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• Create instance of generic type in Java?

  - by David Citron

  Is it possible to create an instance of a generic type in Java? I'm thinking based on what I've seen that the answer is "no" (due to type erasure), but I'd be interested if anyone can see something I'm missing: class SomeContainer<E> { E createContents() { return what??? } } EDIT: It turns out that Super Type Tokens could be used to resolve my issue, but it requires a lot of reflection-based code, as some of the answers below have indicated. I'll leave this open for a little while to see if anyone comes up with anything dramatically different than Ian Robertson's Artima Article.

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• Understanding character encoding in typical Java web app

  - by Marcus

  Some pseudocode from a typical web app: String a = "A bunch of text"; //UTF-16 saveTextInDb(a); //Write to Oracle VARCHAR(15) column String b = readTextFromDb(); //UTF-16 out.write(b); //Write to http response In the first line we create a Java String which uses UTF-16. When you save to Oracle VARCHAR(15) does Oracle also store this as UTF-16? Does the length of an Oracle VARCHAR refer to number of Unicode characters (and not number of bytes)? And then when we write b to the ServletResponse is this being written as UTF-16 or are we by default converting to another encoding like UTF-8?

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• Java: PriorityQueue returning incorrect ordering from custom comparator??

  - by Michael Simpson

  I've written a custom comparator to compare my node classes, but the java priority queue is not returning my items in the correct order. Here is my comparator: public int compare(Node n1, Node n2){ if (n1.getF() > n2.getF()){ return +1; } else if (n1.getF() < n2.getF()){ return -1; } else { // equal return 0; } } Where getF returns a double. However after inserting several Nodes into the priority queue, I print them out using: while(open.size() > 0) { Node t = (Node)(open.remove()); System.out.println(t.getF()); } Which results in: 6.830951894845301 6.830951894845301 6.0 6.0 5.242640687119285 7.4031242374328485 7.4031242374328485 8.071067811865476 Any ideas why this is so? Is my comparator wrong? Thanks. Mike

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• Sort Java List/Map by order in which items are in an XML File

  - by Brandon Smith

  Hello everyone, What I'm looking to do is to sorta a Java List or Map in the order the items are in a XML File. For Example I have a list of function names as so: functionOne functionThree functionTwo The XML File looks like this: <xml> <function>functionOne</function> <function>functionTwo</function> <function>functionThree</function> </xml> So I would like to sort the list so the function names are as so: functionOne functionTwo functionThree Now Im trying to do this for Variables as well, so there are around 500+ unique 'items'. Does anyone have any idea how I can go about doing this? Now for the file that determines that sort order doesn't have to be XML it just what I use the most, it can be anything that can get the job done. Thanks in advance for your time.

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• when to use StringBuilder in java

  - by kostja

  It is supposed to be generally preferable to use a StringBuilder for String concatenation in Java. Is it always the case? What i mean is : Is the overhead of creating a StringBuilder object, calling the append() method and finally toString() smaller then concatenating existing Strings with + for 2 Strings already or is it only advisable for more Strings? If there is such a threshold, what does it depend on (the String length i suppose, but in which way)? And finally - would you trade the readability and conciseness of the + concatenation for the performance of the StringBuilder in smaller cases like 2, 3, 4 Strings?

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• Java: pass an event to another component

  - by John

  Sorry I don't know if this is very clear, but I'm pretty new to Java. So I have a JFrame with a BorderLayout containing a JPanel and a JButton. What I want to do is when something happens in my JPanel, I want for example change the text of the JButton, or enable/disable it. How would I do that? How can I access the JButton from the JPanel? I know some ways of doing it but I don't think they're the best way to do it. What would be the best way to do this? Thanks in advance

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• java.lang.OutOfMemoryError: unable to create new native thread for multiple threads

  - by Kaustubh Ranjan Singh

  static class solver implements Runnable { static calculator(problem){ //Some code if(condition) {solver s = new solver(newproblem); new Thread(s).start();} } Public solver(int newproblem) { this.problem = newproblem ; } public void run() { // TODO Auto-generated method stub calculator(promblem); } } i am having a big array maze of 100x100 and i am trying to solve it and i am getting an error java.lang.OutOfMemoryError: unable to create new native thread(after running a code for some times). How can solve this , How can i use ExecutorService i think that will solve the problem or i want something like thisIf Number of generated threads4K then stop the first 100 threads

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• Java - Reg. Ex. File Question

  - by aloh

  I'm grabbing lines from a text file and sifting line by line using regular expressions. I'm trying to search for blank lines, meaning nothing or just whitespace. However, what exactly is empty space? I know that whitespace is \s but what is a line that is nothing at all? null (\0)? newline (\n)? I tried the test harness in the Java tutorial to try and test to see what an empty space is but no luck so far.

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• Examples of both beautiful and ugly java code?

  - by tputkonen

  I would like to demonstrate how difficult it is for a layman to identify high quality code from flawed code. I'm thinking of doing this with the help of two java methods. Both of the methods should look like they do the same, pretty simple thing. However one of them should have several kind of flaws, for example: iteration with array off by one error string concatenations causing lots of objects to be created (as opposed to StringBuffer in the "good" code, which looks more complicated) possibly null pointer exception (but it should not be trivial to spot) Those are just some examples, all kinds of other issues including bugs and performance related structures are highly appreciated. Methods should be around 10-20 lines of length, and the task they do should be something simple - preferably printing something in an iteration.

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• Can I add and remove elements of enumeration at runtime in Java

  - by Brabster

  It is possible to add and remove elements from an enum in Java at runtime? For example, could I read in the labels and constructor arguments of an enum from a file? @saua, it's just a question of whether it can be done out of interest really. I was hoping there'd be some neat way of altering the running bytecode, maybe using BCEL or something. I've also followed up with this question because I realised I wasn't totally sure when an enum should be used. I'm pretty convinced that the right answer would be to use a collection that ensured uniqueness instead of an enum if I want to be able to alter the contents safely at runtime.

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• generating function arguments in java

  - by aloishis89

  I'm very new to java and am working on my first Android app. I am using the webview demo as a template. I am trying to generate a random integer between 1 and 12 and then call a certain javascript function based on the result. Here's what I have: int number = 1 + (int)(Math.random() * ((12 - 1) + 1)); number = (int) Math.floor(number); String nextQuote = "javascript:wave" + number + "()"; mWebView.loadUrl(nextQuote); So mWebView.loadUrl(nextQuote) will be the same as something like mWebView.loadUrl("javascript:wave1()") I just want to know if what I have here is correct and will work the way I think it will. The application isn't responding as expected and I suspect this bit of code is the culprit.

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• Java Synchronized Write Block

  - by ikurtz

  im new to java and i have a question regarding Synchronized. i have the following code for writing to network (simple implementation for now): public void networkSendData(byte[] data){ try { out.write(data); out.flush(); } catch (IOException e) { } } i was wondering if there is a need for block level Synchronized here as im am writing the whole data at once. or is there a chance for race condition? i ask because the data to write is coming for multiple sources. thank you.

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• Java multidimensional array and scanner novice Q

  - by Max

  I'm new to Java and trying to in essence implement a grid with a character, and if the user inputs 'w' 'a' 's' or 'd' the character moves up/down/left/right within the plane. I created a multidimensional array sized 10x10 public static String[][] grid = new String[10][10]; And then just used a for loop to print "*"s in a 10x10 grid, except for grid[a][b] which is equal to character "A" i.e. my thing to be moved around. That seemed to work alright, then I needed to detect the 'wasd' input from the user so I set up a: Scanner in = new Scanner (System.in); while (in.hasNext()) And I had then: String s = in.next(); char ch = s.charAt(0); switch (ch) but I couldn't make this work, and it wasn't because I didn't complete the "switch" statement, I did, I just see it void copying and pasting the entire thing. I'm sure its incredibly easy slight thing I am missing, can you please point it out for me?

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• Convert php to java/android arrays

  - by lacas

  $tagArray = array( "apples" => 12, "oranges" => 38, "pears" => 10, "mangos" => 24, "grapes" => 18, "bananas" => 56, "watermelons" => 80, "lemons" => 12, "limes" => 12, "pineapples" => 15, "strawberries" => 20, "coconuts" => 43, "cherries" => 20, "raspberries" => 8, "peaches" => 25 ); and how i can do this in java, and how to calling for the first and second params?

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• java decmail string to AS 3.0 conversion procedure

  - by Jack Smith

  Hello, I have a problem with conversion java code to action script 3. Anyone can help with code translation? Thanks. public static short[] decmail_str_to_binary_data(String s) { short[] data = new short[s.length()/2]; for (int i = 0; i < s.length(); i += 2) { char c1 = s.charAt(i); char c2 = s.charAt(i + 1); int comb = Character.digit(c1, 16) & 0xff; comb <<= 4; comb += Character.digit(c2, 16) & 0xff; data[i/2] = (short)comb; } return data; }

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• Count in base 2, 3, 4 etc in Java and output all permutations

  - by tree-hacker

  I want to write a function in Java that takes as input an integer and outputs every possible permutation of numbers up to that integer. For example: f(1) 0 f(2) should output: 00 01 10 11 f(3) should output: 000 001 002 010 011 012 020 021 022 100 .... 220 221 222 That is it should output all 27 permutations of the digits of the numbers 0, 1, 2. f(4) should output 0000 0001 0002 0003 0010 ... 3330 3331 3332 3333 f(4) should output 00000 00001 ... 44443 44444 I have been trying to solve this problem but cannot seem to work out how to do it and keep getting confused by how many loops I need. Does anyone know how to solve this problem? Thanks in advance.

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• how to get a constant in java with class

  - by vincent

  basically I need to get a constant for a class however I have no instance of the object but only its class. In PHP I would do constant(XYZ); Is there a similar way of retrieving a constant in JAVA? I need it to facilitate a dynamic getMethod call Class parameterType = Class.forName(class_name); object.setProperty(field name, field value, parameterType); the set property method would then get the correct method and set the specified property, however I cant get a method which has int as parameter type without using Interger.TYPE

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• Selectively parsing log files using Java

  - by GPX

  I have to parse a big bunch of log files, which are in the following format. SOME SQL STATEMENT/QUERY DB20000I The SQL command completed successfully. SOME OTHER SQL STATEMENT/QUERY DB21034E The command was processed as an SQL statement because it was not a valid Command Line Processor command. EDIT 1: The first 3 lines (including a blank line) indicate an SQL statement executed successfully, while the next three show the statement and the exception it caused. darioo's reply below, suggesting the use of grep instead of Java, works beautifully for a single line SQL statement. EDIT 2: However, the SQL statement/query might not be a single line, necessarily. Sometimes it is a big CREATE PROCEDURE...END PROCEDURE block. Can this problem be overcome using only Unix commands too? Now I need to parse through the entire log file and pick all occurrences of the pair of (SQL statement + error) and write them in a separate file. Please show me how to do this!

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• Network programming on java

  - by maks

  I have written a simple network program on java using sockets. Program has a client and a server. When user types a word on client side, server simply return this word to client. On server side I use Serversocket and bind it to port 4444. Why does not firewall block this connection on my server PC? I ask this question because earlier I wrote this program using corba technology, and firewall on my server PC was blocking the connection to this port; when I disabled the firewall the program worked fine.

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• How does array class work in Java?

  - by oks16

  In Java, array is a class and extends Object. I am curious to know about this special array class. I don't find the class definition anywhere. Doing a getClass().getName() gives strange result. String[] array = new String[]{"one","two"}; System.out.println(array.getClass().getName()); // prints [Ljava.lang.String; I want to understand how array works under the hood. Is the array class definition hardcoded in the JVM? Any resources, books, links on this will be helpful. Thank you.

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• Extracting methods body from a class of Java Source Code

  - by Muhammad Asaduzzaman

  Hi, I want to extract method body from a Java Source Code. Suppose I have the following code: public class A{ public void print(){ System.out.println("Print This thing"); System.out.println("Print This thing"); System.out.println("Print This thing"); } } My objective is not to extract the method name (in this case print) but also the bode of the method(In this case the three print statement inside the print method). Can anyone suggest how can I do so? Is their any library available for doing so.

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• Copying files from one directory to another in Java

  - by user42155

  Hello, I want to copy files from one directory to another (subdirectory) using Java. I have a directory, dir, with text files. I iterate over the first 20 files in dir, and want to copy them to another directory in the dir directory, which I have created right before the iteration. In the code, I want to copy the review (which represents the ith text file or review) to trainingDir. How can I do this? There seems not to be such a function (or I couldn't find). Thank you. boolean success = false; File[] reviews = dir.listFiles(); String trainingDir = dir.getAbsolutePath() + "/trainingData"; File trDir = new File(trainingDir); success = trDir.mkdir(); for(int i = 1; i <= 20; i++) { File review = reviews[i]; }

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• identify the input that is multiple of 11 and odd or even java

  - by Bolor Ch

  i am trying to write code to determine the nature of input using if statement only. The nature of input could be following: a multiple of 11 even or odd. For the code below, when I enter my input, it does not display the result as "input:NOT:ODD". Also how can I check multiple conditions with if statement? (else is not considered) import java.util.Scanner; public class test { public static void main( String args[] ) { Scanner input = new Scanner( System.in ); int x; int EO; int Mult; System.out.print ( "Enter value: " ); x = input.nextInt(); EO = x % 2; Mult = x % 11; if (EO > 0 && Mult > 0) { System.out.printf ("%d:NOT:ODD"); } } }

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• Circle class in java

  - by user292395

  I need to get the Java code for the below problem asap. Create a class Circle that will allow us to define a circle with a specific radius. Your class should include the following methods: A constructor that can create a circle with a specified radius. A method setRadius that sets a specific value as a new radius for an existing circle. A method getRadius that returns the radius of an existing circle. A method toString that returns a String with all the information about an existing circle. A method computeArea that computes the area of a circle A method computePerimeter that computes the perimeter of a circle.

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• How to stop Interruptible Threads in Java

  - by Dr.Lesh

  I have a Java application that I CAN'T EDIT that starts a Thread wich has this run method: public void run(){ while(true){ System.out.println("Something"); } } And at a certain moment I wanna stop it, but if I use thread.interrupt(); it won't work. If I use thread.stop(); it works, but this method is deprecated and its use is discouraged because soon it will be removed from JVM. Does anyone knows how to do it? Thank you.

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