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  • Java Concurrency : Volatile vs final in "cascaded" variables?

    - by Tom
    Hello Experts, is final Map<Integer,Map<String,Integer>> status = new ConcurrentHashMap<Integer, Map<String,Integer>>(); Map<Integer,Map<String,Integer>> statusInner = new ConcurrentHashMap<Integer, Map<String,Integer>>(); status.put(key,statusInner); the same as volatile Map<Integer,Map<String,Integer>> status = new ConcurrentHashMap<Integer, Map<String,Integer>>(); Map<Integer,Map<String,Integer>> statusInner = new ConcurrentHashMap<Integer, Map<String,Integer>>(); status.put(key,statusInner); in case the inner Map is accessed by different Threads? or is even something like this required: volatile Map<Integer,Map<String,Integer>> status = new ConcurrentHashMap<Integer, Map<String,Integer>>(); volatile Map<Integer,Map<String,Integer>> statusInner = new ConcurrentHashMap<Integer, Map<String,Integer>>(); status.put(key,statusInner); In case the it is NOT a "cascaded" map, final and volatile have in the end the same effect of making shure that all threads see always the correct contents of the Map... But what happens if the Map iteself contains a map, as in the example... How do I make shure that the inner Map is correctly "Memory barriered"? Tanks! Tom

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  • Java threadpool functionality

    - by cpf
    Hi stackoverflow, I need to make a program with a limited amount of threads (currently using newFixedThreadPool) but I have the problem that all threads get created from start, filling up memory at alarming rate. I wish to prevent this. Threads should only be created shortly before they are executed. e.g.: I call the program and instruct it to use 2 threads in the pool. The program should create & launch the first 2 Threads immediately (obviously), create the next 2 to wait for the previous 2, and at that point wait until one or both of the first 2 ended executing. I thought about extending executor or FixedThreadPool or such. However I have no clue on how to start there and doubt it is the best solution. Easiest would have my main Thread sleeping on intervals, which is not really good either... Thanks in advance!

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  • java: how to convert a file to utf8

    - by Enrique San Martín
    Hi, i have a file that have some non-utf8 caracters (like "ISO-8859-1"), and so i want to convert that file (or read) to UTF8 encoding, how i can do it? The code it's like this: File file = new File("some_file_with_non_utf8_characters.txt"); /* some code to convert the file to an utf8 file */ ... edit: Put an encoding example

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  • Show users a list of unique items on Java Google App Engine

    - by James
    I've been going round in circles with what must be a very simple challenge but I want to do it the most efficient way from the start. So, I've watched Brett Slatkin's Google IO videos (2008 & 2009) about building scalable apps including http://www.youtube.com/watch?v=AgaL6NGpkB8 and read the docs but as a n00b, I'm still not sure. I'm trying to build an app on GAEJ similar to the original 'hotornot' where a user is presented with an item which they rate. Once they rate it, they are presented with another one which they haven't seen before. My question is this; is it most efficient to do a query up front to grab x items (say 100) and put them in a list (stored in memcache?) or is it better to simply make a query for a new item after each rating. To keep track of the items a user has seen, I'm planning to keep those items' keys in a list property of the user's entity. Does that sound sensible? I've really got myself confused about this so any help would be much appreciated.

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  • Deleting from arraylist and exporting back to .buab file(Java)

    - by Dave
    Ive made an address book. I can currently write to the arraylist and save it back to the .buab file, but I cant delete from the arraylist and export it back to the .buab file? Im pretty much stuck on this. Im able to retrive contacts from the .buab and scroll through them using the JTextFields and buttons ive created. Any help will be dearly appreciated. Ive set up seprate classes for all operations (newcontacts, nextcontact etc). If you need the code posted let me know.. Cheers Dave

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  • Using mail servers in java

    - by sword101
    greeting all i want to use a mail server where the users send emails to it and then i parse this emails then do some action please suggest me what mail server to be used and where to start some links,tutorials,guide is very appreciated .

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  • Sum up values in an array list in java

    - by user1449997
    Hello to all tired and frustrated programmers, There is a problem I can't solve: In my array list there are Object arrays with the length of three and always on the last cell of the array there is a double. I need a loop which goes through the array list and sums up all doubles. (I need also some other thing from the arrays so a general idea how to loop through an array list would be perfect :-)) Any ideas? Thanks for your help, Greetings Demian

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  • Java - SwingWorker - problem in done() method

    - by Yatendra Goel
    I am using javax.swing.SwingWorker for the first time. I want to update a JLabel from the interim result published by the swing worker as follows: publish("Published String"); Now to update the JLabel, I have coded the following: process(List<String> chunks) { if (chunks.size() > 0) { String text = chunks.get(chunks.size() - 1); label.setText(text); } } The above code works but my problem(or to be more specific, my doubt) is as follows: The above swing worker task is an annonymous inner class so it can access label field. But what if I want to make the swing worker class a non-inner class. Should I need to pass label as an argument to the constructor of swing worker class so that the process() method can access. Or Is there any other way? What approach does other developer follow to update UI components from the swing worker class' result when the swing worker class is not an inner class?

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  • How is Java Process.getOutputStream() Implemented?

    - by Amit Kumar
    I know the answer depends on the particular JVM, but I would like to understand how it is usually implemented? Is it in terms of popen (posix)? In terms of efficiency do I need to keep something in mind (other than using a Buffered stream as suggested by the javadoc). I would be interested to know if there is a general reference about implementations of JVMs which answers such questions.

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  • Java Conversion of byte[] into a srting and then back to a byte[]

    - by Sid
    I am working on a proxy server. I am getting data in byte[] which i convert into a string to perform certain operations. Now when i convert this new string back into a byte [] it causes unkonw problems. So mainly its like i need to know how to correctly convert a byte[] into a string and then back into a byte[] again. I tried to just convert the byte[] to string and then back to byte[] again (to make sure thats its not my operations that are causing problems). So its like: // where reply is a byte[] String str= new String(reply,0, bytesRead); streamToClient.write(str.getBytes(), 0, bytesRead); is not equivalent to streamToClient.write(reply, 0, bytesRead); my proxy works fine when i just send the byte[] without any conversion but when i convert it from byte[] to a string and then back to a byte[] its causes problems. can some one please help? =]

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  • Java - FontMetrics without Graphics

    - by subSeven
    Hello! How to get FontMetrics without use Graphics ? I want to get FontMetrics in constructor, now I do this way: BufferedImage bi = new BufferedImage(5, 5, BufferedImage.TYPE_INT_RGB); FontMetrics fm = bi.getGraphics().getFontMetrics(font); int width = fm.stringWidth(pattern); int height = fm.getHeight();

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  • DWR and other possible tools for build java application level Ajax framework

    - by Fazal
    I would assume that this question would have been asked in different ways already, but I could not find one so posting it. Sorry if its a repetition The basic idea which I am trying to explore is to design a common Ajax framework/API for our application. The main requirement is to have a common framework in the product which every module in the application can call whenever it needs to provide Ajax behavior. We basically have a page based application and not everything would be Ajax for sure. Mostly it will be smaller pieces of work which Ajax will handle. We have used GWT and it works great. But we have some constraints because of which certain areas in the application need to build standard jsp and html pages only I know little bit about dwr and it looks very promising (as even answered by some people). I wanted to know what are the other possible frameworks like dwr which I can evaluate too, before making the decision

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  • Java Clock Assignment

    - by Mike S
    For my assignment we are suppose to make a clock. We need variables of hours, minutes, and seconds and methods like setHours/getHours, setMinutes/getMinutes, setSeconds/getSeconds. Now the parts of the assignment that I am having trouble on is that we need a addClock() method to make the sum of two clock objects and a tickDown() method which decrements the clock object and a tick() method that increments a Clock object by one second. Lastly, the part where I am really confused on is, I need to write a main() method in the Clock class to test the functionality of your objects with a separate Tester class with a main() method. Here is what I have so far... public class Clock { private int hr; //store hours private int min; //store minutes private int sec; //store seconds //Default constructor public Clock () { setClock (0, 0, 0); } public Clock (int hours, int minutes, int seconds) { setTimes (hours, minute, seconds); } public void setClock (int hours, int minutes, int seconds) { if(0 <= hours && hours < 24) { hr = hours; } else { hr = 0; } if(0 <= minutes && minutes < 60) { min = minutes; } else { min = 0; } if(0 <= seconds && seconds < 60) { sec = seconds; } else { sec = 0; } } public int getHours ( ) { return hr; } public int getMinutes ( ) { return min; } public int getSeconds ( ) { return sec; } //Method to increment the time by one second //Postcondition: The time is incremented by one second //If the before-increment time is 23:59:59, the time //is reset to 00:00:00 public void tickSeconds ( ) { sec++; if(sec > 59) { sec = 0; tickMinutes ( ); //increment minutes } } public void tickMinutes() { min++; If (min > 59) { min = 0; tickHours(); //increment hours } } public void tickHours() { hr++; If (hr > 23) hr = 0; } }

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  • java singleton instantiation

    - by jurchiks
    I've found three ways of instantiating a Singleton, but I have doubts as to whether any of them is the best there is. I'm using them in a multi-threaded environment and prefer lazy instantiation. Sample 1: private static final ClassName INSTANCE = new ClassName(); public static ClassName getInstance() { return INSTANCE; } Sample 2: private static class SingletonHolder { public static final ClassName INSTANCE = new ClassName(); } public static ClassName getInstance() { return SingletonHolder.INSTANCE; } Sample 3: private static ClassName INSTANCE; public static synchronized ClassName getInstance() { if (INSTANCE == null) INSTANCE = new ClassName(); return INSTANCE; } The project I'm using ATM uses Sample 2 everywhere, but I kind of like Sample 3 more. There is also the Enum version, but I just don't get it. The question here is - in which cases I should/shouldn't use any of these variations? I'm not looking for lengthy explanations though (there's plenty of other topics about that, but they all eventually turn into arguing IMO), I'd like it to be understandable with few words.

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  • Java HashSet using a specified method

    - by threenplusone
    I have a basic class 'HistoryItem' like so: public class HistoryItem private Date startDate; private Date endDate; private Info info; private String details; @Override public int hashCode() { int hash = (startDate == null ? 0 : startDate.hashCode()); hash = hash * 31 + (endDate == null ? 0 : endDate.hashCode()); return hash; } } I am currently using a HashSet to remove duplicates from an ArrayList on the startDate & endDate fields, which is working correctly. However I also need to remove duplicates on different fields (info & details). My question is this. Is there a way to specify a different method which HashSet will use in place of hashCode()? Something like this: public int hashCode_2() { int hash = (info == null ? 0 : info.hashCode()); hash = hash * 31 + (details == null ? 0 : details.hashCode()); return hash; } Set<HistoryItem> removeDups = new HashSet<HistoryItem>(); removeDups.setHashMethod(hashCode_2); Or is there another way that I should be doing this?

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  • Extracting Mail from Microsoft Exchange server 2007 through IMAPS in java

    - by abhishekgem84
    props.put("mail.debug", "true"); props.setProperty("mail.store.protocol","imaps"); props.setProperty("mail.imaps.auth.plain.disable","false"); props.setProperty("mail.imaps.host","Mail3.connect.com"); props.setProperty("mail.imaps.port","135"); props.setProperty("mail.imaps.user","test"); props.setProperty("mail.imaps.pwd","123"); props.setProperty("mail.imaps.ssl.protocols","SSL"); props.setProperty("mail.imaps.socketFactory.class", "javax.net.ssl.SSLSocketFactory"); props.setProperty("mail.imaps.socketFactory.fallback", "false"); props.setProperty("mail.imaps.socketFactory.port", "135"); i have done all this but it still says javax.mail.AuthenticationFailedException: failed to connect, no password specified? kindly help me out thanks

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  • How to combine array list string in java ?

    - by tiendv
    I have some arraylist string with keyword inside like that ! A windows is arraylist string with keyword is bold Struct of window : 9 words before + keyword + 9 words after You can see some window overlaping How to i combine that arraylist to receive like that : Thanks

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  • communication between threads in java

    - by Noona
    How can I make a thread run only if the other thread is running too, meaning, if I return from run in one thread, then I want the other to stop running too, my code looks something like this: ClientMessageHandler clientMessagehandler = new ClientMessageHandler(); ServerMessageHandler serverMessagehandler = new ServerMessageHandler(); Thread thread1 = new Thread(serverMessagehandler); Thread thread2 = new Thread(clientMessagehandler); thread2.start(); thread1.start(); i want to cause thread1 to stop running when thread2 stops running. thanks

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  • Java program has errors, 80 lines of code

    - by user2961687
    I have a problem with a program. It contains a lot of errors in Eclipse. Sorry for my english and thank you in advance. Here is the task: I. Declare a class that contains private fields Jam: String taste, double weight Create constructors containing variables as parameters: (String taste, double weight), (double weight), (String taste). Parameters constructors should initialize class fields. In case the constructor does not provide the necessary parameter, it must be assumed that the field taste must have the value "No Name" and weight - 100.0. Introduce the use of all constructors creating objects that represent three kinds of jams. Note: it must be assumed that the only constructor with two parameters can be assigned to fields of the class. Declare a class Jar that contains the field Jam jam, a dedicated constructor initiating all declared fields and methods: open close isItOpen Next, create an object of class Jar and fill it with the selected type of jam, operations repeat for all the kinds of jams. This is my code this far: public class App { public static void main(String[] args) { Jam strawberry = new Jam("strawberry", 20.45); Jam raspberry = new Jam(40.50); Jam peach = new Jam("peach"); Jar jar_1 = new Jar(); Jar jar_2 = new Jar(); Jar jar_3 = new Jar(); jar_1.open(); jar_1.fillIn(strawberry); jar_2.fillIn(peach); jar_3.fillIn(raspberry); } } class Jam { String taste; double weight; public Jam(String taste, double weight) { this.taste = taste; this.weight = weight; } public Jam(double weight) { this.taste = "No name"; this.weight = weight; } public Jam(String taste) { this.taste = taste; this.weight = 100.0; } } class Jar { public Jam Jam = new Jam(); private String state_jar; public Jar() { Jam.weight = 0; Jam.taste = ""; state_jar = "closed"; } public static String open() { state_jar = open; return state_jar; } public static String close() { state_jar = "closed"; return state_jar; } public static boolean isItOpen() { return state_jar; } public void fillIn(Jam jam) { if (isItOpen == false) open(); this.Jam.weight = jam.weight; this.Jam.taste = jam.taste; this.Jam.close(); } }

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  • how to generate a random String? in java

    - by chandra wibowo
    hi everyone, i have an object called Student, and it has studentName, studentId, studentAddress, etc. for the studentId, i have to generate random string consist of seven numeric charaters, eg. studentId = getRandomId(); studentId = "1234567" <-- from the random generator. and i have to make sure that there is no duplicate id. thanks in advance

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