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  • `return value' from Constructor Exception in Java?

    - by Lajos Nagy
    Take a look that the following code snippet: A a = null try { a = new A(); } finally { a.foo(); // What happens at this point? } Suppose A's constructor throws a runtime exception. At the marked line, am I always guaranteed to get a NullPointerException, or foo() will get invoked on a half constructed instance?

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  • asking the container to notify your application whenever a session is about to timeout in Java

    - by user136101
    Which method(s) can be used to ask the container to notify your application whenever a session is about to timeout?(choose all that apply) A. HttpSessionListener.sessionDestroyed -- correct B. HttpSessionBindingListener.valueBound C. HttpSessionBindingListener.valueUnbound -- correct this is kind of round-about but if you have an attribute class this is a way to be informed of a timeout D. HttpSessionBindingEvent.sessionDestroyed -- no such method E. HttpSessionAttributeListener.attributeRemoved -- removing an attribute isn’t tightly associated with a session timeout F. HttpSessionActivationListener.sessionWillPassivate -- session passivation is different than timeout I agree with option A. 1) But C is doubtful How can value unbound be tightly coupled with session timeout.It is just the callback method when an attribute gets removed. 2) and if C is correct, E should also be correct. HttpSessionAttributeListener is just a class that wants to know when any type of attribute has been added, removed, or replaced in a session. It is implemented by any class. HttpSessionBindingListener exists so that the attribute itself can find out when it has been added to or removed from a session and the attribute class must implement this interface to achieve it. Any ideas…

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  • JAVA Inheritance Generics and Casting.

    - by James Moore
    Hello, I have two classes which both extends Example. public class ClassA extends Example { public ClassA() { super("a", "class"); } .... } public class ClassB extends Example { public ClassB() { super("b", "class"); } .... } public class Example () { public String get(String x, String y) { return "Hello"; } } So thats all very well. So suppose we have another class called ExampleManager. With example manager I want to use a generic type and consequently return that generic type. e.g. public class ExampleManager<T extends Example> { public T getExample() { return new T("example","example"); // So what exactly goes here? } } So where I am returning my generic type how do i get this to actually work correctly and cast Example as either classA or classB? Many Thanks

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  • Java: autofiltering list?

    - by Jason S
    I have a series of items arriving which are used in one of my data structures, and I need a way to keep track of those items that are retained. interface Item {} class Foo implements Item { ... } class Baz implements Item { ... } class StateManager { List<Foo> fooList; Map<Integer, Baz> bazMap; public List<Item> getItems(); } What I want is that if I do the following: for (int i = 0; i < SOME_LARGE_NUMBER; ++i) { /* randomly do one of the following: * 1) put a new Foo somewhere in the fooList * 2) delete one or more members from the fooList * 3) put a new Baz somewhere in the bazMap * 4) delete one or more members from the bazMap */ } then if I make a call to StateManager.getItems(), I want to return a list of those Foo and Baz items, which are found in the fooList and the bazMap, in the order they were added. Items that were deleted or displaced from fooList and bazMap should not be in the returned list. How could I implement this? SOME_LARGE_NUMBER is large enough that I don't have the memory available to retain all the Foo and Baz items, and then filter them.

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  • Sending an Email Using Java

    - by user217029
    try{ Properties props = new Properties(); props.put("mail.smtp.host", "ipc-smtp.bits-pilani.ac.in"); Session sess = Session.getInstance(props, null); sess.setDebug(true); Message msg = new MimeMessage(sess); InternetAddress addressFrom = new InternetAddress("[email protected]"); msg.setFrom(addressFrom); msg.setRecipient(Message.RecipientType.TO, new InternetAddress("[email protected]")); msg.addHeader("MyHeaderName", "myHeaderValue"); msg.setSubject("Test"); msg.setContent("Yippe", "text/plain"); Transport.send(msg); }catch(Exception exp){ exp.printStackTrace(); } The error is javax.mail.MessagingException: 554 The mail was blocked due to zen-spamhaus RBL action This is my college's smtp server.

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  • Java downcasting and is-A has-A relationship

    - by msharma
    HI, I have a down casting question, I am a bit rusty in this area. I have 2 clasess like this: class A{ int i; String j ; //Getters and setters} class B extends A{ String k; //getter and setter} I have a method like this, in a Utility helper class: public static A converts(C c){} Where C are objects that are retireved from the database and then converted. The problem is I want to call the above method by passing in a 'C' and getting back B. So I tried this: B bClasss = (B) Utility.converts(c); So even though the above method returns A I tried to downcast it to B, but I get a runtime ClassCastException. Is there really no way around this? DO I have to write a separate converts() method which returns a B class type? If I declare my class B like: class B { String k; A a;} // So instead of extending A it has-a A, getter and setters also then I can call my existing method like this: b.setA(Utility.converts(c) ); This way I can reuse the existing method, even though the extends relationship makes more sense. What should I do? Any help much appreciated. Thanks.

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  • Strange problem with simple multithreading program in Java

    - by Elizabeth
    Hello, I am just starting play with multithreading programming. I would like to my program show alternately character '-' and '+' but it doesn't. My task is to use synchronized keyword. As far I have: class FunnyStringGenerator{ private char c; public FunnyStringGenerator(){ c = '-'; } public synchronized char next(){ if(c == '-'){ c = '+'; } else{ c = '-'; } return c; } } class ThreadToGenerateStr implements Runnable{ FunnyStringGenerator gen; public ThreadToGenerateStr(FunnyStringGenerator fsg){ gen = fsg; } @Override public void run() { for(int i = 0; i < 10; i++){ System.out.print(gen.next()); } } } public class Main{ public static void main(String[] args) throws IOException { FunnyStringGenerator FSG = new FunnyStringGenerator(); ExecutorService exec = Executors.newCachedThreadPool(); for(int i = 0; i < 20; i++){ exec.execute(new ThreadToGenerateStr(FSG)); } } } EDIT: I also testing Thread.sleep in run method instead for loop.

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  • How to Create Own HashMap in Java?

    - by Taranfx
    I know about hashing algorithm and hashCode() to convert "key" into an equivalent integer (using some mathematically random expression) that is then compressed and stored into buckets. But can someone point me to an implementation or at least data structure that should be used as baseline? I haven't found it anywhere on the web.

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  • Java ternary operator and boxing Integer/int?

    - by Markus
    I tripped across a really strange NullPointerException the other day caused by an unexpected type-cast in the ternary operator. Given this (useless exemplary) function: Integer getNumber() { return null; } I was expecting the following two code segments to be exactly identical after compilation: Integer number; if (condition) { number = getNumber(); } else { number = 0; } vs. Integer number = (condition) ? getNumber() : 0; . Turns out, if condition is true, the if-statement works fine, while the ternary opration in the second code segment throws a NullPointerException. It seems as though the ternary operation has decided to type-cast both choices to int before auto-boxing the result back into an Integer!?! In fact, if I explicitly cast the 0 to Integer, the exception goes away. In other words: Integer number = (condition) ? getNumber() : 0; is not the same as: Integer number = (condition) ? getNumber() : (Integer) 0; . So, it seems that there is a byte-code difference between the ternary operator and an equivalent if-else-statement (something I didn't expect). Which raises three questions: Why is there a difference? Is this a bug in the ternary implementation or is there a reason for the type cast? Given there is a difference, is the ternary operation more or less performant than an equivalent if-statement (I know, the difference can't be huge, but still)?

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  • replace values in a String from a Hashtable in Java

    - by Adnan
    My string looks like; String values = "I am from UK, and you are from FR"; and my hashtable; Hashtable countries = new Hashtable(); countries.put("United Kingdom", new String("UK")); countries.put("France", new String("FR")); What would be the most effective way to change the values in my string with the values from the hashtable accordingly. These are just 2 values to change, but in my case I will have 100+

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  • Java Refreshing a screen

    - by Goutham
    I have a screen in which one of its components is made invisible depending on a boolean value. If the boolean changes after the screen has been created, how do I refresh the screen to take this into account?

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  • Which Queue implementation to use in Java?

    - by devoured elysium
    I need to use a FIFO structure in my application. It needs to have at most 5 elements. I'd like to have something easy to use (I don't care for concurrency) that implements the Collection interface. I've tried the LinkedList, that seems to come from Queue, but it doesn't seem to allow me to set it's maximum capacity. It feels as if I just want at max 5 elements but try to add 20, it will just keep increasing in size to fit it. I'd like something that'd work the following way: XQueue<Integer> queue = new XQueue<Integer>(5); //where 5 is the maximum number of elements I want in my queue. for (int i = 0; i < 10; ++i) { queue.offer(i); } for (int i = 0; i < 5; ++i) { System.out.println(queue.poll()); } That'd print: 5 6 7 8 9 Thanks

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  • Lines don't overlap when they should Java Swing

    - by Sven
    I'm drawing lines in a JFrame on a self made gridPanel. Problem is, I draw the lines between 2 points. When I have a line that is between point 1 and point 2 and a line between point 2 and point 3, the lines should connect. This however isn,t the case, there is a small gap in between, no idea why. But it isn't drawing till the end of the specified point. (start point is correct.) Here is the code of the JFrame: public void initialize(){ this.setLayout(new BorderLayout()); this.setPreferredSize(new Dimension(500, 400)); gridPane = new GridPane(); gridPane.setBackground(Color.WHITE); gridPane.setSize(this.getPreferredSize()); gridPane.setLocation(0, 0); this.add(gridPane,BorderLayout.CENTER); //createSampleLabyrinth(); drawWall(0,5,40,5); //These are the 2 lines that don't connect. drawWall(40,5,80,5); this.pack(); } drawWall calls a method that calls a method in GridPane. The relevant code in gridPane: /** * Draws a wall on this pane. With the starting point being x1, y1 and its end x2,y2. * @param x1 * @param y1 * @param x2 * @param y2 */ public void drawWall(int x1, int y1, int x2, int y2) { Wall wall = new Wall(x1,y1,x2,y2, true); wall.drawGraphic(); wall.setLocation(x1, y1); wall.setSize(10000,10000); this.add(wall, JLayeredPane.DEFAULT_LAYER); this.repaint(); } This method creates a wall and puts it in the Jframe. The relevant code of the wall: public class Wall extends JPanel { private int x1; private int x2; private int y1; private int y2; private boolean black; /** * x1,y1 is the start point of the wall (line) end is x2,y2 * * @param x1 * @param y1 * @param x2 * @param y2 */ public Wall(int x1, int y1, int x2, int y2, boolean black) { this.x1 = x1; this.x2 = x2; this.y1 = y1; this.y2 = y2; this.black = black; setOpaque(false); } private static final long serialVersionUID = 1L; public void drawGraphic() { repaint(); } public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D) g; if(black){ g2.setColor(Color.BLACK); g2.setStroke(new BasicStroke(8)); } else { g2.setColor(Color.YELLOW); g2.setStroke(new BasicStroke(3)); } g2.drawLine(x1, y1, x2, y2); } } So, where am I going wrong? The true/false is to determine if the wall should be black or yellow, nothing to be concerned about.

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  • Implementing Iterable in Java

    - by Artium
    I have the following code public class A extends Iterable<Integer> { ... public Iterator<Integer> iterator() { return new Iterator<Integer>() { A a; public boolean hasNext() { ... } public Integer next() { ... } public void remove(){ ... } }; I would like to initialize the "a" field in the anonymous class with the instance of A that iterator method was called on. Is it possible? Thank you.

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  • How to place images on top of each other in java

    - by Haxed
    Hi, I have a background image of a road, which I have displayed on a JFrame using an ImageIcon. I want to put a car on top of that background image at a certain (x,y) location. I tried using another ImageIcon and the background does not appear when I run the program , however the car does. import javax.swing.ImageIcon; import javax.swing.JFrame; import javax.swing.JLabel; public class Gui extends JFrame { private ImageIcon northcar = new ImageIcon("src/north.gif"); private ImageIcon usIcon = new ImageIcon("src/trafficLight.jpg"); public Gui() { add(new JLabel(usIcon)); // background image does not appear after the i added the northcar label add(new JLabel(northcar)); // this picture can be seen } public static void main(String[] args) { Gui frame = new Gui(); frame.setTitle("TestImageIcon"); frame.setLocationRelativeTo(null); // Center the frame frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setBounds(0, 0, 650, 650); frame.setVisible(true); } } I heard about using a canvas but have no clue. Any ideas ? Many Thanks

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  • export a JOGL applet and embedd into a html page

    - by nkint
    hi guys it is some time that i'm testing opengl with java and JOGL. now i have good result and i wanto to pubblish it on web. but i have some problem. i'm in eclipse, and i'm testing an Applet with JOGL. first of all i have this run time error (but the program works correctly): java.lang.IllegalArgumentException: adding a window to a container at java.awt.Container.checkNotAWindow(Container.java:431) at java.awt.Container.addImpl(Container.java:1039) at java.awt.Container.add(Container.java:365) at AppletHelloWorld.init(AppletHelloWorld.java:30) at sun.applet.AppletPanel.run(AppletPanel.java:424) at java.lang.Thread.run(Thread.java:619) then i found this incredibly clear page and i do what is said, i open html with the browser, the libs are downloaded but it stops at "Starting applet AppletHelloWorld" that is the name i gave to my applet. mayebe i miss something like main function or exporting well the jar? this is my main code: public class AppletHelloWorld extends Applet { public static void main(String[] args) { JFrame fr=new JFrame(); fr.setBounds(0,0,1015,600); fr.add(new AppletHelloWorld()); fr.setVisible(true); } public void init() { setLayout(null); MyJOGLProject canvas = new MyJOGLProject(); //MyJOGLProject extends JFrame canvas.run(); Container c = new Container(); c.add(canvas); add(c); } //....

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  • JADE (Java) - Changing Agent Container

    - by Chad S
    Is there a way to reassign agents to a different container or will I have to create a new container and then create all new instances of the agents within the new container? I have done a lot of searching and can't seem to find anything on container reassignment. Thanks in advance for any info!

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  • Date conversion in Java

    - by llm
    How can I take a string in a format such as: 2008-06-02 00:00:00.0 and convert it to: 02-Jun-2008? Can I somehow take the original string, convert it to a Date object, then use a formatter to get the final output (rather than parsing the string myself)? Thanks!

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  • Nothing happen when refreshing the main Frame (JAVA)

    - by Ams
    Hello everyone, I try to show a ( Logged in ) message when a user is succefully connected but nothing happen when a do a repaint(). you can take a look to the code : public class MainFrame extends JFrame implements ActionListener{ private static final long serialVersionUID = 1L; private static final int FRAME_HEIGHT = 400; private static final int FRAME_WIDTH = 250; private static final String TITLE = new String("TweeX"); private static String TWITTERID = new String(); private static String TWITTERPW = new String(); private boolean logged = false; private JTextField loginField = new JTextField(10); private JPasswordField passField = new JPasswordField(10); private JButton login = new JButton("Connect"); private GridBagConstraints c = new GridBagConstraints(); private String UserStatus = new String("Please login..."); /* * Constructor ! */ MainFrame() { setSize(FRAME_WIDTH, FRAME_HEIGHT); setTitle(TITLE); setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE); setResizable(false); loginUser(); } /* * Login Forms */ protected void loginUser(){ this.setLayout(new GridBagLayout()); //add Login Fiels + Label c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 0; c.insets = new Insets(5,5,5,20); c.gridy = 0; add(new JLabel("Username:"),c); c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 1; c.gridy = 0; add(loginField,c); //add Password Fiels + Label c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 0; c.gridy = 1; add(new JLabel("Password:"),c); c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 1; c.gridy = 1; add(passField,c); //add Login button c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 1; c.gridy = 2; add(login,c); //add listener to login button login.addActionListener((ActionListener) this); c.fill = GridBagConstraints.HORIZONTAL; c.gridx = 1; c.gridy = 3; add(new JLabel(UserStatus),c); setVisible(true); } @Override public void actionPerformed(ActionEvent e) { TWITTERID = loginField.getText(); TWITTERPW = passField.getText(); Twitter twitter = new TwitterFactory().getInstance(TWITTERID,TWITTERPW); logged = true; try { twitter.verifyCredentials(); } catch (TwitterException e1) { logged = false; } } protected void connect(){ if(logged){ UserStatus = "Loged In :)"; repaint(); } } static public void main(String[] argv) { new MainFrame(); } }

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  • How to mix two arrays in Java?

    - by roddik
    Hello. I have some String[] arrays, for example: ['a1', 'a2'] ['b1', 'b2', 'b3', 'b4'] ['c1'] How can I mix them, so that I get ['a1', 'b1', 'c1', 'a2', 'b2', 'b3', 'b4'] (0 element of a, then b, c, 1 element of a, b, c and so on)? Thanks

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  • Joining remote paths in Java

    - by Mickael Marrache
    I'm using the FTP library provided by Apache (commons-net). I want to check if a file exists on the FTP server so I use the listFiles method of FTPClient: ftpClient.listFiles(remoteFileDir + "\\" + fileName); The current directory is the FTP server root directory. So, the value of remoteFileDir is a path relative to this root directory. My question concerns the merge between the remote directory path and the file name. What is the right way to do it? For a local file, I would do: File file = new File(remoteFileDir,fileName); but here it doesn't work since when I call file.getAbsolutePath(), I get an absolute path for the file in the local current directory which is not what I want. Also, I guess the merging has been done according to my local environment. PS: I looked at How are paths determined on a remote machines? but it doesn't help me. Thanks

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  • Unique number generation with Java Server Faces

    - by Buddhika Ariyaratne
    I am developing an application for a medical channelling centre where multiple users reserve bookings for doctors with JSF and JPA. A sequence number is unique to the Doctor, Date and Session. I tried to get a unique sequence number from counting the previous bookings and add one, but if two requests comes at the same time, two bookings get the same number causing trouble to functionality. How can I get unique number in this case? Can I use an application wide bean to generate it? (I thought it is not practicle to get the unique number from the database sequence number as there are several doctors, sessions and daily they have to have different booking number.)

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