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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 Detect Variable Change Using PropertyChangeSupport and PropertyChangeListener

  - by Sam

  I'm trying to print out debug statements when some third party code changes a variable. For example, consider the following: public final class MysteryClass { private int secretCounter; public synchronized int getCounter() { return secretCounter; } public synchronized void incrementCounter() { secretCounter++; } } public class MyClass { public static void main(String[] args) { MysteryClass mysteryClass = new MysteryClass(); // add code here to detect calls to incrementCounter and print a debug message } I don't have the ability to change the 3rd party MysteryClass, so I thought that I could use PropertyChangeSupport and PropertyChangeListener to detect changes to the secretCounter: public class MyClass implements PropertyChangeListener { private PropertyChangeSupport propertySupport = new PropertyChangeSupport(this); public MyClass() { propertySupport.addPropertyChangeListener(this); } public void propertyChange(PropertyChangeEvent evt) { System.out.println("property changing: " + evt.getPropertyName()); } public static void main(String[] args) { MysteryClass mysteryClass = new MysteryClass(); // do logic which involves increment and getting the value of MysteryClass } } Unfortunately, this did not work and I have no debug messages printed out. Does anyone see what is wrong with my implementation of the PropertyChangeSupport and Listener interfaces? I want to print a debug statement whenever incrementCounter is called or the value of secretCounter changes.

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• Question About Eclipse Java Debugger Conditional Breakpoints Inefficiency

  - by Personman

  I just set a conditional breakpoint in Eclipse's debugger with a mildly inefficient condition by breakpoint standards - checking whether a HashMap's value list (8 elements) contains Double.NaN. This resulted in an extremely noticeable slowdown in performance - after about five minutes, I gave up. Then I copy pasted the condition into an if statement at the exact same line, put a noop in the if, and set a normal breakpoint there. That breakpoint was reached in the expected 20-30 seconds. Is there something special that conditional breakpoints do that is different from this, or is Eclipse's implementation just kinda stupid? It seems like they could fairly easily just do exactly the same thing behind the scenes.

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• interface as a method parameter in Java

  - by PeterYu

  Hi all, I had an interview days ago and was thrown a question like this. Q: Reverse a linked list. Following code is given: public class ReverseList { interface NodeList { int getItem(); NodeList nextNode(); } void reverse(NodeList node) { } public static void main(String[] args) { } } I was confused because I did not know an interface object could be used as a method parameter. The interviewer explained a little bit but I am still not sure about this. Could somebody enlighten me?

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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 Sound Clip Looping Frame Position

  - by InsertNickHere

  Hi, I have a little problem with a loopting clip: If you have a soundfile e.g. 20000 samples long, the frame position will not reset after looping, so I get values that are "out of bounds" of the original soundfile. As I want to draw a position marker on my waveform, I'm a bit confused how to achive. At this time I just get myClip.getLongFramePosition() but this does not work as described above. Does anyone have an idea how to fix that? Is there a possibility to count how often a clip was looped before? Regards

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• Java File manipulation

  - by user69514

  So I have an application with a JFileChooser from which I select a file to read. Then I change some words and write a new file. The problem that I am having is that when I write the new file it's saved in the project directory. How do I save it in the same directory as the file that I chose using the JFileChooser. Note: I don't want to use the JFileChooser to choose the location. I just need to save the file in the same directory as the original file that I read.

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• Java Object Creation Error

  - by ikurtz

  package Sartre.Connect4; import javax.swing.*; public class ChatGUI extends JDialog { public ChatGUI(){ setTitle("Chat"); } } when i do this in another class in the same package: ChatGUI chatGUI = new ChatGUI(); i end up with a situation: Cannot Find Symbol please help?

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• ConcurentModificationException in Java HashMap

  - by Bear

  Suppose I have two methods in my classes, writeToMap() and processKey() and both methods are called by multiple threads. writeToMap is a method to write something in hashmap and processKey() is used to do sth based on the keySet of HashMap. Inside processKey, I first copy the originalMap before getting the key set. new HashMap<String, Map<String,String>(originalMap).get("xx").keySet(); But I am still getting ConcurrentModificationException even though I always copy the hashmap. Whats the problem?

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• Strange numbers in java socket output

  - by user293163

  I have small test app: Socket socket = new Socket("jeck.ru", 80); PrintWriter pw = new PrintWriter(socket.getOutputStream(), false); pw.println("GET /ip/ HTTP/1.1"); pw.println("Host: jeck.ru"); pw.println(); pw.flush(); BufferedReader rd = new BufferedReader(new InputStreamReader(socket.getInputStream())); String str; while ((str = rd.readLine()) != null) { System.out.println(str); } It`s output: HTTP/1.1 200 OK Date: Sat, 13 Mar 2010 22:06:51 GMT Content-Type: text/html;charset=utf-8 Transfer-Encoding: chunked Connection: keep-alive Keep-Alive: timeout=5 Server HTTP/1.1 200 OK Date: Sat, 13 Mar 2010 22:06:51 GMT Content-Type: text/html;charset=utf-8 Transfer-Encoding: chunked Connection: keep-alive Keep-Alive: timeout=5 Server: Apache Cache-Control: max-age=0 Expires: Sat, 13 Mar 2010 22:06:51 GMT 123 <!DOCTYPE html> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> <title>??? IP</title> </head> <body> <div style='text-align: center; font: 32pt Verdana;margin-top: 300px'> ??? IP &#151; 94.103.87.153 </div> </body> </html> 0 Whence these numbers (123 an 0) takes?

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• Implementing Clonable in Java

  - by Artium

  In which cases should I use this way: public A clone() throws CloneNotSupportedException { A clone = (A)super.clone(); clone.x= this.x; return clone; } And in which cases should I use that way: public ShiftedStack clone() throws CloneNotSupportedException { return new A(this.x); } What should I do if x is final and I want to use the first way? Regarding the first way, I understand it like this: we clone the super class and up-cast it, leading to some members uninitialized. After this initialize these members. Is my understanding correct? Thank you.

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• Java: Friendlier way to get an instance of FontMetrics

  - by Martijn Courteaux

  Hi people, Is there a friendlier way to get an instance of FontMetrics than FontMetrics fm = Graphics.getFontMetrics(Font); I hate this way because of the following example: If you want to create in a game a menu and you want all the menuitems in the center of the screen you need fontmetrics. But, mostly, menuitems are clickable. So I create an array of Rectangles and all the rectangles fits around the items, so when the mouse is pressed, I can simply use for (int i = 0; i < rects.length; i++) if (rects[i].contains(mouseX, mouseY)) { ... } But to create the rects I also need FontMetrics for their coordinates. So this mean that I have to construct all my rectangles in the paint-method of my menu. So I want a way to get the FontMetrics so I can construct the Rectangles in a method called by the constructor. Hope you understand. Thanks in advance.

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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 phone number validation....

  - by user69514

  Here is my problem: Create a constructor for a telephone number given a string in the form xxx-xxx-xxxx or xxx-xxxx for a local number. Throw an exception if the format is not valid. So I was thinking to validate it using a regular expression, but I don't know if I'm doing it correctly. Also what kind of exception would I have to throw? Do I need to create my own exception? public TelephoneNumber(String aString){ if(isPhoneNumberValid(aString)==true){ StringTokenizer tokens = new StringTokenizer("-"); if(tokens.countTokens()==3){ areaCode = Integer.parseInt(tokens.nextToken()); exchangeCode = Integer.parseInt(tokens.nextToken()); number = Integer.parseInt(tokens.nextToken()); } else if(tokens.countTokens()==2){ exchangeCode = Integer.parseInt(tokens.nextToken()); number = Integer.parseInt(tokens.nextToken()); } else{ //throw an excemption here } } } public static boolean isPhoneNumberValid(String phoneNumber){ boolean isValid = false; //Initialize reg ex for phone number. String expression = "(\\d{3})(\\[-])(\\d{4})$"; CharSequence inputStr = phoneNumber; Pattern pattern = Pattern.compile(expression); Matcher matcher = pattern.matcher(inputStr); if(matcher.matches()){ isValid = true; } return isValid; } Hi sorry, yes this is homework. For this assignments the only valid format are xxx-xxx-xxxx and xxx-xxxx, all other formats (xxx)xxx-xxxx or xxxxxxxxxx are invalid in this case. I would like to know if my regular expression is correct

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• Java Swing popup visibility

  - by Octavio

  Why a popup created like this is shown in front of all windows applications with JRE 1.6.0_18 but it doesn't using 1.6.0_03 ? PopupFactory popupFactory = new PopupFactory(); Popup popup= popupFactory.getPopup(null,new JPanel(),200,200); popup.show();

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• socket problem in java

  - by stefan89

  I have two classes X and Y, like this: class X implements Serializable { int val1; Y val2; } class Y implements Serializable { int val; } I want to transmit an object of type X from a client to server but i can't because the class X has a field of type Y. I replace the field of type Y with a field of type X in class X and it works.

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• Simple Question:Output of below Java program

  - by Abhishek Jain

  public class abc1 { private String s; public abc1(String s){this.s=s;} public static void main(String args[]) { HashSet<Object> hs=new HashSet<Object>(); abc1 a1= new abc1("abc"); abc1 a2= new abc1("abc"); String s1= new String("abc"); String s2= new String("abc"); hs.add(a1); hs.add(a2); hs.add(s1); hs.add(s2); System.out.println(hs.size()); } } Why above program output is 3?

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• Java Applet - ArrayIndexOutOfBoundsException

  - by Dan

  OK so I am getting an ArrayIndexOutofBoundsException. I don't know why. Here's my code: http://www.so.pastebin.com/y5MjD1k3 The thing is when I go to the red brick at board[2][2]... I go there. Then I go up... then I TRY go to back down but that error pops up. Also when I go to the right 8 squares... I ALSO get that error. ALSO, pretend my 2d map is split into FOUR squares... well square one is the top left... if I go ANYWHERE else ... I get that error. What am I doing wrong? Thanks.

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• Java: Are these 2 codes the same?

  - by Kevin Duke

  for (Player p : players) { p.addCard(deck.dealCard()); p.addCard(deck.dealCard()); } and for (int i = 0; i < players.size() ; i++) { Player p = players.get(i); p.addCard(deck.dealCard()); p.addCard(deck.dealCard()); } The second code yeilds a null pointer exception, what can be done to make the bottom one equivalent ?

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• does iterator come under collections in java?

  - by androidbase Praveen

  the iterator is a collection or a kind of control statement use. define plz.

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• java /TableModel of Objects/Update Object"

  - by Tomás Ó Briain

  I've a collection of Stock objects that I'm updating about 10/15 variables for in real-time. I'm accessing each Stock by its ID in the collection. I'm also trying to display this in a JTable and have implemented an AbstractTablemodel. It's not working too well. I've a RowMap that I add each ID to as Stocks are added to the TableModel. To update the prices and variables of all the stocks in the TableModel, I want to send a Stock object to an updateModel(Stock s) method. I can find the relevant row by searching the map, but how do I handle this nicely, so I don't have to start iterating through table columns and comparing the values of the cells to the variables of the object to see whether there are any differences?? Basically, i want to send a Stock object to the TableModel and update cells if there are changes and do nothing if there aren't. Any ideas about how to implement a TableModel that might do this? Any pointeres at all would be appreciated. Thanks.

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• java problem about ResultSet

  - by vipul

  Hello Friends, I want to merge two Resultset Object into third one. If you have any idea then reply.. vipul

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• Java- screen capture behind the application.

  - by Auxiliary

  Is there a way to capture the screen but without the app itself getting shown. I know how to minimize or make a frame invisible but this has to be done real fast cos it's going to happen alot in each second (at least once every second). I want it to look something like the magnifier in Windows Vista/7. (The app can see behind itself.) Thanks in advance.

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• java: converting part of a ByteBuffer to a string

  - by Jason S

  I have a ByteBuffer containing bytes that were derived by String.getBytes(charsetName), where "containing" means that the string comprises the entire sequence of bytes between the ByteBuffer's position() and limit(). What's the best way for me to get the string back? (assuming I know the encoding charset) Is there anything better than the following (which seems a little clunky) byte[] ba = new byte[bbuf.remaining()]; bbuf.get(ba); try { String s = new String(ba, charsetName); } catch (UnsupportedEncodingException e) { /* take appropriate action */ }

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• Java - Regex problem

  - by Yatendra Goel

  I have a list of urls of type http://www.abc.com/pk/ca and http://www.abc.com/pk Now, I want to find out only those urls that ends with /pk or /pk/ and don't have anything in between .com and /pk

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