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  • Stopping looping thread in Java

    - by halfwarp
    I'm using a thread that is continuously reading from a queue. Something like: public void run() { Object obj; while(true) { synchronized(objectsQueue) { if(objectesQueue.isEmpty()) { try { objectesQueue.wait(); } catch (InterruptedException e) { e.printStackTrace(); } obj = objectesQueue.poll(); } } // Do something with the Object obj } } What is the best way to stop this thread? I see two options: 1 - Since Thread.stop() is deprecated, I can implement a stopThisThread() method that uses a n atomic check-condition variable. 2 - Send a Death Event object or something like that to the queue. When the thread fetches a death event it exists. I prefer the 1st way, however, I don't know when to call the stopThisThread() method, as something might be on it's way to the queue and the stop signal can arrive first (not desirable). Any suggestions?

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  • Java generics question

    - by user247866
    So I have 3 classes. Abstract class A Class B extends class A independent Class C In class D that contains the main method, I create a list of instances of class B List<B> b = methodCall(); // the method returns a list of instances of class B Now in class C I have one method that is common to both A and B, and hence I don't want to duplicate it. I want to have one method that takes as input an instance of class A, as follows: public void someMethod(List<A> a) However, when I do: C c = new C(); c.someMethod(b); I get an error that some-method is not applicable for the argument List<B>, instead it's expecting to get List<A>. Is there a good way to fix this problem? Many thanks!

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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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  • Please clarify a few points concerning Java Servlets

    - by EugeneP
    suppose, I use Tomcat as a web container. Is it true that once all the servlets found in a web-app/WEBAPPNAME are init(IALIZED) then every change of a Servlet's property will be seen to every session. So session 1 changes a property userName of a Servlet1 from "user1" to "user2" session 1 is closed. session 2 starts. It will see "user2" as the only value of Servlet1.userName property?? Any change of a Servlet's field will be seen to all subsequent sessions? Are servlets singletons, aren't they?

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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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  • Revalidate and repaint - Java Swing

    - by bosra
    I have a JPanel that I am adding JLabel's to. I then want to remove all the JLabels and add some new ones. So I do the following: panel.removeAll();panel.repaint(); panel.add(new JLabel("Add something new"); panel.revalidate(); This works fine. My problem arises when I start a new thread after this like: panel.removeAll();panel.repaint(); (1)panel.add(new JLabel("Add something new"); panel.revalidate(); //new thread to start - this thread creates new JLabels that should appear under (1) firstProducer.start(); try { firstProducer.join(); } catch (InterruptedException e) { // TODO Auto-generated catch block e.printStackTrace(); } Then the output from the original JLabels is still visible. I have read that the revalidate process is a long running task and hence the firstProducer thread is getting started while the revalidation is going on and a conflict is arising. What is the best way to deal with this?

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  • Java consistent synchronization

    - by ring0
    We are facing the following problem in a Spring service, in a multi-threaded environment: three lists are freely and independently accessed for Read once in a while (every 5 minutes), they are all updated to new values. There are some dependencies between the lists, making that, for instance, the third one should not be read while the second one is being updated and the first one already has new values ; that would break the three lists consistency. My initial idea is to make a container object having the three lists as properties. Then the synchronization would be first on that object, then one by one on each of the three lists. Some code is worth a thousands words... so here is a draft private class Sync { final List<Something> a = Collections.synchronizedList(new ArrayList<Something>()); final List<Something> b = Collections.synchronizedList(new ArrayList<Something>()); final List<Something> c = Collections.synchronizedList(new ArrayList<Something>()); } private Sync _sync = new Sync(); ... void updateRunOnceEveryFiveMinutes() { final List<Something> newa = new ArrayList<Something>(); final List<Something> newb = new ArrayList<Something>(); final List<Something> newc = new ArrayList<Something>(); ...building newa, newb and newc... synchronized(_sync) { synchronized(_sync.a) { _synch.a.clear(); _synch.a.addAll(newa); } synchronized(_sync.b) { ...same with newb... } synchronized(_sync.c) { ...same with newc... } } // Next is accessed by clients public List<Something> getListA() { return _sync.a; } public List<Something> getListB() { ...same with b... } public List<Something> getListC() { ...same with c... } The question would be, is this draft safe (no deadlock, data consistency)? would you have a better implementation suggestion for that specific problem? update Changed the order of _sync synchronization and newa... building. Thanks

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  • Should I use Flash or Java?

    - by cable729
    I want to make some 2d games that I may want to submit to a game site, such as newgrounds.com. Even if I decide not to submit, I'd still like to know which is a better choice. Which has a faster startup time? Which performs faster in a 2d game? Which IDE should I use? Thanks in advance!

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  • Java JPA @OneToMany neededs to reciprocate @ManyToOne?

    - by bguiz
    Create Table A ( ID varchar(8), Primary Key(ID) ); Create Table B ( ID varchar(8), A_ID varchar(8), Primary Key(ID), Foreign Key(A_ID) References A(ID) ); Given that I have created two tables using the SQL statements above, and I want to create Entity classes for them, for the class B, I have these member attributes: @Id @Column(name = "ID", nullable = false, length = 8) private String id; @JoinColumn(name = "A_ID", referencedColumnName = "ID", nullable = false) @ManyToOne(optional = false) private A AId; In class A, do I need to reciprocate the many-to-one relationship? @Id @Column(name = "ID", nullable = false, length = 8) private String id; @OneToMany(cascade = CascadeType.ALL, mappedBy = "AId") private List<B> BList; //<-- Is this attribute necessary? Is it a necessary or a good idea to have a reciprocal @OneToMany for the @ManyToOne? If I make the design decision to leave out the @OneToMany annotated attribute now, will come back to bite me further down.

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  • java alternative for phpMyAdmin

    - by rcourtna
    I find myself working in an environment where all I have access to is Tomcat + MySQL. No Apache, no PHP. I'm looking for a web interface to MySQL that will run under Tomcat (can't be a desktop interface, as the mysql port is not accessible). I'm tried jMyAdmin and javaMyAdmin. I can't get either to work, and both lack documentation and community support. Can anyone make a recommendation please?

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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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  • How to profile object creation in Java?

    - by gooli
    The system I work with is creating a whole lot of objects and garbage collecting them all the time which results in a very steeply jagged graph of heap consumption. I would like to know which objects are being generated to tune the code, but I can't figure out a way to dump the heap at the moment the garbage collection starts. When I tried to initiate dumpHeap via JConsole manually at random times, I always got results after GC finished its run, and didn't get any useful data. Any notes on how to track down excessive temporary object creation are welcome.

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  • Java Recursion Triangle Standing on Tip

    - by user1629075
    I was wondering how to create triangle out of asterisks on its tip rather than on its base. I have the code for making it stand on its base: public static String printTriangle (int count) { if( count <= 0 ) return ""; String p = printTriangle(count - 1); p = p + "*"; System.out.print(p); System.out.print("\n"); return p; } But then I'm stuck on how to have the greatest number of stars on the top, and then the next least, and so on. I was thinking something along the terms of having (count - p) to have the input of rows be subtracted from the amount of decrease, but then i was confused by this idea because p is string. EDIT: I tried changing the position of printTriangle(count - 1) using my original method without iterations and got 1 star per each line; how can I fix this? public class triangles { public static void main(String[] args) { printTriangle(5); } public static String printTriangle (int count) { if( count <= 0 ) return ""; String p = ""; p = p + "*"; System.out.print(p); System.out.print("\n"); p = printTriangle(count - 1); return p; } }

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  • Distinguish between a single click and a double click in Java

    - by user552279
    Hi, I search the forum and see this codes: public void mouseClicked(MouseEvent e) { if (e.getClickCount() == 2) { System.out.println(" and it's a double click!"); wasDoubleClick = true; } else { Integer timerinterval = (Integer) Toolkit.getDefaultToolkit().getDesktopProperty( "awt.multiClickInterval"); timer = new Timer(timerinterval.intValue(), new ActionListener() { public void actionPerformed(ActionEvent evt) { if (wasDoubleClick) { wasDoubleClick = false; // reset flag } else { System.out.println(" and it's a simple click!"); } } }); timer.setRepeats(false); timer.start(); } } but the code runs incorrectly(Sometime it prints out " and it's a single click!" 2 times . It should print out " and it's a double click!"). Can anybody show me why? or can you give me some better ways to do this? Thank you!

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  • Adding file names to an array list in java

    - by Chris G
    Hi I am currently needing to load the contents of a folders filenames to an arraylist I have but I am unsure as how to do this. To put it into perspective I have a folder with One.txt, Two.txt, Three.txt etc. I want to be able to load this list into an arraylist so that if I was to check the arraylist its contents would be : arraylist[0] = One arraylist[1] = Two arraylist[3] = Three If anyone could give me any insight into this it would be much appreciated.

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  • Java generics question with wildcards

    - by Sean
    Just came across a place where I'd like to use generics and I'm not sure how to make it work the way I want. I have a method in my data layer that does a query and returns a list of objects. Here's the signature. public List getList(Class cls, Map query) This is what I'd like the calling code to look like. List<Whatever> list = getList(WhateverImpl.class, query); I'd like to make it so that I don't have to cast this to a List coming out, which leads me to this. public <T> List<T> getList(Class<T> cls, Map query) But now I have the problem that what I get out is always the concrete List<WhateverImpl> passed in whereas I'd like it to be the Whatever interface. I tried to use the super keyword but couldn't figure it out. Any generics gurus out there know how this can be done?

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  • Java Thread - Memory consistency errors

    - by Yatendra Goel
    I was reading a Sun's tutorial on Concurrency. But I couldn't understand exactly what memory consistency errors are? I googled about that but didn't find any helpful tutorial or article about that. I know that this question is a subjective one, so you can provide me links to articles on the above topic. It would be great if you explain it with a simple example.

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  • How to concatenate int values in java?

    - by Shamli
    hi, I have the following values: int a=1; int b=0; int c=2; int d=2; int e=1; How do i concatenate these values so that i end up with a String that is 10221; please note that multiplying a by 10000, b by 1000.....and e by 1 will not working since b=0 and therefore i will lose it when i add the values up. Thnks you in advance.

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  • Android / Java rare and seemingly impossible exception causing force close

    - by Guzba
    Hello all, I have an interesting problem being reported to me from an android application I have published. I have a two-dimensional array that I am iterating through using two for loops like so: for (int i = 0; i < arr.length; ++i) { for (int j = 0; j < arr[i].length; ++j) { if (arr[i][j] != 0) // does stuff } } The problem is, somehow arr[i][j] != 0 is throwing an ArrayIndexOutOfBoundsException. But very rarely. I have thousands of people use the app on a daily basis and get maybe twenty force close reports. Is this something I can't avoid, maybe a problem with the phones memory, etc. or is there something I can do that I haven't thought of yet? Thanks.

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  • Adapting pseudocode to java implementation for finding the longest word in a trie

    - by user1766888
    Referring to this question I asked: How to find the longest word in a trie? I'm having trouble implementing the pseudocode given in the answer. findLongest(trie): //first do a BFS and find the "last node" queue <- [] queue.add(trie.root) last <- nil map <- empty map while (not queue.empty()): curr <- queue.pop() for each son of curr: queue.add(son) map.put(son,curr) //marking curr as the parent of son last <- curr //in here, last indicate the leaf of the longest word //Now, go up the trie and find the actual path/string curr <- last str = "" while (curr != nil): str = curr + str //we go from end to start curr = map.get(curr) return str This is what I have for my method public static String longestWord (DTN d) { Queue<DTN> holding = new ArrayQueue<DTN>(); holding.add(d); DTN last = null; Map<DTN,DTN> test = new ArrayMap<DTN,DTN>(); DTN curr; while (!holding.isEmpty()) { curr = holding.remove(); for (Map.Entry<String, DTN> e : curr.children.entries()) { holding.add(curr.children.get(e)); test.put(curr.children.get(e), curr); } last = curr; } curr = last; String str = ""; while (curr != null) { str = curr + str; curr = test.get(curr); } return str; } I'm getting a NullPointerException at: for (Map.Entry<String, DTN> e : curr.children.entries()) How can I find and fix the cause of the NullPointerException of the method so that it returns the longest word in a trie?

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  • Changing Color with LinearLayout and TextView in Java (Android)

    - by Rob S.
    I'm a relatively new Android developer and I noticed what seems like an oddity to me that I'm hoping someone can explain. I have LinearLayout ll. This line of code fails for me when executed: ll.setBackgroundColor(R.color.white); However this line of code works: ll.setBackgroundResource(R.color.white); I assume its simply because I have white defined in my resources. However, I've also tried passing 0xFFFFFF in setBackgroundColor() and that doesn't work either. Similarly with my TextView text this line of code fails when executed: text.setTextColor(R.color.white); I can see my TextView so I know I initialized it correctly (like my LinearLayout which I can also see). So I guess my question boils down to: How do I properly use LinearLayout.setBackgroundColor() and TextView.setTextColor() ? Thanks a ton in advance. I've read through the docs and tried to find information online via googling and haven't come up with anything.

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