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• Issues in Ajax based applications

  - by Sinuhe

  I'm very interested in developing Ajax based applications. This is, loading almost all of the content of the application via XMLHttpRequest, instead of only some combos and widgets. But if I try to do this form scratch, soon I find some problems without an easy solution. I wonder if there is some framework (both client and server side) to deal with this issues. As far as I know, there isn't (but I've searched mainly in Java world). So I am seriously thinking of doing my own framework, at least for my projects. Therefore, in this question I ask for several things. First, the possible problems of an ajax based development. Then, I'm looking for some framework or utility in order to deal with them. Finally, if there is no framework available, what features must it have. Here are the issues I thought: 1 - JavaScript must be enabled. Security paranoia isn't the only problem: a lot of mobile devices couldn't use the application, too. 2 - Sometimes you need to update more than one DIV (e.g. main content, menu and breadcrumbs). 3 - Unknown response type: when you make an Ajax call, you set the callback function too, usually specifying if expected response is a javascript object or in which DIV put the result. But this fails when you get another type of response: for example when the session has expired and the user must log in again. 4 - Browser's refresh, back and forward buttons can be a real pain. User will expect different behaviors depending on the situation. 5 - When search engines indexes a site, only follow links. Thus, content load by Ajax won't "exist" for who doesn't know about it yet. 6 - Users can ask for open a link in a different window/tab. 7 - Address bar doesn't show the "real" page you are in. So, you can't copy the location and send it to a friend or bookmark the page. 8 - If you want to monetize the site, you can put some advertisings. As you don't refresh entire page and you want to change the ad after some time, you have to refresh only the DIV where the ad is. But this can violate the Terms and Conditions of your ad service. In fact, it can go against AdSense TOS. 9 - When you refresh an entire page, all JavaScript gets "cleaned". But in Ajax calls, all JavaScript objects will remain. 10 - You can't easily change your CSS properties.

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• Get data from aspx.cs page to aspx page.

  - by Brad8118

  So I am using a jquery plug in that allows me to change the order of things in a list by dragging and dropping them. So my goal is to be able to grab a list of my objects (AlertInfo) and using it in a javascript function. I was able to use a json webservice call in a test project to pass the data to the page. But we don't have a webservice page now so I tried to grab it from a aspx.cs page and it hasn't worked. ///Aspx page: $.ajax({ type: "POST", url: "~/Alerts/GetAlerts", data: "{}", contentType: "application/json; charset=utf-8", dataType: "json", success: function (msg) { var data = eval("(" + msg.d + ")"); jQuery.each(data, function (rec) { AlertList[AlertList.length] = new objAlert(this.id, this.title, this.details, JSONDateSerializationFix(this.startdate), JSONDateSerializationFix(this.enddate)); UpdateDisplayList(); }) }, error: function (msg) { alert("BRAD" + msg); } The issue is that the Alerts page in "URL /Alerts/GetAlerts" is Alerts.aspx.cs. I can't figure out if I can use this ajax command to call a method in a aspx.cs page. //Code behind page aspx.cs [WebMethod] //[ScriptMethod(ResponseFormat = ResponseFormat.Json)] public string GetAlerts() { List list = AlertInfo.GetTestAlerts(); return new JavaScriptSerializer().Serialize(list); } public List GetAlertsList() { List list = AlertInfo.GetTestAlerts(); return list; ; } So I was hoping that I could load data into an asp control (dataList) and then grab the data //code behind page protected void Page_Load(object sender, EventArgs e) { dataListAlertList.DataSource = GetAlertsList(); dataListAlertList.DataBind(); } public static List<AlertInfo> GetTestAlerts() { List<AlertInfo> list = new List<AlertInfo>(); list.Add(new AlertInfo("0", "Alert 1 Title", "Alert 1 Detail", "10/10/2010", "10/10/2011")); list.Add(new AlertInfo("1", "Alert 2 Title", "Alert 2 Detail", "10/10/2010", "10/10/2011")); return list; } //.aspx page $(document).ready(function () { var a1 = $("#dataListAlertList").val(); // do fun stuff now. } But I keep getting undefined....

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• WPF DataGrid, Help with Binding to a List<X> where each X has a Dictionary<Y,object> property.

  - by panamack

  I'm building an application which helps someone manage an event and works with data originating from Excel. I want to use the WPF Toolkit DataGrid to display the incoming data but can't guarantee how many Columns there are going to be or what information they will contain. I'd like to have an Info class that stores column information and have each Visitor at my Event own a Dictionary that uses shared references to Info objects for the keys. Here's the general gist: public class Info{ public string Name{get;set;} public int InfoType{get;set;} } public class Visitor{ public Dictionary<Info,object> VisitorInfo {get;set;} } public class Event{ public List<Visitor> Visitors{get;set;} public Event(){ Info i1 = new Info(){ Name = "ID", InfoType = 0};// type 0 for an id Info i2 = new Info(){ Name = "Name", InfoType = 1};// type 1 for a string Info i3 = new Info(){ Name = "City", InfoType = 1}; Visitor v1 = new Visitor(); v1.VisitorInfo.Add(i1, 0); v1.VisitorInfo.Add(i2, "Foo Harris"); v1.VisitorInfo.Add(i3, "Barsville"); Visitor v2 = new Visitor(); ... this.Visitors.Add(v1); this.Visitors.Add(v2); ... } } XAML: <!-- Window1.xaml --> ... <local:Event x:Key="MyEvent"/> ... <wpftk:DataGrid DataContext="{StaticResource MyEvent}" ItemsSource="{Binding Path=Visitors}" /> Disappointingly, DataGrid just sees a collection of Visitors each having a VisitorInfo property and displays one column called VisitorInfo with the string "(Collection)" once for each Visitor. As a workaround I've created a ListTVisitorToDataTableConverter that maps Infos to DataColumns and Visitors to DataRows and used it like this: <wpftk:DataGrid DataContext="{StaticResource Event}" ItemsSource{Binding Path=Visitors, Converter={StaticResource MySmellyListTVisitorToDataTableConverter}}" /> I don't think this is good though, I haven't started trying to convert back yet which I guess I'll need to do if I want to be able to edit any data! How can I do better? Thanks.

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• How to pass operators as parameters

  - by Rodion Ingles

  I have to load an array of doubles from a file, multiply each element by a value in a table (different values for different elements), do some work on it, invert the multiplication (that is, divide) and then save the data back to file. Currently I implement the multiplication and division process in two separate methods. Now there is some extra work behind the scenes but apart from the specific statements where the multiplication/division occurs, the rest of the code is identical. As you can imagine, with this approach you have to be very careful making any changes. The surrounding code is not trivial, so its either a case of manually editing each method or copying changes from one method to the other and remembering to change the * and / operators. After too many close calls I am fed up of this and would like to make a common function which implements the common logic and two wrapper functions which pass which operator to use as a parameter. My initial approach was to use function pointers: MultiplyData(double data) { TransformData(data, &(operator *)); } DivideData(double data) { TransformData(data, &(operator /)); } TransformData(double data, double (*func)(double op1, double op2)) { /* Do stuff here... */ } However, I can't pass the operators as pointers (is this because it is an operator on a native type?), so I tried to use function objects. Initially I thought that multiplies and divides functors in <functional> would be ideal: MultiplyData(double data) { std::multiplies<double> multFunct; TransformData(data, &multFunct); } DivideData(double data) { std::divides<double> divFunct; TransformData(data, &divFunct); } TransformData(double data, std::binary_function<double, double, double> *funct) { /* Do stuff here... */ } As you can see I was trying to use a base class pointer to pass the functor polymorphically. The problem is that std::binary_function does not declare an operator() member for the child classes to implement. Is there something I am missing, or is the solution to implement my own functor heirarchy (which really seems more trouble than it is worth)?

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• Javascript self contained sandbox events and client side stack

  - by amnon

  I'm in the process of moving a JSF heavy web application to a REST and mainly JS module application . I've watched "scalable javascript application architecture" by Nicholas Zakas on yui theater (excellent video) and implemented much of the talk with good success but i have some questions : I found the lecture a little confusing in regards to the relationship between modules and sandboxes , on one had to my understanding modules should not be effected by something happening outside of their sandbox and this is why they publish events via the sandbox (and not via the core as they do access the core for hiding base libary) but each module in the application gets a new sandbox ? , shouldn't the sandbox limit events to the modoules using it ? or should events be published cross page ? e.g. : if i have two editable tables but i want to contain each one in a different sandbox and it's events effect only the modules inside that sandbox something like messabe box per table which is a different module/widget how can i do that with sandbox per module , ofcourse i can prefix the events with the moduleid but that creates coupling that i want to avoid ... and i don't want to package modules toghter as one module per combination as i already have 6-7 modules ? while i can hide the base library for small things like id selector etc.. i would still like to use the base library for module dependencies and resource loading and use something like yui loader or dojo.require so in fact i'm hiding the base library but the modules themself are defined and loaded by the base library ... seems a little strange to me libraries don't return simple js objects but usualy wrap them e.g. : u can do something like $$('.classname').each(.. which cleans the code alot , it makes no sense to wrap the base and then in the module create a dependency for the base library by executing .each but not using those features makes a lot of code written which can be left out ... and implemnting that functionality is very bug prone does anyonen have any experience with building a front side stack of this order ? how easy is it to change a base library and/or have modules from different libraries , using yui datatable but doing form validation with dojo ... ? some what of a combination of 2+4 if u choose to do something like i said and load dojo form validation widgets for inputs via yui loader would that mean dojocore is a module and the form module is dependant on it ? Thanks .

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• T-SQL generated from LINQ to SQL is missing a where clause

  - by Jimmy W

  I have extended some functionality to a DataContext object (called "CodeLookupAccessDataContext") such that the object exposes some methods to return results of LINQ to SQL queries. Here are the methods I have defined: public List<CompositeSIDMap> lookupCompositeSIDMap(int regionId, int marketId) { var sidGroupId = CompositeSIDGroupMaps.Where(x => x.RegionID.Equals(regionId) && x.MarketID.Equals(marketId)) .Select(x => x.CompositeSIDGroup); IEnumerator<int> sidGroupIdEnum = sidGroupId.GetEnumerator(); if (sidGroupIdEnum.MoveNext()) return lookupCodeInfo<CompositeSIDMap, CompositeSIDMap>(x => x.CompositeSIDGroup.Equals(sidGroupIdEnum.Current), x => x); else return null; } private List<TResult> lookupCodeInfo<T, TResult>(Func<T, bool> compLambda, Func<T, TResult> selectLambda) where T : class { System.Data.Linq.Table<T> dataTable = this.GetTable<T>(); var codeQueryResult = dataTable.Where(compLambda) .Select(selectLambda); List<TResult> codeList = new List<TResult>(); foreach (TResult row in codeQueryResult) codeList.Add(row); return codeList; } CompositeSIDGroupMap and CompositeSIDMap are both tables in our database that are represented as objects in my DataContext object. I wrote the following code to call these methods and display the T-SQL generated after calling these methods: using (CodeLookupAccessDataContext codeLookup = new CodeLookupAccessDataContext()) { codeLookup.Log = Console.Out; List<CompositeSIDMap> compList = codeLookup.lookupCompositeSIDMap(5, 3); } I got the following results in my log after invoking this code: SELECT [t0].[CompositeSIDGroup] FROM [dbo].[CompositeSIDGroupMap] AS [t0] WHERE ([t0].[RegionID] = @p0) AND ([t0].[MarketID] = @p1) -- @p0: Input Int (Size = 0; Prec = 0; Scale = 0) [5] -- @p1: Input Int (Size = 0; Prec = 0; Scale = 0) [3] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1 SELECT [t0].[PK_CSM], [t0].[CompositeSIDGroup], [t0].[InputSID], [t0].[TargetSID], [t0].[StartOffset], [t0].[EndOffset], [t0].[Scale] FROM [dbo].[CompositeSIDMap] AS [t0] -- Context: SqlProvider(Sql2005) Model: AttributedMetaModel Build: 3.5.30729.1 The first T-SQL statement contains a where clause as specified and returns one column as expected. However, the second statement is missing a where clause and returns all columns, even though I did specify which rows I wanted to view and which columns were of interest. Why is the second T-SQL statement generated the way it is, and what should I do to ensure that I filter out the data according to specifications via the T-SQL? Also note that I would prefer to keep lookupCodeInfo() and especially am interested in keeping it enabled to accept lambda functions for specifying which rows/columns to return.

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• Can I write a test that succeeds if and only if a statement does not compile?

  - by Billy ONeal

  I'd like to prevent clients of my class from doing something stupid. To that end, I have used the type system, and made my class only accept specific types as input. Consider the following example (Not real code, I've left off things like virtual destructors for the sake of example): class MyDataChunk { //Look Ma! Implementation! }; class Sink; class Source { virtual void Run() = 0; Sink *next_; void SetNext(Sink *next) { next_ = next; } }; class Sink { virtual void GiveMeAChunk(const MyDataChunk& data) { //Impl }; }; class In { virtual void Run { //Impl } }; class Out { }; //Note how filter and sorter have the same declaration. Concrete classes //will inherit from them. The seperate names are there to ensure only //that some idiot doesn't go in and put in a filter where someone expects //a sorter, etc. class Filter : public Source, public Sink { //Drop objects from the chain-of-command pattern that don't match a particular //criterion. }; class Sorter : public Source, public Sink { //Sorts inputs to outputs. There are different sorters because someone might //want to sort by filename, size, date, etc... }; class MyClass { In i; Out o; Filter f; Sorter s; public: //Functions to set i, o, f, and s void Execute() { i.SetNext(f); f.SetNext(s); s.SetNext(o); i.Run(); } }; What I don't want is for somebody to come back later and go, "Hey, look! Sorter and Filter have the same signature. I can make a common one that does both!", thus breaking the semantic difference MyClass requires. Is this a common kind of requirement, and if so, how might I implement a test for it?

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• Simple RSA encryption (Java)

  - by jake blue

  This is simply for fun. This will not be used for any actual encryption. I'm only first year comp sci student and love cryptography. This took a long time to get working. At approximately N = 18, it begins breaking down. It won't encrypt messages properly after that point. I'm not sure why. Any insights? I'd also appreciate any links you could provide me to tutorials or interesting reading about Cryptography. import java.math.BigInteger; import java.security.SecureRandom; /** * Cryptography. * * Generates public and private keys used in encryption and * decryption * */ public class RSA { private final static BigInteger one = new BigInteger("1"); private final static SecureRandom random = new SecureRandom(); // prime numbers private BigInteger p; private BigInteger q; // modulus private BigInteger n; // totient private BigInteger t; // public key private BigInteger e; // private key private BigInteger d; private String cipherText; /** * Constructor for objects of class RSA */ public RSA(int N) { p = BigInteger.probablePrime(N/2, random); q = BigInteger.probablePrime(N/2, random); // initialising modulus n = p.multiply(q); // initialising t by euclid's totient function (p-1)(q-1) t = (p.subtract(one)).multiply(q.subtract(one)); // initialising public key ~ 65537 is common public key e = new BigInteger("65537"); } public int generatePrivateKey() { d = e.modInverse(t); return d.intValue(); } public String encrypt(String plainText) { String encrypted = ""; int j = 0; for(int i = 0; i < plainText.length(); i++){ char m = plainText.charAt(i); BigInteger bi1 = BigInteger.valueOf(m); BigInteger bi2 = bi1.modPow(e, n); j = bi2.intValue(); m = (char) j; encrypted += m; } cipherText = encrypted; return encrypted; } public String decrypt() { String decrypted = ""; int j = 0; for(int i = 0; i < cipherText.length(); i++){ char c = cipherText.charAt(i); BigInteger bi1 = BigInteger.valueOf(c); BigInteger bi2 = bi1.modPow(d, n); j = bi2.intValue(); c = (char) j; decrypted += c; } return decrypted; } }

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• Are Objective-C initializers allowed to share the same name?

  - by NattKatt

  I'm running into an odd issue in Objective-C when I have two classes using initializers of the same name, but differently-typed arguments. For example, let's say I create classes A and B: A.h: #import <Cocoa/Cocoa.h> @interface A : NSObject { } - (id)initWithNum:(float)theNum; @end A.m: #import "A.h" @implementation A - (id)initWithNum:(float)theNum { self = [super init]; if (self != nil) { NSLog(@"A: %f", theNum); } return self; } @end B.h: #import <Cocoa/Cocoa.h> @interface B : NSObject { } - (id)initWithNum:(int)theNum; @end B.m: #import "B.h" @implementation B - (id)initWithNum:(int)theNum { self = [super init]; if (self != nil) { NSLog(@"B: %d", theNum); } return self; } @end main.m: #import <Foundation/Foundation.h> #import "A.h" #import "B.h" int main (int argc, const char * argv[]) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; A *a = [[A alloc] initWithNum:20.0f]; B *b = [[B alloc] initWithNum:10]; [a release]; [b release]; [pool drain]; return 0; } When I run this, I get the following output: 2010-04-26 20:44:06.820 FnTest[14617:a0f] A: 20.000000 2010-04-26 20:44:06.823 FnTest[14617:a0f] B: 1 If I reverse the order of the imports so it imports B.h first, I get: 2010-04-26 20:45:03.034 FnTest[14635:a0f] A: 0.000000 2010-04-26 20:45:03.038 FnTest[14635:a0f] B: 10 For some reason, it seems like it's using the data type defined in whichever @interface gets included first for both classes. I did some stepping through the debugger and found that the isa pointer for both a and b objects ends up the same. I also found out that if I no longer make the alloc and init calls inline, both initializations seem to work properly, e.g.: A *a = [A alloc]; [a initWithNum:20.0f]; If I use this convention when I create both a and b, I get the right output and the isa pointers seem to be different for each object. Am I doing something wrong? I would have thought multiple classes could have the same initializer names, but perhaps that is not the case.

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• NHibernate unable to create SessionFactory

  - by Tyler

  I'm having a bit of trouble setting up NHibernate, and I'm not too sure what the problem is exactly. I'm attempting to save a domain object to the database (Oracle 10g XE). However, I'm getting a TypeInitializationException while trying to create the ISessionFactory. Here is what my hibernate.cfg.xml looks like: <?xml version="1.0" encoding="utf-8"?> <hibernate-configuration xmlns="urn:nhibernate-configuration-2.2" > <session-factory name="MyProject.DataAccess"> <property name="connection.driver_class">NHibernate.Driver.OracleClientDriver</property> <property name="connection.connection_string"> User ID=myid;Password=mypassword;Data Source=localhost </property> <property name="show_sql">true</property> <property name="dialect">NHibernate.Dialect.OracleDialect</property> <property name="proxyfactory.factory_class">NHibernate.ByteCode.LinFu.ProxyFactoryFactory, NHibernate.ByteCode.LinFu</property> <mapping resource="MyProject/Domain/User.hbm.xml"/> </session-factory> </hibernate-configuration> I created a DAO which I will use to persist domain objects to the database. The DAO uses a HibernateUtil class that creates the SessionFactory. Both classes are in the DataAccess namespace along with the Hibernate configuration. This is where the exception is occuring. Here's that class: public class HibernateUtil { private static ISessionFactory SessionFactory = BuildSessionFactory(); private static ISessionFactory BuildSessionFactory() { try { // This seems to be where the problem occurs return new Configuration().Configure().BuildSessionFactory(); } catch (TypeInitializationException ex) { Console.WriteLine("Initial SessionFactory creation failed." + ex); throw new Exception("Unable to create SessionFactory."); } } public static ISessionFactory GetSessionFactory() { return SessionFactory; } } The DataAccess namespace references the NHibernate DLLs. This is virtually the same setup I've used with Hibernate in Java, so I'm not entirely sure what I'm doing wrong here. Any ideas? Edit The innermost exception is the following: "Could not find file 'C:\Users\Tyler\Documents\Visual Studio 2010\Projects\MyProject\MyProject\ConsoleApplication\bin\Debug\hibernate.cfg.xml'." ConsoleApplication contains the entry point where I've created a User object and am trying to persist it with my DAO. Why is it looking for the configuration file there? The actual persisting takes place in the DAO, which is in DataAccess. Also, when I add the configuration file to ConsoleApplication, it still does not find it.

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• Specializing a template on a lambda in C++0x

  - by Tony A.

  I've written a traits class that lets me extract information about the arguments and type of a function or function object in C++0x (tested with gcc 4.5.0). The general case handles function objects: template <typename F> struct function_traits { template <typename R, typename... A> struct _internal { }; template <typename R, typename... A> struct _internal<R (F::*)(A...)> { // ... }; typedef typename _internal<decltype(&F::operator())>::<<nested types go here>>; }; Then I have a specialization for plain functions at global scope: template <typename R, typename... A> struct function_traits<R (*)(A...)> { // ... }; This works fine, I can pass a function into the template or a function object and it works properly: template <typename F> void foo(F f) { typename function_traits<F>::whatever ...; } int f(int x) { ... } foo(f); What if, instead of passing a function or function object into foo, I want to pass a lambda expression? foo([](int x) { ... }); The problem here is that neither specialization of function_traits<> applies. The C++0x draft says that the type of the expression is a "unique, unnamed, non-union class type". Demangling the result of calling typeid(...).name() on the expression gives me what appears to be gcc's internal naming convention for the lambda, main::{lambda(int)#1}, not something that syntactically represents a C++ typename. In short, is there anything I can put into the template here: template <typename R, typename... A> struct function_traits<????> { ... } that will allow this traits class to accept a lambda expression?

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• How do you unit test the real world?

  - by Kim Sun-wu

  I'm primarily a C++ coder, and thus far, have managed without really writing tests for all of my code. I've decided this is a Bad Idea(tm), after adding new features that subtly broke old features, or, depending on how you wish to look at it, introduced some new "features" of their own. But, unit testing seems to be an extremely brittle mechanism. You can test for something in "perfect" conditions, but you don't get to see how your code performs when stuff breaks. A for instance is a crawler, let's say it crawls a few specific sites, for data X. Do you simply save sample pages, test against those, and hope that the sites never change? This would work fine as regression tests, but, what sort of tests would you write to constantly check those sites live and let you know when the application isn't doing it's job because the site changed something, that now causes your application to crash? Wouldn't you want your test suite to monitor the intent of the code? The above example is a bit contrived, and something I haven't run into (in case you haven't guessed). Let me pick something I have, though. How do you test an application will do its job in the face of a degraded network stack? That is, say you have a moderate amount of packet loss, for one reason or the other, and you have a function DoSomethingOverTheNetwork() which is supposed to degrade gracefully when the stack isn't performing as it's supposed to; but does it? The developer tests it personally by purposely setting up a gateway that drops packets to simulate a bad network when he first writes it. A few months later, someone checks in some code that modifies something subtly, so the degradation isn't detected in time, or, the application doesn't even recognize the degradation, this is never caught, because you can't run real world tests like this using unit tests, can you? Further, how about file corruption? Let's say you're storing a list of servers in a file, and the checksum looks okay, but the data isn't really. You want the code to handle that, you write some code that you think does that. How do you test that it does exactly that for the life of the application? Can you? Hence, brittleness. Unit tests seem to test the code only in perfect conditions(and this is promoted, with mock objects and such), not what they'll face in the wild. Don't get me wrong, I think unit tests are great, but a test suite composed only of them seems to be a smart way to introduce subtle bugs in your code while feeling overconfident about it's reliability. How do I address the above situations? If unit tests aren't the answer, what is? Thanks!

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• SAX parser does not resolve filename

  - by phantom-99w

  Another day, another strange error with SAX, Java, and friends. I need to iterate over a list of File objects and pass them to a SAX parser. However, the parser fails because of an IOException. However, the various File object methods confirm that the file does indeed exist. The output which I get: 11:53:57.838 [MainThread] DEBUG DefaultReactionFinder - C:\project\trunk\application\config\reactions\TestReactions.xml 11:53:57.841 [MainThread] ERROR DefaultReactionFinder - C:\project\trunk\application\config\reactions\null (The system cannot find the file specified) So the problem is obviously that null in the second line. I've tried nearly all variations of passing the file as a parameter to the parser, including as a String (both from getAbsolutePath() and entered by hand), as a URI and, even more weirdly, as a FileInputStream (for this I get the same error, except that the entire relative path gets reported as null, so C:\project\trunk\null). All that I can think of is that the SAXParserFactory is incorrectly configured. I have no idea what is wrong, though. Here is the code concerned: SAXParserFactory factory = SAXParserFactory.newInstance(); factory.setValidating(true); try { parser = factory.newSAXParser(); } catch (ParserConfigurationException e) { throw new InstantiationException("Error configuring an XML parser. Given error message: \"" + e.getMessage() + "\"."); } catch (SAXException e) { throw new InstantiationException("Error creating a SAX parser. Given error message: \"" + e.getMessage() + "\"."); } ... for (File f : fileLister.getFileList()) { logger.debug(f.getAbsolutePath()); try { parser.parse(f, new ReactionHandler(input)); //FileInputStream fs = new FileInputStream(f); //parser.parse(fs, new ReactionHandler(input)); //fs.close(); } catch (IOException e) { logger.error(e.getMessage()); throw new ReactionNotFoundException("An error occurred processing file \"" + f + "\"."); } ... } I have made no special provisions to provide a custom SAX parser implementation: I use the system default. Any help would be greatly appreciated!

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• Patterns for dynamic CMS components (event driven?)

  - by CitrusTree

  Sorry my title is not great, this is my first real punt at moving 100% to OO as I've been procedural for more years than I can remember. I'm finding it hard to understand if what I'm trying to do is possible. Depending on people's thoughts on the 2 following points, I'll go down that route. The CMS I'm putting together is quote small, however focuses very much on different types of content. I could easily use Drupal which I'm very comfortable with, but I want to give myself a really good reasons to move myself into design patterns / OO-PHP 1) I have created a base 'content' class which I wish to be able to extend to handle different types of content. The base class, for example, handles HTML content, and extensions might handle XML or PDF output instead. On the other hand, at some point I may wish to extend the base class for a given project completely. I.e. if class 'content-v2' extended class 'content' for that site, any calls to that class should actually call 'content-v2' instead. Is that possible? If the code instantiates an object of type 'content' - I actually want it to instantiate one of type 'content-v2'... I can see how to do it using inheritance, but that appears to involve referring to the class explicitly, I can't see how to link the class I want it to use instead dynamically. 2) Secondly, the way I'm building this at the moment is horrible, I'm not happy with it. It feels very linear indeed - i.e. get session details get content build navigation theme page publish. To do this all the objects are called 1-by-1 which is all very static. I'd like it to be more dynamic so that I can add to it at a later date (very closely related to first question). Is there a way that instead of my orchestrator class calling all the other classes 1-by-1, then building the whole thing up at the end, that instead each of the other classes can 'listen' for specific events, then at the applicable point jump in and do their but? That way the orchestrator class would not need to know what other classes were required, and call them 1-by-1. Sorry if I've got this all twisted in my head. I'm trying to build this so it's really flexible.

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• How do I write object classes effectively when dealing with table joins?

  - by Chris

  I should start by saying I'm not now, nor do I have any delusions I'll ever be a professional programmer so most of my skills have been learned from experience very much as a hobby. I learned PHP as it seemed a good simple introduction in certain areas and it allowed me to design simple web applications. When I learned about objects, classes etc the tutor's basic examnples covered the idea that as a rule of thumb each database table should have its own class. While that worked well for the photo gallery project we wrote, as it had very simple mysql queries, it's not working so well now my projects are getting more complex. If I require data from two separate tables which require a table join I've instead been ignoring the class altogether and handling it on a case by case basis, OR, even worse been combining some of the data into the class and the rest as a separate entity and doing two queries, which to me seems inefficient. As an example, when viewing content on a forum I wrote, if you view a thread, I retrieve data from the threads table, the posts table and the user table. The queries from the user and posts table are retrieved via a join and not instantiated as an object, whereas the thread data is called using my Threads class. So how do I get from my current state of affairs to something a little less 'stupid', for want of a better word. Right now I have a DB class that deals with connection and escaping values etc, a parent db query class that deals with the common queries and methods, and all of the other classes (Thread, Upload, Session, Photo and ones thats aren't used Post, User etc ) are children of that. Do I make a big posts class that has the relevant extra attributes that I retrieve from the users (and potentially threads) table? Do I have separate classes that populate each of their relevant attributes with a single query? If so how do I do that? Because of the way my classes are written, based on what I was taught, my db update row method, or insert method both just take the attributes as an array and update all of that, if I have extra attributes from other db tables in each class then how do I rewrite those methods as obbiously updating automatically like that would result in errors? In short I think my understanding is limited right now and I'd like some pointers when it comes to the fundamentals of how to write more complex classes.

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• Strange Java Socket Behavior (Connects, but Doesn't Send)

  - by Donald Campbell

  I have a fairly complex project that boils down to a simple Client / Server communicating through object streams. Everything works flawlessly for two consecutive connections (I connect once, work, disconnect, then connect again, work, and disconnect). The client connects, does its business, and then closes. The server successfully closes both the object output stream and the socket, with no IO errors. When I try to connect a third time, the connection appears to go through (the ServerSocket.accept() method goes through and an ObjectOutputStream is successfully created). No data is passed, however. The inputStream.readUnshared() method simply blocks. I have taken the following memory precautions: When it comes time to close the sockets, all running threads are stopped, and all objects are nulled out. After every writeUnshared() method call, the ObjectOutputBuffer is flushed and reset. Has anyone encountered a similar problem, or does anyone have any suggestions? I'm afraid my project is rather large, and so copying code is problematic. The project boils down to this: SERVER MAIN ServerSocket serverSocket = new ServerSocket(port); while (true) { new WorkThread(serverSocket.accept()).start(); } WORK THREAD (SERVER) public void run() { ObjectInputBuffer inputBuffer = new ObjectInputBuffer(new BufferedInputStream(socket.getInputStream())); while (running) { try { Object myObject = inputBuffer.readUnshared(); // do work is not specified in this sample doWork(myObject); } catch (IOException e) { running = false; } } try { inputBuffer.close(); socket.close(); } catch (Exception e) { System.out.println("Could not close."); } } CLIENT public Client() { Object myObject; Socket mySocket = new Socket(address, port); try { ObjectOutputBuffer output = new ObjectOutputBuffer(new BufferedOutputStream(mySocket.getOutputStream())); output.reset(); output.flush(); } catch (Exception e) { System.out.println("Could not get an input."); mySocket.close(); return; } // get object data is not specified in this sample. it simply returns a serializable object myObject = getObjectData(); while (myObject != null) { try { output.writeUnshared(myObject); output.reset(); output.flush(); } catch (Exception e) { e.printStackTrace(); break; } // catch } // while try { output.close(); socket.close(); } catch (Exception e) { System.out.println("Could not close."); } } Thank you to everyone who may be able to help!

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• calling startActivity() inside of a instance method - causing a NullPointerException

  - by Cole

  Heya - I'm trying to call startActivity() from a class that extends AsyncTask in the onPostExecute(). Here's the flow: Class that extends AsyncTask: protected void onPostExecute() { Login login = new Login(); login.pushCreateNewOrChooseExistingFormActivity(); } Class that extends Activity: public void pushCreateNewOrChooseExistingFormActivity() { // start the CreateNewOrChooseExistingForm Activity Intent intent = new Intent(Intent.ACTION_VIEW); **ERROR_HERE*** intent.setClassName(this, CreateNewOrChooseExistingForm.class.getName()); startActivity(intent); } And I get this error… every time: 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): FATAL EXCEPTION: main 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): java.lang.NullPointerException 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): at android.content.ContextWrapper.getPackageName(ContextWrapper.java:120) 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): at android.content.ComponentName.(ComponentName.java:62) 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): at android.content.Intent.setClassName(Intent.java:4850) 03-17 16:04:29.579: ERROR/AndroidRuntime(1503): at com.att.AppName.Login.pushCreateNewOrChooseExistingFormActivity(Login.java:47) For iOS developers - I'm just trying to push a new view controller on to a navigational controller's stack a la pushViewController:animated:. Which apparently - is hard to do on this platform. Any ideas? Thanks in advance! UPDATE - FIXED: per @Falmarri advice, i managed to resolve this issue. first of all, i'm no longer calling Login login = new Login(); to create a new login object. bad. bad. bad. no cookie. instead, when preparing to call .execute(), this tutorial suggests passing the applicationContext to the class the executes the AsyncTask, for my purposes, as shown below: CallWebServiceTask task = new CallWebServiceTask(); // pass the login object to the task task.applicationContext = login; // execute the task in the background, passing the required params task.execute(login); now, in onPostExecute(), i can get to my Login objects methods like so: ((Login) applicationContext).pushCreateNewOrChooseExistingFormActivity(); ((Login) applicationContext).showLoginFailedAlert(result.get("httpResponseCode").toString()); ... hope this helps someone else out there! especially iOS developers transistioning over to Android...

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• XmlSerializer.Deserialize blocks over NetworkStream

  - by Luca

  I'm trying to sends XML serializable objects over a network stream. I've already used this on an UDP broadcast server, where it receive UDP messages from the local network. Here a snippet of the server side: while (mServiceStopFlag == false) { if (mSocket.Available > 0) { IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Any, DiscoveryPort); byte[] bData; // Receive discovery message bData = mSocket.Receive(ref ipEndPoint); // Handle discovery message HandleDiscoveryMessage(ipEndPoint.Address, bData); ... Instead this is the client side: IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Broadcast, DiscoveryPort); MemoryStream mStream = new MemoryStream(); byte[] bData; // Create broadcast UDP server mSocket = new UdpClient(); mSocket.EnableBroadcast = true; // Create datagram data foreach (NetService s in ctx.Services) XmlHelper.SerializeClass<NetService>(mStream, s); bData = mStream.GetBuffer(); // Notify the services while (mServiceStopFlag == false) { mSocket.Send(bData, (int)mStream.Length, ipEndPoint); Thread.Sleep(DefaultServiceLatency); } It works very fine. But now i'me trying to get the same result, but on a TcpClient socket, but the using directly an XMLSerializer instance: On server side: TcpClient sSocket = k.Key; ServiceContext sContext = k.Value; Message msg = new Message(); while (sSocket.Connected == true) { if (sSocket.Available > 0) { StreamReader tr = new StreamReader(sSocket.GetStream()); msg = (Message)mXmlSerialize.Deserialize(tr); // Handle message msg = sContext.Handler(msg); // Reply with another message if (msg != null) mXmlSerialize.Serialize(sSocket.GetStream(), msg); } else Thread.Sleep(40); } And on client side: NetworkStream mSocketStream; Message rMessage; // Network stream mSocketStream = mSocket.GetStream(); // Send the message mXmlSerialize.Serialize(mSocketStream, msg); // Receive the answer rMessage = (Message)mXmlSerialize.Deserialize(mSocketStream); return (rMessage); The data is sent (Available property is greater then 0), but the method XmlSerialize.Deserialize (which should deserialize the Message class) blocks. What am I missing?

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• fancybox image sometimes renders outside box

  - by Colleen

  I have the following django template: <script type="text/javascript" src="{{ STATIC_URL }}js/ jquery.fancybox-1.3.4.pack.js"></script> <link rel="stylesheet" href="{{ STATIC_URL }}css/ jquery.fancybox-1.3.4.css" type="text/css" media="screen" /> {% include "submission-form.html" with section="photos" %} <div class="commentables"> {% load thumbnail %} {% for story in objects %} <div class="image {% if forloop.counter|add:"-1"| divisibleby:picsinrow %}left{% else %}{% if forloop.counter| divisibleby:picsinrow %}right{% else %}middle{% endif %}{% endif %}"> {% if story.image %} {% thumbnail story.image size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{% if story.image_url %}{{ story.image_url }}{% else %} {{ story.image.url }}{% endif %}"> </a> {% endthumbnail %} {% else %} {% if story.image_url %} {% thumbnail story.image_url size crop="center" as full_im %} <a rel="gallery" href="{% url post slug=story.slug %}"> <img class="preview" {% if story.title %} alt="{{ story.title }}" {% endif %} src="{{ full_im.url }}" full- image="{{ story.image_url }}"> </a> {% endthumbnail %} {% endif %} {% endif %} </div> {% endfor %} {% if rowid != "last" %} <br style="clear: both" /> {% endif %} {% if not no_more_button %} <p style="text-align: right;" class="more-results"><a href="{% url images school_slug tag_slug %}">more...</a></p> {% endif %} </div> <script> $(document).ready(function(){ function changeattr(e){ var f = $(e.clone()); $(f.children()[0]).attr('src', $(f.children() [0]).attr("full-image")); $(f.children()[0]).attr('height', '500px'); return f[0].outerHTML; } $('.image a').each(function(idx, elem) { var e = $(elem); e.fancybox({ title: $(e.children()[0]).attr('alt'), content: changeattr(e) }); }); }); </script> and I'm occasionally getting weird display errors where the box will either not render anything at all (so it will show up as just a thin white bar, basically) or it will render only about 30 px wide, and position itself halfway down the page. In both cases, if I inspect element, I can see the "shadow" of the full picture, at the right size, with the right url. Image source doesnt' seem to make a difference, I'm getting no errors, and this is happening in both chrome and firefox. Does anyone have any ideas?

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• Adding Insert Row in tableView

  - by user333624

  Hello everyone, I have a tableView that loads its data directly from a Core Data table with a NSFetchedResultsController. I'm not using an intermediate NSMutableArray to store the objects from the fetch results; I basically implemented inside my UITableViewController the protocol method numberOfRowsInSection and it returns the numberOfObjects inside a NSFetchedResultsSectionInfo. id <NSFetchedResultsSectionInfo> sectionInfo = [[fetchedResultsController sections] objectAtIndex:section]; and then I configure the cell content by implementing configureCell:atIndexPath and retrieving object info from the fetchedResultController but right now I have a generic configuration for any object (to avoid complications) cell.textLabel.text = @"categoria"; I also have a NavigationBar with a custom edit button at the right that loads my own selector called customSetEditing. What I'm trying to accomplish is to load an "Insert Cell" at the beginning of the tableView so when I tap it, it creates a new record. This last part is easy to implement the problem is that I dont's seem to be able to load the insert row or any row when I tap on the navigation bar edit button. this is the code for my customSetEditing: - (void) customSetEditing { [super setEditing:YES animated:YES]; [self.tableView setEditing:YES animated:YES]; [[self tableView] beginUpdates]; //[[self tableView] beginUpdates]; UIBarButtonItem *customDoneButtonItem = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemDone target:self action:@selector(customDone)]; [self.navigationItem.rightBarButtonItem release]; self.navigationItem.rightBarButtonItem = customDoneButtonItem; //[categoriasArray insertObject:[NSNull null] atIndex:0]; NSMutableArray *indexPaths = [[NSMutableArray alloc] initWithObjects:[NSIndexPath indexPathForRow:0 inSection:0],nil ]; [self.tableView insertRowsAtIndexPaths:indexPaths withRowAnimation:UITableViewRowAnimationTop]; //[indexPaths release]; [self.tableView reloadData];} Before adding the:[self.tableView reloadData]; I was getting an out of bounds error plus a program crash and although the program is not crashing it is not loading anything. I have seen many examples of similar situations in stackoverflow (by the way is an excellent forum with very helpful and nice people) none of the examples seems to work for me. Any ideas?

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• Django save_m2m() and excluded field

  - by jul

  hi, in a ModelForm I replaced a field by excluding it and adding a new one with the same name, as shown below in AddRestaurantForm. When saving the form with the code shown below, I get an error in form.save_m2m() ("Truncated incorrect DOUBLE value"), which seems to be due to the function to attempt to save the tag field, while it is excluded. Is the save_m2m() function supposed to save excluded fields? Is there anything wrong in my code? Thanks Jul (...) new_restaurant = form.save(commit=False) new_restaurant.city = city new_restaurant.save() tags = form.cleaned_data['tag'] if(tags!=''): tags=tags.split(',') for t in tags: tag, created = Tag.objects.get_or_create(name = t.strip()) tag.save() new_restaurant.tag.add(tag) new_restaurant.save() form.save_m2m() models.py class Tag(models.Model): name = models.CharField(max_length=100, unique=True) class Restaurant(models.Model): name = models.CharField(max_length=50) city=models.ForeignKey(City) category=models.ManyToManyField(Category) tag=models.ManyToManyField(Tag, blank=True, null=True) forms.py class AddRestaurantForm(ModelForm): name = forms.CharField(widget=forms.TextInput(attrs=classtext)) city = forms.CharField(widget=forms.TextInput(attrs=classtext), max_length=100) tag = forms.CharField(widget=forms.TextInput(attrs=classtext), required=False) class Meta: model = Restaurant exclude = ('city','tag') Traceback: File "/var/lib/python-support/python2.5/django/core/handlers/base.py" in get_response 92. response = callback(request, *callback_args, **callback_kwargs) File "/home/jul/atable/../atable/resto/views.py" in addRestaurant 498. form.save_m2m() File "/var/lib/python-support/python2.5/django/forms/models.py" in save_m2m 75. f.save_form_data(instance, cleaned_data[f.name]) File "/var/lib/python-support/python2.5/django/db/models/fields/ related.py" in save_form_data 967. setattr(instance, self.attname, data) File "/var/lib/python-support/python2.5/django/db/models/fields/ related.py" in set 627. manager.add(*value) File "/var/lib/python-support/python2.5/django/db/models/fields/ related.py" in add 430. self._add_items(self.source_col_name, self.target_col_name, *objs) File "/var/lib/python-support/python2.5/django/db/models/fields/ related.py" in _add_items 497. [self._pk_val] + list(new_ids)) File "/var/lib/python-support/python2.5/django/db/backends/util.py" in execute 19. return self.cursor.execute(sql, params) File "/var/lib/python-support/python2.5/django/db/backends/mysql/ base.py" in execute 84. return self.cursor.execute(query, args) File "/var/lib/python-support/python2.5/MySQLdb/cursors.py" in execute 168. if not self._defer_warnings: self._warning_check() File "/var/lib/python-support/python2.5/MySQLdb/cursors.py" in _warning_check 82. warn(w[-1], self.Warning, 3) File "/usr/lib/python2.5/warnings.py" in warn 62. globals) File "/usr/lib/python2.5/warnings.py" in warn_explicit 102. raise message Exception Type: Warning at /restaurant/add/ Exception Value: Truncated incorrect DOUBLE value: 'a'

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• Where are the function literals c++?

  - by academicRobot

  First of all, maybe literals is not the right term for this concept, but its the closest I could think of (not literals in the sense of functions as first class citizens). The idea is that when you make a conventional function call, it compiles to something like this: callq <immediate address> But if you make a function call using a function pointer, it compiles to something like this: mov <memory location>,%rax callq *%rax Which is all well and good. However, what if I'm writing a template library that requires a callback of some sort with a specified argument list and the user of the library is expected to know what function they want to call at compile time? Then I would like to write my template to accept a function literal as a template parameter. So, similar to template <int int_literal> struct my_template {...};` I'd like to write template <func_literal_t func_literal> struct my_template {...}; and have calls to func_literal within my_template compile to callq <immediate address>. Is there a facility in C++ for this, or a work around to achieve the same effect? If not, why not (e.g. some cataclysmic side effects)? How about C++0x or another language? Solutions that are not portable are fine. Solutions that include the use of member function pointers would be ideal. I'm not particularly interested in being told "You are a <socially unacceptable term for a person of low IQ>, just use function pointers/functors." This is a curiosity based question, and it seems that it might be useful in some (albeit limited) applications. It seems like this should be possible since function names are just placeholders for a (relative) memory address, so why not allow more liberal use (e.g. aliasing) of this placeholder. p.s. I use function pointers and functions objects all the the time and they are great. But this post got me thinking about the don't pay for what you don't use principle in relation to function calls, and it seems like forcing the use of function pointers or similar facility when the function is known at compile time is a violation of this principle, though a small one.

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• Architecture Suggestions/Recommendations for a Web Application with Sub-Apps

  - by user579218

  Hello. I’m starting to plan an architecture for a big web application, and I wanted to get suggestions and/or recommendations on where to begin and which technologies and/or frameworks to use. The application will be an Intranet-based web site using Windows authentication, running on IIS and using SQL Server and ASP.NET. It’ll need to be structured as a main/shell application with sub-applications that are “pluggable” based on some configuration settings. The main or shell application is to provide the overall user interface structure – header/footer, dynamically built tabs for each available sub-app, and a content area in which the sub-application will be loaded when the user clicks on the sub-application’s tab. So, on start-up of the main/shell application, configuration information will be queried from a database, and, based on the user and which of the sub-apps are available, the main or shell app would dynamically build tabs (or buttons or something) as a way to access each individual application. On start-up, the content area will be populated with the “home” sub-app. But, clicking on an sub-app tab will cause the content area to be populated with the sub-app corresponding to the tab. For example, we’re going to have a reports application, a display application, and probably a couple other distinct applications. On startup of the main/shell application, after determining who the user is, the main app will query the database to determine which sub-apps the user can use and build out the UI. Then the user can navigate between available sub-apps and do their work in each. Finally, the entire app and all sub-apps need to be a layered design with presentation, service, business, and data access layers, as well as cross-cutting objects for things such as logging, exception handling, etc. Anyway, my questions revolve around where to begin to plan something like this application. What technologies/frameworks would work best in developing a solution for this application? MVC? MVP? WCSF? EF? NHibernate? Enterprise Library? Repository Pattern? Others???? I know all these technologies/frameworks are not used for the same purpose, but knowing which ones to focus on is a little overwhelming. Which ones would be the best choice(s) for a solution? Which ones work well together for an end-to-end design? How would one structure the VS project for something like this? Thanks!

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• How to replace an object in an NSMutableArray at a given index with a new object

  - by shakeelw

  Hi guys. I have an NSMutableArray object(retained, synthesized as all) that is initiated just fine and I can easily add objects to it using the 'addObject:' method. But if I want to replace an object at a certain index with a new one in that NSMutableArray, it doesn't work. For example: ClassA.h @interface ClassA : NSObject { NSMutableArray *list; } @property (nonatomic, copy, readwrite) NSMutableArray *list; end ClassA.m import "ClassA.h" @implementation ClassA @synthesize list; (id)init { [super init]; NSMutableArray *localList = [[NSMutableArray alloc] init]; self.list = localList; [localList release]; //Add initial data [list addObject:@"Hello "]; [list addObject:@"World"]; } // Custom set accessor to ensure the new list is mutable (void)setList:(NSMutableArray *)newList { if (list != newList) { [list release]; list = [newList mutableCopy]; } } -(void)updateTitle:(NSString *)newTitle:(NSString *)theIndex { int i = [theIndex intValue]-1; [self.list replaceObjectAtIndex:i withObject:newTitle]; NSLog((NSString *)[self.list objectAtIndex:i]); // gives the correct output } However, the change remains true only inside the method. from any other method, the NSLog((NSString *)[self.list objectAtIndex:i]); gives the same old value. How can I actually get the old object replaced with the new one at a specific index so that the change can be noticed from within any other method as well. I even modified the method like this, but the result is the same: -(void)updateTitle:(NSString *)newTitle:(NSString *)theIndex { int i = [theIndex intValue]-1; NSMutableArray *localList = [[NSMutableArray alloc] init]; localList = [localList mutableCopy]; for(int j = 0; j < [list count]; j++) { if(j == i) { [localList addObject:newTitle]; NSLog(@"j == 1"); NSLog([NSString stringWithFormat:@"%d", j]); } else { [localList addObject:(NSString *)[self.list objectAtIndex:j]]; } } [self.list release]; //self.list = [localList mutableCopy]; [self setList:localList]; [localList release]; } Please help out guys :)

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• Conceptual inheritance implementation

  - by TheSENDER

  Hi there, I'm writing a spatial data structure and I have a doubt about what's the best NODE implementation. According to my design I have an abstract node entity and three classes which inherit from it: EMPTYNODE, FULLNODE, INTERNALNODE. The first one has no particular data. The second one has 1 reference to a generic element. The third one has 2 references to other nodes. I have found several ways to implement this situation (that I have already coded) but I can't decide what's the best. The first solution that I have found is to use a single class Node that potentially performs all the operation in this way: private static class Node { private Elem elem = null; private Node left = null, right = null; public Elem getElem() { assert isFull(); return elem; } public boolean isEmpty() { return elem == null && left == null; } public boolean isFull() { return elem != null; } public boolean isInternal() { return elem == null && left != null; } } The second solution is to write an explicit division by classes where every class offers only its methods. Obviously in this way we are obliged to perform several casts to the node objects. private static abstract class Node { public abstract boolean isEmpty(); public abstract boolean isFull(); public abstract boolean isInternal(); } private static class FullNode extends Node{ private ITriangle elem; @Override public boolean isEmpty() { return false; } @Override public final boolean isFull() { return true; } @Override public final boolean isInternal() { return false; } public Elem getElem() { return elem; } } The third one solution is to use the inheritance allowing every classes to offer all the methods, but the object type should by check by "isEmpty()" and similar methods. In case of wrong call we'll throw an exception. private static abstract class Node { public abstract boolean isEmpty(); public abstract boolean isFull(); public abstract boolean isInternal(); public abstract Elem getElem(); } private static class Empty extends Node{ @Override public boolean isEmpty() { return true; } @Override public final boolean isFull() { return false; } @Override public final boolean isInternal() { return false; } @Override public Elem getElem() { throw new AssertionError(); } } What do you think about these three solutions? Which one would you use? Any other ideas? Thanks for your help. Every idea will be appreciated.

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