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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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  • Java/swing: console component?

    - by Jason S
    I am looking for a component I can use in Swing that acts as a GUI console which has a text area with scrollbars that can be set to a particular font has an InputStream and an OutputStream that a host application can obtain accepts keyboard input, prints it onto the end of the console text, and sends that input to the InputStream prints the OutputStream text to the end of the console has some kind of FIFO-ish property whereby the amount of text displayed in the console can be limited by automatically discarding the oldest text, when appropriate allows copy (but not cut or paste or any other editing) of the console text to the system clipboard This is kind of like the Console tab in Eclipse. Are there any good libraries that provide this?

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  • Close resources before exiting JFrame and TCP communication in Java

    - by Oz Molaim
    1. I'm writing a chat based application on TCP communication. I'm using NetBeans and I want to add functionality to the default EXIT_ON_CLOSE when exiting JFrame. The reason of course is because I want to clean resources and end threads safely. How can I call a method that clear resources and only then close the JFrame safely and end the process. 2. I need to implement the server side. The server has List/HashMap/Queue of 'Socket' with their chat nick-names. Is there any simple design pattern to do it correctly because I don't want to re-invent the wheel. thanks.

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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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  • Data table with legends in JfreeChart/Java

    - by TiNS
    Hi All, I am using JFreeChart to generate images. I am trying to create barchart like below. I am able to create it successfully without data table. I tried to get more information from the jfreechar forums and found this post. According to the post , its not supported by JfreeChart. Is it still not supported by jfreechart API ? If yes, can I use any other charting (open source) tool to generate chart with data table ? Thanks

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  • Dijkstra's Algorithm explanation java

    - by alchemey89
    Hi, I have found an implementation for dijkstras algorithm on the internet and was wondering if someone could help me understand how the code works. Many thanks private int nr_points=0; private int[][]Cost; private int []mask; private void dijkstraTSP() { if(nr_points==0)return; //algorithm=new String("Dijkstra"); nod1=new Vector(); nod2=new Vector(); weight=new Vector(); mask=new int[nr_points]; //initialise mask with zeros (mask[x]=1 means the vertex is marked as used) for(int i=0;i<nr_points;i++)mask[i]=0; //Dijkstra: int []dd=new int[nr_points]; int []pre=new int[nr_points]; int []path=new int[nr_points+1]; int init_vert=0,pos_in_path=0,new_vert=0; //initialise the vectors for(int i=0;i<nr_points;i++) { dd[i]=Cost[init_vert][i]; pre[i]=init_vert; path[i]=-1; } pre[init_vert]=0; path[0]=init_vert; pos_in_path++; mask[init_vert]=1; for(int k=0;k<nr_points-1;k++) { //find min. cost in dd for(int j=0;j<nr_points;j++) if(dd[j]!=0 && mask[j]==0){new_vert=j; break;} for(int j=0;j<nr_points;j++) if(dd[j]<dd[new_vert] && mask[j]==0 && dd[j]!=0)new_vert=j; mask[new_vert]=1; path[pos_in_path]=new_vert; pos_in_path++; for(int j=0;j<nr_points;j++) { if(mask[j]==0) { if(dd[j]>dd[new_vert]+Cost[new_vert][j]) { dd[j]=dd[new_vert]+Cost[new_vert][j]; } } } } //Close the cycle path[nr_points]=init_vert; //Save the solution in 3 vectors (for graphical purposes) for(int i=0;i<nr_points;i++) { nod1.addElement(path[i]); nod2.addElement(path[i+1]); weight.addElement(Cost[path[i]][path[i+1]]); } }

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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 URLConnection Timeout

    - by Chris
    I am trying to parse an XML file from an HTTP URL. I want to configure a timeout of 15 seconds if the XML fetch takes longer than that, I want to report a timeout. For some reason, the setConnectTimeout and setReadTimeout do not work. Here's the code: URL url = new URL("http://www.myurl.com/sample.xml"); URLConnection urlConn = url.openConnection(); urlConn.setConnectTimeout(15000); urlConn.setReadTimeout(15000); urlConn.setAllowUserInteraction(false); urlConn.setDoOutput(true); InputStream inStream = urlConn.getInputStream(); InputSource input = new InputSource(inStream); And I am catching the SocketTimeoutException. Thanks Chris

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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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  • 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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  • Java style FOR loop in a clojure interpeter ?

    - by Kevin
    I have a basic interpreter in clojure. Now i need to implement for (initialisation; finish-test; loop-update) { statements } inside my interpreter. I will attach my interpreter code I got so far. Any help is appreciated. Interpreter (declare interpret make-env) ;; (def do-trace false) ;; ;; simple utilities (def third ; return third item in a list (fn [a-list] (second (rest a-list)))) (def fourth ; return fourth item in a list (fn [a-list] (third (rest a-list)))) (def run ; make it easy to test the interpreter (fn [e] (println "Processing: " e) (println "=> " (interpret e (make-env))))) ;; for the environment (def make-env (fn [] '())) (def add-var (fn [env var val] (cons (list var val) env))) (def lookup-var (fn [env var] (cond (empty? env) 'error (= (first (first env)) var) (second (first env)) :else (lookup-var (rest env) var)))) ;; -- define numbers (def is-number? (fn [expn] (number? expn))) (def interpret-number (fn [expn env] expn)) ;; -- define symbols (def is-symbol? (fn [expn] (symbol? expn))) (def interpret-symbol (fn [expn env] (lookup-var env expn))) ;; -- define boolean (def is-boolean? (fn [expn] (or (= expn 'true) (= expn 'false)))) (def interpret-boolean (fn [expn env] expn)) ;; -- define functions (def is-function? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'lambda (first expn))))) (def interpret-function (fn [expn env] expn)) ;; -- define addition (def is-plus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '+ (first expn))))) (def interpret-plus (fn [expn env] (+ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define subtraction (def is-minus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '- (first expn))))) (def interpret-minus (fn [expn env] (- (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define multiplication (def is-times? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '* (first expn))))) (def interpret-times (fn [expn env] (* (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define division (def is-divides? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '/ (first expn))))) (def interpret-divides (fn [expn env] (/ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define equals test (def is-equals? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '= (first expn))))) (def interpret-equals (fn [expn env] (= (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define greater-than test (def is-greater-than? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '> (first expn))))) (def interpret-greater-than (fn [expn env] (> (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define not (def is-not? (fn [expn] (and (list? expn) (= 2 (count expn)) (= 'not (first expn))))) (def interpret-not (fn [expn env] (not (interpret (second expn) env)))) ;; -- define or (def is-or? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'or (first expn))))) (def interpret-or (fn [expn env] (or (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define and (def is-and? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'and (first expn))))) (def interpret-and (fn [expn env] (and (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define with (def is-with? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'with (first expn))))) (def interpret-with (fn [expn env] (interpret (third expn) (add-var env (first (second expn)) (interpret (second (second expn)) env))))) ;; -- define if (def is-if? (fn [expn] (and (list? expn) (= 4 (count expn)) (= 'if (first expn))))) (def interpret-if (fn [expn env] (cond (interpret (second expn) env) (interpret (third expn) env) :else (interpret (fourth expn) env)))) ;; -- define function-application (def is-function-application? (fn [expn env] (and (list? expn) (= 2 (count expn)) (is-function? (interpret (first expn) env))))) (def interpret-function-application (fn [expn env] (let [function (interpret (first expn) env)] (interpret (third function) (add-var env (first (second function)) (interpret (second expn) env)))))) ;; the interpreter itself (def interpret (fn [expn env] (cond do-trace (println "Interpret is processing: " expn)) (cond ; basic values (is-number? expn) (interpret-number expn env) (is-symbol? expn) (interpret-symbol expn env) (is-boolean? expn) (interpret-boolean expn env) (is-function? expn) (interpret-function expn env) ; built-in functions (is-plus? expn) (interpret-plus expn env) (is-minus? expn) (interpret-minus expn env) (is-times? expn) (interpret-times expn env) (is-divides? expn) (interpret-divides expn env) (is-equals? expn) (interpret-equals expn env) (is-greater-than? expn) (interpret-greater-than expn env) (is-not? expn) (interpret-not expn env) (is-or? expn) (interpret-or expn env) (is-and? expn) (interpret-and expn env) ; special syntax (is-with? expn) (interpret-with expn env) (is-if? expn) (interpret-if expn env) ; functions (is-function-application? expn env) (interpret-function-application expn env) :else 'error)))

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  • java: relationship of the Runnable and Thread interfaces

    - by Karl Patrick
    I realize that the method run() must be declared because its declared in the Runnable interface. But my question comes when this class runs how is the Thread object allowed if there is no import call to a particular package? how does runnable know anything about Thread or its methods? does the Runnable interface extend the Thread class? Obviously I don't understand interfaces very well. thanks in advance. class PrimeFinder implements Runnable{ public long target; public long prime; public boolean finished = false; public Thread runner; PrimeFinder(long inTarget){ target = inTarget; if(runner == null){ runner = new Thread(this); runner.start() } } public void run(){ } }

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  • Java - Thread problem

    - by Yatendra Goel
    My question is related to all those methods(including Thread.sleep(...)) which throw InterruptedException. I found a statement on Sun's tutorial saying InterruptedException is an exception that sleep throws when another thread interrupts the current thread while sleep is active. Is that means that the interrupt will be ignored if the sleep is not active at the time of interrupt? Suppose I have two threads: threadOne and threadTwo. threadOne creates and starts threadTwo. threadTwo executes a runnable whose run method is something like: public void run() { : : try { Thread.sleep(10 * 1000); } catch (InterruptedException e) { return; } : : : // In the middle of two sleep invocations : : try { Thread.sleep(10 * 1000); } catch (InterruptedException e) { return; } : : } After thread creation, threadOne interrupts threadTwo. Suppose the threadTwo is in the middle of two sleep invocations at the time of interrupt (when no sleep method was active), then will the second sleep method throw InterrupteException as soon as it is invoked? If not, then will this interrupt will be ignored forever? How to be sure that threadTwo will always know about the interrupt (doesn't matter whether its one of the sleep method is active or not)?

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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 - 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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  • Java fixed memory map

    - by juber
    Hi, Is there a simple, efficient Map implementation that allows a limit on the memory to be used by the map. My use case is that I want to allocate dynamically most of the memory available at the time of its creation but I don't want OutOFMemoryError at any time in future. Basically, I want to use this map as a cache, but but I wanna avoid heavy cache implementations like EHCache. My need is simple (at most an LRU algorithm)

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  • generation of random numbers in java

    - by S.PRATHIBA
    Hi all, I want to create 30 tables which consists of the following fields.For example, Service_ID Service_Type consumer_feedback 75 Computing 1 35 Printer 0 33 Printer -1 3 rows in set (0.00 sec) mysql select * from consumer2; Service_ID Service_Type consumer_feedback 42 data 0 75 computing 0 mysql select * from consumer3; Service_ID Service_Type consumer_feedback 43 data -1 41 data 1 72 computing -1 As you can infer from the above tables, i am getting the feedback values.I have generated these consumer_feedback values,Service_ID,Service_Type using the concept of random numbers .I have used the funtion int min1=31;//printer int max1=35;//the values are generated if the Service_Type is printer. int provider1 = (int) (Math.random() * (max1 - min1 + 1) ) + min1; int min2=41;//data int max2 =45 int provider2 = (int) (Math.random() * (max2 - min2 + 1) ) + min2; int min3=71;//computing int max3=75; int provider3 = (int) (Math.random() * (max3 - min3 + 1) ) + min3; int min5 = -1;//feedback values int max5 =1; int feedback = (int) (Math.random() * (max5 - min5 + 1) ) + min5; I need the Service_Types to be distributed uniformly in all the 30 tables.Similarly I need feedback value of 1 to be generated many times other than 0 and -1.Please Help me.

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  • Java NullPointerException when traversing a non-null recordset

    - by Tim
    Hello again - I am running a query on Sybase ASE that produces a ResultSet that I then traverse and write the contents out to a file. Sometimes, this will throw a NullPointerException, stating that the ResultSet is null. However, it will do this after printing out one or two records. Other times, with the same exact input, I will receive no errors. I have been unable to consistently produce this error. The error message is pointing to a line: output.print(rs.getString(1)); It appears to happen when the query takes a little longer to run, for some reason. The recordset returns thus far have been very small (4 to 7 records). Sometimes I'll have to run the app 3 or 4 times, then the errors will just stop, as though the query was getting "warmed up". I've run the query manually and there doesn't appear to be any performance problems. Thanks again!

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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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