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  • Singleton & Multithreading in Java

    - by vivek jagtap
    What is the preferred way to work with Singleton class in multithreaded environment? Suppose if I have 3 thread, and all they try to access getInstance() method of singleton class at the same time - What would happen if no synchronization is maintained? Is it good practice to use synchronized getInstance() method or use synchronized block inside getInstance(). Please advise if there is any other way out.

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  • How to implement a list fold in Java

    - by Peter Kofler
    I have a List and want to reduce it to a single value (functional programing term "fold", Ruby term "inject"), like Arrays.asList("a", "b", "c") ... fold ... "a,b,c" As I am infected with functional programing ideas (Scala), I am looking for an easier/shorter way to code it than sb = new StringBuilder for ... { append ... } sb.toString

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  • Changing Encoding while Xml.parse() with SaxFeedParser Java

    - by zOro
    Hi , I am trying to load hebrew rss using the fllow : Xml.parse(_InputStream, Xml.Encoding.ISO_8859_1 , root.getContentHandler()); taken from ibm site : link text I would like to use other Encoding like "ISO8859_8" rather than : Xml.Encoding.ISO_8859_1, Xml.Encoding.US_ASCII, Xml.Encoding.UTF_16, Xml.Encoding.UTF_8 Thanks a lot!

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  • Why are these lines being skipped? (java)

    - by David
    Here's the relevant bit of the source code: class Dice { String name ; int x ; int[] sum ; ... public Dice (String name) { this.name = name ; this.x = 0 ; this.sum = new int[7] ; } ... public static void main (String[] arg) { Dice a1 = new Dice ("a1") ; printValues (a1) ; } public static void printDice (Dice Dice) { System.out.println (Dice.name) ; System.out.println ("value: "+Dice.x) ; printValues (Dice) ; } public static void printValues (Dice Dice) { for (int i = 0; i<Dice.sum.length; i++) System.out.println ("#of "+i+"'s: "+Dice.sum[i]) ; } } Here is the output: #of 0's: 0 #of 1's: 0 #of 2's: 0 #of 3's: 0 #of 4's: 0 #of 5's: 0 #of 6's: 0 Why didn't these two lines execute inside printDice: System.out.println (Dice.name) ; System.out.println ("value: "+Dice.x) ; if they had then i would expect to see "a1" and "Value: 0" printed at the top of the rows of #of's

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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: serial thread confinement question

    - by denis
    Assume you have a Collection(ConcurrentLinkedQueue) of Runnables with mutable state. Thread A iterates over the Collection and hands the Runnables to an ExecutorService. The run() method changes the Runnables state. The Runnable has no internal synchronization. The above is a repetitive action and the worker threads need to see the changes made by previous iterations. So a Runnable gets processed by one worker thread after another, but is never accessed by more than one thread at a time - a case of serial thread confinement(i hope ;)). The question: Will it work just with the internal synchronization of the ConcurrentLinkedQueue/ExecutorSerivce? To be more precise: If Thread A hands Runnable R to worker thread B and B changes the state of R, and then A hands R to worker thread C..does C see the modifications done by B?

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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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  • [java] Returning the element number of the longest string in an array

    - by JohnRoberts
    Hoookay, so. I'm trying to get the longestS method to take the user-inputted array of strings, then return the element number of the longest string in that array. I got it to the point where I was able to return the number of chars in the longest string, but I don't believe that will work for what I need. My problem is that I keep getting incompatible type errors when trying to figure this out. I don't understand the whole data type thing with strings yet. It's confusing me how I go about return a number of the array yet the array is of strings. The main method is fine, I got stuck on the ???? part. { public static void main(String [] args) { Scanner inp = new Scanner( System.in ); String [] responseArr= new String[4]; for (int i=0; i<4; i++) { System.out.println("Enter string "+(i+1)); responseArr[i] = inp.nextLine(); } int highest=longestS(responseArr); } public static int longestS(String[] values) { int largest=0 for( int i = 1; i < values.length; i++ ) { if ( ????? ) } return largest; } }

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  • Generate a collection of changed lines between two revisions of a file using Java

    - by mchr
    I am writing an eclipse plugin which needs to be able to determine which lines of a file have changed compared to a different version of the same file. Is there an existing class or library which I can use for this task? The closest I have found is org.eclipse.compare.internal.merge.DocumentMerger. This can be used to find the information I need but is in an internal package so is not suitable for me to use. I could copy/paste the source of this class and adapt it to my requirements. However, I am hoping there is an existing library to handle textual comparisons.

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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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  • Java BufferedImage increase width

    - by James Moore
    Hello, I have managed to load in an image using: BufferedImage image = ImageIO.read(out); and place text over it however, I want the text to appear next to the image. How can I increase the image width on the right to allow for space for the text to be drawn on. Or do I have to create a new empty image and insert the existing one? Thanks

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  • java detect clicked buttons

    - by user253530
    I have multiple panels on a JFrame window. I am going to populate each panel differently every time. For example: i start the GUI: (image center panel, right panel, bottom panel). Center panel is populated with 20 buttons, right panel with 10 buttons and bottom panel with 3. second start of the GUI (same gui). Center panel has 50 buttons, right panel has 12 buttons, bottom has 3. So everytime there is a random number of buttons, impossible to be all uniquely named. Given the fact that I don't have a unique name for each button (just a list) I would like to know which buttons were clicked according to the panel they belong to. is that possible?

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  • Getter/Setter (composition, Java, HW)

    - by Crystal
    I have one class called Person that basically looks like: public class Person { String firstName; String lastName; String telephone; String email; public Person() { firstName = ""; lastName = ""; telephone = ""; email = ""; } public Person(String firstName, String lastName, String telephone, String email) { this.firstName = firstName; this.lastName = lastName; this.telephone = telephone; this.email = email; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } .... Using that class, I setup an abstract class called Loan that looks like: public abstract class Loan { public void setClient(Person client) { this.client = client; } public Person getClient() { return client; } public void setLoanId(int nextId) { loanId = nextId; nextId++; } public int getLoanId() { return loanId; } public void setInterestRate(double interestRate) { this.interestRate = interestRate; } public double getInterestRate() { return interestRate; } public void setLoanLength(int loanLength) { this.loanLength = loanLength; } public int getLoanLength() { return loanLength; } public void setLoanAmount(double loanAmount) { this.loanAmount = loanAmount; } public double getLoanAmount(double loanAmount) { return loanAmount; } private Person client; private int loanId; private double interestRate; private int loanLength; private double loanAmount; private static int nextId = 1; } I have to extend the Loan class with CarLoan and it looks like: public class CarLoan extends Loan { public CarLoan(Person client, double vehiclePrice, double downPayment, double salesTax, double interestRate, CAR_LOAN_TERMS length) { super.setClient(client); super.setInterestRate(interestRate); this.client = client; this.vehiclePrice = vehiclePrice; this.downPayment = downPayment; this.salesTax = salesTax; this.length = length; } public void setVehiclePrice(double vehiclePrice) { this.vehiclePrice = vehiclePrice; } public double getVehiclePrice() { return vehiclePrice; } public void setDownPayment(double downPayment) { this.downPayment = downPayment; } public double getDownPayment() { return downPayment; } public void setSalesTax(double salesTax) { this.salesTax = salesTax; } public double getSalesTax() { return salesTax; } public String toString() { return getClass().getName() + "[vehiclePrice = " + vehiclePrice + '\n' + "downPayment = " + downPayment + '\n' + "salesTax = " + salesTax + "]"; } public enum CAR_LOAN_TERMS {TWO_YEAR, THREE_YEAR, SIX_YEAR}; private double vehiclePrice; private double downPayment; private double salesTax; Few questions. (a) Is what I did in the Loan class to setClient correct given what I have in the Person class? (e.g.this.client = client) (b) Can I call super twice in a method? I have to set two attributes from the Loan class from the constructor in the CarLoan class and I thought that would be a way to do it. (c) Do you have to set attributes for enumeration types differently in a constructor or getter/setter methods? I get an error for (this.length = length) in my CarLoan class and I was unsure of how enumeration values should be set. Thanks!

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  • Java: Convenient way to refactor the application.

    - by Harshal
    Hi, Everyone We have an agile enterprise application built on JSP and Servlet without any design strategy. This application was built in early 2002 considering 1000 users. After 2002, we received lots of requests from the marketing partners. Currently, the application has lots of spaghetti code with lots of Ifs and elses. One class has more than 20,000 lines of code with a huge body of functions without abstraction. Now, we need to support billions of records, what we need to do immediately and gradually? We have to refactor the application? Which framework, we need to use? How the usage of the framework will be helpful to the end users? How to convince the leaders to do the refactoring? How to gain the faster response time as compare to the current system?

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  • Java: Tracking a user login session - Session EJBs vs HTTPSession

    - by bguiz
    If I want to keep track of a conversational state with each client using my web application, which is the better alternative - a Session Bean or a HTTP Session - to use? Using HTTP Session: //request is a variable of the class javax.servlet.http.HttpServletRequest //UserState is a POJO HttpSession session = request.getSession(true); UserState state = (UserState)(session.getAttribute("UserState")); if (state == null) { //create default value .. } String uid = state.getUID(); //now do things with the user id Using Session EJB: In the implementation of ServletContextListener registered as a Web Application Listener in WEB-INF/web.xml: //UserState NOT a POJO this this time, it is //the interface of the UserStateBean Stateful Session EJB @EJB private UserState userStateBean; public void contextInitialized(ServletContextEvent sce) { ServletContext servletContext = sce.getServletContext(); servletContext.setAttribute("UserState", userStateBean); ... In a JSP: public void jspInit() { UserState state = (UserState)(getServletContext().getAttribute("UserState")); ... } Elsewhere in the body of the same JSP: String uid = state.getUID(); //now do things with the user id It seems to me that the they are almost the same, with the main difference being that the UserState instance is being transported in the HttpRequest.HttpSession in the former, and in a ServletContext in the case of the latter. Which of the two methods is more robust, and why?

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  • Java, Massive message processing with queue manager (trading)

    - by Ronny
    Hello, I would like to design a simple application (without j2ee and jms) that can process massive amount of messages (like in trading systems) I have created a service that can receive messages and place them in a queue to so that the system won't stuck when overloaded. Then I created a service (QueueService) that wraps the queue and has a pop method that pops out a message from the queue and if there is no messages returns null, this method is marked as "synchronized" for the next step. I have created a class that knows how process the message (MessageHandler) and another class that can "listen" for messages in a new thread (MessageListener). The thread has a "while(true)" and all the time tries to pop a message. If a message was returned, the thread calls the MessageHandler class and when it's done, he will ask for another message. Now, I have configured the application to open 10 MessageListener to allow multi message processing. I have now 10 threads that all time are in a loop. Is that a good design?? Can anyone reference me to some books or sites how to handle such scenario?? Thanks, Ronny

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  • how does serializable work in java?

    - by Karl Trumstedt
    If I have an instance of a class that I store in a session I need to make it serializable. This class has a static variable, will this be serialized in every instance stored? The static variable is a reference to a cache containing a lot of data in the background. Will all of this data be serialized? If so, it seems preferable to make this variable transient and re-fetch the cache instance each time the instance is restored. Maybe not store the cache instance at all in the class. Will the constructor execute when a class is restored from a serialized state? if not is there any other method I can use to re-instate a transient variable?

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  • How to access java classes in a subfolder.

    - by Jacob
    Hi all. I'm trying to make a program that can load an unknown set of plugins from a sub-folder, "Plugins". All of these plugins implement the same interface. What I need to know is how do I find all of the classes in this folder so that I can instantiate and use them?

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  • Java Logger only to file, no screen output!

    - by Tom Taylor
    Hello there SO'ers, I got a quite simple problem but cant find a solution for it. I have a logger with a file handler added, but it still spams to hell out of my console. How could I get the logger to solely route all output to a file, with NO console outputs? Thanks a lot, cru

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  • JAVA-I don't want clicking on my app make other apps lose focus

    - by Auxiliary
    Wow, what a stupid question you might say. But is it possible? I have a mouse move event in a Jpanel and it works even when the application is not in focus, now can I have something like that for the clicking event or something similar. And most importantly I don't want other apps (even something like the main menu) to lose focus when i click on my app. I was the system's event queue but i'm not sure where that would lead me to. Thanks in advance.

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