DynamicText{ private TextPublisher textPblshr = null; public DynamicText(TextPublisher txtPblshr){ this.textPblshr = txtPblshr; } // call textPblshr.notifyListeners whenever text changes } public class TextPublisher implments Publisher{ // implements publisher interface methods } Q1. Is design 1 a SPR violation? Q2. Is composition a better choice here to remove SPR violation as in design 2.

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  • How to access parent window variables from object

    - by Pickle
    I've got an XHTML 1.1 Strict document that is loading another XHTML 1.1 document in an <object> element (as <iframe> isn't part of the XHTML 1.1 spec). I'm having trouble in IE8 (don't care about 6 or 7) with accessing a javascript variable, Lightbox, in the parent window, from the document loaded in the <object>. In Firefox and everywhere I've seen online, I can just use window.parent.Lightbox. In IE8 however, I get it being undefined. window.parent does give me an object but it doesn't have my Lightbox variable. I've also tried window.Lightbox, window.top.Lightbox, and window.top.document.Lightbox, but all return undefined. I should mention I'm using Javascript to set the data property of the <object> - but I don't see how that could affect anything relevant. What Javascript Fu do I need to do to be able to access my Lightbox variable?

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  • Events in Classes (VB.NET)

    - by Otaku
    I find that I write a lot of code within my classes to keep properties in sync with each other. I've read about Events in Classes, but have not been able to wrap my head around how to make them work for what I'm looking for. I could use some advice here. For example, in this one I always want to keep myColor up to date with any change whatsoever in any or all of the Red, Green or Blue properties. Class myColors Private Property Red As Byte Private Property Green As Byte Private Property Blue As Byte Private Property myColor As Color Sub New() myColor = Color.FromArgb(0, 0, 0) End Sub Sub ChangeRed(ByVal r As Byte) Red = r myColor = Color.FromArgb(Red, Green, Blue) End Sub Sub ChangeBlue(ByVal b As Byte) Blue = b myColor = Color.FromArgb(Red, Green, Blue) End Sub End Class If one or more of those changes, I want myColor to be updated. Easy enough as above, but is there a way to work with events that would automatically do this so I don't have to put myColor = Color.FromArgb(Red, Green, Blue) in every sub routine?

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  • domain modeling naming problem

    - by cherouvim
    Hello There are some simple entities in an application (e.g containing only id and title) which rarely change and are being referenced by the more complex entities of the application. These are usually entities such as Country, City, Language etc. How are these called? I've used the following names for those in the past but I'm not sure which is the best way to call them: reference data lookup values dictionaries thanks

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  • What patterns exist for web application development?

    - by DaveDev
    I understand that MVC & MVP are design patterns that are commonly used for web development, as well as ASP.NET WebForms (more of an anti-pattern, really!). What other patterns are used in web application development? I'm not necessarily saying I want to learn/use new patterns just to be different - I do believe there's a lot of value in taking the conventional route - but I think it's good to know what else is out there to be able to properly understand what I'm currently working with. Thanks.

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  • Unserializing an API return object (PHP/Ebay API)

    - by DavidYell
    I have been working with the Ebay api for a project and have found it great. I have however found a problem now, more PHP related. When I read my items from Ebay, I store a bunch of details in the database. Currently, just for the sake of it really, I serialize the whole return object and store it in the database in a related table. The idea being, that when I display my information, I have all the details to hand should I need them. The problem arises in that the pricing information is always in a sub object. [ConvertedAdjustmentAmount] => __PHP_Incomplete_Class Object ( [__PHP_Incomplete_Class_Name] => eBayAmountType [_] => 0 [currencyID] => USD ) As you can see when I unserialize my object, my cunning plan falls foul of the Incomplete class problem. I have checked the following question, without success. http://stackoverflow.com/questions/965611/forcing-access-to-php-incomplete-class-object-properties The main issue lies, as far as I can see, in that the price class is stored in the Ebay api, so how do I recreate it? I have been reading this page, http://uk3.php.net/manual/en/function.unserialize.php and trying to figure out, unserialize_callback_func which I can't figure out either, so any help would be appreciated!

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