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• What is the possible benefit (if any) of allowing recursive contructors?

  - by Penang

  In Java, constructors cannot be recursive. Compile time error: "recursive constructor invocation". Let's assume that we did not have this restriction. Things to keep in mind: The return type of a constructor is void. Since it is a void method you can't harness the complete power of recursion. A constructor can invoke itself (or any other constructor) using this(). But a "call to this must be first statement in constructor" We could use non local data between consecutive calls to still have some possible gain from recursive constructors. Would there be any benefit from allowing recursive constructors?

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• Finding the average of two number using classes and methods

  - by Have alook

  I want to use methods inside class. Q: find the average of two number using classes and methods. import java.util.*; class aaa { int a,b,sum,avrg; void average() { System.out.println("The average is ="+avrg); avrg=(sum/2); } } class ave { public static void main(String args[]){ aaa n=new aaa(); Scanner m=new Scanner(System.in); System.out.println("write two number"); n.a=m.nextInt(); n.b=m.nextInt(); n.average(); } }

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• How can I create a new class instance from a class within a (static) class?

  - by Mervin

  I'm new to Java (have experience with C#), This is what i want to do: public final class MyClass { public class MyRelatedClass { ... } } public class OtherRandomClass { public void DoStuff() { MyRelatedClass data = new MyClass.MyRelatedClass(); } } which gives this error in Eclipse: "No enclosing instance of type BitmapEffects is accessible. Must qualify the allocation with an enclosing instance of type BitmapEffects (e.g. x.new A() where x is an instance of BitmapEffects)." this is possible in C# with static classes , how should it be done here?

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• Notifying JMS Message sender (Web App) after message processed by Listener

  - by blob

  I have a Web application (Flex 4 - Spring Blaze DS - Spring 3.0) which sends out an JMS event to a batch application (Standalone java) I am using JMS infrastrucure provided by Spring (spring JmsTemplate,SimpleMessageListenerContainer,MessageListenerAdapter) with TIBCO EMS. Is there any way by which we can notify a web user once message processing is completed by listener. One of the way to send a response event which will be listened by web application; but how to address following scenario: User1 click on submit - which in turn sends a JMS message Listener on receiving message processes the message (message processing may take 20-30 mins to complete). Listener application sends out another JMS event "Process_complete" As this is a web application; there are n users currently logged into the application. so how to identify a correct user / what if user is already logged off? Is there any way to handle this? Please post your views.

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• Corba sequence<octet> a lot slower than using a socket

  - by Totonga

  I have a corba releated question. In my Java app I use typedef sequence Data; Now I played around with this Data vector. If I am right with the Corba specification sequence will either be converted to xs:base64Binary or xs:hexBinary. It should be an Opaque type and so it should not use any marshalling. I tried different idl styles: void Get(out Data d); Data Get(); but what I see is that moving the data using Corba is a lot slower than using a socket directly. I am fine with a little overhead but it looks for me like tha data is still marshalled. Do I need to somehow configure my orb to suppress the marshalling or did I miss something.

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• Learning how to program real things.

  - by Sean

  How would you guys recommend I actually learn to program real things? I mean, I know how to do basic academic things. I can implement a templated stack/queue/map/etc. data structure in C++ or Java or whatever. I can make a text-based hangman game or whatever. Etc etc. But how can I learn to program something real, something useful? I've done project Euler up to question 100 or so, and I feel like that's given me more mathematical maturity but not programming maturity. Should I buy a book and follow exercises, struggle through interesting projects, etc, ? In short, how did you guys transition from academic exercises to real, fun and/or useful programs?

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• Log RuntimeException thrown from thread created by Spring via the @Async annotation

  - by Eugen

  I'm having some difficulty logging RuntimeException from a thread. My system is: Java 7 (b118), Spring 3.0.5. The threads are not created by hand, but via Spring's @Async annotation, which creates it's own executor behind the scenes, so I don't really have the option of overriding any methods of the thread, FutureTask or anything low level. So my question is if Spring has any support or if there are any best practices for handling (logging) these type of exceptions? Any suggestions are appreciated. Thanks.

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• resizing arrays when close to memory capacity

  - by user548928

  So I am implementing my own hashtable in java, since the built in hashtable has ridiculous memory overhead per entry. I'm making an open-addressed table with a variant of quadratic hashing, which is backed internally by two arrays, one for keys and one for values. I don't have the ability to resize though. The obvious way to do it is to create larger arrays and then hash all of the (key, value) pairs into the new arrays from the old ones. This falls apart though when my old arrays take up over 50% of my current memory, since I can't fit both the old and new arrays in memory at the same time. Is there any way to resize my hashtable in this situation Edit: the info I got for current hashtable memory overheads is from here How much memory does a Hashtable use? Also, for my current application, my values are ints, so rather than store references to Integers, I have an array of ints as my values.

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• How to setup Main class in manifest file in jar produced by NetBeans project

  - by Leni Kirilov

  I have the following problem. I have a Java project in my NetBeans IDE 6.8. When I compile it and it produces a .jar file containing everything possible, the META-INF is not right. It doesn't contain the class to be executed - with main() method. When I click the Run button inside the IDE, everything works. The settings of the project are also set the right way - pointing to a class in my project. I tried adding a folder META-INF with manifest file but I didn't manage. Is there a way to do this manually in NetBeans, because I found that if I add the missing Main class in the manifest, everything works.

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• What should I do if a IOException is thrown?

  - by Roman

  I have the following 3 lines of the code: ServerSocket listeningSocket = new ServerSocket(earPort); Socket serverSideSocket = listeningSocket.accept(); BufferedReader in = new BufferedReader(new InputStreamReader(serverSideSocket.getInputStream())); The compiler complains about all of these 3 lines and its complain is the same for all 3 lines: unreported exception java.io.IOException; In more details, these exception are thrown by new ServerSocket, accept() and getInputStream(). I know I need to use try ... catch .... But for that I need to know what this exceptions mean in every particular case (how should I interpret them). When they happen? I mean, not in general, but in these 3 particular cases.

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• Why element creation requires the document object in DOM?

  - by maayank

  As noted by others, in Java, with the default W3C DOM libraries, one is required to use the Document object a factory to elements, i.e.: import org.w3c.dom.Document; import org.w3c.dom.Element; Document d; Element e; e = d.createElement("tag"); Why is that necessary? Why conceptually a method can't create an XML element without knowing all of the target document? Why I can't just instantiate using 'new' or something to that effect?

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• JFrame.setBackground() not working -- why?

  - by devoured elysium

  JFrame mainFrame = new JFrame(); mainFrame.setSize(100, 100); mainFrame.setBackground(Color.CYAN); mainFrame.setVisible(true); My intent is to create a window with a cyan background. What is wrong with this? My window doesn't get cyan, as I'd expect! Also, could anyone point out why I seem to have all the colors in duplicate (there's a Color.CYAN and a Color.cyan). Is there any difference at all between the two? Maybe the older one was a constant from before there were enums in Java and the second one is from the Enum? Thanks

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• Increasing the JVM maximum heap size for memory intensive applications

  - by Alceu Costa

  I need to run a Java memory intensive application that uses more than 2GB, but I am having problems to increase the heap maximum size. So far, I have tried the following approaches: Setting the -Xmx parameter, e.g. -Xmx3000m. This approaches fails at the creation of the JVM. From what I've googled, it looks like that -Xmx must be less than 2GB. Using the -XX:+AggressiveHeap option. When I try this approach I get an 'Not enough memory' error that tells that the heap size is 1273.4 MB, even though my computer has 8GB of memory. Is there another approach that I can try to increase the maximum heap size of the JVM? Here's a summary of the computer specs: OS: Windows 7 (64 bit) Processor: Intel Core i7 (2.66 GHz) Memory: 8 GB

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• Jquery Retrieving data from webpage and entering it in database.

  - by user279321

  Hello Everyone. I am new to Jquery. I want to read specific data(some div's only) from a webpage and store it in my database. Now I am not able to do so as load functions loads the data into the current page id. My goal is to read specific data and store them in database. So which Jquery function should I use for this? if someone can throw some light on it, it would be really helpful for me. I am using JSP,MySQL,Java.

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• Finding if a path between 2 sides of a game board exists

  - by Meny

  Hi, i'm currently working on a game as an assignment for school in java. the game cuurently is designed for Console. the game is for 2 players, one attacking from north to south, and the other from west to east. the purpose of the game is to build a "bridge"/"path" between the 2 of your sides before your opponent does. for example: A B C D E F 1 _ _ X _ _ _ 1 2 O X X _ _ _ 2 3 O X O O O O 3 4 O X O _ _ _ 4 5 X X _ _ _ _ 5 6 X O _ _ _ _ 6 A B C D E F player that attacks from north to south won (path/bridge from C to A) my problem is, what algorithm would be good to check if the user have managed to create a path (will be checked at the end of each turn). you're help would be very appreciated.

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• Vim Editor is very smart?

  - by Narek

  I am programming in C++ or Java. So I want to use Vim editor, because it is very flexible. I have heard that I can configure the Vim editor to be able to go from object to the definition from function to the definition from class name to the definition Do we have any professional Vim-er that could tell me how exactly to configure Vim for that? Thanks in advance. P.S. In case readers will consider this question is not connected with programming, I would say that this is improving speed of programming. So it is a help to the programmer. So please don't close this question. EDIT: Also I would like to know how vim works with code completion and can vim hint the list of methods that are available for the certain object? If yes, then I would like to know how to configure these options too?

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• libriaries on Android

  - by Will

  In native development, you can produce libraries - .so, .dll etc - that other libraries and programs can share. In Java, you can put your code in a JAR in the classpath and other programs and libraries can use it. So how can you do modular programs in Android? I get the impression from the documentation that each APK is a self-contained island. Can different APKs share code? And how does installation work when you have these external dependencies? Can you specify that you rely on library that is not in your APK and get it installed too? If you can't have 'dynamic linking', can you have 'static linking' by developing your shared code and somehow putting the equiv of a JAR into each APK that requires it?

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• What is the JVM Scheduling algorithm ?

  - by IHawk

  Hello ! I am really curious about how does the JVM work with threads ! In my searches in internet, I found some material about RTSJ, but I don't know if it's the right directions for my answers. I also found this topic in sun's forums, http://forums.sun.com/thread.jspa?forumID=513&threadID=472453, but that's not satisfatory. Can someone give me some directions, material, articles or suggestion about the JVM scheduling algorithm ? I am also looking for information about the default configurations of Java threads in the scheduler, like 'how long does it take for every thread' in case of time-slicing. And this stuff. I would appreciate any help ! Thank you !

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• Benefit of outputting JSON as opposed to plain HTML

  - by Franco

  Hey guys, Just wondering which is best here. I want to output data from a table in my DB then put a lot of this data into a html table on the fly on my page. Im working with Java on the server side. Basically I pull the results form the DB and have the raw data..just what next? There is a chance I may want to take data from multiple tables in order to combine it into one table for my site. I retrieve the results of the query from the DB, now do i create a text from it in the form of json which i can parse as json using jquery upon the return of the object to my browser?(kind of a sub question of this question: Is just using a stringbuilder the correct way to make a json object to output?) Or.. Should i build the HTML as a string and output that to the browser instead? Which is better and why? Thanks in advance!

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• Reading files from an embedded ZIP archive

  - by aix

  I have a ZIP archive that's embedded inside a larger file. I know the archive's starting offset within the larger file and its length. Are there any Java libraries that would enable me to directly read the files contained within the archive? I am thinking along the lines of ZipFile.getInputStream(). Unfortunately, ZipFile doesn't work for this use case since its constructors require a standalone ZIP file. For performance reasons, I cannot copy the ZIP achive into a separate file before opening it. edit: Just to be clear, I do have random access to the file.

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• SmartGWT Server File Browser

  - by jluzwick

  Hi All: I have been looking for a SmartGWT example that would show me how to build a File Browser widget that takes the files from the local server's root directory. The user would be shown the files through the browser which they could then select to perform some processing operations. So far I have thought of using SmartGWT's Tree-Data Binding-Load from Local Data Widget and then grabbing a list of the directories using: new File("\").listFiles(); My Question is: Is there a better way to do this? Has someone already thought of this and has an example of their code that I can see? PS: I'm fairly new to GWT and Web Services but fairly competent with Java. If you believe there is a better way to do this (while still doing this through the web and not using Applets, please tell me). Thanks

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• J2EE: Default values for custom tag attributes

  - by Nick

  So according to Sun's J2EE documentation (http://docs.sun.com/app/docs/doc/819-3669/bnani?l=en&a=view), "If a tag attribute is not required, a tag handler should provide a default value." My question is how in the hell do I define a default value as per the documentation's description. Here's the code: <%@ attribute name="visible" required="false" type="java.lang.Boolean" %> <c:if test="${visible}"> My Tag Contents Here </c:if> Obviously, this tag won't compile because it's lacking the tag directive and the core library import. My point is that I want the "visible" property to default to TRUE. The "tag attribute is not required," so the "tag handler should provide a default value." I want to provide a default value, so what am I missing? Any help is greatly appreciated.

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• Need help understanding the MVC design pattern

  - by Doron Sinai

  Hi, I am trying to find a ood example of MVC design pattern in java. This is what i understood from reading about it, please correct me if I am wrong: I have the Model part which is the logic behind the program, let's say if we have a phonebook, so adding and removing contact from the Array will be the model. The Gui is the view and it contains buttons that upon clicking them, the model is changing. What I am trying to undersand what is the controller part, is it the ActionListeners? how to you seperate those modules in practice. thank you

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• How to do animation using swing and clojure ?

  - by Humberto Pinheiro

  I'm trying to animate a chess piece in a board. First I created a java.util.Timer object that "scheduleAtFixedRate" a TimerTask implemented as a proxy function. So I kept a record of the piece to move (piece-moving-record) and when it's apropriate (when the user move the piece using the mouse) the TimerTask proxy function should be test if the record is not nil and execute the piece-moving function. The piece-moving function just updates the x and y coordinates of the piece, according to a vector pre-calculated. I put a add-watch on the piece-moving-record so when it changes it should repaint the board (canvas). The paint method tests if this piece-moving-record is not nil to paint it. The problem is that the animation doesn't appear. The piece just jump to the destiny, without the movement between. There is some problem with the animation scheme ou there is a better way to do it?

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• Tuning JVM (GC) for high responsive server application

  - by elgcom

  I am running an application server on Linux 64bit with 8 core CPUs and 6 GB memory. The server must be highly responsive. After some inspection I found that the application running on the server creates rather a huge amount of short-lived objects, and has only about 200~400 MB long-lived objects(as long as there is no memory leak) After reading http://java.sun.com/javase/technologies/hotspot/gc/gc_tuning_6.html I use these JVM options -Xms2g -Xmx2g -XX:MaxPermSize=256m -XX:NewRatio=1 -XX:+UseConcMarkSweepGC Result: the minor GC takes 0.01 ~ 0.02 sec, the major GC takes 1 ~ 3 sec the minor GC happens constantly. How can I further improve or tune the JVM? larger heap size? but will it take more time for GC? larger NewSize and MaxNewSize (for young generation)? other collector? parallel GC? is it a good idea to let major GC take place more often? and how?

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