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• When should one let an application crash because of an exception in Java (design issue)?

  - by JVerstry

  In most cases, it is possible to catch exceptions in Java, even unchecked ones. But, it is not necessarily possible to do something about it (for example out of memory). For other cases, the issue I am trying to solve is a design principle one. I am trying to set-up a design principle or a set of rules indicating when one should give up on an exceptional situation, even if it is detected in time. The objective is trying to not crash the application as much as possible. Has someone already brainstormed and communicated about this? I am looking for specific generic cases and possible solutions, or thumb-rules. UPDATE Suggestions so far: Stop running if data coherency can be compromised Stop running if data can be deleted Stop running if you can't do anything about it (Out of memory...) Stop running if key service is not available or becomes unavailable and cannot be restarted If application must be stopped, degrade as gracefully as possible Use rollbacks in db transactions Log as much relevant information as you can Notify the developers

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• On Linux do people chroot a Java Web Application or use IPTables and run as non-root?

  - by Adam Gent

  When you run a Java Servlet Container that you would like to serve both static and dynamic content on port 80 you have the classic question of whether to run the server as: As root in hopefully a chroot jail if you can (haven't gotten this working yet) As a non root user and then use IPTables to forward port 80 to some other port (1024) that the container is running on Both: As a non root user, IPTables, and chroot jail. The problem with opt. 1 is the complexity of chrooting and still the security problems of running root.The problem with opt. 2 is that each Linux distro has a different way of persisting IPTables. Option 3 of course is probably idea but very hard to setup. Finally every distro has the annoying differences in daemon scripts. What do people find as the best distro agnostic solution and are there resources to show how to do this?

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• Struggling with how to compare hours with different time zones in Java?

  - by Riki

  Hi! I have 2 date object in the database that represent the company's working hours. I only need the hours but since I have to save date. it appears like this: Date companyWorkStartHour; Date companyWorkEndHour; start hours: 12-12-2001-13:00:00 finish hours: 12-12-2001-18:00:00 I have the timezone of the company and of the user. (my server may be in another timezone). TimeZone userTimeZone; TimeZone companyTimeZone; I need to check if the user's current time (considering his timezone) is within the company working hours (considering the company's time zone). How can I do it? I am struggling for over a week with Java calendar and with no success!

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• Regex to split a string (in Java) so that spaces are preserved?

  - by david

  I need to split a string (in Java) into individual words ... but I need to preserve spaces. An example of the text I need to split is something like this: ABC . . . . DEF . . . . GHI I need to see "ABC", " . . . .", "DEF", ". . . .", and "GHI". Obviously splitting on the space character \s isn't going to work, as all the spaces get swallowed up as one space. Any suggestions? Thanks

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• Why does Java ArrayList use per-element casting instead of per-array casting?

  - by user1111929

  What happens inside Java's ArrayList<T> (and probably many other classes) is that there is an internal Object[] array = new Object[n];, to which T Objects are written. Whenever an element is read from it, a cast return (T) array[i]; is done. So, a cast on every single read. I wonder why this is done. To me, it seems like they're just doing unnecessary casts. Wouldn't it be more logical and also slightly faster to just create a T[] array = (T[]) new Object[n]; and then just return array[i]; without cast? This is only one cast per array creation, which is usually far less than the number of reads. Why is their method to be preferred? I fail to see why my idea isn't strictly better?

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• Why do C# and Java require everything to be in a class?

  - by Javier Badia

  It seemed like this question should have been asked before, but searching found nothing. I've always wondered what's the point of making us put every bit of code inside a class or interface. I seem to remember that there were some advantages to requiring a main() function like C, but nothing for classes. Languages like Python are, in a way, even more object oriented than Java since they don't have primitives, but you can put code wherever you want. Is this some sort of "misinterpretation" of OOP? After all, you can write procedural code like you would in C and put it inside a class, but it won't be object oriented.

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• How to center string output using printf() and variable width? [java]

  - by Khan

  I'm creating an output in Java using printf() to create table headers. One of the columns needs variable width. Basically it should look like this: //two coords Trial Column Heading 1 (20,30)(30,20) //three coords Trial Column Heading 1 (20,40)(50,10)(90,30) I tried to use: int spacing = numCoords * 7; //size of column printf("Trial %*^s", column, "Column Heading"); But I keep getting output errors when I try to use * or ^ in the conversion statement. Does anyone have any idea what the correct formatting string should be?

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• Is there a way to specify java annotations in antlr grammar files?

  - by Steve B.

  I'm looking for a way to include a few additional strings in output .java files generated from antlr. Is there a comprehensive listing of available directives? For example, given parser output like this: package com.foo.bar; //<-- this can be generated with @header { .... } //antlr generated import org.antlr.runtime.*; ... //<-- is there a way to generate anything here? public class MyParser { //<--- or here? public void f1(){ ... } } Is there a way to generate strings that appear after the import statements (e.g. class-level annotations) or possibly method annotations?

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• How do I replace "this" in Java with something that works.

  - by Luke Alderton

  I'm looking to get the showGUI() method work, the compiler says "this" is not a static variable and cannot be referenced from a static context, what would I use to replace "this"? I've tried test.main (test being the package it's in). The reason I'm using the static method showGUI() is because I need the method to be called from another static method, as well as the startup() method. Below are my two main classes. public class Main extends SingleFrameApplication { @Override protected void startup() { showGUI(); } @Override protected void configureWindow(java.awt.Window root) { } public static Main getApplication() { return Application.getInstance(Main.class); } public static void main(String[] args) { launch(Main.class, args); } public static void showGUI() { show(new GUI(this)); } } public class GUI extends FrameView { public GUI(SingleFrameApplication app) { super(app); initComponents(); } private void initComponents() { //all the GUI stuff is somehow defined here } }

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• Java SWT - dissolve (fade) from one image to the next.

  - by carillonator

  I'm pretty new to Java and SWT, hoping to have one image dissolve into the next. I have one image now in a Label (relevant code): Device dev = shell.getDisplay(); try { Image photo = new Image(dev, "photo.jpg"); } catch(Exception e) { } Label label = new Label(shell, SWT.IMAGE_JPEG); label.setImage(photo); Now I'd like photo to fade into a different image at a speed that I specify. Is this possible as set up here, or do I need to delve more into the org.eclipse.swt.graphics api? Also, this will be a slide show that may contain hundreds of photos, only ever moving forward (never back to a previous image)---considering this, is there something I explicitly need to do in order to remove the old images from memory? Thanks!!

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• What is the right way to implement communication between java objects?

  - by imoschak

  I'm working on an academic project which simulates a rather large queuing procedure in java. The core of the simulator rests within one package where there exist 8 classes, each one implementing a single concept. Every class in the project follows SRP. These classes encapsulate the behavior of the simulator and inter-connect every other class in the project. The problem that has arisen is that most of these 8 classes are, as is logical i think, tightly coupled and each one has to have working knowledge of every other class in this package in order to be able to call methods from it when needed. The application needs only one instance of each class so it might be better to create static fields for each class in a new class and use that to make calls -instead of preserving a reference in each class for every other class in the package (which I'm certain that is incorrect)-, but is this considered a correct design solution? or is there a design pattern maybe that better suits my needs?

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• How do I create a "jumping" circle in Java GUI?

  - by Roman

  I would like to have a red filled circle at some place in my window. After the click on the circle, the circle should jump from the original place to a new one (disappear from the old place and appear on the new one). What would be the best way to do it? I also would like to know how to draw a filled circle in Java. Are there any simple tools to do it? Or may be the easiest way is to use an image created by some other software?

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• How to parse out html links from a huge string with html links and other text (Java).

  - by Robert

  Hello, my question is how would i be able to go through a string and take out only the links and erase all the rest? I thought about using some type of delemiter, but wouldnt know how to go about using it in java. an example of what i am trying to do: this is my String: String myString = "The file is http: // www. .com/hello.txt and the second file is " + "http: // www. .com/hello2.dat"; I would want the output to be: "http: // www. .com/hello.txt http: // www. .com/hello2.dat" or each could be added to an array, separately. I just want some ideas, id like to write the code myself but am having trouble on how to do it. Any help would be awesome.

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• Running Java Server Application 24/7 (NOT on local machine) ...?

  - by Steven

  I am currently experimenting with Java Socket Programming and succeeded in creating a simple application that allows me to send data back and forth between a client and a server. I don't want to power my laptop (on which the server application runs) 24/7. Therefore, of course, no computer (that runs my client application) will be able to communicate with my selfmade server application as long as my laptop is turned off. Now I'd like to know: (1) Is it possible to run the server application on a remote (physical) server, so that I don't need my laptop for that purpose? (2) If yes, do you have any suggestions where I could rent such a remote server? As I said, I'm just curious about how all that stuff works. I don't really something expensive.

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• Need help in translating code from C or C++ to Java.

  - by Vuntic

  From this article. Here's the code: float InvSqrt(float x){ // line 0 float xhalf = 0.5f * x; int i = *(int*)&x; // store floating-point bits in integer i = 0x5f3759d5 - (i >> 1); // initial guess for Newton's method x = *(float*)&i; // convert new bits into float x = x*(1.5f - xhalf*x*x); // One round of Newton's method return x; } ...I can't even tell if that's C or C++. Could someone translate it to Java for me, please? It's (only, I hope) lines 2 and 4 that are confusing me. I'll edit out the confusion about C/C++ from the tags once somebody tells me which it is.

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• Is it possible to pass arithmatic operators to a method in java?

  - by user349611

  Right now I'm going to have to write a method that looks like this: public String Calculate(String Operator, Double Operand1, Double Operand2) { if (Operator.equals("+")) { return String.valueOf(Operand1 + Operand2); } else if (Operator.equals("-")) { return String.valueOf(Operand1 - Operand2); } else if (Operator.equals("*")) { return String.valueOf(Operand1 * Operand2); } else { return "error..."; } } It would be nice if I could write the code more like this: public String Calculate(String Operator, Double Operand1, Double Operand2) { return String.valueOf(Operand1 Operator Operand2); } So Operator would replace the Arithmetic Operators (+, -, *, /...) Does anyone know if something like this is possible in java?

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• Generic type parameter naming convention for Java (with multiple chars)?

  - by chaper29

  In some interfaces i wrote I'd like to name generic type parameter with more than one character to make the code more readable. Something like.... Map<Key,Value> Instead of this... Map<K,V> But when it comes to methods, the type-parameters look like java-classes which is also confusing. public void put(Key key, Value value) This seems like Key and Value are classes. I found or thought of some notations, but nothing like a convention from sun or a general best-practice. Alternatives i guesed of or found... Map<KEY,VALUE> Map<TKey,TValue>

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• How can I represent URL (possibly including query string) as a filename in Java without obscuring the original URL?

  - by jerluc

  Is there any real way to represent a URL (which more than likely will also have a query string) as a filename in Java without obscuring the original URL completely? My first approach was to simply escape invalid characters with arbitrary replacements (for example, replacing "/" with "_", etc). The problem is, as in the example of replacing with underscores is that a URL such as "app/my_app" would become "app_my_app" thus obscuring the original URL completely. I have also attempted to encode all the special characters, however again, seeing crazy %3e %20 etc is really not clear. Thank you for any suggestions.

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• How to validate phone number(US format) in Java?

  - by Maxood

  I just want to know where am i wrong here: import java.io.*; class Tokens{ public static void main(String[] args) { //String[] result = "this is a test".split(""); String[] result = "4543 6546 6556".split(""); boolean flag= true; String num[] = {"0","1","2","3","4","5","6","7","8","9"}; String specialChars[] = {"-","@","#","*"," "}; for (int x=1; x<result.length; x++) { for (int y=0; y<num.length; y++) { if ((result[x].equals(num[y]))) { flag = false; continue; } else { flag = true; } if (flag == true) break; } if (flag == false) break; } System.out.println(flag); } }

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• What is the time complexity of LinkedList.getLast() in Java?

  - by i.

  I have a private LinkedList in a Java class & will frequently need to retrieve the last element in the list. The lists need to scale, so I'm trying to decide whether I need to keep a reference to the last element when I make changes (to achieve O(1)) or if the LinkedList class does that already with the getLast() call. What is the big-O cost of LinkedList.getLast() and is it documented? (i.e. can I rely on this answer or should I make no assumptions & cache it even if it's O(1)?)

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• What are some good practice assignments for learning Java?

  - by HW

  Hello, I am a computer science in my Student Second year. I already know a good deal about C++, Data Structures, File Structures, OOP, etc. I decided to learn Java. I have read couple of books but I know that it takes practice to master any Programming language. I was wondering if anyone knew of some assignments or problems that helped them become good at programming. I am looking for something more challenging than "hello world"s and "3+2=5"s exercises. Thanks, ~HW

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• Is it possible in Java to implement something similar to Object.clone() ?

  - by devoured elysium

  The Object.clone() method in Java is pretty special, as instead of returning a copy of the object that is to be cloned with the Object type, it returns the correct Object type. This can be better described with the following code: class A extends Object { public A clone() { return super.clone(); //although Object.clone() is from the Object class //this super.clone() will return an A object! } } class B extends A { } public static void main(String[] args) { B = new B(); B.clone();//here, although we haven't defined a clone() method, the cloned //object return is of type B! } So, could anyone explain if possible if is there anyway to replicate what happens inside Object.clone()'s method?

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• How expensive is a call to java.util.HashMap.keySet()?

  - by fx42

  I implemented a sparse matrix as List<Map<Integer,Double>>. To get all entries of row i I call list.get(i).keySet(). How expensive is this call? I also used the trove library for an alternative implementation as List<TIntDoubleHashMap>. What's the cost of calling list.get(i).keys(), here? Do you have any further ideas of how to implement an efficient sparse matrix? Or can you provide a list of existing implementations in java?

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• What is the memoy size of a Java object array after it has been created?

  - by brenns10

  This probably doesn't even need asking, but I want to make sure I'm right on this. When you create an array of any object in Java like so: Object[] objArr = new Object[10]; The variable objArr is located in stack memory, and it points to a location in the heap where the array object is located. The size of that array in the heap is equal to a 12 byte object header + 4 (or 8, depending on the reference size) bytes * the number of entries in the array. Is this accurate? My question, then, is as follows. Since the array above is empty, does it take up 12 + 4*10 = 52 bytes of memory in the heap immediately after the execution of that line of code? Or does the JVM wait until you start putting things into the array before it instantiates it? Do the null references in the array take up space?

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• Is it easier to create a chat client using java or PHP?

  - by Mendy

  first off, my goal: Create a chat client that requires the user to log in with a password. Would it be easier to create it with just HTML an PHP or java? EDIT: Sorry, I didn't mention. I have some experience in javascript. I'm afraid that is all. I could and would learn another language. So I want to know which language would be better to learn for this project? also, for the username and password login I'm guessing I would need some mySQL knowledge? Thanks.

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