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  • Improving as a coder with respect to design.

    - by dmarakaki
    As a soon-to-be computer science graduate, I have to come realization that I have a long way to go when it comes to the overall design of an application. After spending many semesters of programming from the hip I have come to appreciate the mulling over the needs of an application before diving head first into the coding portion. My question is to the intermediate and expert programmers, how can I improve in the area of the design phase of development?

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  • Elusive race condition in Java

    - by nasufara
    I am creating a graphing calculator. In an attempt to squeeze some more performance out of it, I added some multithreaded to the line calculator. Essentially what my current implementation does is construct a thread-safe Queue of X values, then start however many threads it needs, each one calculating a point on the line using the queue to get its values, and then ordering the points using a HashMap when the calculations are done. This implementation works great, and that's not where my race condition is (merely some background info). In examining the performance results from this, I found that the HashMap is a performance bottleneck, since I do that synchronously on one thread. So I figured that ordering each point as its calculated would work best. I tried a PriorityQueue, but that was slower than the HashMap. I ended up creating an algorithm that essentially works like this: I construct a list of X values to calculate, like in my current algorithm. I then copy that list of values into another class, unimaginatively and temporarily named BlockingList, which is responsible for ordering the points as they are calculated. BlockingList contains a put() method, which takes in two BigDecimals as parameters, the first the X value, the second the calculated Y value. put() will only accept a value if the X value is the next one on the list to be accepted in the list of X values, and will block until another thread gives it the next excepted value. For example, since that can be confusing, say I have two threads, Thread-1 and Thread-2. Thread-2 gets the X value 10.0 from the values queue, and Thread-1 gets 9.0. However, Thread-1 completes its calculations first, and calls put() before Thread-2 does. Because BlockingList is expecting to get 10.0 first, and not 9.0, it will block on Thread-1 until Thread-2 finishes and calls put(). Once Thread-2 gives BlockingList 10.0, it notify()s all waiting threads, and expects 9.0 next. This continues until BlockingList gets all of its expected values. (I apologise if that was hard to follow, if you need more clarification, just ask.) As expected by the question title, there is a race condition in here. If I run it without any System.out.printlns, it will sometimes lock because of conflicting wait() and notifyAll()s, but if I put a println in, it will run great. A small implementation of this is included below, and exhibits the same behavior: import java.math.BigDecimal; import java.util.concurrent.ConcurrentLinkedQueue; public class Example { public static void main(String[] args) throws InterruptedException { // Various scaling values, determined based on the graph size // in the real implementation BigDecimal xMax = new BigDecimal(10); BigDecimal xStep = new BigDecimal(0.05); // Construct the values list, from -10 to 10 final ConcurrentLinkedQueue<BigDecimal> values = new ConcurrentLinkedQueue<BigDecimal>(); for (BigDecimal i = new BigDecimal(-10); i.compareTo(xMax) <= 0; i = i.add(xStep)) { values.add(i); } // Contains the calculated values final BlockingList list = new BlockingList(values); for (int i = 0; i < 4; i++) { new Thread() { public void run() { BigDecimal x; // Keep looping until there are no more values while ((x = values.poll()) != null) { PointPair pair = new PointPair(); pair.realX = x; try { list.put(pair); } catch (Exception ex) { ex.printStackTrace(); } } } }.start(); } } private static class PointPair { public BigDecimal realX; } private static class BlockingList { private final ConcurrentLinkedQueue<BigDecimal> _values; private final ConcurrentLinkedQueue<PointPair> _list = new ConcurrentLinkedQueue<PointPair>(); public BlockingList(ConcurrentLinkedQueue<BigDecimal> expectedValues) throws InterruptedException { // Copy the values into a new queue BigDecimal[] arr = expectedValues.toArray(new BigDecimal[0]); _values = new ConcurrentLinkedQueue<BigDecimal>(); for (BigDecimal dec : arr) { _values.add(dec); } } public void put(PointPair item) throws InterruptedException { while (item.realX.compareTo(_values.peek()) != 0) { synchronized (this) { // Block until someone enters the next desired value wait(); } } _list.add(item); _values.poll(); synchronized (this) { notifyAll(); } } } } My question is can anybody help me find the threading error? Thanks!

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  • How to intercept touches events on a MKMapView or UIWebView objects?

    - by Martin
    Hello, I'm not sure what i'm doing wrong but I try to catch touches on a MKMapView object. I subclassed it by creating the following class : #import <UIKit/UIKit.h> #import <MapKit/MapKit.h> @interface MapViewWithTouches : MKMapView { } - (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *) event; @end And the implementation : #import "MapViewWithTouches.h" @implementation MapViewWithTouches - (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *) event { NSLog(@"hello"); //[super touchesBegan:touches withEvent:event]; } @end But it looks like when I use this class, I see nothing on the Console : MapViewWithTouches *mapView = [[MapViewWithTouches alloc] initWithFrame:self.view.frame]; [self.view insertSubview:mapView atIndex:0]; Any idea what I'm doing wrong? Heelp! :) Thanks a lot! Martin

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  • How to throttle login attemps in Java webapp?

    - by Jörn Zaefferer
    I want to implement an efficient mechanism to throttle login attemps in my Java web application, to prevent brute-force attacks on user accounts. Jeff explained the why, but not the how. Simon Willison showed an implementation in Python for Django: That doesn't really help me along as I can't use memcached nor Django. Porting his ideas from scratch doesn't seem like a great either - I don't want to reinvent the wheel. I found one Java implementation, though it seems rather naiive: Instead of a LRU cache, it just clears all entries after 15 minutes. EHCache could be an alternative for memcached, but I don't have any experience with it and don't really want to intoduce yet another technology if there are better alternatives for this task. So, whats a good way to implement login throttling in Java?

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  • LINQPad - Dump extension method - I want one!

    - by gav
    Hi, LINQPad is amazing, particularly useful is the Dump() extension methods which renders objects and structs of almost any type, anonymous or not, to the console. Initially, when I moved to Visual Studio 2010, I tried to make my own Dump method using a delegate to get the values to render for anonymous types etc. It's getting pretty complicated though and whilst it was fun and educational at first what I need is a solid implementation. Having checked out the LinqPad code in reflector I am even more assured that I'm not going to get the implementation right. Is there a free library I can include to provide the Dump functionality? Thanks, Gavin

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  • Is MVVM pointless?

    - by joebeazelman
    Is orthodox MVVM implementation pointless? I am creating a new application and I considered Windows Forms and WPF. I chose WPF because it's future-proof and offer lots of flexibility. There is less code and easier to make significant changes to your UI using XAML. Since the choice for WPF is obvious, I figured that I may as well go all the way by using MVVM as my application architecture since it offers blendability, separation concerns and unit testability. Theoretically, it seems beautiful like the holy grail of UI programming. This brief adventure; however, has turned into a real headache. As expected in practice, I’m finding that I’ve traded one problem for another. I tend to be an obsessive programmer in that I want to do things the right way so that I can get the right results and possibly become a better programmer. The MVVM pattern just flunked my test on productivity and has just turned into a big yucky hack! The clear case in point is adding support for a Modal dialog box. The correct way is to put up a dialog box and tie it to a view model. Getting this to work is difficult. In order to benefit from the MVVM pattern, you have to distribute code in several places throughout the layers of your application. You also have to use esoteric programming constructs like templates and lamba expressions. Stuff that makes you stare at the screen scratching your head. This makes maintenance and debugging a nightmare waiting to happen as I recently discovered. I had an about box working fine until I got an exception the second time I invoked it, saying that it couldn’t show the dialog box again once it is closed. I had to add an event handler for the close functionality to the dialog window, another one in the IDialogView implementation of it and finally another in the IDialogViewModel. I thought MVVM would save us from such extravagant hackery! There are several folks out there with competing solutions to this problem and they are all hacks and don’t provide a clean, easily reusable, elegant solution. Most of the MVVM toolkits gloss over dialogs and when they do address them, they are just alert boxes that don’t require custom interfaces or view models. I’m planning on giving up on the MVVM view pattern, at least its orthodox implementation of it. What do you think? Has it been worth the trouble for you if you had any? Am I just a incompetent programmer or does MVVM not what it's hyped up to be?

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  • How do I parse a UTC date format string to local date time?

    - by Brian Scott
    I'm currently using the jQuery fullcalendar plugin but I've came across an issue regarding daylight savings time in Britain. After the daylight savings take effect the hours are being displayed an hour out of phase. The problem appears to be with the plugins parsing function. Can someone please provide me with the funciton to parse a UTC string which includes the 'Z' demonination into a local date time for display?

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  • F# currying efficiency?

    - by Eamon Nerbonne
    I have a function that looks as follows: let isInSet setElems normalize p = normalize p |> (Set.ofList setElems).Contains This function can be used to quickly check whether an element is semantically part of some set; for example, to check if a file path belongs to an html file: let getLowerExtension p = (Path.GetExtension p).ToLowerInvariant() let isHtmlPath = isInSet [".htm"; ".html"; ".xhtml"] getLowerExtension However, when I use a function such as the above, performance is poor since evaluation of the function body as written in "isInSet" seems to be delayed until all parameters are known - in particular, invariant bits such as (Set.ofList setElems).Contains are reevaluated each execution of isHtmlPath. How can best I maintain F#'s succint, readable nature while still getting the more efficient behavior in which the set construction is preevaluated. The above is just an example; I'm looking for a general pattern that avoids bogging me down in implementation details - where possible I'd like to avoid being distracted by details such as the implementation's execution order since that's usually not important to me and kind of undermines a major selling point of functional programming.

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  • How to access the Principal from a Java service object without using FlexContext?

    - by Marplesoft
    We're building some Java objects that are exposed via BlazeDS to our flex client application. So basically the BlazeDS messagebroker servlet instantiates and invokes methods on these objects in response to client requests. Works great. We're using app server-based authentication and have set up a security constraint on the <destination> elements in the remoting-config.xml file element to prevent unauthenticated clients from being able to access these remote java objects. Again, works fine. However, there are several places within the implementation of these java objects where we want to get the currently logged on user's username. Right now we are doing this via FlexContext.getUserPrincipal(), which gives access to this but we have a nagging concern that we don't like the idea that the implementation of these objects (the service layer) has a hard dependency on a BlazeDS class. But we're not sure how else to get access to this. The same applies to accessing the ServletContext and such. Any ideas?

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  • Objective C message dispatch mechanism

    - by Dolphin
    I am just staring to play around with Objective C (writing toy iPhone apps) and I am curious about the underlying mechanism used to dispatch messages. I have a good understanding of how virtual functions in C++ are generally implemented and what the costs are relative to a static or non-virtual method call, but I don't have any background with Obj-C to know how messages are sent. Browsing around I found this loose benchmark and it mentions IMP cached messages being faster than virtual function calls, which are in turn faster than a standard message send. I am not trying to optimize anything, just get deeper understanding of how exactly the messages get dispatched. How are Obj-C messages dispatched? How do Instance Method Pointers get cached and can you (in general) tell by reading the code if a message will get cached? Are class methods essentially the same as a C function (or static class method in C++), or is there something more to them? I know some of these questions may be 'implementation dependent' but there is only one implementation that really counts.

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  • Invalid receiver type 'NSUInteger'

    - by CJ
    I have a Core Data entity whose header file looks like this: @interface MyEntity : NSManagedObject { } @property (nonatomic, retain) NSNumber * index; @end And it's implementation file looks like this: @implementation MyEntity @dynamic index; @end Now, I have a piece of code that looks like this: NSArray* selectedObects = [myEntityArrayController selectedObjects]; NSUInteger theIndex = [[[selectedObects objectAtIndex:0] index] unsignedIntegerValue]; The 'myEntityArrayController' object is a NSArrayController which manages all entities of MyEntity. This code executes correctly, however XCode always gives the warning "Invalid receiver type 'NSUInteger'" for the last line of code. For some reason, XCode thinks that the index method returns a NSUInteger. I'm not sure why it thinks this, because 'objectAtIndex' returns an object of type 'id'. I've cleaned the project several times, and these warnings have hung around for a while. Any suggestions are appreciated.

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  • Gecko/Firefox support for HTML5 Notifications

    - by jAndy
    Hi Folks, I'm wondering if there is any build-in support for the HTML5 Notification feature in Gecko browsers so far? Maybe some hidden developer thingy ? I'm aware of WebKits window.webkitNotifications which works great, so, is there a Firefox implementation ? update After searching and reading some w3c HTML5 specs, I'm maybe a little bit off here. I can't find any Notification feature anywhere there. Am I facing wrong facts here? Is that just a "very own webkit implementation" ?

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  • Will OpenGL give me any FPS improvement over CoreAnimation for scrolling a large image?

    - by Ben Roberts
    Hi, I'm considering re-writing the menu system of my iPhone app to use Open GL just to improve the smoothness of scrolling a big image (480x1900px) across the screen. I'm looking at doing this as a way to improve on using the method/solution as described here (http://stackoverflow.com/questions/1443140/smoother-uiview). This solution was a big improvement over the previous implementation but it's still not perfect and as this is the first thing the user will see I'd like it to be as flawless as possible. Will switching to OpenGL give me the sort of smooth scrolling I'm looking for? I've stayed clear of OpenGL until now as this is my first app and core animation has handled everything else I've thrown at it well enough, would be good to know if this alternative implementation is likely to work! thanks

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  • Need good RDLC examples/samples

    - by Sachin
    I am in evaluation phase of report tool. I prefer RDLC for the same. But I need some examples/samples available in the wild which can guide us on using the RDLC off the shelf. I would be looking for examples from as simple as list of data and as complex as using matrix, calculation, grouping, etc. This will help us to make a reference point if anytime we get stuck up somewhere.

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  • Difference between implementing an interface and applying an attribute in C#

    - by RemotecUk
    This might be a stupid question but Ill ask anyway, I was reading "OOP Demystified: A Self-Teaching Guide by Jim Keogh and Mario Giannini" chapter 11 which covers interfaces. The examples in this book are C++. I noticed that C++ uses ISerializable to make a class serializable which you would implement where as in C# you just attribute the class with the [Serializable] attribute. What is the key difference here? Is it that with an interface you must provide the implementation where as if you attribute something the compiler will work out the implementation for you? I guess that with the [Serializable] attribute the .Net framework uses reflection to make the serialized object from the actual object. That said is it possible in that case to have an [Disposable] attribute or using my theory above the framework wont know how to actually dispose of an object hence you have to do it yourself? Would be grateful for a clarification. Thanks.

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  • Greenspun's 10th rule in Perl?

    - by DVK
    Greenspun's Tenth Rule of Programming is a common aphorism in computer programming and especially programming language circles. It states: Any sufficiently complicated C or Fortran program contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp. The questions are, 1) Would you consider this to be true of Perl interpreter? Only objective arguments please (e.g. which features of Common Lisp are implemented within the interpreter) 2) Independently, does there exist a Lisp (or at least a n ad hoc, informally-specified, bug-ridden, slow implementation of half of Common Lisp) implemented entirely in Perl?

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  • apache cxf: c error

    - by robinmag
    I tried to deploy the sample application from this tutorial: http://united-coders.com/phillip-steffensen/developing-a-simple-soap-webservice-using-spring-30-apache-cxf-226-and-maven-2 In eclipse everything works fine, but i went to troubles when deploying the service. The cxf-java2ws-plugin throws a ClassNotFoundException for org.springframework.core.NestedRuntimeException. Here is my maven pom build plugin (copy from the example) <plugin> <groupId>org.apache.cxf</groupId> <artifactId>cxf-java2ws-plugin</artifactId> <version>${cxf.version}</version> <dependencies> <dependency> <groupId>org.apache.cxf</groupId> <artifactId>cxf-rt-frontend-jaxws</artifactId> <version>${cxf.version}</version> </dependency> <dependency> <groupId>org.apache.cxf</groupId> <artifactId>cxf-rt-frontend-simple</artifactId> <version>${cxf.version}</version> </dependency> </dependencies> <executions> <execution> <id>process-classes</id> <phase>process-classes</phase> <configuration> <className>vn.vdconline.ws.sso.PersonService</className> <genWsdl>true</genWsdl> <verbose>true</verbose> </configuration> <goals> <goal>java2ws</goal> </goals> </execution> </executions> </plugin> Please tell me if i missed any thing. Thank you!

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  • ORA-22835 using JPA (Buffer too small)

    - by Kenneth
    I am trying to persist an Entity with a @Lob annotated String field. The content of that fiels if bigger than the 40k buffer size limit. The first problem I had was related to the setString method used internally by the JPA implementation (Hibernate in my case) and the Oracle JDBC Driver. This problem was solved adding <property name="hibernate.connection.SetBigStringTryClob" value="true"/> to my persistence.xml file. Then, the error changed to a ORA-22835 error (the buffer is too small). ¿Is there any way that JPA solves this problem without going to a low-level implementation? ¿Any suggestions?

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