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• How to get back to auto-completion after misspelling of a methodname in Eclipse?

  - by Jonas

  When I am coding Java in Eclipse I like the auto-completion feature. With that I mean the popup with method-names that comes when you start typing in a method name for an object. Or maybe it's called something different, i.e. method-suggestions? But the popup is hidden if I misspells a method name, and it doesn't come back if I delete the misspelled part of the method name. Is there any way to get back the popup after a misspelling without starting to type in the hole methodname again?

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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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• When to trash hashmap contents to avoid performance degradation?

  - by Jack

  Hello, I'm woking on Java with a large (millions) hashmap that is actually built with a capacity of 10.000.000 and a load factor of .75 and it's used to cache some values since cached values become useless with time (not accessed anymore) but I can't remove useless ones while on the way I would like to entirely empty the cache when its performance starts to degrade. How can I decide when it's good to do it? For example, with 10 millions capacity and .75 should I empty it when it reaches 7.5 millions of elements? Because I tried various threshold values but I would like to have an analytic one. I've already tested the fact that emping it when it's quite full is a boost for perfomance (first 2-3 algorithm iterations after the wipe just fill it back, then it starts running faster than before the wipe) Thanks

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• ArrayList without repetition

  - by tuxou

  Hi i'm using arraylist in java and i need to add integers during 10 iteration (integer is got randomly from an array of integers named arrint) without any repetion: for (int i =0; i<10; ++i) array.add(integer); and then add in the same array 20 other integers for the same array of integer(arrint) during 20 iteration without repetion for (int i =0; i<10; ++i) array.add(integer); but repetition is permited between the 10 first integers and the 20 integers thank you

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• How easy would it be to refactor a small JSP/Servlet/JDBC project to SpringMVC/Hibernate

  - by John

  With reference to this post, I am considering starting a new web-based Java project. Since I don't know Spring/Hibernate I was concerned if it's a bad plan to start learning them while creating a new project, especially since it will slow down the early development. One idea I had was to write a prototype using tech I do know, namely JSP/Servlets/JDBC, since I can get this running much quicker with my current knowledge. I could then throw the whole thing away and start over with Spring, etc, but I'd like to consider how easy it would be to refactor a smallish project from JSP/Servlets/JDB to SpringMVC/Hibernate? My DB could of course be re-used but what about other code... would I expect to save most of it plugged into an MVC framework, or is the paradigm shift big enough this would cause more trouble than it avoids? Please use the other question for more general advice on choosing technologies

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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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• Alternatives libraries for loading PNG images

  - by Robert

  My java J2SE application is reading a lot of (png) images from the web and some of them use features such as a transparency color for true-color images (tRNS section) that Sun's/Oracle's PNGImageReader implementation simply ignores. Therefore the common solution for loading via ImageIO.read(...); does not work for me as it relies on this incomplete PNGImageReader implementation. Does anybody know a png reader implementation that can read all forms of PNG images correctly - those with color table or true-color and alpha transparency or transparent color? As it is for a GPL project it should be a non-commercial one that can be included without licensing problems into the app. Edit: My be this question was too specific. Therefore let be redesign my question: Who knows alternative implementations and libraries that are able to load PNG files? I will then test the implementations for their capabilities to load some test png images. Edit2: The end result have to be a BufferedImage

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• Template engine recommendations

  - by alex

  I'm looking for a template engine. Requirements: Runs on a JVM. Java is good; Jython, JRuby and the like, too... Can be used outside of servlets (unlike JSP) Is flexible wrt. to where the templates are stored (JSP and a lot of people require the templates to be stored in the FS). It should provide a template loading interface which one can implement or something like that Easy inclusion of parameterized templates- I really like JSP's tag fragments Good docs, nice code, etc., the usual suspects I've looked at JSP- it's nearly perfect except for the servlet and filesystem coupling, Stringtemplate- I love the template syntax, but it fails on the filesystem coupling, the documentation is lacking and template groups and stuff are confusing, GXP, TAL, etc. Ideas, thoughts? Alex

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• sql jdbc getgeneratedkeys with mysql returns column "id" not found

  - by iamrohitbanga

  I want to retrieve the most recently updated value in the table using an insert query. these are the datatypes in my sql table. int(11) // primary key auto increment, not being assigned by sqlQuery varchar(30) timestamp // has a default value. but i am explicit assigning it using CURRENT_TIMESTAMP varchar(300) varchar(300) varchar(300) int(11) varchar(300) // java code statement.executeUpdate(sqlQuery, Statement.RETURN_GENERATED_KEYS); ResultSet rs = statement.getGeneratedKeys(); System.out.println("here: " + rs.getMetaData().getColumnCount()); System.out.println("here1: " + rs.getMetaData().getColumnName(1)); // none of the following 3 works System.out.println("id: " + rs.getInt(1)); System.out.println("id: " + rs.getInt("GENERATED_KEY")); System.out.println("id: " + rs.getInt("id")); for a bit of background see this

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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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• 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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• Output of System.out.println(object)

  - by Shaarad Dalvi

  I want to know what exactly the output tells when I do the following : class data { int a=5; } class main { public static void main(String[] args) { data dObj=new data(); System.out.println(dObj); } } I know it gives something related to object as the output in my case is data@1ae73783. I guess the '1ae73783' is a hex number. I also did some work around and printed System.out.println(dObj.hashCode()); I got number 415360643. I got an integer value. I don't know what hashCode() returns, still out of curiosity, when I converted 1ae73783 to decimal, I got 415360643! That's why I am curious that what exactly is this number?? Is this some memory location of Java's sandbox or some other thing? Any light on this matter will be helpful..thanks! :)

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• How can I leverage String constants in an XML file?

  - by jayshao

  I'd like to enforce standardized keys by storing them as static final String variables on a Java class, and either referencing or statically importing them, to use them as values in either XML, Strings, Methods, Annotations, etc. Does anyone know a good way to have Maven insert (like filtering) values like StringKeys.SOME_KEY into an XML file? e.g. something like <element value="${StringKeys.SOME_KEY}"/> or similar - the main idea is to enforce commonality and prevent key mis-alignment. Or an alternative solution to accomplish the same - with some semantic that if a non-existant String is referenced, that it fails during build? Bonus points if it works in C# as well.

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• Generating Ant build file for a project in eclipse

  - by Negai

  Hi everyone. I have a project written in Java using eclipse and I would like to generate an Ant build file for it. How can it be done? I constantly need to keep it synchronized with the source code, so that I could promote it along with the code to the common repository when with the project structure is modified. I'm sorry for a stupid question like this. I've browsed across the forums, but still no luck. Thanks in advance.

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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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• 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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• Circular Tally Counter Not Rolling Over

  - by chris ward

  I was practicing my java and was trying to make a simple counter with rollover at max, but for some reason it isn't rolling over. Any advice? public class HandTallyCounter { private int max; private int count; public HandTallyCounter(int max) { this.max = max; count = 0; } public void click() { if (count++ > max) { count = 0; } } public int getCount() { return count; } public void reset() { count = 0; } }

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• Adapting methods which return true/false

  - by James P.

  What's the best practise when adapting C-style functions which return a true/false to Java? Here's a simple method to illustrate where the problem lies. public static boolean fileNameEndsWithExtension( String filename, String fileExtension) { return filename.endsWith( fileExtension ); } Note that there's probably a more elegant way of filtering files (feel free to comment on this). Anyway, if filename is a null value, does one: Return a false if filename is null? If so, how does one go about distinguishing between the case where filename is null and the case where the String or file name doesn't end with a given file extension? Change the return type to the wrapper class Boolean which allows a null value. Throw an Exception and force the programmer to make sure that a null value is never passed to the method? Use another solution?

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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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• Android XML file doesn't save

  - by Shane

  I'm writing a game with LibGDX, and I'm trying to save an XML file, but there's always an exception (java.io.FileNotFoundException: /data/Slugfest/teams/Team1.xml: open failed: ENOENT (No such file or directory)) when saving the file. This code saves the file. public void save() { try { TransformerFactory transformerFactory = TransformerFactory.newInstance(); Transformer transformer = transformerFactory.newTransformer(); DOMSource source = new DOMSource(doc); StreamResult result; if (Gdx.app.getType() == ApplicationType.Android) { result = new StreamResult(new File("/data/Slugfest/teams/" + name + ".xml")); } else { result = new StreamResult(new File(name + ".xml")); } transformer.transform(source, result); Gdx.app.log("Slugfest", "File saved."); } catch (TransformerException tfe) { Gdx.app.log("Slugfest", tfe.getLocalizedMessage()); } } My manifest file includes the WRITE/READ_EXTERNAL_STORAGE permissions, by the way.

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• how to get a specific class object

  - by user1275129

  I am still quite new to class concept. I have a problem here. for example, i have a class called class p{ int id; p(id){ //constructor } } now, i want to make instances of class p. After i have all the instances. i want to build a function get_p(pid) to return the specific class p which id=pid, how can i write this part? I have read c++ and java class tutorials, but i cant sort this out. please help me.

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• When to stop following the advice of static code analysis?

  - by bananeweizen

  I do use static code analysis on a project with more than 100.000 lines of Java code for quite a while now. I started with Findbugs, which gave me around 1500 issues at the beginning. I fixed the most severe over time and started using additional tools like PMD, Lint4J, JNorm and now Enerjy. With the more severe issues being fixed, there is a huge number of low severity issues. How do you handle these low priority issues? Do you try fixing all of them? Or only in newly written code? Do you regularly disable certain rules? (I found that I do on nearly any of the available tools). And if you ignore or disable rules, do you document those? What do your managers say about "leaving some thousand low priority issues not fixed"? Do you use (multiple) tool specific comments in the code or is there any better way?

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• Running a method after the constructor of any derived class

  - by Alexey Romanov

  Let's say I have a Java class abstract class Base { abstract void init(); ... } and I know every derived class will have to call init() after it's constructed. I could, of course, simply call it in the derived classes' constructors: class Derived1 extends Base { Derived1() { ... init(); } } class Derived2 extends Base { Derived2() { ... init(); } } but this breaks "don't repeat yourself" principle rather badly (and there are going to be many subclasses of Base). Of course, the init() call can't go into the Base() constructor, since it would be executed too early. Any ideas how to bypass this problem? I would be quite happy to see a Scala solution, too.

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• How can I create or assign a method to temp (WindowAdapter)?

  - by Doug Hauf

  I want to create an instance of the WindowAdapter and put my method for windowClosing in it and then sent the temp into the f.addWindowListener(temp) can this be done. Java will not let me create an instance of WindowAdapter like below. WindowAdapter temp = new WindowAdapter(); <-- Does not compile How could this be done? Code: public static void main(String args[]) { setLookFeel(); JFrame f = new JFrame("Hello World Printer..."); WindowAdapter temp; f.addWindowListener(new WindowAdapter() { public void windowClosing(WindowEvent e) { System.exit(0); } }); JButton printButton = new JButton("Print Hello World"); printButton.addActionListener(new HelloWorldPrinter()); f.add("Center", printButton); f.pack(); f.setVisible(true); }

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• Tomcat servlet-api.jar problem

  - by CitadelCSCadet

  I am running a web application using Tomcat and Java Servlets, JSP's, etc. I am aware that in order to use Servlets, it is dependent on the Servlet-api.jar file. Initially I placed this jar file in the WEB-INF/lib/ directory. This has worked fine for me for months during the developmental phase. When we put the application onto the server space we are using, we started seeing wierd problems showing up in the Catalina.out file telling us that there was dependency problems with the servlet-api.jar file. I am aware that tomcat has this jar file in its container, and that I should remove it from the WEB-INF/lib/ directory. I have tried this and it does not work. What do I have to do when I remove this jar file from the local files and allow it to depend on tomcats servlet-api.jar file.

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