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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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• StringBufferInputStream Question in Java

  - by JJG

  I want to read an input string and return it as a UTF8 encoded string. SO I found an example on the Oracle/Sun website that used FileInputStream. I didn't want to read a file, but a string, so I changed it to StringBufferInputStream and used the code below. The method parameter jtext, is some Japanese text. Actually this method works great. The question is about the deprecated code. I had to put @SuppressWarnings because StringBufferInputStream is deprecated. I want to know is there a better way to get a string input stream? Is it ok just to leave it as is? I've spent so long trying to fix this problem that I don't want to change anything now I seem to have cracked it. @SuppressWarnings("deprecation") private String readInput(String jtext) { StringBuffer buffer = new StringBuffer(); try { StringBufferInputStream sbis = new StringBufferInputStream (jtext); InputStreamReader isr = new InputStreamReader(sbis, "UTF8"); Reader in = new BufferedReader(isr); int ch; while ((ch = in.read()) > -1) { buffer.append((char)ch); } in.close(); return buffer.toString(); } catch (IOException e) { e.printStackTrace(); return null; } }

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• Java - JPA - @Version annotation

  - by Yatendra Goel

  I am new to JPA. I am cofused about the @Version annotation. How it works? I have googled it and found various answers whose extract is as follows: JPA uses a version field in your entities to detect concurrent modifications to the same datastore record. When the JPA runtime detects an attempt to concurrently modify the same record, it throws an exception to the transaction attempting to commit last. But still I am not sure how it works? ================================================================================== Also as from the following lines: You should consider version fields immutable. Changing the field value has undefined results. Does it mean that we should declare our version field as final

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• What is the use of reflection in Java/C# etc

  - by zengr

  I was just curious, why should we use reflection in the first place? // Without reflection Foo foo = new Foo(); foo.hello(); // With reflection Class cls = Class.forName("Foo"); Object foo = cls.newInstance(); Method method = cls.getMethod("hello", null); method.invoke(foo, null); We can simply create an object and call the class's method, but why do the same using forName, newInstance and getMthod functions? To make everything dynamic?

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• Java Null Pointer Exception :-(

  - by John-Michael Reed

  I've got a Null Pointer Exception in my main that just won't go away and I'm totally out of ideas. The error is on the line "Board[x][y].color = 2;" in which Board is a public, static array of piece objects that contain instance variables like the one "color" that is being set to 2 in the above statement. Pieces is not static - that is there are many different copies of pieces, each with its own data, but only one board. The array has been initialized and defined as both public Piece[][] Board = new Piece[8][8] and public static Piece[][] Board = new Piece[8][8], but no matter how I mess around with it (getting rid of static, putting the variable in another object, etc.), I can't seem to get the error to go away. Help?

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• java overloaded method

  - by Sean Nguyen

  Hi, I have an abstract template method: class abstract MyTemplate { public void something(Object obj) { doSomething(obj) } protected void doSomething(Object obj); } class MyImpl extends MyTemplate { protected void doSomething(Object obj) { System.out.println("i am dealing with generic object"); } protected void doSomething(String str) { System.out.println("I am dealing with string"); } } public static void main(String[] args) { MyImpl impl = new MyImpl(); impl.something("abc"); // --> this return "i am dealing with generic object" } How can I print "I am dealing with string" w/o using instanceof in doSomething(Object obj)? Thanks,

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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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• 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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• 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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• Output Parameter in java

  - by soclose

  Hi, could u give me a sample code of using output parameter in function? I tried to google it but just found it in function. I'd like to use this output value in another function. I will use this code in Android.

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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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• 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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• 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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• 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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• Java - 'continue' loop iteration after certain timeout period

  - by Matt

  Is there a way to exit ('continue;') a loop iteration after a certain timeout period? I have a loop that will run gathering data from the web and then use this data to make a calculation. The data become obsolete after about 1 to 2 seconds so if the loop iteration takes longer than 1 second then i want it to 'continue' to the next iteration. Sometimes gathering the data can take time but sometimes the calculation can take longer than 1 second so a HTTP timeout won't work for what i need. Also, while doing the calculation the thread i am using is blocked so i cannot check System.currentTimeMillis(); Is there a way to use another Thread to check the time and force the original for loop to continue.

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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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• 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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• 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 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's Swing Threading

  - by nevets1219

  My understanding is that if I start up another thread to perform some actions, I would need to SwingUtilities.invokeAndWait or SwingUtilities.invokeLater to update the GUI while I'm in said thread. Please correct me if I'm wrong. What I'm trying to accomplish is relatively straightforward: when the user clicks submit, I want to (before performing any actions) disable the submit button, perform the action, and at the end of the action re-enable the button. My method to perform the action updates the GUI directly (displays results) when it gets the results back. This action basically queries a server and gets some results back. What I have so far is: boolean isRunning = false; synchronized handleButtonClick() { if ( isRunning == false ) { button.setEnabled( false ); isRunning = true; doAction(); } } doAction() { new Thread() { try { doAction(); // Concern A } catch ( ... ) { displayStackTrace( ... ); // Concern B } finally { SwingUtilities.invokeLater ( /* simple Runnable to enable button */ ); isRunning = false; } } } For both of my concerns above, do I would have to use SwingUtilities.invokeAndWait since they both will update the GUI? All GUI updates revolve around updating JTextPane. Do I need to in my thread check if I'm on EDT and if so I can call my code (regardless of whether it updates the GUI or not) and NOT use SwingUtilities.invokeAndWait?

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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 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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• java objects, shared variables

  - by raven

  hello, I have a simple question here. If I declare a variable inside an object which was made [declared] in the main class, like this: public static int number; ( usually I declare it like this : private int number; ) can it be used in a different object which was also made [declared] in the main class? btw I do not care about security atm, I just want to make something work, don't care about protection)

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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 /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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