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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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• 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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• How to keep Hibernate mapping use under control as requirements grow

  - by David Plumpton

  I've worked on a number of Java web apps where persistence is via Hibernate, and we start off with some central class (e.g. an insurance application) without any time being spent considering how to break things up into manageable chunks. Over time as features are added we add more mappings (rates, clients, addresses, etc.) and then amount of time spent saving and loading an insurance object and everything it connects to grows. In particular you get close to a go-live date and performance testing with larger amounts of data in each table is starting to demonstrate that it's all too slow. Obviously there are a number of ways that we could attempt to partition things up, e.g. map only the client classes for the client CRUD screens, etc., which would have been better to get in place earlier rather than trying to work it in at the end of the dev cycle. I'm just wondering if there are recommendations about ways to handle/mitigate this.

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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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• Recursive Binary Search Tree Insert

  - by Nick Sinklier

  So this is my first java program, but I've done c++ for a few years. I wrote what I think should work, but in fact it does not. So I had a stipulation of having to write a method for this call: tree.insertNode(value); where value is an int. I wanted to write it recursively, for obvious reasons, so I had to do a work around: public void insertNode(int key) { Node temp = new Node(key); if(root == null) root = temp; else insertNode(temp); } public void insertNode(Node temp) { if(root == null) root = temp; else if(temp.getKey() <= root.getKey()) insertNode(root.getLeft()); else insertNode(root.getRight()); } Thanks for any advice.

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• XHTML editor wanted!

  - by stolsvik

  I need an editor that outputs clean HTML, and preferably strict XHTML with CSS, as I am going to display the resulting pages as help pages in a java application using xhmtlrenderer ("Flying Saucer") which apparently very much prefers XHTML with CSS 2.1. Having searched stackoverflow for HTML editor and XHTML editor, I've found several mentioned. Aptana is one, Eclipse's Web Tools is another, and KompoZer is a third. I am going to try these out. With this question, I ask whether there are other good options that would satisfy my requirements, and if there are any one of these mentioned or new ones that you'd recommend in particular, that I should focus a bit extra on?

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• How to implement GCM in Google AppEngine?

  - by Broken System

  I'm a PHP Developer and Web Designer. In my work a partner asked me if I could set up a Google Cloud Messaging server. I read the documentation but couldn't find a clear tutorial to set up this server. I got no knowleadge about Java so it makes my job harder. I could "compile" a war file using ant as GCM Demo Tutorial says. But I can't deploy it to my AppEngine server to try it out (It's my first time using AppEngine too). Could you give me some steps to create my own GCM server? Sorry about my bad english. Thanks!

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• What is a good practice to access class attributes in class methods?

  - by Clem

  I always wonder about the best way to access a class attribute from a class method in Java. Could you quickly convince me about which one of the 3 solutions below (or a totally different one :P) is a good practice? public class Test { String a; public String getA(){ return this.a; } public setA(String a){ this.a = a; } // Using Getter public void display(){ // Solution 1 System.out.println(this.a); // Solution 2 System.out.println(getA()); // Solution 3 System.out.println(this.getA()); } // Using Setter public void myMethod(String b, String c){ // Solution 1 this.a = b + c; // Solution 2 setA(b + c); // Solution 3 this.setA(b + c); } }

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• Should GSON ignore varying types if they're not used in a class?

  - by loeschg

  I'm making an API call that returns JSON which has a particular field which either returns false or a map depending on content. It's a field that I don't care about. I expected GSON to ignore this particular field, though it doesn't seem to be. The object generation fails with the following message: com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_OBJECT at line 1 column 403560 I've seen this particular question (Gson deserialize json with varying value types). I want to make sure I need to make a custom deserializer before doing so. I'm wondering if I may have another issue. edit: Example: "anonymous_flag": { } vs "anonymous_flag": "yes" Another Edit: I actually had the field in my model object... I was referencing the wrong class. Judge away :)

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• Catching errors in ANTLR and finding parent

  - by Andreas

  I have found out that I can catch errors during parsing by overwriting displayRecognitionError, but how do I find the parent "node" of this error? ex. if I have the grammar: prog: stat expr; stat: STRING; expr: INTEGER; And give it the input "abc def". Then I will get an error at "def" which should be an integer. At this point I then want to get the parent which is "expr" (since it fails inside the INTEGER part) and it's parent "prog". Kind of like printing stack trace in java. I tried to look at the node from RecognitionException parsed to displayRecognitionError, but it is null, and using CommonErrorNode the parent is null. Should I maybe take a completely different approach?

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• executing a class in remote jvm

  - by redzedi

  Hi All, I have a small test class that i want to run on a particular jvm that's already up and running (basically it's an web application running on Tomcat) . The reason i want to do this is i want to execute a small test class (with the main method and all) within that jvm so that i get the same environment (loaded and initialized classes) for my test class. Is it possible to indicate that ,say through a jvm parameter, that it should not initialize a new vm to execute my class but instead go and execute on the remote vm and show me the result here, on my console. So the local jvm acts as a kind of thin proxy ? I am not aware in case there are some tools that should make this possible ... also heard somewhere that java 6 jvm comes with an option like this , is that true ?? Please help me. Thanks,

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• I'm having a hard time calling the variable from first frame.

  - by iownfloppydisk

  here is how should my program be. In first frame, there is a textfield1 where a user input text and when he press a button, a new frame will be display with a textfield2 that displays the inputted text from the textfield1. please help me with the syntax. i'm still a beginner in java. much thanks guys. First Frame: textfield= new JTextField(); textfield.setPreferredSize( new Dimension(200,30) ) ; textfield.setSize( textfield.getPreferredSize() ) ; textfield.setLocation(95,198) ; textfield.setSize(175,28); cont.add(textfield); public void actionPerformed(ActionEvent e) { this.setVisible(false); new Frame2().setVisible(true); //displays the 2nd frame right? } now i don't know what to do on my 2nd frame or where to start because i can't get the variable from the first frame

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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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• 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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• Validate an XML File Against Multiple Schema Definitions

  - by Jon

  I'm trying to validate an XML file against a number of different schemas (apologies for the contrived example): a.xsd b.xsd c.xsd c.xsd in particular imports b.xsd and b.xsd imports a.xsd, using: <xs:include schemaLocation="b.xsd"/> I'm trying to do this via Xerces in the following manner: XMLSchemaFactory xmlSchemaFactory = new XMLSchemaFactory(); Schema schema = xmlSchemaFactory.newSchema(new StreamSource[] { new StreamSource(this.getClass().getResourceAsStream("a.xsd"), "a.xsd"), new StreamSource(this.getClass().getResourceAsStream("b.xsd"), "b.xsd"), new StreamSource(this.getClass().getResourceAsStream("c.xsd"), "c.xsd")}); Validator validator = schema.newValidator(); validator.validate(new StreamSource(new StringReader(xmlContent))); but this is failing to import all three of the schemas correctly resulting in cannot resolve the name 'blah' to a(n) 'group' component. I've validated this successfully using Python, but having real problems with Java 6.0 and Xerces 2.8.1. Can anybody suggest what's going wrong here, or an easier approach to validate my XML documents?

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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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• BlackBerry project version number

  - by Seva Alekseyev

  I have a BlackBerry Java project in Eclipse. It has version number written down in four different spots: in the project properties, under "BlackBerry Project Settings/General" in the JAD file, under MIDlet-1 in the JAD file, under MIDlet-Version in the ALX file, under <version> And they seem uncorrelated. Changing either of these affects none of the rest. The third one is what the users sees during over-the-air setup and under Options/Advanced. Questions - why do we need all these? Are there contexts where numbers 1, 2, 4 come up? It's my understanding that the ALX is generated during compilation - where does the version # come from? Is there a way to learn at least one of those programmatically (without signing the app)?

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• C++: Create abstract class with abstract method and override the method in a subclass

  - by Martijn Courteaux

  Hi, How to create in C++ an abstract class with some abstract methods that I want to override in a subclass? How should the .h file look? Is there a .cpp, if so how should it look? In Java it would look like this: abstract class GameObject { public abstract void update(); public abstract void paint(Graphics g); } class Player extends GameObject { @Override public void update() { // ... } @Override public void paint(Graphics g) { // ... } } // In my game loop: for (int i = 0; i < objects.size(); i++) { objects.get(i).update(); } for (int i = 0; i < objects.size(); i++) { objects.get(i).paint(g); } Translating this code to C++ is enough for me.

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• How useful is a 'pure' MVC implementation?

  - by Matt1776

  I work at a company that provides custom made 'CRM'-like software. We are currently redesigning/redeveloping the software with the hopes that it will look more modern and be easier to develop and customize for future clients. Currently it takes a long time to customize each new application. There is a presumption that the reason it takes so long is because of the amount of business logic that is present in the 'view' layer. To some extent I can vouch for this being true, but symptoms don't always reliably point out a cause. There was a suggestion that if we just move the business logic to the controller layer and use pure view (we use java J2EE and struts) as in implementing struts tags instead of calling the bean layer and iterating objects right on the jsp - etc etc. Before I start advocating we go forward with this, I wanted to get a feeling for what other people thought. Does a "pure" implementation of the MVC (especially emphasis on decoupling the controller and the view) provide a cleaner, easier to develop and change code base?

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• Netbeans GUI Form corrupted?

  - by hubpages-my

  I was making a java gui application through netbeans IDE. All was working fine but yesterday when I opened the form in Netbeans, it said that the form had been corrupted and it could only open that form in read-only mode. When I opened that form in read-only mode, I found that the code was incomplete as severals lines of coding wasn't there at the end. Could anybody tell me how to recover the form. The form is perfect in lookwise but the generated code is incomplete. The code is too long to write it again. The Netbeans told me to get help from http://wiki.netbeans.org/FormGuardedBlockError but I didn't find any helpful information there.

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• How to use Map element as text of a JComboBox

  - by llm

  I am populating a JComboBox (using addItem()) with all the elements of a collection. Each element in the collection is a HashMap (so its a ComboBox of Hashmaps..). My question is - Given that I need each item to be a HashMap how do I set the text to apear in the combobox on the GUI? It needs to be the value of a certain key in the map. Normally if I am populating a combobox with my own type, I would just overide the toString() method...but I am not sure how to acheive this since I am using a Java HashMap. Any ideas (if possible, without implementing my own HashMap)? Update: It seems like there isn't anyway to avoid having the object int the JComboBox overide toString() if I want custom functionality..I wish there was a way to (1) specify the objects to be loaded into the JComboBox and (2) specify how these objects are to appear in the GUI.

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• Manage duplicate jar files using ant and ivy.

  - by lawardy

  HI, I am using ant build script to build my java app and utilizing Ivy to manage its dependency. As my application is dependent/subset of other application(Main App), when running it, I set the classpath to point to lib jars of the Main App. As I tried to build the kit using ant and ivy, it includes/retrieve all the jars that is required as specified in ivy.xml The problem is, some of the jar files are duplicated with with jars file in the main app folders. Is there a way to set the classpath in build.xml to ignore retrieving jar files that already exist in the main app's lib folder. Thanks

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• tomcat multithreading problem

  - by jutky

  Hi all I'm writing a java application that runs in Tomcat, on a multi-core hardware. The application executes an algorithm and returns the answer to the user. The problem is that even when I run two requests simultaneously, the tomcat process uses at most one CPU core. As far as I understand each request in Tomcat is executed in separate thread, and JVM should run each thread on separate CPU core. What could be the problem that bounds the JVM or Tomcat to use no more than one core? Thanks in advance.

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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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• 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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