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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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• 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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• 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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• 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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• collsion issues with quadtree [on hold]

  - by QuantumGamer

  So i implemented a Quad tree in Java for my 2D game and everything works fine except for when i run my collision detection algorithm, which checks if a object has hit another object and which side it hit.My problem is 80% of the time the collision algorithm works but sometimes the objects just go through each other. Here is my method: private void checkBulletCollision(ArrayList object) { quad.clear(); // quad is the quadtree object for(int i=0; i < object.size();i++){ if(object.get(i).getId() == ObjectId.Bullet) // inserts the object into quadtree quad.insert((Bullet)object.get(i)); } ArrayList<GameObject> returnObjects = new ArrayList<>(); // Uses Quadtree to determine to calculate how many // other bullets it can collide with for(int i=0; i < object.size(); i++){ returnObjects.clear(); if(object.get(i).getId() == ObjectId.Bullet){ quad.retrieve(returnObjects, object.get(i).getBoundsAll()); for(int k=0; k < returnObjects.size(); k++){ Bullet bullet = (Bullet) returnObjects.get(k); if(getBoundsTop().intersects(bullet.getBoundsBottom())){ vy = speed; bullet.vy = -speed; } if(getBoundsBottom().intersects(bullet.getBoundsTop())){ vy = -speed; bullet.vy = speed; } if(getBoundsLeft().intersects(bullet.getBoundsRight())){ vx =speed; bullet.vx = -speed; } if(getBoundsRight().intersects(bullet.getBoundsLeft())){ vx = -speed; bullet.vx = speed; } } } } } Any help would be appreciated. Thanks in advance.

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• Physics in my game confused after restructuring the Game loop

  - by Julian Assange

  Hello! I'm on my way with making a game in Java. Now I have some trouble with an interpolation based game loop in my calculations. Before I used that system the calculation of a falling object was like this: Delta based system private static final float SPEED_OF_GRAVITY = 500.0f; @Override public void update(float timeDeltaSeconds, Object parentObject) { parentObject.y = parentObject.y + (parentObject.yVelocity * timeDeltaSeconds); parentObject.yVelocity -= SPEED_OF_GRAVITY * timeDeltaSeconds; ...... What you see here is that I used that delta value from previous frame to the current frame to calculate the physics. Now I switched and implement a interpolation based system and I actually left the current system where I used delta to calculate my physics. However, with the interpolation system the delta time is removed - but now are my calculations screwed up and I've tried the whole day to solve this: Interpolation based system private static final float SPEED_OF_GRAVITY = 500.0f; @Override public void update(Object parentObject) { parentObject.y = parentObject.y + (parentObject.yVelocity); parentObject.yVelocity -= SPEED_OF_GRAVITY; ...... I'm totally clueless - how should this be solved? The rendering part is solved with a simple prediction method. With the delta system I could see my object be smoothly rendered to the screen, but with this interpolation/prediction method the object just appear sticky for one second and then it's gone. The core of this game loop is actually from here deWiTTERS Game Loop, where I trying to implement the last solution he describes. Shortly - my physics are in a mess and this need to be solved. Any ideas? Thanks in advance!

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• How to implement RLE into a tilemap?

  - by Smallbro

  Currently I've been using a 3D array for my tiles in a 2D world but the 3D side comes in when moving down into caves and whatnot. Now this is not memory efficient and I switched over to a 2D array and can now have much larger maps. The only issue I'm having now is that it seems that my tiles cannot occupy the same space as a tile on the same z level. My current structure means that each block has its own z variable. This is what it used to look like: map.blockData[x][y][z] = new Block(); however now it works like this map.blockData[x][y] = new Block(z); I'm not sure why but if I decide to use the same space on say the floor below it wont allow me to. Does anyone have any ideas on how I can add a z-axis to my 2D array? I'm using java but I reckon the concept carries across different languages. Edit: As Will posted, RLE sounds like the best method for achieving a fast 3D array. However I'm struggling to understand how I would even start to implement it? Would I create a 4D array the 4th being something which controls how many to skip? Or would the x-axis simply change altogether and have large gaps in between - for example [5][y][z] would skip 5 tiles? Is there something really obvious here which I am missing? The number of z levels I'm trying to have is around 66, it would be preferably that I can have up to or more than 1000 in x and y.

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• 2D grid with multiple types of objects

  - by Alexandre P. Levasseur

  This is my first post here in programmers.stackexchange (I'm a regular on SO). I hope this isn't too general. I'm trying a simple project to learn Java from something I've seen done in the past. Basically, it's an AI simulation where there are herbivorous and carnivorous creatures and both must try to survive. The part I am trying to come up with is that of the board itself. Let's assume very simple rules. The board must be of size X by Y and only one element can be in one place at one time. For example, a critter cannot be in the same tile as a food block. There can be obstacles (rocks, trees..), there can be food, there can be critters of any type. Assuming these rules, what would be one good way to represent this situation ? This is what I came up with and want suggestions if possible: Use multiple levels of inheritance to represent all the different possible objects (AbstractObject - (NonMovingObject - (Food, Obstacle) , MovingObject - Critter - (Carnivorous, Herbivorous))) and use polymorphism in a 2D array to store the instances and still have access to lower level methods. Many thanks. Edit: Here is the graphic representation of the structure I have in mind.

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