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• Create a string with n characters.

  - by C. Ross

  Is there a way in java to create a string with a specified number of a specified character? In my case I would need to create a string with 10 spaces. My current code is: StringBuffer outputBuffer = new StringBuffer(length); for (int i = 0; i < length; i++){ outputBuffer.append(" "); } return outputBuffer.toString(); Is there a better way to accomplish the same thing. In particular I'd like something that is fast (in terms of execution).

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• Why are mainframes still around?

  - by ThaDon

  It's a question you've probably asked or been asked several times. What's so great about Mainframes? The answer you've probably been given is "they are fast" "normal computers can't process as many 'transactions' per second as they do". Jeese, I mean it's not like Google is running a bunch of Mainframes and look how many transactions/sec they do! The question here really is "why?". When I ask this question to the mainframe devs I know, they can't answer, they simply restate "It's fast". With the advent of Cloud Computing, I can't imagine mainframes being able to compete both cost-wise and mindshare-wise (aren't all the Cobol devs going to retire at some point, or will offshore just pickup the slack?). And yet, I know a few companies that still pump out net-new Cobol/Mainframe apps, even for things we could do easily in say .NET and Java. Anyone have a real good answer as to why "The Mainframe is faster", or can point me to some good articles relating to the topic?

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• How to test if an array contains a pair of numbers whose product is odd?

  - by John

  How can I write a function that takes an array of integers and returns true if their exists a pair of numbers whose product is odd? What are the properties of odd integers? And of course, how do you write this function in Java? Also, maybe a short explanation of how you went about formulating an algorithm for the actual implementation. Yes, this is a function out of a textbook. No, this is not homework—I'm just trying to learn, so please no "do your own homework comments."

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• Solr adding document cycle & wait on response issue

  - by user1585896

  I am trying to send http post request to Solr for adding 50000 documents (all individual request one after another in while loop). I am using DefaultHttpClient in java to connect to Solr and when I use execute method on my HttpPost Solr takes 3 to 4 ms to response. I have commit=false, autoCommit=false, autoSoftCommit=false. My question is why it takes that much time to response and why cycle it follows to add new document. Basically I want to send add request but do not want to commit to see how many request can Solr handle without doing any kind of commits(without having to do any disk access). My guess is with above parameter tuned off I should be hitting Solr about 10000 times every second, but my result is 300 times a second. I am generating random data to add in my code.

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• Can I write to different jetty databases using JPA that is using the same "entity class"

  - by Per

  I am using Java persistance and there EntityManager class and have it assigned to storage a class object that shall be written to the database. My problem is that I want to write to different databases using the same storage class. My solution to that was to write a StorageManagerfactory that has a Map holding all EntityManagers. The solution looked good until I looked at the databases and realized that all information (undepending of the Map, which gets the correct value) was written to the same database (one of the initialised in the Map). So my question is: Can I write to different databases using JPA that is using the same storage class (the class holding the structure of my database)? Thanks

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• reading unicode

  - by user121196

  I'm using java io to retrieve text from a server that might output character such as é. then output it using System.err, they turn out to be '?'. I am using UTF8 encoding. what's wrong? int len=0; char[]buffer=new char[1024]; OutputStream os = sock.getOutputStream(); InputStream is = sock.getInputStream(); os.write(query.getBytes("UTF8"));//iso8859_1")); Reader reader = new InputStreamReader(is, Charset.forName("UTF-8")); do{ len = reader.read(buffer); if (len0) { if(outstring==null)outstring=new StringBuffer(); outstring.append(buffer,0,len); } }while(len0); System.err.println(outstring);

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• How can I make a recursive version of my iterative method?

  - by user247679

  Greetings. I am trying to write a recursive function in Java that prints the numbers one through n. (n being the parameter that you send the function.) An iterative solution is pretty straightforward: public static void printNumbers(int n){ for(int i = 1; i <= n; i++){ System.out.println(i); i++; } As a novice programmer, I'm having troubles figuring out how a recursive version of this method would work. Any bright ideas? Thanks for reading my problem!

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• Suspend TimerTask until the next execution

  - by user1052518

  I am using a TimerTask to run some periodic tasks, the task being processing a set of files. I have a requirement where if the number of files to be processed exceeds a pre-determined limit, the thread suspends execution and waits till the next cycle to start processing the files again. Is there a way to suspend the TimerTask until the next execution period or do I have to extend the TimerTask class to achieve this functionality? I saw there is a TimerTask.cancel method, but this will cancel all further executions of this thread. I don't want this to happen. I just want the thread to be suspended until the next execution period. I don't have the luxury of moving to any of the other concurrent classes in Java as our framework uses TimerTask, and I have to stick with it. Any suggestions, pointers or tips are greatly appreciated. thanks, Asha

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• In Eclipse, how do I change the default modifiers in the class/type template?

  - by gustafc

  Eclipse's default template for new types (Window Preferences Code Style Code Templates New Java Files) looks like this: ${filecomment} ${package_declaration} ${typecomment} ${type_declaration} Creating a new class, it'll look something like this: package pkg; import blah.blah; public class FileName { // Class is accessible to everyone, and can be inherited } Now, I'm fervent in my belief that access should be as restricted as possible, and inheritance should be forbidden unless explicitly permitted, so I'd like to change the ${type_declaration} to declare all classes as final rather than public: package pkg; import blah.blah; final class FileName { // Class is only accessible in package, and can't be inherited } That seems easier said than done. The only thing I've found googling is a 2004 question on Eclipse's mailing list which was unanswered. So, the question in short: How can I change the default class/type modifiers in Eclipse? I'm using Eclipse Galileo (3.5) if that matters.

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• How do I synchronize GUI-Elements?

  - by anonymous2500

  Hello, I have a little problem with my java-program. I wanna use Observer, to synchronize two GUIs. But I can't synchronize the JComponent / JButton elements. For example: I have a GUI-Class which implements the Observer-Class: public class GUI extends JFrame implements Observer I have a second "GUI"-Class which extends the JButton-Class and makes changes on a specific Button-Element. public class Karte extends JButton{ ... this.setEnabled(false); ... How do I synchronize this Button via Observable? I have already tried to use "extends Observable" in this class, but the "setEnabled()" method is explicit for the JButton-Class, which is not Observable! Can someone help? Thanks.

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• Python - Create a list with initial capacity

  - by Claudiu

  Code like this often happens: l = [] while foo: #baz l.append(bar) #qux This is really slow if you're about to append thousands of elements to your list, as the list will have to constantly be re-initialized to grow. (I understand that lists aren't just wrappers around some array-type-thing, but something more complicated. I think this still applies, though; let me know if not). In Java, you can create an ArrayList with an initial capacity. If you have some idea how big your list will be, this will be a lot more efficient. I understand that code like this can often be re-factored into a list comprehension. If the for/while loop is very complicated, though, this is unfeasible. Is there any equivalent for us python programmers?

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• How can I delete a specific element in priority queue?

  - by Yuan

  import java.util.*; public class test4 { public static void main(String[] args){ PriorityQueue[] P = new PriorityQueue[10]; P[1] = new PriorityQueue<ClassEntry>(); P[1].add(new ClassEntry(1.2,1)); P[1].add(new ClassEntry(1.5,2)); P[1].add(new ClassEntry(1.2,3)); P[1].add(new ClassEntry(10,4)); P[1].remove(new ClassEntry(10,4));//I can't delete this object??? System.out.println(P[1].size()); ClassEntry ce = (ClassEntry) P[1].peek(); System.out.println(P[1].size()); System.out.println(ce.sim+"\t"+ce.index); } } Why i can't delete (10,4)? Can somebody teach how to implement...thanks!

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• What's the purpuse behind wildcards and how are they different from generics?

  - by Nazgulled

  Hi, I never heard about wildcars until a few days ago and after reading my teacher's Java book, I'm still not sure about what's it for and wwhy would I need to use it. Let's say I have a super class Animal and few sub classes like Dog, Cat, Parrot, etc... Now I need to have a list of animals, my first thought would be something like: List<Animal> listAnimals Instead, my colleagues are recommending something like: List<? extends Animal> listAnimals Why should I use wildcards instead of simple generics? Let's say I need to have a get/set method, should I use the former or the later? How are they so different?

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• how to use a wav file in eclipse

  - by AlphaAndOmega

  I've been trying to add audio to a project I've been doing. I found some code on here for html that is also supposed to work with file but it keeps saying "Exception in thread "main" javax.sound.sampled.UnsupportedAudioFileException: could not get audio input stream from input file at javax.sound.sampled.AudioSystem.getAudioInputStream(Unknown Source) at LoopSound.main(LoopSound.java:15)" the code public class LoopSound { public static void main(String[] args) throws Throwable { File file = new File("c:\\Users\\rabidbun\\Pictures\\10177-m-001.wav"); Clip clip = AudioSystem.getClip(); // getAudioInputStream() also accepts a File or InputStream AudioInputStream wav = AudioSystem.getAudioInputStream( file ); clip.open(wav); // loop continuously clip.loop(-1000); SwingUtilities.invokeLater(new Runnable() { public void run() { // A GUI element to prevent the Clip's daemon Thread // from terminating at the end of the main() JOptionPane.showMessageDialog(null, "Close to exit!"); } }); } } What is wrong with the code?

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• how to represent negative number to array of integers ?

  - by stdnoit

  I must convert string of 1324312321 to array of integers in java this is fine. I could use integer parseint and string substring method but how do I repesent -12312312 to my original array of integer.. the fact that - is a char / string and convert to array of integer would alter the value ( even though I convert - to integer-equivalent , it would change the rest of 12312312) it must be an array of integers and how should I convert negative numbers and still keeep the same value somehow reminding me of two complements trick but i dont think i need to go down to binary level in my program.. any other trick for doing this? thanks!

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• FTP - way to programatically determine time remaining or bytes transfered?

  - by ciaranarcher

  Hi all I want to make an FTP connection (ideally using Coldfusion 8, but Java is fine too) that will copy a file to a remote server. Crucially however, I want to know how many bytes have been transferred so I can give some feedback to the user. Is this possible, and if so what FTP API would you recommend as I understand the Sun implementation may be a bit lacking. Thanks in advance! PS: I could have another process/thread poll the file size on the destination machine via a web service call while the file is being transferred but it's not ideal. Getting it through FTP API would be the neatest solution.

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• How should I be storing objects that I wish to access in reverse order of the way I placed them in

  - by andrew hicks

  I'm following this guide here: http://www.mazeworks.com/mazegen/mazetut/index.htm Or more specficially create a CellStack (LIFO) to hold a list of cell locations set TotalCells = number of cells in grid choose a cell at random and call it CurrentCell set VisitedCells = 1 while VisitedCells < TotalCells find all neighbors of CurrentCell with all walls intact if one or more found choose one at random knock down the wall between it and CurrentCell push CurrentCell location on the CellStack make the new cell CurrentCell add 1 to VisitedCells else pop the most recent cell entry off the CellStack make it CurrentCell endIf endWhile Im writing this in java, My problem is. How should I be storing my visited cells, So that I can access them from reverse order of when I placed them in. Like this? List<Location> visitedCells = new ArrayList<Location>(); Then do I grab with visitedCells.get(visitedCells.size()-1)? Location stores the x, y and z. Not something Im trying to ask you.

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• What is the absolute fastest way to implement a concurrent queue with ONLY one consumer and one producer?

  - by JohnPristine

  java.util.concurrent.ConcurrentLinkedQueue comes to mind, but is it really optimum for this two-thread scenario? I am looking for the minimum latency possible on both sides (producer and consumer). If the queue is empty you can immediately return null AND if the queue is full you can immediately discard the entry you are offering. Does ConcurrentLinkedQueue use super fast and light locks (AtomicBoolean) ? Has anyone benchmarked ConcurrentLinkedQueue or knows about the ultimate fastest way of doing that? Additional Details: I imagine the queue should be a fair one, meaning the consumer should not make the consumer wait any longer than it needs (by front-running it) and vice-versa.

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• errorPage directive works, but error-page in web.xml doesn't?

  - by danb

  I get jsp exceptions causing a forward to my error page when I put this at the top of my JSPs... <%@ page errorPage="/error.page" %> but when I try to do it globally with web.xml like so: <error-page> <exception-type>java.lang.Throwable</exception-type> <location>/error.page</location> </error-page> I just get a blank page... I've also tried putting /error.jsp in the location element.. but no love with that either.. I am triggering an exception with a jsp that just contains this: <%if(true)throw new RuntimeException("test exception");%> I do see the exception in the console from tomcat but I just can't get that error page to show without a directive on every jsp... am I missing something simple here?

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• Need help on how to setup package structure and project

  - by jax

  I am new to Java and am making a license generator. This is my current setup. com.example.licensegenerator.client (used by the client application) :LicenseLoader (no Main method) :LicenseDownloader (no Main method) com.example.licensegenerator.server.keys (used by the server) :ProductKeyGenerator(Main method) com.example.licensegenerator.server.license (used on the server also) :LicenseGenerator(Main method) com.example.licensegenerator.lib (Shared classes between client and server) :Contants (no main) Now I have a few questions. Is it OK to have multiple Main() methods in a single project? Will I be able to compile them to different .jar files? (In this case I need two different jars for the server) Is there a better way to setup the packages? And a totally unrelated question, with exceptions, is it better to handle them right then and there or throw them and let the main method catch them all (the program cannot recover if an error occurs)

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• Parallelism on two duo-core processor system

  - by Qin

  I wrote a Java program that draw the Mandelbrot image. To make it interesting, I divided the for loop that calculates the color of each pixel into 2 halves; each half will be executed as a thread thus parallelizing the task. On a two core one cpu system, the performance of using two thread approach vs just one main thread is nearly two fold. My question is on a two dual-core processor system, will the parallelized task be split among different processor instead of just utilize the two core on one processor? I suppose the former scenario will be slower than the latter one simply because the latency of communicating between 2 CPU over the motherboard wires. Any ideas? Thanks

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• Is there an automatic way to remove debugging methods for a release build?

  - by Lewis

  Note: This is an extension of an earlier question I asked here: Do additional function/method definitions increase a program's memory footprint? When I write a class, I usually end up writing several testing/debugging methods, used to make sure the class works as it should, or for printing data to help with debugging, or for unit testing, etc. Is there an easy/automatic way to make a release without these methods, or do I need to manually delete the extra code any time I want to compile a release version? I ask this question both from a C++ and a Java perspective. I'm using Code::Blocks and Eclipse as IDEs, if that plays into the answer somehow.

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• How to export javascript generated report to pdf?

  - by Parhs

  Hello... I made a reporting engine with javascript for my project... The problem is with printing.. Although with page-break and css i can produce a good looking report, i want to export that report to a pdf in order to be printed better without ,the url,page title and other stuff that browsers add. Note in Chrome there isnt page setup!!! I am using java for server side. i think sending via ajax the Html of the report somehow and return a url for the pdf report maybe... i am looking for a good tool for this thank you

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• What the best way to convert from String to HashMap?

  - by eugenn

  I would like to serialize a Java HashMap to string representation. The HashMap will contains only primitive values like string and integer. After that this string will be stored to db. How to restore back the HashMap? Is it make sense to use BeanUtils and interface Converter or use JSON? For example: List list = new ArrayList(); list.add(new Long(1)); list.add(new Long(2)); list.add(new Long(4)); Map map = new HashMap(); map.put("cityId", new Integer(1)); map.put("name", "test"); map.put("float", new Float(-3.2)); map.put("ids", list); map.toString() -> {float=-3.2,ids=[1, 2, 4],name=test,cityId=1} map.toJSON -> {"float":-3.2,"ids":[1,2,4],"name":"test","cityId":1}

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• Creating immutable objects from javabean

  - by redzedi

  Hi All, I am involved in this project where we are building on good bit of legacy code. I have a particular situation about one big java bean object which has to be transferred over wire. So my first thought was to make it immutable and serializable to do the trick .At this point I am faced with a few difficult choices :- 1 Ideally I want some way to automatically generate an immutable, serializable version of this class. I dont have the scope to refactor or alter this class in any way and i would really really hate to have to copy paste the class with a different name ?? 2 Assuming that i gave up on 1 i.e i actually chose to duplicate code of the HUGE javabean class , i still will be in the unsavoury situation of having to write a constructor with some 20-25 parameters to make this class immutable. what is a better way to make a class immutable other than constructor injection ?? Thanks and Regards,

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