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  • Java : Singleton class instances in a Web based Application

    - by Preethi Jain
    I have this Singleton class inside a Web Application . public class MyDAO { private static MyDAO instance; private MyDAO() { } public static MyDAO getInstance() { if (instance == null) { instance = new MyDAO(); } return instance; } I will access it this way public void get_Data() { MyDAO dao = MyDAO.getInstance(); } How many Objects of MyDAO class will be created if there are 3 Users accessing the Application ?? Will there be one instance of MyDAO per User ??

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  • JAVA: How do I accept input during a certain period of time only

    - by sebrock
    Im trying to do a simple game where I continually need input from players. This needs to happen during a certain time period only. Everything that is sent after that will be discarded. After the time period a new game starts. So: Start game Wait for input from all players during 10 seconds 10 secs no more input Calculate the winner and do some stuff Goto 1. I was thinking using a timer and timertask to keep track of time and maybe use a boolean variable that changes from "open" to "closed" after 10 seconds? Please give me some advise on this.

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  • java - Depth First Search - Perform DFS on a tree

    - by DJDonaL3000
    Im trying to perform DFS on a Minimum Spanning Tree which contains 26 nodes. Nodes are named 'A' to 'Z' and the tree is undirected. I have an empty function called DFS here that I am trying to write, which (i presume) takes in the tree (a 2D array) a startNode (randomly selected node 'M') and the endNode (randomly selected node 'Z'). The weights of connected nodes are identified in the 2D array parameter, but how do I actually get started visiting nodes? All that is required is to print each nodeName in the order of the DFS traversal. Do I need to create a Node_class for each node in the 2d array??

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  • Java, merging two arrays evenly

    - by user2435044
    What would be the best way to merge two arrays of different lengths together so they are evenly distributed in the new array? Say I have the following arrays String[] array1 = new String[7]; String[] array2 = new String[2]; String[] mergedArray = new String[array1.length + array2.length]; I would want mergedArray to have the following elements array1 array1 array1 array2 array1 array1 array1 array2 array1 but if I were to change the size of the arrays to String[] array1 = new String[5]; String[] array2 = new String[3]; String[] mergedArray = new String[array1.length + array2.length]; then I would want it to be array1 array2 array1 array2 array1 array2 array1 array1 basically if it can be helped each array2 element shouldn't be touching each other; exception if array2 has a size larger than array1.

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  • Java: How to make this Serializable?

    - by Hasslarn
    I dont know that much about Serializable, but i need this class to be. How to achieve it? package helpers; public class XY implements Comparable<XY> { public int x; public int y; public XY (int x, int y) { this.x = x; this.y = y; } public int compareTo( XY other ) { String compare1 = this.x + "-" + this.y; String compare2 = other.x + "-" + other.y; return compare1.compareTo( compare2 ); } public String toString() { return this.x + "-" + this.y; } } As of now i cant send it as an object with outputstream..I´ve tried just to implement Serializable but it doesnt do the trick!

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  • what databases can be used with java?

    - by aadersh patel
    I am doing an undergrad final project, and need to justify my choice of MySQL for the database element of my project. Truth is, it's the only one I can really use, and hence I went for it. What other database systems could I have used? Any advantages and disadvantages of these over MySQL?

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  • High performance distributed asynchronous RPC in java

    - by unludo
    I would like to do RPC to a list of clients with the following requirements: the server does not know the clients (implies a kind of broker?) and the cleints do not know the server there may be several clients - they share the load to treat the RPC The RPC is asynchronous very fast (round-trip < 1ms) optional : offers a fail-over mechanism. It can be done with underlying tools which are not really intended for that (Hazelcast is an example). What would you use for such requirements? Thanks!

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  • Java Variable Initialization

    - by Samuel Brainard
    Here's a piece of code I wrote. public class cube { private int length; private int breadth; private int height; private int volume; private int density; private int weight; public cube(int l,int b,int h, int d) { length=l; breadth=b; height=h; density=d; } public void volmeShow(){ volume=length*breadth*height; System.out.println("The Volume of the cube is "+this.volume); So if I implement the above cube class like this, public class cubeApp { public static void main(String[] args){ cube mycube = new cube(5,6,9,2); mycube.volumeShow(); I get an output that tells me Volume is 270. But I get an output that says Volume is 0 if I define the volume variable like this: public class cube { private int length; private int breadth; private int height; private int volume=length*breadth*height; private int density; private int weight; public cube(int l,int b,int h, int d) { length=l; breadth=b; height=h; density=d; } public void volmeShow(){ System.out.println("The Volume of the cube is "+this.volume); Can somebody please explain why this is happening? Thanks, Samuel.

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  • Java connecting to Http which method to use?

    - by jax
    I have been looking around at different ways to connect to URLs and there seem to be a few. My requirements are to do POST and GET queries on a URL and retrieve the result. I have seen URL class DefaultHttpClient class And there were some others in apache commons which method is best?

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  • Java - Does null variable require space in memory

    - by Yatendra Goel
    class CheckStore { private String displayText; private boolean state; private String meaningfulText; private URL url; public CheckStore(String text, boolean state) { this.displayText = text; this.state = state; } : : } As I am initializing only two variables (displayText and state) in the constructor, Will the rest two variables (meaningfulText and url which will have the value null) will require space in memory to store null value. Q1. I think they will require space. If they will, then how much memory does a null value takes in the memory (like int takes 4 bytes). Q2. How much space a string takes in memory. I guess it will depend on the length of the string. So how much space a string takes of how much length?

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  • Java combine parents of two large inheritance chains

    - by Soylent Green
    I have two parent classes in a huge project, let's say ClassA and ClassB. Each class has many subclasses, which in turn have many subclasses, which in turn have many subclasses, etc. My task is to "marry" these two "families" so that both inherit from a SINGLE parent. I need to essentially make ClassA and ClassB one class (parent) to both of their combined subclasses (children). ClassA and ClassB both currently implement Serializable. I am currently trying to make both inheritance chains inherit from ClassA, and then copy all functions and data members from ClassB into ClassA. This is tedious, and I think a terrible solution. What would be the CORRECT way to solve this problem?

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  • how to convert BigInteger to String in java

    - by Bipul
    i converted a string to BigInteger as follows: Scanner sc=new Scanner(System.in); System.out.println("enter the message"); String msg=sc.next(); byte[] bytemsg=msg.getBytes(); BigInteger m=new BigInteger(bytemsg); now i want my string back.i m using m.toString() method but not getting desired result. why??? what is bug in it and what is its remedy.

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  • java - find out the type of class which implements of other classes

    - by Johnzzz
    i have a kind of specific problem, let's say, that i have public interface A { } //------------------------------ public class B implements A{ static int countx = 0; } //---------------------------------- public class C implements A{ static int county = 0; } //---------------------------------- public class Arc { public A from; public A to; //======================================== and now I have an object a (which is an instance of Arc) and I want to find out whether it is an instance of B or C and get to the atributes countX or countY (stg like a.from.countX) any ideas? :)

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  • this and super in java

    - by abson
    this and super are keywords isn't it, then how can we use them for passing arguments to constructors the same way as with a method?? In short how is it that these can show such distinct behaviours??

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  • Deferred printing in Java

    - by Bober02
    I have a specific issue with general console printing and I was wondering whether anyone has a solution for it. I am trying to print a dataTable which would look like sth like this: Table ---------------------- Name |Surname | ---------------------- Mike |Mikhailowish| Rafaello|Mirena | and so on. In order to print the border of the bar I need to know what the maximum length of each column value is. I don't want to go through the whole database to find that out and then again to print it. I would rather like to do sth like: System.out.printLater(s); //herejust leave a pointer to a StringBuilder you will build ... s.append("--------"); ... System.out.printAllDeferred(); I understand the above is probably in 99.99999% chances impossible, but perhaps you guys have a clever way of achieving the above?

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  • How to load modules in Java

    - by gerardorn
    I'm building a server that loads modules. Each module is a .jar file. Inside the jar there's all the classes that the module needs. The server needs to read the jar, find the main class (it doesnt have the main method on it is just the class that makes the module work but not as a different program), build an object of that class and store it in a vector of modules so that it can acces to a specific module depending on the job to be done. How can I do that? As far as the server, it is listening and receiving request, but there's no modules to delegate the job to, since my vector of modules is empty.

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  • Exception and Inheritance in JAVA

    - by user1759950
    Suppose we have this problem public class Father{ public void method1(){...} } public class Child1 extends Father{ public void method1() throws Exception{ super.method1(); ... } } Child1 extends Father and override method1 but given implementation Child1.method1 now throws a exception, this wont compile as override method can't throw new exceptions. What is the best solution? Propagate the required exception to the Father.. to me this is against encapsulation, inheritance and general OOP ( the father potentially throw and exception that will never happen ) Use a RuntimeException instead? This solution wont propagate the Exception to the father but I read In Oracle docs and others sources states class of exceptions should be used when "Client code cannot do anything" this is not that case, this exception will b useful to recover blablabla ( why is wrong to use RuntimeException instead? ) Other.. thanks, Federico

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  • Java - Thread - Problem in one of the Sun's tutorial

    - by Yatendra Goel
    I was reading this Sun's tutorial on Thread. I found a block of code there which I think can be replaced by a code of fewer lines. I wonder why Sun's expert programmers followed that long way when the task can be accomplished with a code of fewer lines. I am asking this question so as to know that if I am missing something that the tutorial wants to convey. The block of code is as follows: t.start(); threadMessage("Waiting for MessageLoop thread to finish"); //loop until MessageLoop thread exits while (t.isAlive()) { threadMessage("Still waiting..."); //Wait maximum of 1 second for MessageLoop thread to //finish. t.join(1000); if (((System.currentTimeMillis() - startTime) > patience) && t.isAlive()) { threadMessage("Tired of waiting!"); t.interrupt(); //Shouldn't be long now -- wait indefinitely t.join(); } } threadMessage("Finally!"); I think that the above code can be replaced by the following: t.start(); t.join(patience); // InterruptedException is thrown by the main method so no need to handle it if(t.isAlive()) { // t's thread couldn't finish in the patience time threadMessage("Tired of waiting!"); t.interrupt(); t.join(); } threadMessage("Finally!");

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  • How do develop a Java web application without having to deploy all the time

    - by ChrisSmith..zzZZ
    I've set up a simple Eclipse 3.5/Jetty 6.1 web app which returns hello world. It works. This is on Windows and uses the "Jetty Generic Server Adapter". I have auto deployment working so that it deploys after changes periodically. How do I go about setting it up so that if I change any static content it doesn't have to redeploy i.e I can just hit F5 to see the changes straight away. For minor HTML changes it's quite unusable waiting 20-30 seconds for a deployment.

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  • java.lang.classcastExcption

    - by Tara Singh
    Hi, I have an array list of objects in my application. private static ArrayList<Player> userList=new ArrayList<Player>(); In my application, I am converting this list to byte array and then sending it to other clients. At client When I am trying to cast it back to the ArrayList, its giving me casting error. I am doing this in client side after receiving this list as byte array: ArrayList<Player> pl = (ArrayList<Player>) toObject(receivedByteArray); where toObject is my function to convert the byte array to object; Any Suggestions please !!! Thanks.

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  • Relational Database arrays (H2, Java)

    - by Daddy Warbox
    I seem to have two options on how to implement arrays, and I want to know which I should go with: Use the ARRAY data type and (from what I understand) effectively serialize data objects into the database (which in my case are just wrapped primitive types; don't know of another way to make this work). Use a separate table and map with foreign keys for each array item. If you have experience with this (especially with H2), which would you recommend?

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