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  • Java Download Concurrent Data

    - by xger86x
    Hi, i'm developing an app which download map tiles around different places in a city. To do this, i have one thread for each place in which i select the tiles and create a thread to download each. Well, the question is how to avoid creating a thread for a tile that already exists in the thread pool. Should not just check if the file exists, since it is possible that the thread for that tile already exists (other place already need that tile) but the file has not been created- Any idea? Thanks

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  • Java's Swing Threading

    - by nevets1219
    My understanding is that if I start up another thread to perform some actions, I would need to SwingUtilities.invokeAndWait or SwingUtilities.invokeLater to update the GUI while I'm in said thread. Please correct me if I'm wrong. What I'm trying to accomplish is relatively straightforward: when the user clicks submit, I want to (before performing any actions) disable the submit button, perform the action, and at the end of the action re-enable the button. My method to perform the action updates the GUI directly (displays results) when it gets the results back. This action basically queries a server and gets some results back. What I have so far is: boolean isRunning = false; synchronized handleButtonClick() { if ( isRunning == false ) { button.setEnabled( false ); isRunning = true; doAction(); } } doAction() { new Thread() { try { doAction(); // Concern A } catch ( ... ) { displayStackTrace( ... ); // Concern B } finally { SwingUtilities.invokeLater ( /* simple Runnable to enable button */ ); isRunning = false; } } } For both of my concerns above, do I would have to use SwingUtilities.invokeAndWait since they both will update the GUI? All GUI updates revolve around updating JTextPane. Do I need to in my thread check if I'm on EDT and if so I can call my code (regardless of whether it updates the GUI or not) and NOT use SwingUtilities.invokeAndWait?

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  • synchronized in java - Proper use

    - by ZoharYosef
    I'm building a simple program to use in multi processes (Threads). My question is more to understand - when I have to use a reserved word synchronized? Do I need to use this word in any method that affects the bone variables? I know I can put it on any method that is not static, but I want to understand more. thank you! here is the code: public class Container { // *** data members *** public static final int INIT_SIZE=10; // the first (init) size of the set. public static final int RESCALE=10; // the re-scale factor of this set. private int _sp=0; public Object[] _data; /************ Constructors ************/ public Container(){ _sp=0; _data = new Object[INIT_SIZE]; } public Container(Container other) { // copy constructor this(); for(int i=0;i<other.size();i++) this.add(other.at(i)); } /** return true is this collection is empty, else return false. */ public synchronized boolean isEmpty() {return _sp==0;} /** add an Object to this set */ public synchronized void add (Object p){ if (_sp==_data.length) rescale(RESCALE); _data[_sp] = p; // shellow copy semantic. _sp++; } /** returns the actual amount of Objects contained in this collection */ public synchronized int size() {return _sp;} /** returns true if this container contains an element which is equals to ob */ public synchronized boolean isMember(Object ob) { return get(ob)!=-1; } /** return the index of the first object which equals ob, if none returns -1 */ public synchronized int get(Object ob) { int ans=-1; for(int i=0;i<size();i=i+1) if(at(i).equals(ob)) return i; return ans; } /** returns the element located at the ind place in this container (null if out of range) */ public synchronized Object at(int p){ if (p>=0 && p<size()) return _data[p]; else return null; }

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  • Java: JButton actionListener explantation

    - by MrDoctorProfessorTyler
    I am extremely confused as to how a JButton works. I have read over the oracle documents of the JButton, but I have failed to see how a JButton can have an actionlistener added to it. I have really always wondered how things like JFrames and all that can have things like .addMouseListener and all that. Can anyone explain how a JButton can have an actionListener added to it like the .addActionListener(...) syntax?

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  • Issue with WatchService in java 7

    - by user972590
    I'm using jdk7's WatchService API for monitoring the folder on file system.I'm sending a new file through email to that folder, when the file comes into that folder i m triggering the ENTRY_CRATE option. its working fine. But the issue is its generating two events of ENTRY_CREATE instead of one event which i'm invoking. BELOW IS THE CODE: Path dir = Paths.get(/var/mail); WatchService watcher = dir.getFileSystem().newWatchService(); dir.register(watcher, StandardWatchEventKinds.ENTRY_CREATE); System.out.println("waiting for new file"); WatchKey watckKey = watcher.take(); List<WatchEvent<?>> events = watckKey.pollEvents(); System.out.println(events.size()); for(WatchEvent<?> event : events){ if(event.kind() == StandardWatchEventKinds.ENTRY_CREATE){ String fileCreated=event.context().toString().trim(); } } In the above code I'm gettng the events size as 2. Can any one please help me in finding out the reason why i'm getting two events.

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  • Is HashMap in Java collision safe

    - by changed
    Hi I am developing a parser that needs to put key value pairs in hashmap. But a key can have multiple values which i can do in this way HashMap<String,ArrayList<String>> . But what happens if number of keys are very large and it start matching with other key's hashcode. Will that rewrite previous key's value ? thanks -devSunday

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  • Java: Are these 2 codes the same?

    - by Kevin Duke
    for (Player p : players) { p.addCard(deck.dealCard()); p.addCard(deck.dealCard()); } and for (int i = 0; i < players.size() ; i++) { Player p = players.get(i); p.addCard(deck.dealCard()); p.addCard(deck.dealCard()); } The second code yeilds a null pointer exception, what can be done to make the bottom one equivalent ?

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  • Java File manipulation

    - by user69514
    So I have an application with a JFileChooser from which I select a file to read. Then I change some words and write a new file. The problem that I am having is that when I write the new file it's saved in the project directory. How do I save it in the same directory as the file that I chose using the JFileChooser. Note: I don't want to use the JFileChooser to choose the location. I just need to save the file in the same directory as the original file that I read.

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  • implementing keepalives with Java

    - by Bilal
    Hi All, I am biulding a client-server application where I have to implement a keepalive mechanism in order to detect that the client has crashed or not. I have separate threads on both client and server side. the client thread sends a "ping" then sleeps for 3 seconds, while the server reads the BufferedInput Stream and checks whether ping is received, if so it makes the ping counter eqauls zero, else it increments the counter by +1, the server thread then sleeps for 3 seconds, if the ping counter reaches 3, it daclares the client as dead. The problem is that when the server reads the input stream, its a blocking call, and it blocks untill the next ping is received, irrespective of how delayed it is, so the server never detects a missed ping. any suggestions, so that I can read the current value of the stream and it doesn't block if there is nothing on the incoming stream. Thanks,

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  • Collision detection in Java game?

    - by Chetan
    I am developing a game in which I have the problem of collision detection of moving images. The game has a spaceship and number of asteroids (obstacles). I want to detect the collision between them. How can I do this?

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  • Java Swing: JTextArea columns question

    - by battousai622
    How do i put the text in specific columns with jTextArea? private javax.swing.JTextArea jTextArea1; jTextArea1.setColumns(4); jTextArea1.insert(price, 0); //column 1 jTextArea1.insert(cost, 0); //column 2 jTextArea1.insert(quantity, 0); //column ect.. jTextArea1.insert(itemName, 0); jTextArea1.insert("\n", 0);

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  • delete item from array in java

    - by davit-datuashvili
    hello can anybody tell me what is wrong here? i want delete item from array but it shows me error ArrayIndexOutBound exception public class delete{ public static void main(String[]args){ int i; //delete item from array int k[]=new int[]{77,99,44,11,00,55,66,33,10}; //delete 55 int searchkey=55; int nums=k.length; for ( i=0;i<nums;i++) if (k[i]==searchkey) break; for (int t=i;t<nums;t++) k[t]=k[t+1]; nums--; for (int m=0;m<nums;m++){ System.out.println(k[m]); } } }

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  • Output Parameter in java

    - by soclose
    Hi, could u give me a sample code of using output parameter in function? I tried to google it but just found it in function. I'd like to use this output value in another function. I will use this code in Android.

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  • Java - 'continue' loop iteration after certain timeout period

    - by Matt
    Is there a way to exit ('continue;') a loop iteration after a certain timeout period? I have a loop that will run gathering data from the web and then use this data to make a calculation. The data become obsolete after about 1 to 2 seconds so if the loop iteration takes longer than 1 second then i want it to 'continue' to the next iteration. Sometimes gathering the data can take time but sometimes the calculation can take longer than 1 second so a HTTP timeout won't work for what i need. Also, while doing the calculation the thread i am using is blocked so i cannot check System.currentTimeMillis(); Is there a way to use another Thread to check the time and force the original for loop to continue.

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  • Java, let CookieHandler work on only one instance.

    - by user435657
    Hi all, I don't know how CookieHandler works system wide, I did view the source of CookieHandler but found no more information except the get/set methods. Where do TCP/HTTP connections use instance of CookieHandler, which I set by CookieHandler.setDefault(...) Which source file I should refer to? URLConnection & HttpURLConnection don't seem have things to do with it. Help, thanks in advance.

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  • Java - understanding servlets

    - by Trup
    I am working on a homework project that should implement a board game between 2 clients over an HttpServlet. I have couple of questions: 1) I read that HttpServlets must be stateless, however, for the sake of the game, I have to keep a lot of state(whose turn it is, the state of the board, etc). Do I have to keep this in the clients? Does the HttpServlet indeed have to be stateless, i.e. have no fields that track state? 2) I know that the clients will talk to the servlet via the doGet/doPost methods, but how can the servlet talk to the clients(for example, if player 1 just made a move and sent it to the servlet, the servlet has to tell client 2 what the move was). Thank you Also, if you can point me to a useful, simple example of a similar code online, I would be very grateful

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  • What is an Enterprise Java Bean really?

    - by HDave
    On the Tomcat FAQ it says: "Tomcat is not an EJB server. Tomcat is not a full J2EE server." But if I: use Spring to supply an application context annotate my entities with JPA annotations (and use Hibernate as a JPA provider) configure C3P0 as a connection pooling data source annotate my service methods with @Transactional (and use Atomikos as JTA provider) Use JAXB for marshalling and unmarshalling and possibly add my own JNDI capability then don't I effectively have a JEE application server? And then aren't my beans EJBs? Or is there some other defining characteristic? What is it that a JEE compliant app server gives you that you can't easily/readily get from Tomcat with some 3rd party subsystems?

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  • Web Form based login in Java

    - by BrunoLM
    How can I block access to the site if a user is not logged in? Under web.xml Security I checked Form authentication then I selected Login and Error page, but I don't know how to block the access and redirect the user to the login page. Do I need a filter? If so, how can I get the login url I specified? And how should I call the validation method? I saw in some examples this code <form method=post action="j_security_check"> <input type="text" name="j_username" /> <input type="password" name="j_password" /> </form> What does it do?

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  • Java reflection framework and security

    - by Jijoy
    Hi , Assume I have a singleton class in an external lib to my application. But still I can create instances of that particular class using reflection. Like this Class clas = Class.forName(Private.class.getName()); for(Constructor c : clas.getDeclaredConstructors()){ c.setAccessible(true); Private p = (Private) c.newInstance(); System.out.println(p); } How can I restrict this ? . Thanks J

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  • Java - JPA - @Version annotation

    - by Yatendra Goel
    I am new to JPA. I am cofused about the @Version annotation. How it works? I have googled it and found various answers whose extract is as follows: JPA uses a version field in your entities to detect concurrent modifications to the same datastore record. When the JPA runtime detects an attempt to concurrently modify the same record, it throws an exception to the transaction attempting to commit last. But still I am not sure how it works? ================================================================================== Also as from the following lines: You should consider version fields immutable. Changing the field value has undefined results. Does it mean that we should declare our version field as final

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  • finding out memory allocation hotspots in java

    - by Zamir
    Our GC is working hard and we have some pauses that we want to decrease. We have some memory allocation issues that we want to solve before or while we are tweaking with the actual JVM GC args. I would like to know which objects are making the GC sweat: is there a way to know which objects are evacuated every time the GC is working? is there a way to know which objects are moved between areas every time the GC is working? Is there a way to know which objects are in Eden area? I am working extensively with Jprofiler and Memory Analyzer. I would like to get this information on a running application in my staging environment.

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