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  • Right method to build a online whiteboard - JAVA

    - by Nikhar Sharma
    I am building a whiteboard, which would have a server(teacher) and clients(students). Teacher would draw something on his side, which will be shown exactly same to the students. I want to know which component i should use to do the drawing? i am currently drawing on JPanel . I want the screen of Server gets copied on the clients, so for that what could be the right method to do this? option1: i save the JPanel as image, and send thru socket, and loads it on the screen of client, also it always saves the background image only, not what the user has drawn onto it. OR option2: both server and client JPanel dimensions are same, so i just send the new coordinates drawn everytime thru socket, with some protocol to understand whether it is rubber or pencil.. Any help would be appreciated.

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  • Memory allocation in java

    - by Girish
    class Someobject { int i=10; } public class OtherObject { public static void main(String args[]) { Someobject obj=new Someobject(); System.out.println(obj.i); } } Please tell me in which section of the memory: This entire code will load. Where will someobject will be stored. Where will obj will be stored Where will i be stored. Thanks every one in advance.

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  • Writing to an already existing file using FileWriter Java

    - by delo
    Is there anyway I can write to an already existing file using Filewriter For example when the user clicks a submit button: FileWriter writer = new FileWriter("myfile.csv"); writer.append("LastName"); writer.append(','); writer.append("FirstName"); writer.append('/n'); writer.append(LastNameTextField.getText()); writer.append(','); writer.append(FirstNameTextField.getText()); I want to be able to write new data into the already existing myfile.csv without having to recreate a brand new one every time

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  • Cast element in Java For Each statement

    - by Carl Summers
    Is it possible (or even advisable) to cast the element retrieved from a for each statement in the statement itself? I do know that each element in list will be of type . I.E.: List<BaseType> list = DAO.getList(); for(<SubType> element : list){ // Cannot convert from element type <BaseType> to <SubType> ... } rather than: List <BaseType> list = DAO.getList(); for(<BaseType> el : list){ <SubType> element = (<SubType>)el; ... }

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  • Java generics: Illegal forward reference

    - by Arian
    Given a generic interface interface Foo<A, B> { } I want to write an implementation that requires A to be a subclass of B. So I want to do class Bar<A, B super A> implements Foo<A, B> { } // --> Syntax error or class Bar<A extends B, B> implements Foo<A, B> { } // --> illegal forward reference But the only solution that seems to work is this: class Bar<B, A extends B> implements Foo<A, B> { } which is kind of ugly, because it reverses the order of the generic parameters. Are there any solutions or workarounds to this problem?

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  • JAVA: Build XML document using XPath expressions

    - by snoe
    I know this isn't really what XPath is for but if I have a HashMap of XPath expressions to values how would I go about building an XML document. I've found dom-4j's DocumentHelper.makeElement(branch, xpath) except it is incapable of creating attributes or indexing. Surely a library exists that can do this? Map xMap = new HashMap(); xMap.put("root/entity/@att", "fooattrib"); xMap.put("root/array[0]/ele/@att", "barattrib"); xMap.put("root/array[0]/ele", "barelement"); xMap.put("root/array[1]/ele", "zoobelement"); would result in: <root> <entity att="fooattrib"/> <array><ele att="barattrib">barelement</ele></array> <array><ele>zoobelement</ele></array> </root>

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  • Java Enum List from Class

    - by DD
    How do I go from a Class object to a list of enums generically? i.e. public static <T extends Enum> List<T> getList(Class<T> clazz) I cant find a way to get to the values() method

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  • Class unloading in java

    - by java_geek
    When a classloader is garbage collected, are the classes loaded by it unloaded? When the JVM is running is verbose mode, all the loaded classes are o/p. Similarly will the JVM log when it unloads a class? I wrote a custom class loader to test this, but could not see any verbose log for unloading of the classes. CustomClassLoader loader = new CustomClassLoader(new URL[]{}, CustomClassLoader.class.getClassLoader()); loader.addURL("D:\workspace\ClassLoaderTest\implementation.jar"); Class c = null; try { c = Class.forName("Horse",false,loader); if (c != null) { try { Animal animal = (Animal)c.newInstance(); animal.eat(); } catch(Exception ex) { ex.printStackTrace(); } } } catch(Exception e) { e.printStackTrace(); } loader = null; byte[] b = new byte[58*1024*1024]; System.gc(); ClassLoadingMXBean clBean = ManagementFactory.getClassLoadingMXBean(); System.out.println("Number of classes currently loaded " + clBean.getLoadedClassCount()); System.out.println("Number of classes loaded totally " + clBean.getTotalLoadedClassCount()); System.out.println("Number of classes unloaded " + clBean.getUnloadedClassCount()); Even the ClassLoadingMXBean gives number of unloaded classes as 0. How can i know that a class is unloaded when the class loader is GCed?

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  • copy file from one location to another location in linux using java program

    - by Mouli
    Using JSP am trying to move customer logo into another location in linux but its not working. thanks in advance Here is my program String customerLogo = request.getParameter("uploadCustomerLogo").trim(); StringBuffer absoluteFolderPath = new StringBuffer(); absoluteFolderPath.append("/zoniac"); absoluteFolderPath.append("/Companies/"); absoluteFolderPath.append("companyCode/"); absoluteFolderPath.append("custom/"); String destination = absoluteFolderPath.toString(); File sourcefile = new File(customerLogo); File destfile = new File(destination+sourcefile.getName()); FileUtils.copyFile(sourcefile,destfile);

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  • Insert code into a method - Java

    - by DutrowLLC
    Is there a way to automatically insert code into a method? I have the following and I would like to insert the indicated code: public class Person { Set<String> updatedFields = new LinkedHashSet<String>(); String firstName; public String getFirstName(){ return firstName; } boolean isFirstNameChanged = false; // Insert public void setFirstName(String firstName){ if( !isFirstNameChanged ){ // Insert isFirstNameChanged = true; // Insert updatedFields.add("firstName"); // Insert } // Insert this.firstName = firstName; } } I'm also not sure if I can the subset of the method name as a string from inside the method itself as indicated on the line where I add the fieldName as a string into the set of updated fields: updatedFields.add("firstName");. And I'm not sure how to insert fields into a class where I add the boolean field that tracks if the field has been modified or not before (for efficiency to prevent having to manipulate the Set): boolean isFirstNameChanged = false; It seems to most obvious answer to this would be to use code templates inside eclipse, but I'm concerned about having to go back and change the code later.

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  • Mix of Arabic and English causes problems in JTextArea (Java)

    - by Dan
    I have a JTextArea which displays HTML of an Arabic web page. So it's essentially a mix of English and Arabic. In the JTextArea, with columns set to 30, certain text just disappears instead of wrapping properly. The weird thing is that if I copy the invisible text and paste it into Notepad, then I can see it in Notepad. If I change the number of columns to 40, everything displays fine. Any ideas?

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  • Regex help -- cleaning up whitespace -- Java

    - by FarmBoy
    I'm trying to view the text of HTML files in a reasonable way. After I remove all of the markup and retain only the visible text, I obtain a String that looks something like this: \n\n\n\n \n\n\n \n\n \n Title here \n\n\n \n\n \n\n Menu Item 1 \n\n \n\n Menu Item 2 \n\n\n \n\n you get the point. I would like to use String.replaceAll(String regex, String regex) to replace any whitespace substring that contains more than two occurances of \n with "\n\n". Any ideas?

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  • Java - Thread problem

    - by Yatendra Goel
    My question is related to all those methods(including Thread.sleep(...)) which throw InterruptedException. I found a statement on Sun's tutorial saying InterruptedException is an exception that sleep throws when another thread interrupts the current thread while sleep is active. Is that means that the interrupt will be ignored if the sleep is not active at the time of interrupt? Suppose I have two threads: threadOne and threadTwo. threadOne creates and starts threadTwo. threadTwo executes a runnable whose run method is something like: public void run() { : : try { Thread.sleep(10 * 1000); } catch (InterruptedException e) { return; } : : : // In the middle of two sleep invocations : : try { Thread.sleep(10 * 1000); } catch (InterruptedException e) { return; } : : } After thread creation, threadOne interrupts threadTwo. Suppose the threadTwo is in the middle of two sleep invocations at the time of interrupt (when no sleep method was active), then will the second sleep method throw InterrupteException as soon as it is invoked? If not, then will this interrupt will be ignored forever? How to be sure that threadTwo will always know about the interrupt (doesn't matter whether its one of the sleep method is active or not)?

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  • Java Runtime.freeMemory() returning bizarre results when adding more objects

    - by Sotirios Delimanolis
    For whatever reason, I wanted to see how many objects I could create and populate a LinkedList with. I used Runtime.getRuntime().freeMemory() to get the approximation of free memory in my JVM. I wrote this: public static void main(String[] arg) { Scanner kb = new Scanner(System.in); List<Long> mem = new LinkedList<Long>(); while (true) { System.out.println("Max memory: " + Runtime.getRuntime().maxMemory() + ". Available memory: " + Runtime.getRuntime().freeMemory() + " bytes. Press enter to use more."); String s = kb.nextLine(); if (s.equals("m")) for (int i = 0; i < 1000000; i++) { mem.add(new Long((new Random()).nextLong())); } } } If I write in m, the app adds a million Long objects to the list. You would think the more objects (to which we have references, so can't be gc'ed), the less free memory. Running the code: Max memory: 1897725952. Available memory: 127257696 bytes. m Max memory: 1897725952. Available memory: 108426520 bytes. m Max memory: 1897725952. Available memory: 139873296 bytes. m Max memory: 1897725952. Available memory: 210632232 bytes. m Max memory: 1897725952. Available memory: 137268792 bytes. m Max memory: 1897725952. Available memory: 239504784 bytes. m Max memory: 1897725952. Available memory: 169507792 bytes. m Max memory: 1897725952. Available memory: 259686128 bytes. m Max memory: 1897725952. Available memory: 189293488 bytes. m Max memory: 1897725952. Available memory: 387686544 bytes. The available memory fluctuates. How does this happen? Is the GC cleaning up other things (what other things are there on the heap to really clean up?), is the freeMemory() method returning an approximation that's way off? Am I missing something or am I crazy?

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  • Sending files from server to client in Java

    - by Lee Jacobson
    Hi, I'm trying to find a way to send files of different file types from a server to a client. I have this code on the server to put the file into a byte array: File file = new File(resourceLocation); byte[] b = new byte[(int) file.length()]; FileInputStream fileInputStream; try { fileInputStream = new FileInputStream(file); try { fileInputStream.read(b); } catch (IOException ex) { System.out.println("Error, Can't read from file"); } for (int i = 0; i < b.length; i++) { fileData += (char)b[i]; } } catch (FileNotFoundException e) { System.out.println("Error, File Not Found."); } I then send fileData as a string to the client. This works fine for txt files but when it comes to images I find that although it creates the file fine with the data in, the image won't open. I'm not sure if I'm even going about this the right way. Thanks for the help.

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  • How a thread should close itself in Java?

    - by Roman
    This is a short question. At some point my thread understand that it should suicide. What is the best way to do it: Thread.currentThread().interrupt(); return; By the way, why in the first case we need to use currentThread? Is Thread does not refer to the current thread?

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  • Timeout with GAE Java

    - by user242153
    Hi, I am having some issues with an app I have deployed on GAE. Specifically, I am intermittently running into the DeadlineExceededException where the server is not responding within the 30 seconds required. What is odd is that the code is not overly complex, it should run in milliseconds. My guess is that the delay is in dealing with the persistence manager and accessing the datastore. 2 questions: 1) What is the best way to track where all of the CPU time on the server is being used up? Log files do not seem helpful and to make things more complicated the code runs very fast when I am running it locally 2) Any tips / best practices in dealing with the 30 second exception? What are the biggest drivers of this? Datastore? HTTP requests / responses? Thanks

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  • Shallow Copy in Java

    - by Vilius
    Hello there! I already know, what a shallow copy is, but I'm not able to impliment it. Here's a short example. public class Shallow { String name; int number; public Shallow (String name, int number) { this.name = name; this.number = number; } } Test the implementation ... public class ShallowTest { public static void main (String[] args) { Shallow shallow = new Shallow("Shallow", 123); Shallow shallowClone = new Shallow(shallow); shallowClone.name = 'Peter'; shallowClone.number = 321; System.out.println(shallow.name + " - " + shallow.number); } } As I purpose, just the reference of the non primitive datatype String would be copied, so that by calling "shallowClone.name = 'Peter';" I would also change the name of "shallow". Am I right? But somehow, it just does not want to work ....

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  • Create java sandbox based on security policies

    - by argos
    I need to create environment to run potentially untrusted code. Program allowed to connect to preconfigured address:port and nothing else (even read the system time). I have compiled the class whitelist. I'd searched similar questions, but found only template that based on SecurityManager which AFAIK is deprecated. Can anybody give me a simple sample how to run code in sandbox based on security policies and AccessController?

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