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  • What's the best way to keep java app data stored redundantly in a file?

    - by Bijan
    If I have systems that are based on realtime data, how can I ensure that all the information that is current is redundantly stored in a file? So that when the program starts again, it uses this information to initialize itself back to where it was when it closed. I know of xstream and HSQLDB. but wasn't sure if this was the best option for data that needs to be a literal carbon copy.

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  • Java- FileWriter/BufferedWriter - appending to end of a text file?

    - by KP65
    I've done this before once, I'm trying to replicate what I did so far and this is what I've got: try { BufferedWriter writer = new BufferedWriter(new FileWriter("file.P", true)); System.out.println("entered"); if (!(newUserName.isEmpty()) || (newUserPass.isEmpty())){ writer.newLine(); writer.write("hellotest123"); writer.close(); } It seems to find file.P, which is just a txt file, but it doesn't seem to append anything onto it? It enters the code and passes the IF statement fine, but nothing is appended to the text file? I'm slightly stuck!

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  • What's a reasonable way to mutate a primitive variable from an anonymous Java class?

    - by Steve
    I would like to write the following code: boolean found = false; search(new SearchCallback() { @Override void onFound(Object o) { found = true; } }); Obviously this is not allowed, since found needs to be final. I can't make found a member field for thread-safety reasons. What is the best alternative? One workaround is to define final class MutableReference<T> { private T value; MutableReference(T value) { this.value = value; } T get() { return value; } void set(T value) { this.value = value; } } but this ends up taking a lot of space when formatted properly, and I'd rather not reinvent the wheel if at all possible. I could use a List<Boolean> with a single element (either mutating that element, or else emptying the list) or even a Boolean[1]. But everything seems to smell funny, since none of the options are being used as they were intended. What is a reasonable way to do this?

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  • Are there any new/updated Java web development frameworks to watch for?

    - by predhme
    I know recently Spring 3.0 was released which brought about a nice new set of features and ease of web development with their MVC package. However are there any new frameworks on the horizon and/or new versions of other frameworks that a web developer should have their eyes on? I heard about the Stripes framework, but it seems as though development has stopped. It also seems grails has a new release coming out as well which that looks like it is just an update to support the new features in the latest groovy release.

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  • Whats a good API for generating reports for a java web application?

    - by Ahmad
    I have a J2EE application that has a lot of reports, the situation now is the following: Report filters' values are sent to the application over DWR mainly, the data are retrieved from Oracle DB throw DB procedures and returned back to the client. However, some customization is required every now and then (new data filters, columns, ordering, ...), and these changes are painful to implement since we need to modify the JSPs, DB Procedures, the application itself, ... What API do you recommend to use for such reports?

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  • Java: Combination of recursive loops which has different FOR loop inside; Output: FOR loops indexes

    - by vvinjj
    currently recursion is fresh & difficult topic for me, however I need to use it in one of my algorithms. Here is the challenge: I need a method where I specify number of recursions (number of nested FOR loops) and number of iterations for each FOR loop. The result should show me, something simmilar to counter, however each column of counter is limited to specific number. ArrayList<Integer> specs= new ArrayList<Integer>(); specs.add(5); //for(int i=0 to 5; i++) specs.add(7); specs.add(9); specs.add(2); specs.add(8); specs.add(9); public void recursion(ArrayList<Integer> specs){ //number of nested loops will be equal to: specs.size(); //each item in specs, specifies the For loop max count e.g: //First outside loop will be: for(int i=0; i< specs.get(0); i++) //Second loop inside will be: for(int i=0; i< specs.get(1); i++) //... } The the results will be similar to outputs of this manual, nested loop: int[] i; i = new int[7]; for( i[6]=0; i[6]<5; i[6]++){ for( i[5]=0; i[5]<7; i[5]++){ for(i[4] =0; i[4]<9; i[4]++){ for(i[3] =0; i[3]<2; i[3]++){ for(i[2] =0; i[2]<8; i[2]++){ for(i[1] =0; i[1]<9; i[1]++){ //... System.out.println(i[1]+" "+i[2]+" "+i[3]+" "+i[4]+" "+i[5]+" "+i[6]); } } } } } } I already, killed 3 days on this, and still no results, was searching it in internet, however the examples are too different. Therefore, posting the programming question in internet first time in my life. Thank you in advance, you are free to change the code efficiency, I just need the same results.

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  • why is a minus sign prepended to my biginteger?

    - by kyrogue
    package ewa; import java.io.UnsupportedEncodingException; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; import java.util.logging.Level; import java.util.logging.Logger; import java.math.BigInteger; /** * * @author Lotus */ public class md5Hash { public static void main(String[] args) throws NoSuchAlgorithmException { String test = "abc"; MessageDigest md = MessageDigest.getInstance("MD5"); try { md.update(test.getBytes("UTF-8")); byte[] result = md.digest(); BigInteger bi = new BigInteger(result); String hex = bi.toString(16); System.out.println("Pringting result"); System.out.println(hex); } catch (UnsupportedEncodingException ex) { Logger.getLogger(md5Hash.class.getName()).log(Level.SEVERE, null, ex); } } } i am testing conversion of byte to hex and when done, the end result has a minus sign on the beginning of the string, why does this happen? i have read the docs and it says it will add a minus sign, however i do not understand it. And will the minus sign affect the hash result? because i am going to implement it to hash password stored on my database

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  • Need help with strange Class#getResource() issue

    - by Andreas_D
    I have some legacy code that reads a configuration file from an existing jar, like: URL url = SomeClass.class.getResource("/configuration.properties"); // some more code here using url variable InputStream in = url.openStream(); Obviously it worked before but when I execute this code, the URL is valid but I get an IOException on the third line, saying it can't find the file. The url is something like "file:jar:c:/path/to/jar/somejar.jar!configuration.properties" so it doesn't look like a classpath issue - java knows pretty well where the file can be found.. The above code is part of an ant task and it fails while the task is executed. Strange enough - I copied the code and the jar file into a separate class and it works as expected, the properties file is readable. At some point I changed the code of the ant task to URL url = SomeClass.class.getResource("/configuration.properties"); // some more code here using url variable InputStream in = SomeClass.class.getResourceAsStream("/configuration.properties"); and now it works - just until it crashes in another class where a similiar access pattern is implemented.. Why could it have worked before, why does it fail now? The only difference I see at the moment is, that the old build was done with java 1.4 while I'm trying it with Java 6 now. Workaround Today I installed Java 1.4.2_19 on the build server and made ant to use it. To my totally frustrating surprise: The problem is gone. It looks to me, that java 1.4.2 can handle URLs of this type while Java 1.6 can't (at least in my context/environment). I'm still hoping for an explanation although I'm facing the work to rewrite parts of the code to use Class#getRessourceAsStream which behaved much more stable...

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  • Why is my program freezing when I use a method? (Java)

    - by user2915567
    When I use a boolean method in the Main body, my program freezes and stops working. I've tried putting the method at different places but the exact same thing happens - it freezes. The method is really simple and well-written, I'm not sure what's causing the problem. P.S. The method is on the bottom of the code. Thanks for your help! Edit: That was a dumb question now that I look at it. Thanks again everyone! public static void main(String[] args) { Scanner keyboard = new Scanner(System.in); int stringNumber = 0; String[] stringArray = new String[10]; for (int i = 0; i <= stringArray.length; i++) { boolean itemExists = false; boolean AddItem = AddItem(); if (AddItem == true) { out.println("\nEnter a string"); String input = keyboard.next(); if (i > 0) { for (int j = 0; j < stringArray.length; j++) { if (input.equalsIgnoreCase(stringArray[j])) { itemExists = true; out.println("Item \"" + input + "\" already exists."); break; } } } if (itemExists == false) { stringArray[stringNumber] = input; out.println("\"" + stringArray[stringNumber] + "\"" + " has been stored.\n"); } else { out.println("Try again."); i--; } PrintArray(stringArray); stringNumber++; } } } // This is the method I was talking about // public static boolean AddItem() { Scanner keyboard = new Scanner(System.in); int input = keyboard.nextInt(); out.println("If you want to add an item, Press 1"); if (input == 1) { return true; } else { out.println("Invalid input."); return false; } }

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  • Getting a cryptic error on my simple Java program, comparing char[i] to a string.

    - by Sergio Tapia
    public static void ejemplosString(String palabra){ char[] letras = palabra.toCharArray(); int contadorVocales = 0; for (int i = 0; i < letras.length; i++) { if (char[i] == 'a') { contadorVocales++; } if (char[i] == "e") { contadorVocales++; } if (char[i] == "i") { contadorVocales++; } if (char[i] == "o") { contadorVocales++; } if (char[i] == "u") { contadorVocales++; } } } Getting the error on every If statement. Any guidance?

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  • Creating lib file from java jar using mingw/gcj

    - by Xinus
    I am trying to convert jar file to native lib file using mingw-gcj under windows platform it is thrown me the error as C:\testDir\libs>gcj -o glassfish-embedded-all-3.1-SNAPSHOT.lib glassfish-embedde d-all-3.1-SNAPSHOT.jar com/sun/codemodel/JAnonymousClass.java: In class 'com.sun.codemodel.JAnonymousCl ass': com/sun/codemodel/JAnonymousClass.java: In constructor '(com.sun.codemodel.JClas s,com.sun.codemodel.JCodeModel)': In file included from com/sun/codemodel/CodeWriter.java:15, from <built-in>:3: com/sun/codemodel/JAnonymousClass.java:21: error: verification failed at PC=3: c onstant pool index out of range com/sun/codemodel/JAnonymousClass.java:21: confused by earlier errors, bailing o ut Is it related to some java 1.6 issue or bug in gcj /

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  • Java - How to declare table[i][j] elements as instance variables?

    - by JDelage
    All, I am trying to code a Connect4 game. For this, I have created a P4Game class and a P4Board class which represents the i X j dimensions of the Connect4 board. In P4Game, I have the following: public class P4Game{ //INSTANCE VARIABLES private int nbLines; private int nbColumns; private P4Board [][] position; //CONSTRUCTOR public P4Game(int nbLines, int nbColumns){ this.nbColumns = nbColumns; this.nbLines = nbLines; P4Board [][] position = new P4Board [nbLines][nbColumns]; //Creates the table to receive the instances of the P4Board object.*/ for (int i=0; i<nbLines; i++){ for (int j=0; j<nbColumns; j++){ this.position[i][j] = new P4Board(i,j); //Meant to create each object at (line=i, column=j) } } } This causes a NullPointerException in the nested loops where I mention this.position[i][j]. I reference those objects in other methods of this class so I need them to be instance variables. I suppose the exception is due to the fact that I have not listed the table element position[i][j] as an instance variable at the beginning of the class. my question to people here is (1) is my assumption correct, and if so (2) what would be the syntax to declare instance variables of this form? Thank you all for your help with what I realize is a very basic question. Hopefully it will also benefit other newbies. Cheers, JDelage

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  • How do you raise a Java BigInteger to the power of a BigInteger without doing modular arithmetic?

    - by angstrom91
    I'm doing some large integer computing, and I need to raise a BigInteger to the power of another BigInteger. The .pow() method does what I want, but takes an int value as an argument. The .modPow method takes a BigInteger as an argument, but I do not want an answer congruent to the value I'm trying to compute. My BigInteger exponent is too large to be represented as an int, can someone suggest a way to work around this limitation?

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  • How to preview any files inside a Java application ?

    - by Studer
    I'm using a JTree to browse the content of a folder and I want that when a user click on a file, the software shows a preview of it (a screenshot of its first page). The files are mostly Office documents and PDF. I manage to do it for PDF file using a module downloaded from Sun, but I'd like to know if there is a way to do it using any software (JARs preferably) or even the built-in Windows API. I was thinking of converting the file to PDF then do a preview of this PDF but this isn't optimal. Any ideas ?

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