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• Java Memory Management

  - by Tara Singh

  I am designing a client-server chat application in Java. This is a secure application where the messages are exchanged using cryptographic algorithms. I have one server and it can support many clients. My problem is that when one client logs on the server it works fine, but when another user logs into the system, the server starts giving me bad padding exceptions for the encrypted text. I am not able to figure out the problem, according to my logic, when new connection request to server is made, the server creates a thread for listening to the client. Is it possible that once the instance of thread class is created, it does all the processing correctly for the first client, but not for the second client because the variables in server listener thread class already have some previous value, and thus the encrypted text is not decrypted properly? Please advise how I can make this process more robust so that the number of clients does not affect how well the server functions.

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• Java exception reading xml file

  - by xain

  Hi, I have the following xml file: <?xml version="1.0" encoding="UTF-8"?> <c1> <c2 id="0001" n="CM" urlget="/at/CsM" urle="/E/login.jsp"> </c2> <c2 id="0002" n="C2M" urlget="/a2t/CsM" urle="/E2/login.jsp"> </c2> </c1> I'm trying to load c2's attributes this way: Document d = DocumentBuilderFactory.newInstance() .newDocumentBuilder() .parse("epxy.xml"); Element c1 = d.getDocumentElement(); Element c2 = (Element)c1.getFirstChild(); while (c2 != null) { ... c2 = (Element)c2.getNextSibling(); } But I get the exception java.lang.ClassCastException: org.apache.xerces.dom.DeferredTextImpl incompatible with org.w3c.dom.Element in the line Element c2 = (Element)c1.getFirstChild(); before the loop. Any hints ? Thanks.

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• Java: Local Enums

  - by bruno conde

  Today, I found myself coding something like this ... public class LocalEnums { public LocalEnums() { } public void foo() { enum LocalEnum { A,B,C }; // .... // class LocalClass { } } } and I was kind of surprised when the compiler reported an error on the local enum: The member enum LocalEnum cannot be local Why can't enums be declared local like classes? I found this very useful in certain situations. In the case I was working, the rest of the code didn't need to know anything about the enum. Is there any structural/design conflict that explains why this is not possible or could this be a future feature of Java?

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• Extending Throwable in Java

  - by polygenelubricants

  Java lets you create an entirely new subtype of Throwable, e.g: public class FlyingPig extends Throwable { ... } Now, very rarely, I may do something like this: throw new FlyingPig("Oink!"); and of course elsewhere: try { ... } catch (FlyingPig porky) { ... } My questions are: Is this a bad idea? And if so, why? What could've been done to prevent this subtyping if it is a bad idea? Since it's not preventable (as far as I know), what catastrophies could result? If this isn't such a bad idea, why not? How can you make something useful out of the fact that you can extends Throwable?

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• Writing out BMP files with DataBuffer.TYPE_FLOAT or DataBuffer.TYPE_DOUBLE in java

  - by Basil Dsouza

  Hi Guys, I had a problem working with the image classes in java. I am creating a buffered image with DataBuffer.TYPE_DOUBLE. This all works fine in memory (I think). But the problem starts when I try to write it using ImageIO.write. Initially I was getting no exception at all and instead was only getting an empty output file for my troubles.. After a bit of poking around in the code, i found out that the bmp writer doesnt support writing type_double type of files. From: BMPImageWriterSpi.canEncodeImage: if (dataType < DataBuffer.TYPE_BYTE || dataType > DataBuffer.TYPE_INT) return false; So my question is, does anyone have a way of writing out those kind of images to disk? any documentation or tutorial, or link would be helpful. Thanks, Basil Dsouza

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• Java portable media detection

  - by quosoo

  I would like to write a piece of java code that synchronizes files between local hard drive and a usb storage. I would like to have a different synchronization configuration depending on which usb storage is plugged in and I would like to have apropriate configuration to be selected automatically rather than chosen by the user. I've just read the JSR-80 and jUSB documentation as well as a bunch of articles and SO posts, but all of those are very old and it seems that since that time (around 2005) all the efforts have been abandoned especially for Windows platform, while the OS-independence is quite important to me (at least Windows and Linux need to be supported). Do I really need to use any of the USB APIs to recognize external drives that are connected to the system? I need something that is more unique than file path, drive letter or drive label... And if yes which one would you recommend (unless I missed something jUnit is actually the only for which Windows support exists).

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• Books recommendation to learn about java networking

  - by elec

  In order to cover for my (glaring) lack of knowledge in the basics of networking, I'm looking for a book which would ideally cover: - 1 or 2 chapters on the transport layer: tcp, udp... - 1 or 2 chapters on the application layer: http, dns... - rest of the book would be devoted to pratical way of sending data across the wire using Java-related technologies. This would involve discussions about existing products (eg. hessian, protobuf, thrift, tibco...) , performances comparisons, case studies...etc.. Does such a book exist ?

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• java keytool question

  - by user384706

  Hi, I created a java keystore programmatically of type jks (i.e. default type). It is initially empty so I created a DSA certificate. keytool -genkey -alias myCert -v -keystore trivial.keystore How can I see the public and private keys? I.e. is there a command that prints the private key of my certificate? I could only find keytool -certreq which in my understanding prints the certificate as a whole: -----BEGIN NEW CERTIFICATE REQUEST----- MIICaTCCAicCAQAwZTELMAkGA1UEBhMCR1IxDzANBgNVBAgTBkdyZWVjZTEPMA0GA1UEBxMGQXRo BQADLwAwLAIUQZbY/3Qq0G26fsBbWiHMbuVd3VICFE+gwtUauYiRbHh0caAtRj3qRTwl -----END NEW CERTIFICATE REQUEST----- I assume this is the whole certificate. How can I see private (or public key) via keytool? Thank you

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• What causes "java.lang.IncompatibleClassChangeError: vtable stub"?

  - by JimN

  What causes "java.lang.IncompatibleClassChangeError: vtable stub"? In our application, we have seen this error pop up randomly and very seldom (just twice so far, and we run it a lot). It is not readily reproducible, even when restarting the app, using the same jvm/jars without rebuilding. As for our build process, we clean all classes/jars and rebuild them, so it's not the same problem as others have encountered where they made a change in one class and didn't recompile some other dependent classes. This is unlike some of the other questions related to IncompatibleClassChangeError -- none of them mention "vtable stub". In fact, there are surprisingly few google results when searching for "IncompatibleClassChangeError "vtable stub"".

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• Java Polymorphism - Selecting correct method based on subtype

  - by Marty Pitt

  Hi Given the following Class and Service layer signatures: public class PersonActionRequest { PersonVO person // ... other fields } public class MyServiceLayerClass { public void requestAction(PersonActionRequest request) { PersonVO abstractPerson = request.getPerson(); // call appropriate executeAction method based on subclass of PersonVO } private void executeAction(PersonVO person) {} private void executeAction(EmployeeVO employee) {} private void executeAction(ManagerVO manager) {} private void executeAction(UnicornWranglerVO unicornWrangler) {} } As discussed here, java will select the best method based on type info at compile time. (Ie., it will always select executeAction(PersonVO person) ). What's the most appropriate way to select the correct method? The internet tells me that using instanceof gets me slapped. However, I don't see the appropraite way to select the method without explictly casting abstractPerson to one of the other concrete types. Thanks Marty

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• java/swing: Shape questions: serializing and combining

  - by Jason S

  I have two questions about java.awt.Shape. Suppose I have two Shapes, shape1 and shape2. How can I serialize them in some way so I can save the information to a file and later recreate it on another computer? (Shape is not Serializable but it does have the getPathIterator() method which seems like you could get the information out but it would be kind of a drag + I'm not sure how to reconstruct a Shape object afterwards.) How can I combine them into a new Shape so that they form a joint boundary? (e.g. if shape1 is a large square and shape2 is a small circle inside the square, I want the combined shape to be a large square with a small circular hole)

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• Use of Java constructors in persistent entities

  - by Mr Morgan

  Hello I'm new to JPA and using persistence in Java anyway and I have two questions I can't quite work out: I have generated tags as: @JoinColumn(name = "UserName", referencedColumnName = "UserName") @ManyToOne(optional = false) private User userName; @JoinColumn(name = "DatasetNo", referencedColumnName = "DatasetNo") @ManyToOne(optional = false) private Dataset datasetNo; But in one of the constructors for the class, no reference is made to columns UserName or DatasetNo whereas all other columns in the class are referenced in the constructor. Can anyone tell me why this is? Both columns UserName and DatasetNo are 'foreign keys' on the entity Visualisation which corresponds to a database table of the same name. I can't quite work out the ORM. And when using entity classes, or POJO, is it better to have class variables like: private User userName; Where an instance of a class is specified or simply the key of that class instance like: private String userName; Thanks Mr Morgan.

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• Play wave file using AudioFormat in java

  - by angelina

  Dear all, I m getting following exception while running my code on linux operating system.This code works fine on windows operating system.below is the exception and code used. java.lang.IllegalArgumentException: No line matching interface Clip supporting format PCM_SIGNED unknown sample rate, 16 bit, stereo, 4 bytes/frame, big-endian is supported. AudioFormat format = sourceaudio.getFormat(); format = new AudioFormat( AudioFormat.Encoding.PCM_SIGNED, format.getSampleRate(), format.getSampleSizeInBits() * 2, format.getChannels(), format.getFrameSize() * 2, format.getFrameRate(), true); AudioFileFormat.Type targettype = AudioFileFormat.Type.WAVE; AudioInputStream targetaudiostream = AudioSystem.getAudioInputStream(format, sourceaudio); sourceaudio.close(); targetaudiostream.close(); System.out.println("55555555"); URL url = new URL("http://localhost:8084/newvideo/PCMfile.wav"); Clip clip = AudioSystem.getClip(); AudioInputStream ais = AudioSystem.getAudioInputStream(url); clip.open(ais); System.out.println("seconds: " + (clip.getMicrosecondLength() / 1000000));

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• augment the factory pattern in java

  - by TP

  I am trying to use a factory pattern to create a QuestionTypeFactory where the instantiated classes will be like MultipleChoice, TrueFalseQuestion etc. The factory code looks something like this class QuestionFactory { public enum QuestionType { TrueFalse, MultipleChoice, Essay } public static Question createQuestion(QuestionType quesType) { switch (quesType) { case TrueFalse: return new TrueFalseQuestion(); case MultipleChoice: return new MultipleChoiceQuestion(); case Essay: return new EssayQuestion(); } throw new IllegalArgumentException("Not recognized."); } } This works ok for now. If I want to add another question type I will need to modify the factory class and I do not want to do that. How can I set it up so that each question class registers itself with the Factory so that when I add a new question type, I do not have to change the code for the factory? I am a bit new to java and am not sure how to do this.

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• groovy call private method in Java super class

  - by Jeff Storey

  I have an abstract Java class MyAbstractClass with a private method. There is a concrete implementation MyConcreteClass. public class MyAbstractClass { private void somePrivateMethod(); } public class MyConcreteClass extends MyAbstractClass { // implementation details } In my groovy test class I have class MyAbstractClassTest { void myTestMethod() { MyAbstractClass mac = new MyConcreteClass() mac.somePrivateMethod() } } I get an error that there is no such method signature for somePrivateMethod. I know groovy can call private methods but I'm guessing the problem is that the private method is in the super class, not MyConcreteClass. Is there a way to invoke a private method in the super class like this (other than using something like PrivateAccessor)? thanks Jeff

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• What to do of exceptions when implementing java.lang.Iterator

  - by Vincent Robert

  The java.lang.Iterator interface has 3 methods: hasNext, next and remove. In order to implement a read-only iterator, you have to provide an implementation for 2 of those: hasNext and next. My problem is that these methods does not declare any exceptions. So if my code inside the iteration process declares exceptions, I must enclose my iteration code inside a try/catch block. My current policy has been to rethrow the exception enclosed in a RuntimeException. But this has issues because the checked exceptions are lost and the client code no longer can catch those exceptions explicitly. How can I work around this limitation in the Iterator class? Here is a sample code for clarity: class MyIterator implements Iterator { @Override public boolean hasNext() { try { return implementation.testForNext(); } catch ( SomethingBadException e ) { throw new RuntimeException(e); } } @Override public boolean next() { try { return implementation.getNext(); } catch ( SomethingBadException e ) { throw new RuntimeException(e); } } ... }

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• 2d String Array NullPointerException (java)

  - by Student01

  I am currently creating a java application in which I have a 2d array which I want to get some data into. I am creating the 2d array as such String[][] addressData; and then when I am trying to put data in I am using reference the exact position in the 2d array I want to enter the data into e.g addressData[0][0] = "String Data"; The program compiles yet when I run I get a NullPointerException error. Am I using the wrong method to enter data into this 2d array?

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• How can something like BOSH be implemented using Java Servlets

  - by John

  BOSH (Bidirectional-streams Over Synchronous HTTP) is a sneaky way of implementing 2-way client-server communication in situations where true server-push is not allowed, most obviously to let a server push data to a browser client without having to use client polling. It works by the client sending a request to the server, and the server doesn't respond immediately... rather it remembers the request but only responds when it has some data to send. When this happens the client immediately sends another request so there is virtually always a 'stored request' sitting on the server ready to push data to the client. At least, that's how I think it works! My question is how you can do this using a Java EE stack i.e standard servlets. I'm rusty with them, is it allowed to send no response and cache the request somehow? If anyone could share (pseudo) code what a servlet class for handling BOSH might look like, I'd be real grateful.

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• How to write a virtual conditional breakpoint in java

  - by Phuong Nguyen de ManCity fan

  I'm sorry if the question title may mis-inform you, but I cannot find a proper word to explain that. If you ever working with .NET, you would know that there is Assert class that will automatically wake up and attach debugger if necessary and then have debugger (Visual Studio) pausing at the Assert command, given the Assert command failed. Given I'm running a java program and having debugger connected, then how can I have debugger to break on certain condition without manually setup a break point? I'm expecting something like that: void doSomeThing(String x){ if (x==null) breakDebuggerNow(); }

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• mp3 downsampling / compression in java

  - by veenit33

  Well, i was looking forward to modify the bit rate of a mp3 file in java. I want to downsample(change its bit rate) the mp3 file from 256/384 kbps to say 64/128 kbps.. (I guess this is the only way one can achieve mp3 compression..or is there any other way.?) I searched for LameOnJ but that website is temperoraly down and so im not able to get the license file which we need to download in every 2 days. Is this possible using JMF..? What are the other option i have..? Regards, Veenit Shah

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• Working with arrays of lists pattern in java

  - by Mad Wombat

  I am writing a card game in java where I need to spread cards from a deck into several columns until I have fixed amount of cards left. This is how I do this. public class Column extends ArrayList {} List deck = Cards.createNewDeck(); Column[] columns = new Column[10]; int c = 0; while (deck.size() 50) { if (c == 10) { c = 0; } if (columns[c] == null) { columns[c] = new Column(); } columns[c].add(Cards.dealTopCard(deck)); c += 1; } This somehow seems clunky. Is there a more readable/comprehensive way of doing the same thing?

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• Creating database desktop application with data manipulation in Netbeans using Java Persistence

  - by Lulu

  It's my first time to use Persistence in developing a Java program because I usually connect via JDBC. I read that for large amounts of data, it is best to use persistence. I tried playing with the CRUD example of Netbeans. It's not very helpful thought because it only connects to the DB and allows addition and deletion of records. I need something that will allow me to manipulate the data like if the value from column C1 of table T1 is such, it will retrieve data from table t2. In short, I need to apply conditions before knowing what to retrieve exactly. The example in CRUD example already has a specific table to retrieve and only acts like a database manager. How is it possible to retrieve a specific item first then from this, will determine the next steps to be done. I'm also using embedded JavaDB/Derby as my database (also my first time to use because I usually use remote mysql)

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• Eclipse Java Profiler

  - by Jeff Storey

  I know this has been asked before, but I have not found anything recent that really gives a good answer. I'm trying to find a free profiler for eclipse that works well. I would like a graphical breakdown of execution time in particular. I've tried TPTP but have had no luck at all with GUI apps (it took almost a minute for a GUI app to start and was virtually unusable on screen - it uses a lot of Java OpenGL, so I'm not sure if it has to do with that). I liked YourKit, but unfortunately it's not free. I even tried switching to NetBeans since they have a built in profiler. If anyone has had success with particular profilers (even if it was TPTP), I'd like to hear about it. Any recommendations would be greatly appreciated. thanks, Jeff

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• Assigning a default value to a final variable in case of an exception in Java

  - by frenetisch applaudierend

  Why won't Java let me assign a value to a final variable in a catch block after setting the value in the try block, even if it is not possible for the final value to be written in case of an exception. Here is an example that demonstrates the problem: public class FooBar { private final int foo; private FooBar() { try { int x = bla(); foo = x; // In case of an exception this line is never reached } catch (Exception ex) { foo = 0; // But the compiler complains // that foo might have been initialized } } private int bla() { // You can use any of the lines below, neither works // throw new RuntimeException(); return 0; } } The problem is not hard to work around, but I would like to understand why the compiler does not accept this. Thanks in advance for any inputs!

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• Java - binary compatibility of abstract class & subclasses

  - by thSoft

  In Java, I define an abstract class with both concrete and abstract methods in it, and it has to be subclassed independently by third-party developers. Just to be sure: are there any changes I could make to the abstract class that are source compatible with their classes but not binary compatible? In other words: after they have compiled their subclasses, could I change the abstract class - apart from e.g. adding an abstract method to it or removing a protected method from it that is called by subclasses, which are of course source incompatible - in a way that could force them to recompile their subclasses?

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