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  • Java: Interface vs Abstract Class (regarding fields)

    - by lifeR00t
    From what I have gathered, I want to force a class to use particular private fields (and methods) I need an abstract class because an interface only declares public/static/final fields and methods. Correct?? I just started my first big java project and want to make sure I'm not going to hurt myself later :)

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  • Naming conventions for complex getters in Java

    - by Simon
    Hi there! I was reading this C# article about the usage of properties and methods. It points out why and when to use properties or methods. Properties are meant to be used like fields, meaning that properties should not be computationally complex or produce side effects I was asking myself how you could express this difference in Java, where you only use getters for the retrieval of data. What is your opinion?

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  • Java: Making concurrent MySQL queries from multiple clients synchronised

    - by Misha Gale
    I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC. The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license. I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question: public LicenseKey Acquire() throws SmartLaunchException, SQLException { Connection conn = SmartLaunchDB.getConnection(); int PCID = SmartLaunchDB.getCurrentPCID(); conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE"); String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1"; PreparedStatement statement = conn.prepareStatement(sql); statement.setInt(1, this.id); ResultSet results = statement.executeQuery(); if (results.next()) { int licenseID = results.getInt("ID"); sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?"; statement = conn.prepareStatement(sql); statement.setInt(1, PCID); statement.setInt(2, licenseID); statement.executeUpdate(); statement.close(); conn.commit(); conn.createStatement().execute("UNLOCK TABLES"); return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType")); } else { throw new SmartLaunchException("All licenses of type " + this.name + "are in use"); } }

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  • what are the Ways to create a gui to install a java app

    - by scot
    Hi, I have a java web application . Currently the application is installed in a cli mode [on windows/linux /solaris] by running sh or cmd scripts. I would like to create a GUI for the App installation so that user can edit info in gui for the installation. Any pointers/Best practices for developing gui would be helpful. thank you

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  • Dynamically find other hosts in a LAN in Java

    - by Federico Cristina
    A while ago I developed a little LAN chat app. in Java which allows chatting with other hosts, send images, etc. Although it was created just for fun, now it's being used where I work. Currently, there is no "chat server" on the app. where each client registers, updates it's status, etc. (I liked the idea of symmetric design and not depending on a server running on some other machine). Instead, each host is a client/server which has a hosts.properties file with the hostname of the other hosts, and - for instance - broadcasts to each one of them when sending a massive message/image/whatever. In the beginning there were just a couple of hosts, so this hosts.properties file wasn't an issue. But as the amount of users increased, the need of updating that file was a bit daunting. So now I've decided to get rid of it, and each time the app. starts, dynammically find the other active hosts. However, I cannot find the correct way of implement this. I've tried starting different threads, each one of them searching for other hosts in a known range of IP addresses. Something like this (simplified for the sake of readability): /** HostsLocator */ public static void searchForHosts(boolean waitToEnd) { for (int i=0; i < MAX_IP; i+= MAX_IP / threads) { HostsLocator detector = new HostsLocator(i, i+(MAX_IP / threads - 1)); // range: from - to new Thread(detector).start(); } } public void run() { for (int i=from; i<=to; i++) findHosts( maskAddress + Integer.toString(i) ); } public static boolean findHosts(String IP) { InetAddress address = InetAddress.getByName(IP); if ( address.isReachable(CONNECTION_TIME_OUT) ) // host found! } However: With a single thread and a low value in CONNECTION_TIME_OUT (500ms) I get wrong Host Not Found status for for hosts actually active. With a high value in CONNECTION_TIME_OUT (5000ms) and only one single thread takes forever to end With several threads I've also found problems similar like the first one, due to collisions. So... I guess there's a better way of solving this problem but I couldn't find it. Any advice? Thanks!

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  • Building vs. Compiling (Java)

    - by sixtyfootersdude
    Thinking that the answer to this is pretty obvious but here it goes: When I am working on a small project for school (in java) I "compile" it. On my coop we are using ant to "build" our project. I think that compiling is a subset of building. Is this correct? What is the difference between building and compiling?

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  • Excel In Java Web Application

    - by Anand
    Hi I have a java web application running on windows currently. I may host it in future in a Linux Server. My application allows people to upload data. I want to display the data they have uploaded in an excel file and render it in a portion of my webpage. How do I go about this ?

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  • Java ME right to left TextField label

    - by Nimrod Shory
    Hello, I'm writing a Java ME app and i need the label for the text field to be right aligned.. I've tried setting the layout of the TextField to Item.RIGHT but it doesn't seem to affect anything when running on my test phone (Nokia E71). The text inside the TextField is right aligned and Lists and such items are desplayed correctly (RTL aligned) but the TextField label isn't. Any ideas? Thanks.

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  • java on all platforms

    - by noname
    if you wanna code a desktop application in java for windows, mac and linux, will the code be the same for all of them? and you just change the GUI so that the Windows application will be more Windows-like and so on? how does it work without digging into details?

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  • Java loading user-specified classes at runtime

    - by user349043
    I'm working on robot simulation in Java (a Swing application). I have an abstract class "Robot" from which different types of Robots are derived, e.g. public class StupidRobot extends Robot { int m_stupidness; int m_insanityLevel; ... } public class AngryRobot extends Robot { float m_aggression; ... } As you can see, each Robot subclass has a different set of parameters. What I would like to do is control the simulation setup in the initial UI. Choose the number and type of Robots, give it a name, fill in the parameters etc. This is one of those times where being such a dinosaur programmer, and new to Java, I wonder if there is some higher level stuff/thinking that could help me here. So here is what I've got: (1) User Interface Scrolling list of Robot types on the left. "Add " and "<< Remove" buttons in the middle. Default-named scrolling list of Robots on the right. "Set Parameters" button underneath. (So if you wanted an AngryRobot, you'd select AngryRobot on the left list, click "Add" and "AngryRobot1" would show up on the right.) When selecting a Robot on the right, click "Set Parameters..." button which would call yet another model dialog where you'd fill in the parameters. Different dialog called for each Robot type. (2) Data structures an implementation As an end-product I think a HashMap would be most convenient. The keys would be Robot types and the accompanying object would be all of the parameters. The initializer could just retrieve each item one and a time and instantiate. Here's what the data structures would look like: enum ROBOT_TYPE {STUPID, ANGRY, etc} public class RobotInitializer { public ROBOT_TYPE m_type; public string m_name; public int[] m_int_params; public float[] m_float_params; etc. The initializer's constructor would create the appropriate length parameter arrays based on the type: public RobotInitializer(ROBOT_TYPE type, int[] int_array, float[] float_array, etc){ switch (type){ case STUPID: m_int_params = new int[STUPID_INT_PARAM_LENGTH]; System.arraycopy(int_array,0,m_int_params,0,STUPID_INT_PARAM_LENGTH); etc. Once all the RobotInitializers are instantiated, they are added to the HashMap. Iterating through the HashMap, the simulation initializer takes items from the Hashmap and instantiates the appropriate Robots. Is this reasonable? If not, how can it be improved? Thanks

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  • Put Java Threading Class into a separate class

    - by erlord
    Consider following SWT code example: http://dev.eclipse.org/viewcvs/index.cgi/org.eclipse.swt.snippets/src/org/eclipse/swt/snippets/Snippet151.java?view=co How can I separate the inline defined class? Thread thread = new Thread() { public void run() { ... } }; I want to define a separate class which updates the table just like it does here. How do I pass the list back to the table? Example code?

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  • Java portal architectural considerations

    - by Woot4Moo
    Currently there exists a need to create an application that will serve 5 different customers each requiring their own specific URL and content repository. My question is when designing this application what should my considerations be for protecting the content of the individual customers while meeting the requirements of the unique URL. The system will be sitting on Windows with a postgres database and java as the implementation language.

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  • Easy XML Serializer for Java

    - by sdoca
    I'm looking for a way to serialize Java objects into XML for use by a RESTful web service. I don't have an XSD. I have looked at the following: JAXB - fairly heavy weight with annotations required on classes and also an ObjectFactory class and/or a jaxb.index file Simple - requires annotations but no other config classes/files. Unfortunately it can't serialize Sets. XStream - no annotations etc. required, but doesn't support generics Does anyone else have any suggestions?

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  • K-Means Algorithm and java code

    - by Thandar
    Hi all, I need to calculate for grouping objects according to their size. I got k-means algorithms in java which calculate mostly for classifying according to their two or more features and the results are not satisfy for me.I only want to calculate for grouping objects based on one feature.Pseudocode or code would be helpful, too. Thanks u all for helping.

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  • Java - Incrementing through IP addresses in String format

    - by Matt
    I'm new to java and i'm trying to find a way of incrementing through an user input IP address range. For example from 192.168.0.1 to 192.168.0.255. However the way my application works at the moment is the take the from and to ip addresses as a String. Is there a way I can increment through all the ip addresses the user input from and to? Hope this makes sense and please dont flame me, I have looked for an answer!

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  • wany assignments for java

    - by HW
    Hello, I am a computer science Student Second year ,and i know good deal about c++,Data Structure, File Structure,OOP etc. I decided to learn java i have read couple of books but i know u need practice to master any Programming language so i wonder if anyone could give me the assignments"only the questions not the solution" so that i could solve them as i am getting bored of "hello world"s and "3+2=5"s kinda stuff thanks, ~HW

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  • java mapping for oracle number type

    - by aauser
    I have database with oracle number type. Could there any possible issues with using java Double when updating table with numeric column. Should i switch BigDecimal or Double is approriate type. As I fas as I know, BigDecimal is used when arbitrary precision required, when deal with prices for example. Consider simple API updatePrice(Double d) Should i persist double as is or BigDecimal.valueOf(d) is preferable ?

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  • Java development in Ubuntu

    - by Veera
    I'm a newbie to Linux systems and recently I started using Ubuntu 10.04. When I do java development in Windows, I usually keep my project files under some drive (D: for example) and under my development folder, such as D:\projects\myproj. But I'm bit confused with Ubuntu's folder structure. So, I just want to know how do you organize your projects in Ubuntu? Under which folder do we keep our projects file?

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