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• Use of Java [Interfaces / Abstract classes]

  - by Samuel

  Hello, Lately i decided to take a look at Java so i am still pretty new to it and also to the approach of OO programming, so i wanted to get some things straight before learning more, (i guess it's never to soon to start with good practices). I am programming a little 2D game for now but i think my question applies to any non trivial project. For the simplicity i'll provide examples from my game. I have different kinds of zombies, but they all have the same attributes (x, y, health, attack etc) so i wrote an interface Zombie which i implement by WalkingZombie, RunningZombie TeleportingZombie etc. Is this the best thing to do? Am i better of with an abstract class? Or with a super class? (I am not planning to partially implement functions - therefor my choice for an interface instead of an abstract class) I have one class describing the main character (Survivor) and since it is pretty big i wanted to write an interface with the different functions, so that i can easily see and share the structure of it. Is it good practice? Or is it simply a waste of space and time? I hope this question will not be rated as subjective because i thought that experienced programmers won't disagree about this kind of topic since the use of interfaces / super classes / abstract classes follows logical rules and is thereby not simply a personal choice. Thank you for your time -Samuel

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• post increment operator java

  - by srandpersonia

  I can't make heads or tails of the following code from "java puzzlers" by joshua bloch. public class Test22{ public static void main(String args[]){ int j=0; for(int i=0;i<100;i++){ j=j++; } System.out.println(j); //prints 0 int a=0,b=0; a=b++; System.out.println(a); System.out.println(b); //prints 1 } } I can't get the part where j prints 0. According to the author, j=j++ is similar to temp=j; j=j+1; j=temp; But a=b++ makes b 1. So it should've evaluated like this, a=b b=b+1 By following the same logic, shouldn't j=j++ be evaluated as, j=j j=j+1 Where does the temp come into picture here? Any explanations would be much appreciated. << I'm breaking my head over this. ;) Thanks in advance.

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• Java program will read from database, but not write to it

  - by ck1221

  I have a Java program that successfully connects to a mysql database that is hosted on godaddy's server. I can read from that db with out issue, however, when I try to write to it with INSERT or UPDATE for example, the query does not execute. I am using the 'admin' account that I set up through godaddy, I realize this is not the root account. I have checked and verified that the connection is not read only, and have logged out of phpmyadmin while the query ran. I'm not sure what else I can try or if anyone has experienced this issue. Maybe a setting to the connection I have failed to set? Or maybe its not possible since the db is hosted on godaddy's servers? Any help is great! Thanks. Here is some relevant code: Connection to db: Connection con; public DBconnection(String url, String user, String pass) { try { Class.forName("com.mysql.jdbc.Driver").newInstance(); con = DriverManager.getConnection (url,user,pass); if(!con.isClosed()) System.out.println("connecton open"); } catch (InstantiationException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} catch (ClassNotFoundException e) {e.printStackTrace();} catch (SQLException e) {e.printStackTrace();} } Send Query: public ResultSet executeQuery(String query) { ResultSet rs = null; try { Statement stmt = (Statement) con.createStatement(); rs = stmt.executeQuery(query); //while(rs.next()) //System.out.println(rs.getString("ticket_num")); } catch (SQLException e) {} return rs; } Insert Query (works in phpmyadmin): conn.executeQuery("INSERT INTO tickets VALUES(55555,'12/01/2012','me','reports','test','','','0','Nope')");

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• Using same log4j logger in standalone java application

  - by ktaylorjohn

  I have some code which is a standalone java application comprising of 30+ classes. Most of these inherit from some other base classes. Each and every class has this method to get and use a log4j logger public static Logger getLogger() { if (logger != null) return logger; try { PropertyUtil propUtil = PropertyUtil.getInstance("app-log.properties"); if (propUtil != null && propUtil.getProperties() != null) PropertyConfigurator.configure(propUtil.getProperties ()); logger = Logger.getLogger(ExtractData.class); return logger; } catch (Exception exception) { exception.printStackTrace(); } } A) My question is whether this should be refactored to some common logger which is initialized once and used across by all classes? Is that a better practice? B) If yes, how can this be done ? How can I pass the logger around ? C) This is actually being used in the code not as Logger.debug() but getLogger().debug(). What is the impact of this in terms of performance?

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• Does implementing a Java class under an open-source library's package name constitutes a modificatio

  - by Minyu

  Greetings, I understand that this is not the best forum for getting legal advices. But I am still wondering if anybody has some experience around this topic, or can point me to relevant resources that explain it. I have tried to search it online but could not find clear answers. Assuming I am using a Java open-source library from "somecompany", specifically its entities in the "org.somecompany.somepackage". During development, I found that I need to access some of the internals of that package that are not exposed through its public interface. So I thought maybe creating my own classes under the same "org.somecompany.somepackage" name would make that easier. My question is: does this constitutes a modification of the open-source library? The fully qualified name of my classes look like "org.somecompany.somepackage.myclass". It does seem an "extension" of the original library, even though I am not modifying the original library source or binary. I am also not adding my classes to the open-source jar files. Any advice or pointer to resources will be appreciated!

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• Java jList add item based on combobox selection

  - by Stephan Badert

  I have a csv file that is being loaded in my programme. It contains citys and areas and some other stuff (not important here). Once the csv is selected I load the data into several comboboxes. 1 Thing is not working, I have a combobox containing all the citys and now I need to list all the areas for that country based on selection from the combobox. Here is the event: private void cboProvinciesItemStateChanged(java.awt.event.ItemEvent evt) { //System.out.println(Arrays.asList(gemeentesPerProvincie(gemeentes))); invullenListProvincie(gemeentes); } Here is the method: private void invullenListProvincie(ArrayList<Gemeentes> gemeentes) { Gemeentes gf = (Gemeentes) cboProvincies.getSelectedItem(); DefaultListModel model = new DefaultListModel(); JList list = new JList(model); for (Gemeentes gemeente : gemeentesPerProvincie(gemeentes)) { model.addElement(gemeente); } lstGemeentes.setModel(model); } and this is the method to filter all the areas that equal the selection from the combobox: private ArrayList<Gemeentes> gemeentesPerProvincie(ArrayList<Gemeentes> gemeentes) { String GemPerProv = (String) cboProvincies.getSelectedItem(); ArrayList<Gemeentes> selectie = new ArrayList<Gemeentes>(); for (Gemeentes gemeente : gemeentes) { if (gemeente.getsProvincie().equals(GemPerProv)) { selectie.add(gemeente); } } return selectie; } I am convinced the error is the way I am trying to add items to the jList gemeentesPerProvincie(), I have tried so many things already. I really hope someone can see what i am clearly missing...

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• Transferring HashMap between client and server using Sockets (JAVA)

  - by sar

  I am working on a JAVA project in which there are multiple terminals. These terminals act as client and servers. For example if there are 3 terminals A,B and C.Then at any given point in time one of them say A, will be a client making broadcast request. The other two terminals, B and C, will reply. I am using sockets to make them communicate. Once the reply is received from all the other terminals A will check the pool of channels to see if any one of the channel is free. It takes up the free channel and making it availabilty false. The channelpool is implemented using HashMAp: HashMap channelpool = new HashMap(); channelpool = 1=true, 2=true, 3=false, 4=true, 5=true, 6=true, 7=true, 8=true, 9=true, 10=true So initially all the channels are true, any terminal can take any channel. But once the channel is taken it is set to false for the period of use and then reset to true. Now this Hashmap has to be shared among the distributed terminals. Also it should be kept up to date. I can not used a shared resource among the terminals to store the HashMap.Can someone tell me an easy way to transfer the HashMap between the terminals. I will appreciate if someone could point me to a website which discusses this.

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• UTF-8 support in java application

  - by jacekn

  I'm having trouble with UTF-8. common.jsp <%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%> typical.jsp <%@ include file="common.jsp" %> Page Head <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> Form <form id="screenObject" accept-charset="UTF-8" action="/SiteAdmin/articleHeaderEdit?articleId=15" method="post"> I enter non latin1 characters into a text field and click Save. Validator complains about another field and stops the submission. This never gets to the database, so database ability to handle UTF-8 is not in this picture. The page redisplays with appropriate error but the text that had been entered is all messed up. All non latin1 characters are converted to some gibberish. I'm using Spring 3 MVC, in case that matters... Attempts Adding this to my view resolver didn't help: <property name="contentType" value="text/html;charset=UTF-8" />

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• Sorting a very large text file in Java

  - by Alice

  Hi, I have a large text file I need to sort in Java. The format is: word [tab] frequency [new line] The algorithm for sorting is: Read some of the file, filtering for purlely alphabetic words. Once you have X number of alphabetic words, call Collections.sort and write the result to a file. Repeat until you have finished reading the file. Start reading two sorted files, comparing line by line for the word with higher frequency, and writing at the same time to a new file as to not load much into your memory Repeat until all files are merged into one large file Right now I've divided the large file into smaller ones (sorted by descending frequency) with 10,000 lines each. I know I need to somehow merge these files back together, but I'm not sure how to go about this. I've created a LinkedList to keep track of all the files created. The algorithm says to compare each line in the two files, except I've tried a case where , say file1 = 8,6,5,3,1 and file2 = 9,8,8,8,8. Then if I compare them line by line I would get file3 = 9,8,8,6,8,5,8,3,8,1 which is incorrectly sorted (they should be in decreasing order). I think I'm misunderstanding some part of the algorithm. If someone could point out what I should do instead, I'd greatly appreciate it. Thanks. edit: Yes this is an assignment. We aren't allowed to increase memory unfortunately :(

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• [java] tcp socket communication [send and recieve help]

  - by raven

  hello, I am creating a Chat in java. I have a method (onMouseRelease) inside an object that creates a tcp server and waits for a socket ServerSocket server = new ServerSocket(port); Socket channel = server.accept(); now I want to make a thread that will loop and read data from the socket, so that once the user on the other side sent me a string, I will extract the data from the socket [or is it called packet? sry I am new to this], and update a textbox to add the additional string from the socket [or packet?]. I have no idea how to READ (extract) the information from the scoket [/packet] and then update it into a JTextArea which is called userOutput. And how to Send a string to the other client, so that it will also could read the new data and update its JTextArea. (from what I know, for a 2 sided tcp communication you need one computer to host a server and the other to connect [as a client] and once the connection is set the client can also recieve new information from the socket. Is that true? and please tell me how ) Any help appreciated !! I know this is abit long but I have searched allot and didn't understand [I saw something like printwriter but failed to understand].

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• Java/Python: Integration, problem with looping updating text

  - by Jivings

  Hello! Basically I have a script in Python that grabs the text from an open window using getWindowText() and outputs it to the screen. The python loops so as the text in the window changes, it outputs the changes, so the output of the python will always be up to date with the window text. I'm trying to access this text in my Java program by executing the python script as a process and reading the text it outputs using a buffered reader. For some reason this works fine for the first block of text, but will not read any more after this, it wont read any updates to the text as the python outputs it. Can someone shed some light on this? I'm about to try and use Jython, but I'd really like to know what the problem is here... try { Runtime r = Runtime.getRuntime(); Process p = r.exec("cmd /c getText.py"); BufferedReader br = new BufferedReader( new InputStreamReader(p.getInputStream())); int line; while (true) { line = br.read(); System.out.print((char) line); } } catch (Exception e) { e.printStackTrace(); }

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• Quick question regarding MVC framework for a java game

  - by Jason

  I have a question regarding the use of the MVC framework for a blackjack game im writing in java. As I understand, my model would be a class that stores the state of the game, and more specifically, each players hand. Within the model class, I would have an array of listeners, which would be notified each time the game state changes (ie a player has drawn a new card). These listeners would be my viewer, which is the class that handles the display of the game. This class would implement the ActionListener interface, and each time the model changes, it would call/"notify" my viewer's actionPerformed method. My question is as follows: I intend to have two JPanels, each devoted to displaying the respective player's hand. As a player draws a new card, a new ImageIcon would be added to the panel. My question is how would the viewer class know what card has been added to the player's hand? I suppose I could store the player's hand before a notify event, and then upon the notification event, compare the new state with the old, to find out what has changed. I'm a complete novice here, so I could be completely wrong, but it seems a bit tedious to do this. Is there a more efficient/common way of detecting what has changed in the model between the before and after state? Another solution would be to redraw the entire panel each time the notification occurs? Would this be a better idea?

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• How to round a number to n decimal places in Java

  - by Alex Spurling

  What I'd like is a method to convert a double to a string which rounds using the half-up method. I.e. if the decimal to be rounded is a 5, it always rounds up the previous number. This is the standard method of rounding most people expect in most situations. I also would like only significant digits to be displayed. That is there should not be any trailing zeroes. I know one method of doing this is to use the String.format method: String.format("%.5g%n", 0.912385); returns: 0.91239 which is great, however it always displays numbers with 5 decimal places even if they are not significant: String.format("%.5g%n", 0.912300); returns: 0.91230 Another method is to use the DecimalFormatter: DecimalFormat df = new DecimalFormat("#.#####"); df.format(0.912385); returns: 0.91238 However as you can see this uses half-even rounding. That is it will round down if the previous digit is even. What I'd like is this: 0.912385 -> 0.91239 0.912300 -> 0.9123 What is the best way to achieve this in Java?

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• Problem with Java Scanner sc.nextLine();

  - by Jonathan B

  Hi, sry about my english :) Im new to Java programming and i have a problem with Scanner. I need to read an Int, show some stuff and then read a string so i use sc.nextInt(); show my stuff showMenu(); and then try to read a string palabra=sc.nextLine(); Some one told me i need to use a sc.nextLine(); after sc.nextInt(); but i dont understand why do you have to do it :( Here is my code: public static void main(String[] args) { // TODO code application logic here Scanner sc = new Scanner(System.in); int respuesta = 1; showMenu(); respuesta = sc.nextInt(); sc.nextLine(); //Why is this line necessary for second scan to work? switch (respuesta){ case 1: System.out.println("=== Palindromo ==="); String palabra = sc.nextLine(); if (esPalindromo(palabra) == true) System.out.println("Es Palindromo"); else System.out.println("No es Palindromo"); break; } } Ty so much for your time and Help :D

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• Adding functions to Java class libraries

  - by Eric

  I'm using a Java class library that is in many ways incomplete: there are many classes that I feel ought to have additional member functions built in. However, I am unsure of the best practice of adding these member functions. Lets call the insufficient base class A. class A { public A(/*long arbitrary arguments*/) { //... } public A(/*long even more arbitrary arguments*/) { //... } public int func() { return 1; } } Ideally, I would like to add a function to A. However, I can't do that. My choice is between: class B extends A { //Implement ALL of A's constructors here public int reallyUsefulFunction() { return func()+1; } } and class AddedFuncs { public int reallyUsefulFunction(A a) { return a.func()+1; } } The way I see it, they both have advantages and disadvantages. The first choice gives a cleaner syntax than the second, and is more logical, but has problems: Let's say I have a third class, C, within the class library. class C { public A func() { return new A(/*...*/); } } As I see it, there is no easy way of doing this: C c; int useful = c.func().reallyUsefulFunction(); as the type returned by C.func() is an A, not a B, and you can't down-cast. So what is the best way of adding a member function to a read-only library class?

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• Replacing XML in File from "Document" in Java

  - by poeschlorn

  Hi, after processing my first steps in working with XML in java I am now at the point where I want to update some data in my XML/GPX file... Reaplacing it in my "Document" data type works great :) How here comes the question: how can I store the changed "document"-model back to my file? Do I have to do this by using the standart file-functions (via steams and so on) oder is the a more elegant way to do this? ;-) Here's the code I already worked out, maybe that could help. (the method getParsedXML is just puting the conversion from the file into an extra method) Document tmpDoc = getParsedXML(currentGPX); //XML Parsind tests: // Access to tag attribute <tag attribut="bla"> System.out.println(tmpDoc.getElementsByTagName("wpt").item(0).getAttributes().getNamedItem("lat").getTextContent()); // Access to the value of an child element <a><CHILD>ValueOfChild</CHILD></a> System.out.println(tmpDoc.getElementsByTagName("wpt").item(0).getChildNodes().item(5).getTextContent()); // Replacing access to tag attribute tmpDoc.getElementsByTagName("wpt").item(0).getAttributes().getNamedItem("lat").setTextContent("139.921055008"); System.out.println(tmpDoc.getElementsByTagName("wpt").item(0).getAttributes().getNamedItem("lat").getTextContent()); // Replacing access to child element value tmpDoc.getElementsByTagName("wpt").item(0).getChildNodes().item(5).setTextContent("Cala Sant Vicenç - Mallorca 2"); System.out.println(tmpDoc.getElementsByTagName("wpt").item(0).getChildNodes().item(5).getTextContent());

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• Java JCheckBox ArrayList help needed

  - by user2929626

  I'm new to Java and struggling with something which I'm sure must have a simple answer but I can't seem to find it. I have an array of checkbox objects defined as: ArrayList<JCheckBox> checkBoxList A JPanel is created with a grid layout and the checkboxes are added to the JPanel and the ArrayList: for (int i = 0; i < 256; i++) { JCheckBox c = new JCheckBox(); c.setSelected(false); checkBoxList.add(c); mainPanel.add(c); } Yes, there are 256 checkboxes! The panel is added to a JFrame and eventually the GUI is displayed. The user can select any combination of the 256 checkboxes. My class implements Serializable and this ArrayList of checkboxes can be saved and restored using 'Save' and 'Load' GUI buttons. My code to load the saved object is as below: public class LoadListener implements ActionListener { public void actionPerformed(ActionEvent a) { try { // Prompt the user for a load file JFileChooser fileLoad = new JFileChooser(); fileLoad.showOpenDialog(mainFrame); // Create a object/file input stream linking to the selected file ObjectInputStream is = new ObjectInputStream(new FileInputStream(fileLoad.getSelectedFile())); // Read the checkBox array list checkBoxList = (ArrayList<JCheckBox>) is.readObject(); is.close(); } catch (Exception ex) { ex.printStackTrace(); } } On loading the ArrayList object, the values of the checkboxes are correctly populated, however I want to update the checkboxes on the GUI to reflect this. Is there an easy way to do this? I assumed as the array of checkboxes had the correct values that I could just repaint the panel / frame but this doesn't work. I'd like to understand why - does my loaded array of checkbox objects no longer reflect the checkbox objects on the GUI? Any help would be much appreciated. Thanks!

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• Error compiling basic java code

  - by Michael Younani

  New to java. Practicing coding by following a book. Heres my code: class Motorcycle { //Three instance variables - make and color are strings. while a boolean refers to TRUE OR FLASE(in this case off or on) String make; String color; boolean engineState; void startEngine() { if (engineState == true) System.out.print("The engine is already on."); else { engineState = true; System.out.print("The engine is now on."); } void showAtts() { System.out.print("This motorcycle is a " + color + " " + make); if (engineState ==true) System.out.print("The engine is on."); else System.out.print("The engine is off."); } } } When I compile I get 2 errors: 1) illegal start of expression 2) ; expected I can't pin point the problem. If anyone can direct me or hint me please do.

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• Java: over-typed structures? To have many types in Object[]?

  - by HH

  Term over-type structure = a data structure that accepts different types, can be primitive or user-defined. I think ruby supports many types in structures such as tables. I tried a table with types 'String', 'char' and 'File' in Java but errs. How can I have over-typed structure in Java? How to show types in declaration? What about in initilization? Suppose a structure: INDEX VAR FILETYPE //0 -> file FILE //1 -> lineMap SizeSequence //2 -> type char //3 -> binary boolean //4 -> name String //5 -> path String Code import java.io.*; import java.util.*; public class Object { public static void print(char a) { System.out.println(a); } public static void print(String s) { System.out.println(s); } public static void main(String[] args) { Object[] d = new Object[6]; d[0] = new File("."); d[2] = 'T'; d[4] = "."; print(d[2]); print(d[4]); } } Errors Object.java:18: incompatible types found : java.io.File required: Object d[0] = new File("."); ^ Object.java:19: incompatible types found : char required: Object d[2] = 'T'; ^

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• Highlighting Changes in Java

  - by Buzz Lightyear

  Basically, i have done my program so that it will display differences in strings and display the whole line. I want to highlight (in a colour) the differences in the line. Example: Original at line 5 <rect x="60.01" width="855.38" id="rect_1" y="-244.35" height="641.13" style="stroke-width: 1; stroke: rgb(0, 0, 0); fill: none; "/> Edited at line 5 <rect x="298.43" width="340.00" y="131.12" height="380.00" id="rect_1" style="stroke-width: 1; stroke: rgb(0, 0, 0); fill: rgb(255, 102, 0); "/> In this example, the width is different from the 'original' from the 'edited' version. I would like to be able to highlight that difference and any other difference. My code so far: Patch patch = DiffUtils.diff(centralFile, remoteFile); StringBuffer resultsBuff = new StringBuffer(remoteFileData.length); for (Delta delta : patch.getDeltas()) { resultsBuff.append("Original at line " + delta.getOriginal().getPosition() + "\n"); for (Object line : delta.getOriginal().getLines()) { resultsBuff.append(" " + line + "\n"); } resultsBuff.append("Edited at line " + delta.getRevised().getPosition() + "\n"); for (Object line : delta.getRevised().getLines()) { resultsBuff.append(" " + line + "\n"); } resultsBuff.append("\n"); } return resultsBuff.toString(); } That will display two whole lines like the example before (the original and the edited version) I want to be able to highlight the changes that have actually been made, is there any way to do this in Java?

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• Inheritance concept java..help

  - by max

  Hi everyone. I'd be very grateful if someone could help me to understand the inheritance concept in Java. Is the following code an example of that? I mean the class WavPanel is actually a subclass of JPanel which acts as a superclass. Is that correct? If so it means that "what JPanel has, also WavPanel but it is more specific since through its methods you can do something else". Am I wrong? thank you. Max import javax.swing.JPanel; class WavPanel extends JPanel { List<Byte> audioBytes; List<Line2D.Double> lines; public WavPanel() { super(); setBackground(Color.black); resetWaveform(); } public void resetWaveform() { audioBytes = new ArrayList<Byte>(); lines = new ArrayList<Line2D.Double>(); repaint(); } }

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• Java Socket - how to catch Exception of BufferedReader.readline()

  - by Hasan Tahsin

  I have a Thread (let's say T1) which reads data from socket: public void run() { while (running) { try { BufferedReader reader = new BufferedReader( new InputStreamReader(socket.getInputStream()) ); String input = reader.readLine(); } catch (IOException e) { e.printStackTrace(); } } } Another Thread (lets say T2) try to finish the program in one of its method. Therefore T2 does the following: T1.running = false; socket.close(); Here is this scenario for which i couldn't find a solution: T1 is active and waiting for some input to read i.e. blocking. context switching T2 is active and sets running to false, closes the socket context switching because T1 was blocking and T2 closed the socket, T1 throws an Exception. What i want is to catch this SocketException. i can't put a try/catch(SocketException) in T1.run(). So how can i catch it in T1's running-method? If it's not possible to catch it in T1's running, then how can i catch it elsewhere? PS: "Another question about the Thread Debugging" Normally when i debug the code step by step, i lose the 'active running line' on a context switch. Let's say i'm in line 20 of T1, context switch happens, let's assume the program continues from the 30.line of T2, but the debugger does not go/show to the 30.line of T2, instead the 'active running line' vanishes. So i lose the control over the code. I use Eclipse for Java and Visual Studio for C#. So what is the best way to track the code while debugging on a context switch ?

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• java.awt.Desktop.open doesn’t work with PDF files?

  - by Jason S

  It looks like I cannot use Desktop.open() on PDF files regardless of location. Here's a small test program: package com.example.bugs; import java.awt.Desktop; import java.io.File; import java.io.IOException; public class DesktopOpenBug { static public void main(String[] args) { try { Desktop desktop = null; // Before more Desktop API is used, first check // whether the API is supported by this particular // virtual machine (VM) on this particular host. if (Desktop.isDesktopSupported()) { desktop = Desktop.getDesktop(); for (String path : args) { File file = new File(path); System.out.println("Opening "+file); desktop.open(file); } } } catch (IOException e) { e.printStackTrace(); } } } If I run DesktopOpenBug with arguments c:\tmp\zz1.txt c:\tmp\zz.xml c:\tmp\ss.pdf (3 files I happen to have lying around) I get this result: (the .txt and .xml files open up fine) Opening c:\tmp\zz1.txt Opening c:\tmp\zz.xml Opening c:\tmp\ss.pdf java.io.IOException: Failed to open file:/c:/tmp/ss.pdf. Error message: The parameter is incorrect. at sun.awt.windows.WDesktopPeer.ShellExecute(Unknown Source) at sun.awt.windows.WDesktopPeer.open(Unknown Source) at java.awt.Desktop.open(Unknown Source) at com.example.bugs.DesktopOpenBug.main(DesktopOpenBug.java:21) What the heck is going on? I'm running WinXP, I can type "c:\tmp\ss.pdf" at the command prompt and it opens up just fine. edit: if this is an example of Sun Java bug #6764271 please help by voting for it. What a pain. :(

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• Java: How ArrayList memory management

  - by cka3o4nik

  In my Data Structures class we have studies the Java ArrayList class, and how it grows the underlying array when a user adds more elements. That is understood. However, I cannot figure out how exactly this class frees up memory when lots of elements are removed from the list. Looking at the source, there are three methods that remove elements: [code] public E remove(int index) { RangeCheck(index); modCount++; E oldValue = (E) elementData[index]; int numMoved = size - index - 1; if (numMoved 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // Let gc do its work return oldValue; } public boolean remove(Object o) { if (o == null) { for (int index = 0; index < size; index++) if (elementData[index] == null) { fastRemove(index); return true; } } else { for (int index = 0; index < size; index++) if (o.equals(elementData[index])) { fastRemove(index); return true; } } return false; } private void fastRemove(int index) { modCount++; int numMoved = size - index - 1; if (numMoved 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // Let gc do its work } {/code] None of them reduce the datastore array. I even started questioning if memory free up ever happens, but empirical tests show that it does. So there must be some other way it is done, but where and how? I checked the parent classes as well with no success.

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• Vectors or Java arrays for Tetris?

  - by StackedCrooked

  I'm trying to create a Tetris-like game with Clojure and I'm having some trouble deciding the data structure for the playing field. I want to define the playing field as a mutable grid. The individual blocks are also grids, but don't need to be mutable. My first attempt was to define a grid as a vector of vectors. For example an S-block looks like this: :s-block { :grids [ [ [ 0 1 1 ] [ 1 1 0 ] ] [ [ 1 0 ] [ 1 1 ] [ 0 1 ] ] ] } But that turns out to be rather tricky for simple things like iterating and painting (see the code below). For making the grid mutable my initial idea was to make each row a reference. But then I couldn't really figure out how to change the value of a specific cell in a row. One option would have been to create each individual cell a ref instead of each row. But that feels like an unclean approach. I'm considering using Java arrays now. Clojure's aget and aset functions will probably turn out to be much simpler. However before digging myself in a deeper mess I want to ask ideas/insights. How would you recommend implementing a mutable 2d grid? Feel free to share alternative approaches as well. Source code current state: Tetris.clj (rev452)

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