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• Regular expression: who's greedier?

  - by polygenelubricants

  My primary concern is with the Java flavor, but I'd also appreciate information regarding others. Let's say you have a subpattern like this: (.*)(.*) Not very useful as is, but let's say these two capture groups (say, \1 and \2) are part of a bigger pattern that matches with backreferences to these groups, etc. So both are greedy, in that they try to capture as much as possible, only taking less when they have to. My question is: who's greedier? Does \1 get first priority, giving \2 its share only if it has to? What about: (.*)(.*)(.*) Let's assume that \1 does get first priority. Let's say it got too greedy, and then spit out a character. Who gets it first? Is it always \2 or can it be \3? Let's assume it's \2 that gets \1's rejection. If this still doesn't work, who spits out now? Does \2 spit to \3, or does \1 spit out another to \2 first?

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• To "null" or not to "null" my class's attributes

  - by Helper Method

  When I write a class in Java, I like to initialize the attributes which are set to a default value directly and attributes which are set by the caller in the constructor, something like this: public class Stack<E> { private List<E> list; private size = 0; public Stack(int initialCapacity) { list = new ArrayList<E>(initialCapacity); } // remainder omitted } Now suppose I have a Tree class: public class Tree<E> { private Node<E> root = null; // no constructor needed, remainder omitted } Shall I set the root attribute to null, to mark that it is set to null by default, or omit the null value?

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• Loss of precision - int -> float or double

  - by stan

  I have an exam question i am revising for and the question is for 4 marks "In java we can assign a int to a double or a float". Will this ever loose infromation and why? I have put that because ints are normally of fixed length or size - the precision for sotring data is finite, where storing information in floating point can be infinite, essentially we loose infromation because of this Now i am a little sketchy as to whetehr or not i am hitting the right areas here. I very sure it will loose precision but i cant exactly put my finger on why. Can i getsome help please Thanks

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• Special characters incongruence

  - by Enrique

  Hello I'm building a Spring MVC web application that runs on Tomcat 6.0.20 and JDK 1.6.0_19. When I send some special characters through an HTML form some of them are stored as question marks ? For example these symbols are stored correctly: €, á, é, í, ‰, etc But some symbols are replaced with ? like: £, ?, ? MySQL tables charset is utf-8. My jsp also use utf-8 <%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %> I have included org.springframework.web.filter.CharacterEncodingFilter in web.xml as suggested here When I debug the POST request when sending 3 characters €a£ with firebug I get: %E2%82%ACa%E2%82%A4 which is correct since E2 82 AC is the code for € and E2 82 A4 is the code for £ but £ is stored as ? in the database. When I save £ directly into the database it is displayed correctly in the webpage. How can I fix this?

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• Should I start to use CSS 3 & HTML 5?

  - by LeonixSolutions

  I fear this may sound subjective, sorry. I am wondering how "safe" it is to use CSS3 & HTML5 in a commercial app. I really want the power that they give, but am obviously wary that they are not completely standardized. If it helps any I can probably enforce the use of Chrome as the browser; I can likely offer FireFox as an alternative. I personally do not want to let the user choose their own browser and can probably enforce my choice in a corporate environment which is already heavily biased towards Google. I suppose that if I can enforce a Chrome only policy & carefully test before release then my only worry is that some "behaviour" may change in future. Would you risk it, or would play safe (or go with an alternative, such as a Java app, forgetting the browser)?

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• How should I be storing objects that I wish to access in reverse order of the way I placed them in

  - by andrew hicks

  I'm following this guide here: http://www.mazeworks.com/mazegen/mazetut/index.htm Or more specficially create a CellStack (LIFO) to hold a list of cell locations set TotalCells = number of cells in grid choose a cell at random and call it CurrentCell set VisitedCells = 1 while VisitedCells < TotalCells find all neighbors of CurrentCell with all walls intact if one or more found choose one at random knock down the wall between it and CurrentCell push CurrentCell location on the CellStack make the new cell CurrentCell add 1 to VisitedCells else pop the most recent cell entry off the CellStack make it CurrentCell endIf endWhile Im writing this in java, My problem is. How should I be storing my visited cells, So that I can access them from reverse order of when I placed them in. Like this? List<Location> visitedCells = new ArrayList<Location>(); Then do I grab with visitedCells.get(visitedCells.size()-1)? Location stores the x, y and z. Not something Im trying to ask you.

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• Forward web request for directory index ('/') to an index.htm page in JBoss 4.0.5

  - by The Pretender

  I am using JBoss 4.0.5.GA to run a set of java applications. One of them is a web frontend, using Spring 1.4. URL mappings are configured in a way that 'fake' pages from request URLs are mapped to controllers. That means that when someone requests /index.htm, there's no actual 'index.htm' on disk, and that request maps to a specific conroller which then renders a jsp view. So the problem is as follows: I need to tell JBoss to somehow forward all requests for directory indices to corresponding 'index.htm' URLs like so: / ? /index.htm; /news/ ? /news/index.htm; /foo/bar/baz/ ? /foo/bar/baz/index.htm and so on. I can't use Tomcat's welcome-file-list feature because it looks for those files on disk, while all 'index.htm's are fake and don't actually exist on disk.

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• When is it useful to define your own delegates instead of using the generics?

  - by Carlos

  I've been going through some old code, where I came across some custom defined delegates, which are used thus: private delegate void ListenDelegate(UdpClient listener, bool multicast); private void ListenOn(UdpClient listener, bool multicast) { new ListenDelegate(_ListenLoop).BeginInvoke(listener, multicast, null, null); } With some of the new .NET framework versions, you can do the following: private void ListenOn(UdpClient listener, bool multicast) { new Action<UdpClient, bool>(_ListenLoop).BeginInvoke(listener, multicast, null, null); } This ought to be exactly the same. Is there any point in defining your own delegates, when the generic delegates seem to do the same job with less space? Or have I missed something about the generics that makes them not equivalent?

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• Is concatenating with an empty string to do a string conversion really that bad?

  - by polygenelubricants

  Let's say I have two char variables, and later on I want to concatenate them into a string. This is how I would do it: char c1, c2; // ... String s = "" + c1 + c2; I've seen people who say that the "" + "trick" is "ugly", etc, and that you should use String.valueOf or Character.toString instead. I prefer this construct because: I prefer using language feature instead of API call if possible In general, isn't the language usually more stable than the API? If language feature only hides API call, then even stronger reason to prefer it! More abstract! Hiding is good! I like that the c1 and c2 are visually on the same level String.valueOf(c1) + c2 suggests something is special about c1 It's shorter. Is there really a good argument why String.valueOf or Character.toString is preferrable to "" +? Trivia: in java.lang.AssertionError, the following line appears 7 times, each with a different type: this("" + detailMessage);

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• PgJDBC: "no suitable driver found" when following tutorial, why?

  - by Celeritas

  I'm writing a Java program that queries a PostgreSQL database. I'm following this example and have trouble here: connection = DriverManager.getConnection( "jdbc:postgresql://127.0.0.1:5432/testdb", "mkyong", "123456"); According to the JavaDoc for DriverManager the first string is "a database url of the form jdbc:subprotocol:subname. When I connect to the server I type in psql -h dataserv.abc.company.com -d app -U emp24 and give the password qwe123 (for example sake). What should the first argument of getConnection be? I've tried connection = DriverManager.getConnection( "jdbc:postgresql://dataserv.abc.company.com", "emp24", "qwe123"); and get the run time error: no suitable driver found. I've download JDBC4 Postgresql Driver, Version 9.2-1000.

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• filter log file by defining regexes

  - by fmpdmb

  I have some HUGE log files (50Mb; ~500K lines) I need to start filtering some of the crap out of. The log files are being produced using log4j and have the basic pattern of: [log-level] date-time class etc, etc log-message I'm looking for a way that I can identify a regex start and regex end (or something similar) that will filter out the matching entries from the file so I can more easily wade through these massive files. I'm sure I could write a java program to accomplish this task, but I thought I'd ask the community before going down that path. Thanks in advance.

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• Is it a good idea to use only a key to encrypt an entire (small) filesystem?

  - by Fernando Miguélez

  This question comes as part of my doubts presented on a broader question about ideas implementing a small encrypted filesystem on Java Mobile phones (J2ME, BlackBerry, Android). Provided the litte feedback received, considering the density of the question, I decided to divide those doubts into small questions. So to sum up I plan to "create" an encrypted filesystem for for mobile phones (with the help of BoucyCastle or a subset of JCE), providing an API that let access to them in a transparent way. Encryption would be carried out on a file basis (not blocks). My question is this: Is it a good idea to use only a simmetric key (maybe AES-256) to encrypt all the files (they wouldn't be that many, maybe tens of them) and store this key in a keystore (protected by a pin) or would you rather encrypt each file with an on-the-fly generated key stored alongside each file, encrypting that key with the "master" key stored on the keystore? What are the benefits/drawbacks of each approach?

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• How to start an Open Source Software development

  - by harigm

  I have an idea to start a Open Source Software using Java and PHP, I am so sure that software will help many individuals in their daily routine. Can Any one suggest me how to start the process ? Do we need to register some where and submit our idea for an approval before we start development? Any license that we need to get? How to invite the people for the open source development community, if they are interested? If any people who contributes Do we need to get any agreement signed off? once the Open Source product is stabilized who will have the ownership?

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• Stealing the contents of another application's tree view

  - by User1

  I have an application with a very large TreeView control in Java. I want to get the contents of the tree control in a list (just strings not a JList) of XPath-like elements of leaves only. Here's an example root |-Item1 |-Item1.1 |-Item1.1.1 (leaf) |-Item1.2 (leaf) |-Item2 |-Item2.1 (leaf) Would output: /Item1/Item1.1/Item1.1.1 /Item1/Item1.2 /Item2/Item2.1 I don't have any source code or anything handy like that. Is there I tool I can use to dig into the Window item itself and pull out this data? I don't mind if there are a few post-processing steps because typing it in by hand is my only other option.

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• Ideally How many connections I have to open ?

  - by ranjith-kumar-u

  Hi All, Recently I attended interview in java, the interviewer asked a question like below: I have a request which go throgh A,B,C modules and response go back throgh A , in module A I need to talk to database and again in module C I need to talk to database, so in this situation how many connections you will open and where do you close those connections? My Answer: I said that in module A I will open a connection and I will close it then and there, then control go to module B then module C, in module C again I will open one more connection and i will close it again. then he asked me again another question I want to open one connection per one request processing, how can i do this?

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• How to get the action from the HttpServlet request to dispatch to multiple pages

  - by JFB

  I am using the Page Controller pattern. How could I use the same controller for two different pages by detecting the request action and then dispatching according to the result? Here is my code: account.jsp <form name="input" action="<%=request.getContextPath() %>/edit" method="get"> <input type="submit" value="Modifier" /> </form> Account Servlet public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { System.out.println("received HTTP GET"); String action = request.getParameter("action"); if (action == null) { // the account page dispatch(request, response, "/account"); } else if (action == "/edit") { // the popup edit page dispatch(request, response, "/edit"); } protected void dispatch(HttpServletRequest request, HttpServletResponse response, String page) throws javax.servlet.ServletException, java.io.IOException { RequestDispatcher dispatcher = getServletContext() .getRequestDispatcher(page); dispatcher.forward(request, response); } }

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• Teacher confused about MVC?

  - by Gideon

  I have an assignment to create a game in java using MVC as a pattern. The thing is that stuff I read about MVC aren't really what the teacher is telling me. What I read is that the Model are the information objects, they are manipulated by the controllers. So in a game the controller mutates the placement of the objects and check if there is any collision etc. What my teacher told me is that I should put everything that is universal to the platform in the models and the controllers should only tell the model which input is given. That means the game loop will be in a model class, but also collision checks etc. So what I get from his story is that the View is the screen, the Controller is the unput handeler, and the Model is the rest. Can someone point me in the right direction?

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• Recommendation for tool/framework that follows the naked objects pattern?

  - by Marcus Munzert

  I am searching for a tool/framework that follows the naked objects pattern and is written in Java. I know about tools like for instance JMatter, Naked Objects and Domain Object Explorer. That's not exactly what I am searching for, though. Open source would be great, but doesn't need to be. My intention is to use that tool/framework for the purpose of model-driven software development to do the modeling part. Ideally, such a tool/framework would provide the option to use JPA to store/load objects.

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• Checking for 2 exepcted values in Junit

  - by Swift-Tuttle

  Hi, I have a java program which throws an exception with 2 different messages for 2 different scenarios and I want the Junit test case to check for equality for both of these messages. As an example - public void amethod() { // do some processing if(scenario1 == true) { throw new MySystemException("An error occured due to case 1 being incorrect."); } else if(scenario2 == true) { throw new MySystemException("An error occured as case 2 could not be found"); } } Now the JUnit for this would be something like- public void testAMethod() { // do something assertEquals("Expected", "Actual"); } As I understand, in this above example, if I use the Scenario1 exception message the junit will fail when an exception is thrown for Scenario2 and vice versa. I would like to know if there is any other way provided in Junit by which I can use this one test method and check for both the messages for the test to pass? Something like an OR, if possible to provide the "Expected" value with both these expected message. I hope my query is clear enough. Thanks

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• help implementing All Nearest Smaller Values algorithm

  - by davit-datuashvili

  http://en.wikipedia.org/wiki/All_nearest_smaller_values this is site of the problem and here is my code but i have some trouble to implement it import java.util.*; public class stack{ public static void main(String[]args){ int x[]=new int[]{ 0, 8, 4, 12, 2, 10, 6, 14, 1, 9, 5, 13, 3, 11, 7, 15 }; Stack<Integer> st=new Stack<Integer>(); for (int a:x){ while (!st.empty() && st.pop()>=a){ System.out.println( st.pop()); if (st.empty()){ break; } else{ st.push(a); } } } } } and here is pseudo code from site S = new empty stack data structure for x in the input sequence: while S is nonempty and the top element of S is greater than or equal to x: pop S if S is empty: x has no preceding smaller value else: the nearest smaller value to x is the top element of S push x onto S

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• Behavior of nested finally in Exceptions

  - by kuriouscoder

  Hello: Today at work, I had to review a code snippet that looks similar to this mock example. package test; import java.io.IOException; import org.apache.log4j.Logger; public class ExceptionTester { public static Logger logger = Logger.getLogger(ExceptionTester.class); public void test() throws IOException { new IOException(); } public static void main(String[] args) { ExceptionTester comparator = new ExceptionTester(); try { try { comparator.test(); } finally { System.out.println("Finally 1"); } } catch(IOException ex) { logger.error("Exception happened" ex); // also close opened resources } System.out.println("Exiting out of the program"); } } It's printing the following output.I expected an compile error since the inner try did not have a catch block. Finally 1 Exiting out of the program I do not understand why IOException is caught by the outer catch block. I would appreciate if anyone can explain this, especially by citing stack unwinding process

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• Google app engine sessions now supported???

  - by user246114

  Hi, I thought google app engine did not support sessions (last time I checked was a few months ago). Now I was searching again for it and saw this: http://code.google.com/appengine/docs/java/config/appconfig.html#Enabling_Sessions says it supports: javax.servlet.http.HttpSession does this mean we have servlet session support now? If so, does anyone have an example of using this? I wanted to create my own User class and support user login and session management (I know app engine already supports this for google users, but wanted my own users for various requirements) Thanks!

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• Hibernate and Child Objects (add versus clear)

  - by tyndall

  Lets say I have domain model with Automobile and Wheels. Wheels is a List of Wheels Wheels has a many-to-one relationship to Automobile. If I get an object back from Hibernate and it has 4 wheels. I take that object remove the 4 wheels and add 4. And then Save. If I ask Hibernate for the object again and it returns an auto with 8 wheels... what are we doing wrong? I don't have access to the source for a few days but want to give our Java devs a push in the right direction. Thanks.

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• How to set the component size with GridLayout? Is there a better way?

  - by Blackbam

  Hello guys, I am working on a larger GUI with Java and I am becoming angry on Layout Managers. I have a "Settings-Panel" with a variable number of JComponents in it (Labels, Buttons, JSpinners, JSliders,...). I just want the following: JLabel <- JComponent JLabel <- JComponent JLabel <- JComponent ... My Panel has a size of 500px, so that there is enough space for a lot of components. Unfortunatly the GridLayout always stretches the size of the Components to the whole Panel, even if I set a MaximumSize for every component. It looks stupid if there are only two buttons each with a height of 250px. I tried flow Layout, but i cannot figure out a way to make new lines properly. I tried BoxLayout.Y_AXIS, but the Components are always centered, and Label and Component are not in the same line. Does anybody know a good and short way with LayoutManagers to handle this properly?

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• Full GC real time is much more that user+sys times

  - by Stas

  Hi. We have a Web Java based application running on JBoss with allowed maximum heap size of about 1.2 GB (total machine physical memory is 2 GB). At some point the application stops responding (to clients) for several minutes. After some analysis we found out that the culprit is the Full GC. Here's an excerpt from the verbose GC log: 74477.402: [Full GC [PSYoungGen: 3648K-0K(332160K)] [PSOldGen: 778476K-589497K(819200K)] 782124K-589497K(1151360K) [PSPermGen: 102671K-102671K(171328K)], 646.1546860 secs] [Times: user=3.84 sys=3.72, real=646.17 secs] What I don't understand is how is it possible that the real time spent on Full GC is about 11 minutes (646 seconds), while user+sys times are just 7.5 seconds. 7.5 seconds sound to me much more logical time to spend for cleaning <200 MB from the old generation. Where does all the other time go? Thanks a lot.

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