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• Can an application affect TCP retransmits

  - by sipwiz

  I'm troubleshooting some communications issues and in the network traces I am occasionally coming across TCP sequence errors. One example I've got is: Server to Client: Seq=3174, Len=50 Client to Server: Ack=3224 Server to Client: Seq=3224, Len=50 Client to Server: Ack=3224 Server to Client: Seq=3274, Len=10 Client to Server: Ack=3224, SLE=3274, SRE=3284 Packets 4 & 5 are recorded in the trace (which is from a router in between the client and server) at almost exactly the same time so they most likely crossed in transit. The TCP session has got out of sync with the client missing the last two transmissions from the server. Those two packets should have been retransmitted but they weren't, the next log in the trace is a RST packet from the Client 24 seconds after packet 6. My question is related to what could be responsible for the failure to retransmit the server data from packets 3 & 5? I would assume that the retransmit would be at the operating system level but is there anyway the application could influence it and stop it being sent? A thread blocking or put to sleep or something like that?

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• C++ wxWidgets Event (Focus) Handling

  - by Wallter

  Due to comments I added the following code (in BasicPanel) Connect(CTRL_ONE, wxEVT_KILL_FOCUS, (wxObjectEventFunction)&BasicPanel::OnKillFocus); Connect(CTRL_TWO,wxEVT_KILL_FOCUS, (wxObjectEventFunction)&BasicPanel::OnKillFocus); Connect(CTRL_THREE, wxEVT_KILL_FOCUS, (wxObjectEventFunction)&BasicPanel::OnKillFocus); Connect(CTRL_FOUR, wxEVT_KILL_FOCUS, (wxObjectEventFunction)&BasicPanel::OnKillFocus); Connect(CTRL_FIVE, wxEVT_KILL_FOCUS, (wxObjectEventFunction)&BasicPanel::OnKillFocus); (enums) CTRL_NAME = wxID_HIGHEST + 5, // 6004 CTRL_ADDRESS = wxID_HIGHEST + 6, // 6005 CTRL_PHONENUMBER = wxID_HIGHEST + 7, // 6006 CTRL_SS = wxID_HIGHEST + 8, // 6007 CTRL_EMPNUMBER = wxID_HIGHEST + 9 // 6008 (The OnKillFocus Function - the declaration is included as suggested) void BasicPanel::OnKillFocus(wxFocusEvent& event) { switch (event.GetId()) { case 6004: ... break; ... ... ... } All of these added to the code do nothing when the user changes focus on which text box they are using... Q1:I am using wxWidgets (C++) and have come accost a problem that i can not locate any help. I have created several wxTextCtrl boxes and would like the program to update the simple calculations in them when the user 'kills the focus.' I could not find any documentation on this subject on the wxWidgets webpage and Googling it only brought up wxPython. The two events i have found are: EVT_COMMAND_KILL_FOCUS - EVT_KILL_FOCUS for neither of which I could find any snippet for. Could anyone give me a short example or lead me to a page that would be helpful? Q2:Would i have to create an event to handle the focus being killed for each of my 8 wxTextCtrl boxes? In the case that i have to create a different event: How would i get each event to differentiate from each other? I know i will have to create new wxID's for each of the wxTextCtrl boxes but how do I get the correct one to be triggered? class BasicPanel : public wxPanel { ... wxTextCtrl* one; wxTextCtrl* two; wxTextCtrl* three; wxTextCtrl* four; ... }

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• How to control dojo widget classes, or how to get fine-grained control of style

  - by djna

  I am creating a UI with dojo that is emulting some aspects of an existing thick client application. I need to programatically put two different menu bars on the same page. I have successfully created the two menu bars using new dijit.Menu(); new dijit.MenuItem(); and so on. Now I want to give them slightly different presentation styles. As I'm going to have quite a few pages of this sort my first thought is to use different CSS style classes. And that's the problem: when I create the Menu and it's items we get quite a set of HTML objects, each with CSS classes specified by dojo, and the classes are the same for the items associated with either menu. How can I best get specific control for any one menu? I could determine the dojo-generated ids for each item, and specify styles by ids, but that seems like hard work. Is there an sensible way to control the classes defined by dojo, or a nice CSS way to select only the items associated with one menu bar?

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• Using LINQ to map dynamically (or construct projections)

  - by CodeGrue

  I know I can map two object types with LINQ using a projection as so: var destModel = from m in sourceModel select new DestModelType {A = m.A, C = m.C, E = m.E} where class SourceModelType { string A {get; set;} string B {get; set;} string C {get; set;} string D {get; set;} string E {get; set;} } class DestModelType { string A {get; set;} string C {get; set;} string E {get; set;} } But what if I want to make something like a generic to do this, where I don't know specifically the two types I am dealing with. So it would walk the "Dest" type and match with the matching "Source" types.. is this possible? Also, to achieve deferred execution, I would want it just to return an IQueryable. For example: public IQueryable<TDest> ProjectionMap<TSource, TDest>(IQueryable<TSource> sourceModel) { // dynamically build the LINQ projection based on the properties in TDest // return the IQueryable containing the constructed projection } I know this is challenging, but I hope not impossible, because it will save me a bunch of explicit mapping work between models and viewmodels.

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• Recursive problem...

  - by Chronos

  Guys, I'm new to java and multithreading..... and I have a following problem: I have two classes running in two different threads. Class A and Class B. Class A has the method "onNewEvent()". Once that method is invoked, it will ask class B to do some work. As soon as B finishes the work it invokes onJobDone() method of the class A. Now... here comes the problem... what I want is to create new job within onJobDone() method and to send it again to B. here is what I do (pseudo code) ... in the sequence of execution A.onNewEvent(){ //create job //ask B to do it B.do() } B.do{ // Do some stuff A.jobDone() } A.onJobDOne(){ B.do() //doItAgain // print message "Thank you for doing it" } The problem is... that message "Thank you for doing it" never gets printed... in fact, when onJobDone() method is invoked, it invokes B.do()... because B.do() is very fast, it invokes onJobDone() immediatly... so execution flow never comes to PRINT MESSAGE part of code... I suppose this is one of the nasty multithreading problems.... any help would be appreciated.

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• URL equals and checking Internet access

  - by James P.

  On http://java.sun.com/j2se/1.5.0/docs/api/java/net/URL.html it states that: Compares this URL for equality with another object. If the given object is not a URL then this method immediately returns false. Two URL objects are equal if they have the same protocol, reference equivalent hosts, have the same port number on the host, and the same file and fragment of the file. Two hosts are considered equivalent if both host names can be resolved into the same IP addresses; else if either host name can't be resolved, the host names must be equal without regard to case; or both host names equal to null. Since hosts comparison requires name resolution, this operation is a blocking operation. Note: The defined behavior for equals is known to be inconsistent with virtual hosting in HTTP. According to this, equals will only work if name resolution is possible. Since I can't be sure that a computer has internet access at a given time, should I just use Strings to store addresses instead? Also, how do I go about testing if access is available when requested?

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• SqlCE DB occasionally freezes on one handheld, not another

  - by Michael

  I have two types of custom handhelds which are similar, but slightly different, each running the same WinForm application and a WinCE database: Type 1: WinCE 4.2, 400 mhz, 93244 kb Type 2: WinCE 5.0, 520 mhz, 84208 kb Type 1 will happily proceed through a large batch db operation (initiated) by the app, by Type 2 will consistently begin c-r-a-w-l-i-n-g (for several to many cycles) at around the 200 cycle mark. As several points it will begin running normally and then crawl again. The app does several db op's (inserts, updates and selects, no deletes). To simplify my situation, I've built a small test app which essentially does this: command_s.CommandText = "select dvr from vr where vid = 2211250"; command_u.CommandText = "update pvr set LocationID=81 where Status='OK' and vri = 27861"; while(going) { command_s.ExecuteScalar(); command_u.ExecuteNonQuery(); } and set it off running on the two units side by side. Sure enough, the slower (400 mhz) unit is outpacing the faster (520 mhz) unit (it's about 5000 cycles ahead right now) and I can see noticable pauses on the 520 mhz unit. What is causing this?

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• Codeigniter: Library function--I'm stuck

  - by Kevin Brown

  I have a library function that sets up my forms, and submits data. They're long, and they work, so I'll spare you reading my code. :) I simply need a way for my functions to determine how to handle the data. Until now, the function did one thing: Submit a report for the current user. NOW, the client has requested that an administrator also be able to complete a form--this means that the form would be filled out, and it would CREATE a user at the same time, whereas the current function EDITS and is accessed by an EXISTING user. Do I need a separate function to do essentially the same thing? How do I make one function perform two tasks? One to update a user, and if there is no user, create one. Current controller: function survey() { $id = $this->session->userdata('id'); $data['member'] = $this->home_model->getUser($id); //Convert the db Object to a row array $data['manager'] = $data['member']->row(); $manager_id = $data['manager']->manager_id; $data['manager'] = $this->home_model->getUser($manager_id); $data['manager'] = $data['manager']->row(); $data['header'] = "Home"; $this->survey_form_processing->survey_form($this->_container,$data, $method); } Current Library: function survey_form($container) { //Lots of validation stuff $this->CI->validation->set_rules($rules); if ( $this->CI->validation->run() === FALSE ) { // Output any errors $this->CI->validation->output_errors(); } else { // Submit form $this->_submit(); } $this->CI->load->view($container,$data); The submit function is huge too. Basically says, "Update table with data where user_id=current user" I hope this wasn't too confusing. I'll create two functions if need be, but I'd like to keep redundancy down! }

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• Do all the HTML5 storage systems work together ?

  - by azera

  While there are a lot of good stuff about html5, one thing I don't get is the redondant storage mechanism, first there is localstorage and sessionstorage, which are key value stores, one is for one instance of the app ("one tab"), and the other works for all the instances of that application so they can share data. Both are saved when you close your browser and have a limited size (usually 5MB), that's great and everything would be nice if we stopped there. But then there is the "Web SQL Database", which has the same security system as the localstorage, the same size limit, the same everything except it works like/is sqlite, with tables and sql syntax and all of that. And the bummer is, they don't work on the same data at all ! This is not two way to access your data, this is really two storage for every html 5 app out there (not created by default yes, but still you see my point). What I would like to know is, is there a reason for both of this mechanisms to exist at the same time ? Or did they just look at sql and nosql movement to pick the best then went "screw it let's add both !" ? Why not implement local/session storage as a table inside web sql db ?

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• Why does SQLite take such a long time to fetch the data?

  - by Derk

  I have two possible queries, both giving the result set I want. Query one takes about 30ms, but 150ms to fetch the data from the database. SELECT id FROM featurevalues as featval3 WHERE featval3.feature IN (?,?,?,?) AND EXISTS ( SELECT 1 FROM product_to_value, product_to_value as prod2, features, featurevalues WHERE product_to_value.feature = features.int AND product_to_value.value = featurevalues.id AND features.id = ? AND featurevalues.id IN (?,?) AND product_to_value.product = prod2.product AND prod2.value = featval3.id ) Query two takes about 3ms -this is the one I therefore prefer-, but also takes 170ms to fetch the data. SELECT ( SELECT prod2.value FROM product_to_value, product_to_value as prod2, features, featurevalues WHERE product_to_value.feature = features.int AND product_to_value.value = featurevalues.id AND features.id = ? AND featurevalues.id IN (?,?) AND product_to_value.product = prod2.product AND prod2.value = featval3.id ) as id FROM featurevalues as featval3 WHERE featval3.feature IN (?,?,?,?) The 170ms seems to be related to the number of rows from table featval3. After an index is used on featval3.feature IN (?,?,?,?), 151 items "remain" in featval3. Is there something obvious I am missing regarding the slow fetching? As far as I know everything is properly indexed.. I am confused because the second query only takes a blazing 3ms to run.

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• Can't seem to get .Union to work (merging 2 array's together, exclude duplicates)

  - by D. Veloper

  I want to combine two array's, excluding duplicates. I am using a custom class: public class ArcContact : IEquatable<ArcContact> { public String Text; public Boolean Equals(ArcContact other) { if (Object.ReferenceEquals(other, null)) return false; if (Object.ReferenceEquals(this, other)) return true; return Text.Equals(other.Text); } public override Int32 GetHashCode() { return Text == null ? 0 : Text.GetHashCode(); } } I implemented and the needed IEquatable interface as mentioned in this msdn section. I only want to check the Text property of the ArcContact class and make sure an Array of ArcContact have an unique Text. Here I pasted the code that I use, as you can see I have method with two parameters, array's to combine and below that the code I got from the previous mentioned msdn section. internal static class ArcBizz { internal static ArcContact[] MergeDuplicateContacts(ArcContact[] contacts1, ArcContact[] contacts2) { return (ArcContact[])contacts1.Union(contacts2); } internal static IEnumerable<T> Union<T>(this IEnumerable<T> a, IEnumerable<T> b); } What am I doing wrong?

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• center div tags inside parent div

  - by Senthilnathan

  I have two div tags inside a parent div. I want to display the two divs in same line and centered. Below is the html code. <div id="parent"> <div id="addEditBtn" style="display:inline-block; vertical-align: middle; width:20px; cursor:pointer;" class="ui-state-default ui-corner-all"> <span class="ui-icon ui-icon-pencil"></span></div> <div id="deleteBtn" style="display:inline-block; vertical-align: middle; width:20px; cursor:pointer;" class="ui-state-default ui-corner-all"> <span class="ui-icon ui-icon-trash"></span></div> </div> I tried with "display:inline-block; vertical-align: middle;" but its getting aligned left. Please help me out to centered the div tags inside the parent div.

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• Is it possible to guarantee a unique id for multiple items using the same id variable at a point in

  - by Scarface

  First of all, do not be overwhelmed by the long code, I just put it for reference...I have a function that preg_replaces content and puts it in a jquery dialog box with a matching open-link. For example, if there is a paragraph with two matches, they will be put inside two divs, and a jquery dialog function will be echoed twice; one for each div. While this works for one match, if there are multiple matches, it does not. I am not sure how to distribute unique ids at a point in time for each of the divs and matching dialog open-scripts. Keep in mind, I removed the preg replace function since it kind of complicates the problem. If anyone has any ideas, they will be greatly appreciated. <?php $id=uniqid(); $id2=uniqid(); echo "<div id=\"$id2\"> </div>"; ?> $.ui.dialog.defaults.bgiframe = true; $(function() { $("<?php echo"#$id2"; ?>").dialog({hide: 'clip', modal: true ,width: 600,height: 350,position: 'center', show: 'clip',stack: true,title: 'title', minHeight: 25, minWidth: 100, autoOpen: false}); $('<?php echo"#$id"; ?>').click(function() { $('<?php echo"#$id2"; ?>').dialog('open'); }) .hover( function(){ $(this).addClass("ui-state-hover"); }, function(){ $(this).removeClass("ui-state-hover"); } ).mousedown(function(){ $(this).addClass("ui-state-active"); }) .mouseup(function(){ $(this).removeClass("ui-state-active"); }); });

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• Updating target workbook - extracting data from source workbook

  - by Allan

  My question is as follows: I have given a workbook to multiple people. They have this workbook in a folder of their choice. The workbook name is the same for all people, but folder locations vary. Let's assume the common file name is MyData-1.xls. Now I have updated the workbook and want to give it to these people. However when they receive the new one (let's call it MyData-2.xls) I want specific parts of their data pulled from their file (MyData-1) and automatically put into the new one provided (MyData-2). The columns and cells to be copied/imported are identical for both workbooks. Let's assume I want to import cell data (values only) from MyData-1.xls, Sheet 1, cells B8 through C25 ... to ... the same location in the MyData-2.xls workbook. How can I specify in code (possibly attached to a macro driven import data now button) that I want this data brought into this new workbook. I have tried it at my own location by opening the two workbooks and using the copy/paste-special with links process. It works really well, but It seems to create a hard link between the two physical workbooks. I changed the name of the source workbook and it still worked. This makes me believe that there is a "hard link" between the tow and that this will not allow me to give the target (MyData-2.xls) workbook to others and have it find their source workbook.

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• Is it a good idea to use an integer column for storing US ZIP codes in a database?

  - by Yadyn

  From first glance, it would appear I have two basic choices for storing ZIP codes in a database table: Text (probably most common), i.e. char(5) or varchar(9) to support +4 extension Numeric, i.e. 32-bit integer Both would satisfy the requirements of the data, if we assume that there are no international concerns. In the past we've generally just gone the text route, but I was wondering if anyone does the opposite? Just from brief comparison it looks like the integer method has two clear advantages: It is, by means of its nature, automatically limited to numerics only (whereas without validation the text style could store letters and such which are not, to my knowledge, ever valid in a ZIP code). This doesn't mean we could/would/should forgo validating user input as normal, though! It takes less space, being 4 bytes (which should be plenty even for 9-digit ZIP codes) instead of 5 or 9 bytes. Also, it seems like it wouldn't hurt display output much. It is trivial to slap a ToString() on a numeric value, use simple string manipulation to insert a hyphen or space or whatever for the +4 extension, and use string formatting to restore leading zeroes. Is there anything that would discourage using int as a datatype for US-only ZIP codes?

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• How to play simultaneous multiply audio sources in Silverlight

  - by Shurup

  I want to play simultaneous multiply audio sources in Silverlight. So I've created a prototype in Silverlight 4 that should play a two mp3 files containing the same ticks sound with an intervall 1 second. So these files must be sounded as one sound if they will be played together with any whole second offsets (0 and 1, 0 and 2, 1 and 1 seconds, etc.) I my prototype I use two MediaElement (me and me2) objects. DateTime startTime; private void Play_Clicked(object sender, RoutedEventArgs e) { me.SetSource(new FileStream(file1), FileMode.Open))); me2.SetSource(new FileStream(file2), FileMode.Open))); var timer = new DispatcherTimer { Interval = TimeSpan.FromMilliseconds(1) }; timer.Tick += RefreshData; timer.Start(); } First file should be played at 00:00 sec. and the second in 00:02 second. void RefreshData(object sender, EventArgs e) { if(me.CurrentState != MediaElementState.Playing) { startTime = DateTime.Now; me.Play(); return; } var elapsed = DateTime.Now - startTime; if(me2.CurrentState != MediaElementState.Playing && elapsed >= TimeSpan.FromSeconds(2)) { me2.Play(); ((DispatcherTimer)sender).Stop(); } } The tracks played every time different and not simultaneous as they should (as one sound).

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• In a Tab Bar based app a controller release data of the other ! !

  - by Flodev03

  Hi all ! I've made a ViewBased app, in the app delegate i've set a UITabBarCotntroller, in the app i have different view Controller two of them displays text in a UITextView and labels, the other one is my "ShakeController" a UIViewController in which i've set a UIAcelerometerDelegate, in it i create a instance of UIAccelerometer, in the method which manages the shake everything works fine, in this controller i have also set a UIImageView to make a simple animation, in the view Did Load method i set my imageView.animation to an array of UIImage. My problem is : when the app is launched i use the ViewControllers and everything work fine, but when i tap the ShakeController item in the tab bar and then when i come back to the other controllers the label looks like : label and textView like : Lorem ipsum..... the text of UItextView in IB. I have noticed thaht if i comment the initialisation of my imageView to the array of image i can navigate the items (from a view controller to another) without the label change and stay what i want them to be. Notice that the two controllers are in a UINavigationController. (i use @proprety (nonnatomic, retain) then @synthesize ... then releqse in the dealloc for the labels textview and my uiimageView) Do not know what to do thanks to all

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• Hibernate collection multiple types

  - by CaptainAwesomePants

  I have a class Player that contains a list of Accessory objects. There are two kinds of Accessories. SocketedAccessories have a list of SocketJewels, and MagicAccessories have a list of MagicEnchantments. At the database level, there is a players table that represents the player, and an accessories table that contains a list of accessories. Accessories have a type field that indicates whether they are socketed or magical, and the columns that are only used by one type are just left blank by entries of the other type. There are socket_jewels and magic_enchantments tables, representing the socket jewels or the magic enchantments on each accessory. I am trying to figure out the correct way to map this with Hibernate. One way would be for the player to have two lists of accessories, one for SocketedAccessories and one for MagicAccessories. That seems undesirable, though. What I want is a way to specify that player should have a field List<Accessory> accessories that contains both types of thing. Is there a way to tell Hibernate, in either hbm.xml or annotations, to do this?

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• MS SQL find and replace in TEXT field

  - by incubushead

  I have a database in MS SQL 2005 that was brought up from SQL 2000 and is still using TEXT type fields instead of varchar(max). I need to find and replace a string of characters in the text field but all of the examples of how to do this that I have found don't seem like they would work for me. It seems the UPDATETEXT command requires that the two parameters "insert_offset" and "delete_length" be set explicitly but the string i am searching for could show up in the text at any point or even at several points in the same cell. My understanding of these two parameters is that the string im searching for will always be in the same place, so that insert_offset is the number of spaces into the text that the UPDATETEXT command will start replacing text. Example: Need to find: &lt;u&gt; and Replace it with: <u> Text field example: *Everyone in the room was <b>&lt;u&gt;tired&lt;/u&gt;.</b><br>Then they woke <b>&lt;u&gt;up&lt;/u&gt;. Can anyone help me out with this? THANKS!

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• Pro ASP.Net MVC 3 Entity Framework Sports Store tutorial

  - by gary7

  Following the tutorial in the book "Pro ASP.Net MVC 3 Entity Framework" in Chapter 9 - Image Uploads section; asks that the Product class be updated with two new columns - public byte ImageData, and public string ImageType. It also directs that the database be updated with these two columns via the server explorer. After these updates, the discussion directs that the Entity Framework Conceptual Model be updated via the SportsStore.EDMX file. This file does not exist in the source code for the project, and was not used in the project to begin with. Obvious errata for the book. Adding the ADO.NET Entity Data Model to the Project then overrides the EFProduct reposistory (conceptual model used throughout the project) which inherits from the interface IProductsRepository; and results in errors within the mapping. If the project is debugged after the columns are added, an error is thrown related to the new added columns. Has anyone resolved this issue in the project? I haven't found any solutions so far. Thanks!

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• Updating section in ConfigParser (or an alternative)

  - by lyrae

  I am making a plugin for another program and so I am trying to make thing as lightweight as possible. What i need to do is be able to update the name of a section in the ConfigParser's config file. [project name] author:john doe email: [email protected] year: 2010 I then have text fields where user can edit project's name, author, email and year. I don't think changing [project name] is possible, so I have thought of two solutions: 1 -Have my config file like this: [0] projectname: foobar author:john doe email: [email protected] year: 2010 that way i can change project's name just like another option. But the problem is, i would need the section # to be auto incremented. And to do this i would have to get every section, sort of, and figure out what the next number should be. The other option would be to delete the entire section and its value, and re-add it with the updated values which would require a little more work as well, such as passing a variable that holds the old section name through functions, etc, but i wouldn't mind if it's faster. Which of the two is best? or is there another way? I am willing to go with the fastest/lightweight solution possible, doesn't matter if it requires more work or not.

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• What is the best practice for including third party jar files in a Java program?

  - by ZoFreX

  I have a program that needs several third-party libraries, and at the moment it is packaged like so: zerobot.jar (my file) libs/pircbot.jar libs/mysql-connector-java-5.1.10-bin.jar libs/c3p0-0.9.1.2.jar As far as I know the "best" way to handle third-party libs is to put them on the classpath in the manifest of my jar file, which will work cross-platform, won't slow down launch (which bundling them might) and doesn't run into legal issues (which repackaging might). The problem is for users who supply the third party libraries themselves (example use case, upgrading them to fix a bug). Two of the libraries have the version number in the file, which adds hassle. My current solution is that my program has a bootstrapping process which makes a new classloader and instantiates the program proper using it. This custom classloader adds all .jar files in lib/ to its classpath. My current way works fine, but I now have two custom classloaders in my application and a recent change to the code has caused issues that are difficult to debug, so if there is a better way I'd like to remove this complexity. It also seems like over-engineering for what I'm sure is a very common situation. So my question is, how should I be doing this?

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• How to access child div elements under a given condition with javascript?

  - by hlovdal

  My main question is to calculate the last alert message, but any other information is also welcome. I am trying to learn javascript (to use with greasemonkey later), but I am struggling a bit to grasp the DOM and how to process it. <html> <head> <script type="text/javascript"> function my_test() { var elements = document.getElementsByTagName("div"); // prints "found [object HTMLCollection] with length 8" alert("found " + elements + " with length " + elements.length); // prints "0:[object HTMLDivElement]" alert("0:" + elements[0]); // how to calculate the following? alert("for intereting one is AAAA and three is CCCC"); } </script> </head> <body> <div class="interesing"> <div class="one">AAAA</div> <div class="two">BBBB</div> <div class="three">CCCC</div> </div> <div class="boring"> <div class="one">1111</div> <div class="two">2222</div> <div class="three">3333</div> </div> <input type="button" onclick="my_test()" value="my test" </body> </html> So elements is now an array of elements and I can access each of them individually. But where can I find what methods/properties these elements have?

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• Java downcasting dilemma

  - by Shades88

  please have a look at this code here. class Vehicle { public void printSound() { System.out.print("vehicle"); } } class Car extends Vehicle { public void printSound() { System.out.print("car"); } } class Bike extends Vehicle{ public void printSound() { System.out.print("bike"); } } public class Test { public static void main(String[] args) { Vehicle v = new Car(); Bike b = (Bike)v; v.printSound(); b.printSound(); Object myObj = new String[]{"one", "two", "three"}; for (String s : (String[])myObj) System.out.print(s + "."); } } Executing this code will give ClassCastException saying inheritance.Car cannot be cast to inheritance.Bike. Now look at the line Object myObj = new String[]{"one", "two", "three"};. This line is same as Vehicle v = new Car(); right? In both lines we are assigning sub class object to super class reference variable. But downcasting String[]myObj is allowed but (Bike)v is not. Please help me understand what is going on around here.

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• Behavior of <- NULL on lists versus data.frames for removing data

  - by Ananda Mahto

  Many R users eventually figure out lots of ways to remove elements from their data. One way is to use NULL, particularly when you want to do something like drop a column from a data.frame or drop an element from a list. Eventually, a user comes across a situation where they want to drop several columns from a data.frame at once, and they hit upon <- list(NULL) as the solution (since using <- NULL will result in an error). A data.frame is a special type of list, so it wouldn't be too tough to imagine that the approaches for removing items from a list should be the same as removing columns from a data.frame. However, they produce different results, as can be seen in the example below. ## Make some small data--two data.frames and two lists cars1 <- cars2 <- head(mtcars)[1:4] cars3 <- cars4 <- as.list(cars2) ## Demonstration that the `list(NULL)` approach works cars1[c("mpg", "cyl")] <- list(NULL) cars1 # disp hp # Mazda RX4 160 110 # Mazda RX4 Wag 160 110 # Datsun 710 108 93 # Hornet 4 Drive 258 110 # Hornet Sportabout 360 175 # Valiant 225 105 ## Demonstration that simply using `NULL` does not work cars2[c("mpg", "cyl")] <- NULL # Error in `[<-.data.frame`(`*tmp*`, c("mpg", "cyl"), value = NULL) : # replacement has 0 items, need 12 Switch to applying the same concept to a list, and compare the difference in behavior. ## Does not fully drop the items, but sets them to `NULL` cars3[c("mpg", "cyl")] <- list(NULL) # $mpg # NULL # # $cyl # NULL # # $disp # [1] 160 160 108 258 360 225 # # $hp # [1] 110 110 93 110 175 105 ## *Does* drop the `list` items while this would ## have produced an error with a `data.frame` cars4[c("mpg", "cyl")] <- NULL # $disp # [1] 160 160 108 258 360 225 # # $hp # [1] 110 110 93 110 175 105 The main questions I have are, if a data.frame is a list, why does it behave so differently in this scenario? Is there a foolproof way of knowing when an element will be dropped, when it will produce an error, and when it will simply be given a NULL value? Or do we depend on trial-and-error for this?

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