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  • Validating IPv4 string in Java

    - by Mat Banik
    Bellow method is validating if string is correct IPv4 address it returns true if it is valid. Any improvements in regex and elegance would be very appreciated: public static boolean validIP(String ip) { if (ip == null || ip.isEmpty()) return false; ip = ip.trim(); if ((ip.length() < 8) & (ip.length() > 15)) return false; try { Pattern pattern = Pattern.compile("^(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)$"); Matcher matcher = pattern.matcher(ip); return matcher.matches(); } catch (PatternSyntaxException ex) { return false; } }

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  • Help with float numbers in Java

    - by Alvin
    Hi, Could anyone please me why the output of the following programme is not " different different"? public static void main(String[] args) { float f1=3.2f; float f2=6.5f; if(f1==3.2) System.out.println("same"); else System.out.println("different"); if(f2==6.5) System.out.println("same"); else System.out.println("different"); } o/p :different same

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  • java web templates across multiple WAR files

    - by Casey
    I have a multi WAR web application that was designed badly. There is a single WAR that is responsible for handling some authorization against a database and defines a standard web page using a jsp taglib. The main WAR basically checks the privileges of the user and than based on that, displays links to the context path of the other deployed WARS. Each of the other deployed WARs includes this custom tag lib. I am working on redesigning this application, and one of the nice things that I want to retain is that we have other project teams that have developed these WAR modules that "plug into" our current system to take advantage of other things we have to offer. I am not entirely sure how to handle the page templates though. I need a templating system that would be easy enough to use across multiple wars (I was thinking of jsp fragments??). I really only need to define a consistent header and main navigation section. Whatever else is displayed on the page is up to the individual web project. Any suggestions? I hope that this is clear, if not I can elaborate more.

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  • java increase xmx dynamically at runtime

    - by Tomer
    Hi, I have a jvm server in my machine, now I want to have 2 apservers of mine sitting in same machine, however I want the standby one to have a really low amount of memory allocated with xmx because its passive, one the main server (active) goes down I want to allocate more memory to my passive server which is already up without restarting it (I have have them both having too much xmx - note they would consume memory at startup and I cant allow possibility of outOfMemory). So I want passive - low xmx once active goes down I want my passive to receive much more xmx. is there a way for me to achieve that. Thanks

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  • How to end a thread in java?

    - by beagleguy
    hi all, I have 2 pools of threads ioThreads = (ThreadPoolExecutor)Executors.newCachedThreadPool(); cpuThreads = (ThreadPoolExecutor)Executors.newFixedThreadPool(numCpus); I have a simple web crawler that I want to create an iothread, pass it a url, it will then fetch the url and pass the contents over to a cpuThread to be processed and the ioThread will then fetch another url, etc... At some point the IO thread will not have any new pages to crawl and I want to update my database that this session is complete. How can I best tell when the threads are all done processing and the program can be ended?

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  • [Java] Safe way of exposing keySet().

    - by Jake
    This must be a fairly common occurrence where I have a map and wish to thread-safely expose its key set: public MyClass { Map<String,String> map = // ... public final Set<String> keys() { // returns key set } } Now, if my "map" is not thread-safe, this is not safe: public final Set<String> keys() { return map.keySet(); } And neither is: public final Set<String> keys() { return Collections.unmodifiableSet(map.keySet()); } So I need to create a copy, such as: public final Set<String> keys() { return new HashSet(map.keySet()); } However, this doesn't seem safe either because that constructor traverses the elements of the parameter and add()s them. So while this copying is going on, a ConcurrentModificationException can happen. So then: public final Set<String> keys() { synchronized(map) { return new HashSet(map.keySet()); } } seems like the solution. Does this look right?

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  • Java String object creation

    - by Ajay
    Hi, 1) What is difference in thers two statements: String s1 = "abc"; and String s1 = new String("abc") 2) as i am not using new in first statement, how string object will be created Thanks

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  • Java/Android get array from xml

    - by Ashley
    I have a list of longitude and longitude points in an xml file that is used throughout my application. I find my self repeating this code to get points often and think there must be a better way? String[] mTempArray = getResources().getStringArray(R.array.stations); int len = mTempArray.length; mStationArray = new ArrayList<Station>(); for(int i = 0; i < len; i++){ Station s = new Station(); String[] fields = mTempArray[i].split("[\t ]"); s.setValuesFromArray(fields); Log.i("ADD STATION", ""+s); mStationArray.add(s); } XML is in the format of: <?xml version="1.0" encoding="utf-8"?> <resources> <array name="stations"> <item> <name>Station name</name> <longitude>1111111</longitude> <latitude>11111</latitude> <code>1</code> </item> And another (possible) problem is that to get just one station I have to get all of them and pull the one I want from the array. Is this going to be considerably slower? Can I make this array consistent throughout the app? (But keeping the separate Intent methodology)

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  • Covariant return types in Java enums

    - by Kelvin Chung
    As mentioned in another question on this site, something like this is not legal: public enum MyEnum { FOO { public Integer doSomething() { return (Integer) super.doSomething(); } }, BAR { public String doSomething() { return (String) super.doSomething(); } }; public Object doSomething(); } This is due to covariant return types apparently not working on enum constants (again breaking the illusion that enum constants are singleton subclasses of the enum type...) So, how about we add a bit of generics: is this legal? public enum MyEnum2 { FOO { public Class<Integer> doSomething() { return Integer.class; } }, BAR { public Class<String> doSomething() { return String.class; } }; public Class<?> doSomething(); } Here, all three return Class objects, yet the individual constants are "more specific" than the enum type as a whole...

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  • Extracting user-friendly exception details in Java

    - by Jon
    I've got a J2EE web application that I'm working on and when an exception occurs, I'd like to get some basic details about the exception and log it. The message that I'm logging should be pretty basic, something that might mean something to the people running the web server(s). Would using e.getMessage() be the best thing to log? Thanks.

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  • Assigning int to byte in java?

    - by user303218
    int val =233; byte b = (byte)val; System.out.println(b); I have a simple case, like one integer with some value & i want to convert that value & place in the byte type for output. But in this case negative value is coming? How to successfully place the int value to byte type.

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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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  • 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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  • 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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  • Java - Thread safety of ArrayList constructors

    - by andy boot
    I am looking at this piece of code. This constructor delegates to the native method "System.arraycopy" Is it Thread safe? And by that I mean can it ever throw a ConcurrentModificationException? public Collection<Object> getConnections(Collection<Object> someCollection) { return new ArrayList<Object>(someCollection); } Does it make any difference if the collection being copied is ThreadSafe eg a CopyOnWriteArrayList? public Collection<Object> getConnections(CopyOnWriteArrayList<Object> someCollection) { return new ArrayList<Object>(someCollection); }

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  • Java - Syntax Question: What is <? super T>

    - by aloh
    I'm having trouble understanding the following syntax: public class SortedList< T extends Comparable< ? super T> > extends LinkedList< T > I see that class SortedList extends LinkedList. I just don't know what T extends Comparable< ? super T> means. My understanding of it so far is that type T must be a type that implements Comparable...but what is "< ? super T "?

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  • java: how to parse html-like xml

    - by Yang
    I have an html-like xml, basically it is html. I need to get the elements in each . Each element looks like this: <line tid="744476117"> <attr>1414</attr> <attr>31</attr><attr class="thread_title">title1</attr><attr>author1</attr><attr>date1</attr></line> My code is as below, it does recognize that there are 50 in the file, but it gives me NULLPointException when parsing NodeList fstNmElmntLst = fstElmnt.getElementsByTagName("attr"); Any idea why this is happening? The same code has been used for other applications without problems. DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); DocumentBuilder db = dbf.newDocumentBuilder(); InputSource is = new InputSource(); is.setCharacterStream(new StringReader(cleanxml)); Document doc = db.parse(is); doc.getDocumentElement().normalize(); System.out.println("Root element " + doc.getDocumentElement().getNodeName()); NodeList nodeLst = doc.getElementsByTagName("line"); for (int s = 0; s < nodeLst.getLength(); s++) { System.out.println(nodeLst.getLength()); Node fstNode = nodeLst.item(s); if (fstNode.getNodeType() == Node.ELEMENT_NODE) { Element fstElmnt = (Element) fstNode; NodeList fstNmElmntLst = fstElmnt.getElementsByTagName("attr"); Element fstNmElmnt = (Element) fstNmElmntLst.item(0); NodeList fstNm = fstNmElmnt.getChildNodes(); System.out.println("attr : " + ((Node) fstNm.item(0)).getNodeValue()); } }

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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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  • When can Java produce a NaN (with specific code question)

    - by Brent
    I'm a bit perplexed by some code I'm currently writing. I am trying to preform a specific gradient descent (main loop included below) and depending on the initial conditions I will alternatively get good looking results (perhaps 20% of the time) or everything becomes NaN (the other 80% of the time). However it seems to me that none of the operations in my code could produce NaN's when given honest numbers! My main loop is: // calculate errors delta = m1 + m2 - M; eta = f1 + f2 - F; for (int i = 0; i < numChildren; i++) { epsilon[i] = p[i]*m1+(1-p[i])*m2+q[i]*f1+(1-q[i])*f2-C[i]; } // use errors in gradient descent // set aside differences for the p's and q's float mDiff = m1 - m2; float fDiff = f1 - f2; // first update m's and f's m1 -= rate*delta; m2 -= rate*delta; f1 -= rate*eta; f2 -= rate*eta; for (int i = 0; i < numChildren; i++) { m1 -= rate*epsilon[i]*p[i]; m2 -= rate*epsilon[i]*(1-p[i]); f1 -= rate*epsilon[i]*q[i]; f2 -= rate*epsilon[i]*(1-q[i]); } // now update the p's and q's for (int i = 0; i < numChildren; i++) { p[i] -= rate*epsilon[i]*mDiff; q[i] -= rate*epsilon[i]*fDiff; } This behavior can be seen when we have rate = 0.01; M = 30; F = 30; C = {15, 25, 35, 45}; with the p[i] and q[i] chosen randomly uniformly between 0 and 1, m1 and m2 chosen randomly uniformly to add to M, and f1 and f2 chosen randomly uniformly to add up to F. Does anyone see anything that could create these NaN's?

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