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• Greek/latin scientific JLabel in Java Swing application

  - by MartinStettner

  For a scientific application I want to design an input form which lets the user enter certain parameters. Some of them are designated using greek letters, some of them have latin letters. The parameter names should be displayed using ordinary JLabel controls. On Windows, the Tahoma font (which is used for Labels by default) contains both latin and greek letters, so I simply set the Text property of the label to a greek (unicode) string and everything works fine. I'm wondering if this works also without modifications on Linux and OSX systems resp. for which Java/OS versions this would work. Also I'm curious if there's an easy way to show subscripts in labels ("\eta_0" in TeX), but this is not that important for my application ...

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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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• time required in java

  - by Amol

  i would like to know how much time is required to execute the conditional loops individually. Like if i had an option to use "if else", "while","for", or "foreach" loop then which loop would get executed faster.I know the difference would be very small and most would say that it would not matter but if i were to have a program which would access thousands of data then this point would come into picture. I would like to know if i declare a variable in the beginning in java and if i were to declare it just before i use it will it make any difference. will the total time required be reduced? if yes then which is practically used (the one in which variables are declared in the beginning or where they are just declared b4 they are used)?

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• Passing parametrized ArrayList to a function in java

  - by user150505

  I have the following fucntion. func(ArrayList <String> name) { ........ } The function fills the ArrayList. (I don't want to return the ArrayList) However, in the caller function the ArrayList obtained has all items of ArrayList as null. For eg. 1 name = new ArrayList[num]; 2 func(name); 3 System.out.println(name[0]); I get NullPointerException at line 3. Is this because of line 1, i.e. I am not parametrizing? If yes, is there another way this can be done? Because java does not allow creating a generic array of parametrized ArrayList.

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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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• Java == operator. "Invalid assignment operator"

  - by Tom

  Hi, I was trying to write a simple method boolean validate(MyObject o) { return o.getPropertyA() == null && o.getPropertyB()==null; } And got a strange error on the == null part. Maybe my Java is rusty after a season in PLSQL. Consider this: Integer i = 4; i ==null; //compile error: Syntax error on token ==. Invalid assignment operator. Integer i2 = 4; if (i==null); //No problem How can this be ? Any explanation ? Im using jdk160_05.

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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: Parse Australian Street Addresses

  - by bguiz

  Looking for a quick and dirty way to parse Australian street addresses into its parts: 3A/45 Jindabyne Rd, Oakleigh, VIC 3166 should split into: "3A", 45, "Jindabyne Rd" "Oakleigh", "VIC", 3166 Suburb names can have multiple words, as can street names. See: http://stackoverflow.com/questions/1739746/parse-a-steet-address-into-components Has to be in Java, cannot make http requests (e.g. to web APIs). EDIT: Assume that format specified is always followed. I have no issue with spitting incorrectly formatted strings back at the user with a message telling them to follow the format (which I've described above).

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• Run java thread at specific times

  - by rmarimon

  I have a web application that synchronizes with a central database four times per hour. The process usually takes 2 minutes. I would like to run this process as a thread at X:55, X:10, X:25, and X:40 so that the users knows that at X:00, X:15, X:30, and X:45 they have a clean copy of the database. It is just about managing expectations. I have gone through the executor in java.util.concurrent but the scheduling is done with the scheduleAtFixedRate which I believe provides no guarantee about when this is actually run in terms of the hours. I could use a first delay to launch the Runnable so that the first one is close to the launch time and schedule for every 15 minutes but it seems that this would probably diverge in time. Is there an easier way to schedule the thread to run 5 minutes before every quarter hour?

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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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• 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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• 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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• SocketTimeOutException while creating socket, java

  - by Sunil Kumar Sahoo

  Hi All, I have created a sample java socket application. I used Socket s = new Socket(ip, port) Now it works fine. but when my internet connection is very slow that time after a long interval (even if sometimes after 2 minutes) i used to get SocketTimeOutException in that line. means it gives that error while creating socket. I want the exception should be handled properly means if internet connection is very slow then if that error occurs it happens very late now . I want if this type of error occurs then it should be caught very fast means the error should not come at such a delay interval of time rather it should come immediately. How to achieve this. Thanks Sunil Kumar Sahoo

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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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• 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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• 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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• 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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• 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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• 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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• 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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• When using the getInstance() method of the abstract java.text.NumberFormat class, what is the actual

  - by iamchuckb

  This question expands upon the one at abstract-class-numberformat-very-confused-about-getinstance. I feel that this question is different enough to merit being asked on its own. In the answers to that question, it was stated that a code statement such as NumberFormat en = NumberFormat.getInstance(Locale.US); returns an object that is a subclass of the java.text.NumberFormat class. It makes sense to me why the return type can't be just an instance of NumberFormat since that is an abstract class. Rather, it was stated that the returned object is at least an instance of NumberFormat, but actually something else. My question is this: what specifically is the class of the object that is returned? In the Sun documentation the only subclasses I see are ChoicesFormat and DecimalFormat. Is there some sort of behind the scenes compiler voodoo going on here? Thanks in advance!

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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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• 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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• 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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• 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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