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  • Garbage Collection Java

    - by simion
    On the slides i am revising from it says the following; Live objects can be identified either by maintaining a count of the number of references to each object, or by tracing chains of references from the roots. Reference counting is expensive – it needs action every time a reference changes and it doesn’t spot cyclical structures, but it can reclaim space incrementally. Tracing involves identifying live objects only when you need to reclaim space – moving the cost from general access to the time at which the GC runs, typically only when you are out of memory. I understand the principles of why reference counting is expensive but do not understand what "doesn’t spot cyclical structures, but it can reclaim space incrementally." means. Could anyone help me out a little bit please? Thanks

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  • Java: when to use static methods

    - by KP65
    Hello, I am wondering when to use static methods? Say If i have a class with a few getters and setters, a method or two, and i want those methods only to be invokable on an instance object of the class. Does this mean i should use a static method? e.g Obj x = new Obj(); x.someMethod or Obj.someMethod (is this the static way?) I'm rather confused!

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  • Java - Removing duplicates in an ArrayList

    - by Will
    I'm working on a program that uses an ArrayList to store Strings. The program prompts the user with a menu and allows the user to choose an operation to perform. Such operations are adding Strings to the List, printing the entries etc. What I want to be able to do is create a method called removeDuplicates().This method will search the ArrayList and remove any duplicated values. I want to leave one instance of the duplicated value(s) within the list. I also want this method to return the total number of duplicates removed. I've been trying to use nested loops to accomplish this but I've been running into trouble because when entries get deleted, the indexing of the ArrayList gets altered and things don't work as they should. I know conceptually what I need to do but I'm having trouble implementing this idea in code. Here is some pseudo code: start with first entry; check each subsequent entry in the list and see if it matches the first entry; remove each subsequent entry in the list that matches the first entry; after all entries have been examined, move on to the second entry; check each entry in the list and see if it matches the second entry; remove each entry in the list that matches the second entry; repeat for entry in the list Here's the code I have so far: public int removeDuplicates() { int duplicates = 0; for ( int i = 0; i < strings.size(); i++ ) { for ( int j = 0; j < strings.size(); j++ ) { if ( i == j ) { // i & j refer to same entry so do nothing } else if ( strings.get( j ).equals( strings.get( i ) ) ) { strings.remove( j ); duplicates++; } } } return duplicates; }

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  • How to strip specific contents of a String in Java

    - by user2974877
    So I have a string, and I want to strip out some parts of it using, for example, the firt and last characters of the "interesting" part. String dirty = "$!$!%!%$something interesting&!!$!%$something interesting2"; And the output something like: String clean = "something interesting:something interesting2"; Note: The code needs to work without knowing the random part, changing everytime the program runs. I researched and only found code that does it, but only knowing the random segment.

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  • Java/Swing: Problem with key listener

    - by Mike
    I have a search dialog with a JTextField that's being used as a search box. When the user types something, it searches the DB, shows the result in a JTable and selects the first item in it. If the first result is what they were looking for, I want to let them quickly accept the dialog, by pressing Enter (while the JTextField is focused). So I added a KeyListener to the JTextField and it's working OK. Now the problem: The user opens can open the dialog by pressing Enter when a "Search" button on the dialog's parent frame is focused. The dialog is displayed and the JTextField gets the keyReleased event (from the Enter key that displayed it), so it shows up and closes. If the user holds Enter down, then the JTextField receives the keyPressed, keyTyped and keyReleased events. How can I fix without resorting to ugly workarounds? Platform is Windows 7 x64, btw. Test NetBeans project here: http://dl.dropbox.com/u/6354360/KeyListenerTest.zip Thanks.

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  • Social Media Java Design Problem

    - by jboyd
    I need to put something together quickly that will take blog posts and place them on social media sites, the requirements are as follows: Blog Entries are independent records that already exist, they have a published date and a modified date, the blog entry application cannot be changed, at least not substantially A new blog entry, or update needs to be sent to social media sites I currently do not need to update or delete social media communications if the blog entry is edited, or deleted, though I may need to later My design problems here are as follows: how do I know the status of each update how can I figure out what blog entry updates and postings have already been sent out? how can I quickly poll the blog entry table for postings that haven't yet been sent out? Avoiding looking at each Entry record from the DB as an object and asking if it's been sent already. That would be too slow. I cannot hook into any Blog Entry update code, my only option would be to create a trigger that an update queues something to be processed I'm looking for general guiding principles here, the biggest problem I'm having is coming up with any reasonable way to figure out if a blog entry should be sent to our social media sites in the first place

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  • Profiling a Java Spring application

    - by niklassaers
    Hi guys, I have a Spring application that I believe has some bottlenecks, so I'd like to run it with a profiler to measure what functions take how much time. Any recommendations to how I should do that? I'm running STS, the project is a maven project, and I'm running Spring 3.0.1 Cheers Nik

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  • non blocking client server chat application in java using nio

    - by Amith
    I built a simple chat application using nio channels. I am very much new to networking as well as threads. This application is for communicating with server (Server / Client chat application). My problem is that multiple clients are not supported by the server. How do I solve this problem? What's the bug in my code? public class Clientcore extends Thread { SelectionKey selkey=null; Selector sckt_manager=null; public void coreClient() { System.out.println("please enter the text"); BufferedReader stdin=new BufferedReader(new InputStreamReader(System.in)); SocketChannel sc = null; try { sc = SocketChannel.open(); sc.configureBlocking(false); sc.connect(new InetSocketAddress(8888)); int i=0; while (!sc.finishConnect()) { } for(int ii=0;ii>-22;ii++) { System.out.println("Enter the text"); String HELLO_REQUEST =stdin.readLine().toString(); if(HELLO_REQUEST.equalsIgnoreCase("end")) { break; } System.out.println("Sending a request to HelloServer"); ByteBuffer buffer = ByteBuffer.wrap(HELLO_REQUEST.getBytes()); sc.write(buffer); } } catch (IOException e) { e.printStackTrace(); } finally { if (sc != null) { try { sc.close(); } catch (Exception e) { e.printStackTrace(); } } } } public void run() { try { coreClient(); } catch(Exception ej) { ej.printStackTrace(); }}} public class ServerCore extends Thread { SelectionKey selkey=null; Selector sckt_manager=null; public void run() { try { coreServer(); } catch(Exception ej) { ej.printStackTrace(); } } private void coreServer() { try { ServerSocketChannel ssc = ServerSocketChannel.open(); try { ssc.socket().bind(new InetSocketAddress(8888)); while (true) { sckt_manager=SelectorProvider.provider().openSelector(); ssc.configureBlocking(false); SocketChannel sc = ssc.accept(); register_server(ssc,SelectionKey.OP_ACCEPT); if (sc == null) { } else { System.out.println("Received an incoming connection from " + sc.socket().getRemoteSocketAddress()); printRequest(sc); System.err.println("testing 1"); String HELLO_REPLY = "Sample Display"; ByteBuffer buffer = ByteBuffer.wrap(HELLO_REPLY.getBytes()); System.err.println("testing 2"); sc.write(buffer); System.err.println("testing 3"); sc.close(); }}} catch (IOException e) { e.printStackTrace(); } finally { if (ssc != null) { try { ssc.close(); } catch (IOException e) { e.printStackTrace(); } } } } catch(Exception E) { System.out.println("Ex in servCORE "+E); } } private static void printRequest(SocketChannel sc) throws IOException { ReadableByteChannel rbc = Channels.newChannel(sc.socket().getInputStream()); WritableByteChannel wbc = Channels.newChannel(System.out); ByteBuffer b = ByteBuffer.allocate(1024); // read 1024 bytes while (rbc.read(b) != -1) { b.flip(); while (b.hasRemaining()) { wbc.write(b); System.out.println(); } b.clear(); } } public void register_server(ServerSocketChannel ssc,int selectionkey_ops)throws Exception { ssc.register(sckt_manager,selectionkey_ops); }} public class HelloClient { public void coreClientChat() { Clientcore t=new Clientcore(); new Thread(t).start(); } public static void main(String[] args)throws Exception { HelloClient cl= new HelloClient(); cl.coreClientChat(); }} public class HelloServer { public void coreServerChat() { ServerCore t=new ServerCore(); new Thread(t).start(); } public static void main(String[] args) { HelloServer st= new HelloServer(); st.coreServerChat(); }}

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  • Access files (.wav) in java package

    - by Highmastdon
    I want to access my .wav files which are in a package inside my project. For example I got two packages: package program package sounds From inside the program/something.class I'd like to play the sounds/asound.wav. How is this possible. clip.open(AudioSystem.getAudioInputStream(new File(filename))); clip.start(); //.... something inbetween clip.stop(); Here filename is C:\\projects\\something\\sounds\\, but how is it possible to just give a relative path to the asound.wav in the package?

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  • Getting pattern string from java SimpleDateFormat

    - by D Lawson
    I have a SimpleDateFormat object that I retrieve from some internationalization utilities. Parsing dates is all fine and good, but I would like to be able show a formatting hint to my users like "MM/dd/yyyy". Is there a way to get the formatting pattern from a SimpleDateFormat object?

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  • Java: Making concurrent MySQL queries from multiple clients synchronised

    - by Misha Gale
    I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC. The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license. I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question: public LicenseKey Acquire() throws SmartLaunchException, SQLException { Connection conn = SmartLaunchDB.getConnection(); int PCID = SmartLaunchDB.getCurrentPCID(); conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE"); String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1"; PreparedStatement statement = conn.prepareStatement(sql); statement.setInt(1, this.id); ResultSet results = statement.executeQuery(); if (results.next()) { int licenseID = results.getInt("ID"); sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?"; statement = conn.prepareStatement(sql); statement.setInt(1, PCID); statement.setInt(2, licenseID); statement.executeUpdate(); statement.close(); conn.commit(); conn.createStatement().execute("UNLOCK TABLES"); return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType")); } else { throw new SmartLaunchException("All licenses of type " + this.name + "are in use"); } }

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  • Java Switch Incompatible Types Boolean Int

    - by ikurtz
    i have the following class: public class NewGameContract { public boolean HomeNewGame = false; public boolean AwayNewGame = false; public boolean GameContract(){ if (HomeNewGame && AwayNewGame){ return true; } else { return false; } } } when i try to use it like so: if (networkConnection){ connect4GameModel.newGameContract.HomeNewGame = true; boolean status = connect4GameModel.newGameContract.GameContract(); switch (status){ case true: break; case false: break; } return; } i am getting the error: incompatible types found: boolean required: int on the following switch (status) code. what am i doing wrong please?

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  • Java: using generic wildcards with subclassing

    - by gibberish
    Say I have a class Foo, a class A and some subclass B of A. Foo accepts A and its sublclasses as the generic type. A and B both require a Foo instance in their constructor. I want A's Foo to be of type A , and B's Foo to be of type B or a superclass of B. So in effect, So I only want this: Foo<X> bar = new Foo<X>; new B(bar); to be possible if X is either A, B, or a both subclass of A and superclass of B. So far this is what I have: class Foo<? extends A>{ //construct } class A(Foo<A> bar){ //construct } class B(Foo<? super B> bar){ super(bar); //construct } The call to super(...) doesn't work, because <A> is stricter than <? super B>. Is it somehow possible to use the constructor (or avoid code duplication by another means) while enforcing these types? Edit: Foo keeps a collection of elements of the generic parameter type, and these elements and Foo have a bidirectional link. It should therefore not be possible to link an A to a Foo.

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  • Building "isolated" and "automatically updated" caches (java.util.List) in Java.

    - by Aidos
    Hi Guys, I am trying to write a framework which contains a lot of short-lived caches created from a long-living cache. These short-lived caches need to be able to return their entier contents, which is a clone from the original long-living cache. Effectively what I am trying to build is a level of transaction isolation for the short-lived caches. The user should be able to modify the contents of the short-lived cache, but changes to the long-living cache should not be propogated through (there is also a case where the changes should be pushed through, depending on the Cache type). I will do my best to try and explain: master-cache contains: [A,B,C,D,E,F] temporary-cache created with state [A,B,C,D,E,F] 1) temporary-cache adds item G: [A,B,C,D,E,F] 2) temporary-cache removes item B: [A,C,D,E,F] master-cache contains: [A,B,C,D,E,F] 3) master-cache adds items [X,Y,Z]: [A,B,C,D,E,F,X,Y,Z] temporary-cache contains: [A,C,D,E,F] Things get even harder when the values in the items can change and shouldn't always be updated (so I can't even share the underlying object instances, I need to use clones). I have implemented the simple approach of just creating a new instance of the List using the standard Collection constructor on ArrayList, however when you get out to about 200,000 items the system just runs out of memory. I know the value of 200,000 is excessive to iterate, but I am trying to stress my code a bit. I had thought that it might be able to somehow "proxy" the list, so the temporary-cache uses the master-cache, and stores all of it's changes (effectively a Memento for the change), however that quickly becomes a nightmare when you want to iterate the temporary-cache, or retrieve an item at a specific index. Also given that I want some modifications to the contents of the list to come through (depending on the type of the temporary-cache, whether it is "auto-update" or not) and I get completly out of my depth. Any pointers to techniques or data-structures or just general concepts to try and research will be greatly appreciated. Cheers, Aidos

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  • Java copyOf method problem with an Array of Objects

    - by Greg
    elementData = Arrays.copyOf(elementData, newCapacity); Gives error: "The method copyOf(Object[], int) is undefined for the type Arrays" This was not a problem on my home computer, but at my school's it gives the error above. I'm guessing it's running an older JRE version - any workaround? Thanks

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  • Well written open source java projects

    - by Algorist
    I want to improve my design and programming skills by understanding design & code of open source projects. I downloaded hadoop,groovy but they are very difficult to follow. I am not having a clue of how to follow this code without having a high level overview of the design. Any suggestions?? Thank you.

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  • Why is run() not immediately called when start() called on a thread object in java

    - by pi
    Or is it? I have a thread object from: Thread myThread = new Thread(pObject); Where pObject is an object of a class implementing the Runnable interface and then I have the start method called on the thread object like so: myThread.start(); Now, my understanding is that when start() is called, the JVM implicitly (and immediately) calls the run() method which may be overridden (as it is in my case) However, in my case, it appears that the start() method is not called immediately (as desired) but until the other statements/methods are completed from the calling block i.e. if I had a method after the start() call like so: myThread.start(); doSomethingElse(); doSomthingElse() gets executed before the run() method is run at all. Perhaps I am wrong with the initial premise that run() is always called right after the start() is called. Please help! The desired again is making executing run() right after start(). Thanks.

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  • Java serialization testing

    - by Jeff Storey
    Does anyone know if there is a library that exists to help test if an object graph is fully serializable? It would probably be as simple as writing it out and reading it back in, but I figured someone must have abstracted this already - I just can't find it.

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  • Java: how to read BufferedReader faster

    - by Cata
    Hello, I want to optimize this code: InputStream is = rp.getEntity().getContent(); BufferedReader reader = new BufferedReader(new InputStreamReader(is)); String text = ""; String aux = ""; while ((aux = reader.readLine()) != null) { text += aux; } The thing is that i don't know how to read the content of the bufferedreader and copy it in a String faster than what I have above. I need to spend as little time as possible. Thank you

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