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  • Garbage Collection Java

    - by simion
    On the slides i am revising from it says the following; Live objects can be identified either by maintaining a count of the number of references to each object, or by tracing chains of references from the roots. Reference counting is expensive – it needs action every time a reference changes and it doesn’t spot cyclical structures, but it can reclaim space incrementally. Tracing involves identifying live objects only when you need to reclaim space – moving the cost from general access to the time at which the GC runs, typically only when you are out of memory. I understand the principles of why reference counting is expensive but do not understand what "doesn’t spot cyclical structures, but it can reclaim space incrementally." means. Could anyone help me out a little bit please? Thanks

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  • help with reflections and annotations in java

    - by Yonatan
    Hello Internet ! I'm having trouble with doubling up on my code for no reason other than my own lack of ability to do it more efficiently... `for (Method curr: all){ if (curr.isAnnotationPresent(anno)){ if (anno == Pre.class){ for (String str : curr.getAnnotation(Pre.class).value()){ if (str.equals(method.getName()) && curr.getReturnType() == boolean.class && curr.getParameterTypes().length == 0){ toRun.add(curr); } } } if (anno == Post.class) { for (String str : curr.getAnnotation(Post.class).value()){ if (str.equals(method.getName()) && curr.getReturnType() == boolean.class && curr.getParameterTypes().length == 0){ toRun.add(curr); } } } } }` anno is a parameter - Class, and Pre and Post are my annotations, both have a value() which is an array of strings. Of course, this is all due to the fact that i let Eclipse auto fill code that i don't understand yet.

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  • Java - how to tell class of an object?

    - by lkm
    Given a method that accepts as a parameter a certain supertype. Is there any way, within that method, to determine the actual class of the object that was passed to it? I.e. if a subtype of the allowable parameter was actually passed, is there a way to find out which type it is? If this isn't possible can someone explain why not (from a language design perspective)? Thanks Update: just to make sure I was clear void doSomething(MyType myType) { //determine if myType is MyType OR one of its subclasses } Since the method signature specifies the parameter as being MyType, then how can one tell if the object is actually a subtype of MyType (and which one).

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  • What's wrong with my logic (Java syntax)

    - by soda
    I'm trying to make a simple program that picks a random number and takes input from the user. The program should tell the user if the guess was hot (-/+ 5 units) or cold, but I never reach the else condition. Here's the section of code: public static void giveHint (int guess) { int min = guess - 5; int max = guess + 5; if ((guess > min) && (guess < max)) { System.out.println("Hot.."); } else { System.out.println("Cold.."); } }

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  • In Java, does return trump finally?

    - by jonny five
    If I have a try/catch block with returns inside it, will the finally block be called? For example: try { something(); return success; } catch (Exception e) { return failure; } finally { System.out.println "i don't know if this will get printed out." } I know I can just type this in an see what happens (which is what I'm about to do, actually) but when I googled for answers nothing came up, so I figured I'd throw this up as a question. Thanks!

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  • Modifying annotation attribute value at runtime in java

    - by Lici
    Hi folks: some methods in our model pojos have been annotated like this: @Column(name="cli_clipping_id", updatable=false, columnDefinition = "varchar(" + ModelUtils.ID_LENGTH + ") COLLATE utf8_bin") columnDefinition attribute is database vendor dependant, so when trying to drop schema in HSQLDB using Hibernate it fails: [ERROR] 16 jun 12:58:42.480 PM main [org.hibernate.tool.hbm2ddl.SchemaExport] Unexpected token: COLLATE in statement [create table cms.edi_editorial_obj (edi_uuid varchar(23) COLLATE ] To fix this, i'm thinking on this solution (but don't want to spend time if it isn't possible) , at runtime, for each method column annotated: Get @Column annotation Create a copy of the column annotation, setting columnDefinition null using javaassist. set column method annotation to the copy column annotation object overriding the old one (i don't know it this is possible) Is it possible to "hack" these methods this way? Any help would be much appreciated ...

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  • Java Constructor Style (Check parameters aren't null)

    - by Peter
    What are the best practices if you have a class which accepts some parameters but none of them are allowed to be null? The following is obvious but the exception is a little unspecific: public class SomeClass { public SomeClass(Object one, Object two) { if (one == null || two == null) { throw new IllegalArgumentException("Parameters can't be null"); } //... } } Here the exceptions let you know which parameter is null, but the constructor is now pretty ugly: public class SomeClass { public SomeClass(Object one, Object two) { if (one == null) { throw new IllegalArgumentException("one can't be null"); } if (two == null) { throw new IllegalArgumentException("two can't be null"); } //... } Here the constructor is neater, but now the constructor code isn't really in the constructor: public class SomeClass { public SomeClass(Object one, Object two) { setOne(one); setTwo(two); } public void setOne(Object one) { if (one == null) { throw new IllegalArgumentException("one can't be null"); } //... } public void setTwo(Object two) { if (two == null) { throw new IllegalArgumentException("two can't be null"); } //... } } Which of these styles is best? Or is there an alternative which is more widely accepted? Cheers, Pete

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  • Java Programming Homework

    - by user1427476
    Write a program to read in a file containing the names of the employ of a company and store them in an array of Strings named Employ []. Read another file containing the Salaries of each employ written in the same order and store them in another array of integers named salary[] (contents of both files are shown below). Finally create a file stating for example. Mr. XYZ receive $75,500.00 per year (Note that salary is stored as integer but displayed here as double with a comma separating thousands. This formatting needs to be done using DecimalFormat Class)

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  • Well written open source java projects

    - by Algorist
    I want to improve my design and programming skills by understanding design & code of open source projects. I downloaded hadoop,groovy but they are very difficult to follow. I am not having a clue of how to follow this code without having a high level overview of the design. Any suggestions?? Thank you.

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  • Java generics question

    - by user247866
    So I have 3 classes. Abstract class A Class B extends class A independent Class C In class D that contains the main method, I create a list of instances of class B List<B> b = methodCall(); // the method returns a list of instances of class B Now in class C I have one method that is common to both A and B, and hence I don't want to duplicate it. I want to have one method that takes as input an instance of class A, as follows: public void someMethod(List<A> a) However, when I do: C c = new C(); c.someMethod(b); I get an error that some-method is not applicable for the argument List<B>, instead it's expecting to get List<A>. Is there a good way to fix this problem? Many thanks!

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  • How to close a java swing application from the code

    - by hstoerr
    What is the proper way to terminate a Swing application from the code, and what are the pitfalls? I'd tried to close my application automatically after a timer fires. But just calling dispose() on the JFrame didn't do the trick - the window vanished but the application did not terminate. However when closing the window with the close button, the application does terminate. What should I do?

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  • Why is run() not immediately called when start() called on a thread object in java

    - by pi
    Or is it? I have a thread object from: Thread myThread = new Thread(pObject); Where pObject is an object of a class implementing the Runnable interface and then I have the start method called on the thread object like so: myThread.start(); Now, my understanding is that when start() is called, the JVM implicitly (and immediately) calls the run() method which may be overridden (as it is in my case) However, in my case, it appears that the start() method is not called immediately (as desired) but until the other statements/methods are completed from the calling block i.e. if I had a method after the start() call like so: myThread.start(); doSomethingElse(); doSomthingElse() gets executed before the run() method is run at all. Perhaps I am wrong with the initial premise that run() is always called right after the start() is called. Please help! The desired again is making executing run() right after start(). Thanks.

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  • Java Inheritance - Getting a Parameter from Parent Class

    - by Aaron
    I'm trying to take one parameter from the parent class of Car and add it to my array (carsParked), how can i do this? Parent Class public class Car { protected String regNo; //Car registration number protected String owner; //Name of the owner protected String carColor; /** Creates a Car object * @param rNo - registration number * @param own - name of the owner **/ public Car (String rNo, String own, String carColour) { regNo = rNo; owner = own; carColor = carColour; } /** @return The car registration number **/ public String getRegNo() { return regNo; } /** @return A String representation of the car details **/ public String getAsString() { return "Car: " + regNo + "\nColor: " + carColor; } public String getColor() { return carColor; } } Child Class public class Carpark extends Car { private String location; // Location of the Car Park private int capacity; // Capacity of the Car Park - how many cars it can hold private int carsIn; // Number of cars currently in the Car Park private String[] carsParked; /** Constructor for Carparks * @param loc - the Location of the Carpark * @param cap - the Capacity of the Carpark */ public Carpark (String locations, int room) { location = locations; capacity = room; } /** Records entry of a car into the car park */ public void driveIn() { carsIn = carsIn + 1; } /** Records the departure of a car from the car park */ public void driveOut() { carsIn = carsIn - 1; } /** Returns a String representation of information about the carpark */ public String getAsString() { return location + "\nCapacity: " + capacity + " Currently parked: " + carsIn + "\n*************************\n"; } }

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  • Strange problem with simple multithreading program in Java

    - by Elizabeth
    Hello, I am just starting play with multithreading programming. I would like to my program show alternately character '-' and '+' but it doesn't. My task is to use synchronized keyword. As far I have: class FunnyStringGenerator{ private char c; public FunnyStringGenerator(){ c = '-'; } public synchronized char next(){ if(c == '-'){ c = '+'; } else{ c = '-'; } return c; } } class ThreadToGenerateStr implements Runnable{ FunnyStringGenerator gen; public ThreadToGenerateStr(FunnyStringGenerator fsg){ gen = fsg; } @Override public void run() { for(int i = 0; i < 10; i++){ System.out.print(gen.next()); } } } public class Main{ public static void main(String[] args) throws IOException { FunnyStringGenerator FSG = new FunnyStringGenerator(); ExecutorService exec = Executors.newCachedThreadPool(); for(int i = 0; i < 20; i++){ exec.execute(new ThreadToGenerateStr(FSG)); } } } EDIT: I also testing Thread.sleep in run method instead for loop.

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  • Why are (almost) all the on-line games written in ActionScript (Flash) not Java?

    - by MasterPeter
    I absolutely love good defender games (e.g. Gemcraft, Protector: reclaiming the throne) as they can be intellectually quite challenging; it's like playing chess but a little less thinking a bit more action. Sadly, there are not that many good ones out there and I thought I would create one myself and share it with the rest of the world by making it available on-line. I have never worked with ActionScript but when it comes to on-line games, this is the main choice. I have tried to find a decent 2D game in the form of a Java applet but to no avail. Why is this so? I could write the game, most comfortably, in Delphi for Win32 but then people would need to download the executable, which could deter some form downloading it, and also it would only work on Windows. I am also familiar with Java, having worked with Java for the last four years or so. Although I don't have much experience with games programming. Should I note be deterred by the fact that all online games are written for in Flash and create my defender game as a Java applet, or should I consider learning ActionScript and games development for the ActionScript Virtual Machine (AS3 looks very much like Java... but still, it's an entirely new technology to me and I might never use it professionally.) Could you, please, just answer the the question in the title? Why Flash, not Java applets? Is it only 'politics'?

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  • Java Hibernate id auto increment

    - by vinise
    Hy I'v a little problem with hibernate on netbeans. I've a table with an Auto increment id : CREATE TABLE "DVD" ( "DVD_ID" INT not null primary key GENERATED ALWAYS AS IDENTITY (START WITH 1, INCREMENT BY 1), "TITLE" VARCHAR(150), "COM" LONG VARCHAR, "COVER" VARCHAR(150) ); But this auto increment is not properly detected with Reverse Engineering. I get a map file with this : <id name="dvdId" type="int"> <column name="DVD_ID" /> <generator class="assigned" /> </id> i've looked on google and on this site ... foud some stuf but i'm still stuck.. i've tried to add insert="false" update="false" on the map file but i get back : Caused by: org.xml.sax.SAXParseException: Attribute "insert" must be declared for element type "id". Anny help will be pleased Vincent

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  • Confusion in RegExp Reluctant quantifier? Java

    - by Dusk
    Hi, Could anyone please tell me the reason of getting an output as: ab for the following RegExp code using Relcutant quantifier? Pattern p = Pattern.compile("abc*?"); Matcher m = p.matcher("abcfoo"); while(m.find()) System.out.println(m.group()); // ab and getting empty indices for the following code? Pattern p = Pattern.compile(".*?"); Matcher m = p.matcher("abcfoo"); while(m.find()) System.out.println(m.group());

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  • Java - Removing duplicates in an ArrayList

    - by Will
    I'm working on a program that uses an ArrayList to store Strings. The program prompts the user with a menu and allows the user to choose an operation to perform. Such operations are adding Strings to the List, printing the entries etc. What I want to be able to do is create a method called removeDuplicates().This method will search the ArrayList and remove any duplicated values. I want to leave one instance of the duplicated value(s) within the list. I also want this method to return the total number of duplicates removed. I've been trying to use nested loops to accomplish this but I've been running into trouble because when entries get deleted, the indexing of the ArrayList gets altered and things don't work as they should. I know conceptually what I need to do but I'm having trouble implementing this idea in code. Here is some pseudo code: start with first entry; check each subsequent entry in the list and see if it matches the first entry; remove each subsequent entry in the list that matches the first entry; after all entries have been examined, move on to the second entry; check each entry in the list and see if it matches the second entry; remove each entry in the list that matches the second entry; repeat for entry in the list Here's the code I have so far: public int removeDuplicates() { int duplicates = 0; for ( int i = 0; i < strings.size(); i++ ) { for ( int j = 0; j < strings.size(); j++ ) { if ( i == j ) { // i & j refer to same entry so do nothing } else if ( strings.get( j ).equals( strings.get( i ) ) ) { strings.remove( j ); duplicates++; } } } return duplicates; }

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