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  • Help to understand the abstract factory pattern

    - by Chobeat
    I'm learning the 23 design patterns of the GoF. I think I've found a way to understand and simplify how the Abstract Factory works but I would like to know if this is a correct assumption or if I am wrong. What I want to know is if we can see the result of the Abstract Factory method as a matrix of possible products where there's a Product for every "Concrete Factory" x "AbstractProduct" where the Concrete Factory is a single implementation among the implementations of an AbstractFactory and an AbstractProduct is an interface among the interfaces to create Products. Is this correct or am I missing something?

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  • Moving AI in a multiplayer game

    - by Smallbro
    I've been programming a multiplayer game and its coming together very nicely. It uses both TCP and UDP (UDP for movement and TCP for just about everything else). What I was wondering was how I would go about sending multiple moving AI without much lag. At first I used TCP for everything and it was very slow when people moved. I'm currently using a butchered version of this http://corvstudios.com/tutorials/udpMultiplayer.php for my movement system and I'm wondering what the best method of sending AI movements is. By movements I mean the AI chooses left/right/up/down and the player can see this happening. Thanks.

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  • Evolution has no access to couchdb

    - by berkes
    Evolution gives the error "Cannot open addressbook". "We were unable to open this addressbook. This either means you have entered an incorrect URI, or the server is unreachable". "Details: Operation not permitted". (rough translation from Dutch). Enabling verbose logging in (desktop)couchdb tells me roughly the same: [info] [<0.7875.1>] 127.0.0.1 - - 'PUT' /contacts/ 400 [debug] [<0.7875.1>] httpd 400 error response: {"error":"invalid_consumer","reason":"Invalid consumer (key or signature method)."} It seems that evolution tries to fetch the contacts, then couchdb denies access, and then evolution fails to do a proper oauth. This is on Ubuntu 10.10, with its default dektopcouch 1.0.1. Any hints on where to start would be most appreciated :)

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  • Collision Error

    - by Manji
    I am having trouble with collision detection part of the game. I am using touch events to fire the gun as you will see in the video. Note, the android icon is a temporary graphic for the bullets When ever the user touches (represented by clicks in the video)the bullet appears and kills random sprites. As you can see it never touches the sprites it kills or kill the sprites it does touch. My Question is How do I fix it, so that the sprite dies when the bullet hits it? Collision Code snippet: //Handles Collision private void CheckCollisions(){ synchronized(mSurfaceHolder){ for (int i = sprites.size() - 1; i >= 0; i--){ Sprite sprite = sprites.get(i); if(sprite.isCollision(bullet)){ sprites.remove(sprite); mScore++; if(sprites.size() == 0){ mLevel = mLevel +1; currentLevel++; initLevel(); } break; } } } } Sprite Class Code Snippet: //bounding box left<right and top>bottom int left ; int right ; int top ; int bottom ; public boolean isCollision(Beam other) { // TODO Auto-generated method stub if(this.left>other.right || other.left<other.right)return false; if(this.bottom>other.top || other.bottom<other.top)return false; return true; } EDIT 1: Sprite Class: public class Sprite { // direction = 0 up, 1 left, 2 down, 3 right, // animation = 3 back, 1 left, 0 front, 2 right int[] DIRECTION_TO_ANIMATION_MAP = { 3, 1, 0, 2 }; private static final int BMP_ROWS = 4; private static final int BMP_COLUMNS = 3; private static final int MAX_SPEED = 5; private HitmanView gameView; private Bitmap bmp; private int x; private int y; private int xSpeed; private int ySpeed; private int currentFrame = 0; private int width; private int height; //bounding box left<right and top>bottom int left ; int right ; int top ; int bottom ; public Sprite(HitmanView gameView, Bitmap bmp) { this.width = bmp.getWidth() / BMP_COLUMNS; this.height = bmp.getHeight() / BMP_ROWS; this.gameView = gameView; this.bmp = bmp; Random rnd = new Random(); x = rnd.nextInt(gameView.getWidth() - width); y = rnd.nextInt(gameView.getHeight() - height); xSpeed = rnd.nextInt(MAX_SPEED * 2) - MAX_SPEED; ySpeed = rnd.nextInt(MAX_SPEED * 2) - MAX_SPEED; } private void update() { if (x >= gameView.getWidth() - width - xSpeed || x + xSpeed <= 0) { xSpeed = -xSpeed; } x = x + xSpeed; if (y >= gameView.getHeight() - height - ySpeed || y + ySpeed <= 0) { ySpeed = -ySpeed; } y = y + ySpeed; currentFrame = ++currentFrame % BMP_COLUMNS; } public void onDraw(Canvas canvas) { update(); int srcX = currentFrame * width; int srcY = getAnimationRow() * height; Rect src = new Rect(srcX, srcY, srcX + width, srcY + height); Rect dst = new Rect(x, y, x + width, y + height); canvas.drawBitmap(bmp, src, dst, null); } private int getAnimationRow() { double dirDouble = (Math.atan2(xSpeed, ySpeed) / (Math.PI / 2) + 2); int direction = (int) Math.round(dirDouble) % BMP_ROWS; return DIRECTION_TO_ANIMATION_MAP[direction]; } public boolean isCollision(float x2, float y2){ return x2 > x && x2 < x + width && y2 > y && y2 < y + height; } public boolean isCollision(Beam other) { // TODO Auto-generated method stub if(this.left>other.right || other.left<other.right)return false; if(this.bottom>other.top || other.bottom<other.top)return false; return true; } } Bullet Class: public class Bullet { int mX; int mY; private Bitmap mBitmap; //bounding box left<right and top>bottom int left ; int right ; int top ; int bottom ; public Bullet (Bitmap mBitmap){ this.mBitmap = mBitmap; } public void draw(Canvas canvas, int mX, int mY) { this.mX = mX; this.mY = mY; canvas.drawBitmap(mBitmap, mX, mY, null); } }

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  • How can I gain access to a player instance in a Minecraft mod?

    - by Andrew Graber
    I'm creating Minecraft mod with a pickaxe that takes away experience when you break a block. The method for taking away experience from a player is addExperience on EntityPlayer, so I need to get an instance of EntityPlayer for the player using my pickaxe when the pickaxe breaks a block, so that I can remove the appropriate amount of experience. My pickaxe class currently looks like this: public class ExperiencePickaxe extends ItemPickaxe { public ExperiencePickaxe(int ItemID, EnumToolMaterial material){ super(ItemID, material); } public boolean onBlockDestroyed(ItemStack par1ItemStack, World par2World, int par3, int par4, int par5, int par6, EntityLiving par7EntityLiving) { if ((double)Block.blocksList[par3].getBlockHardness(par2World, par4, par5, par6) != 0.0D) { EntityPlayer e = new EntityPlayer(); // create an instance e.addExperience(-1); } return true; } } Obviously, I cannot actually create a new EntityPlayer since it is an abstract class. How can I get access to the player using my pickaxe?

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  • Using Mock for event listeners in unit-testing

    - by phtrivier
    I keep getting to test this kind of code (language irrelevant) : public class Foo() { public Foo(Dependency1 dep1) { this.dep1 = dep1; } public void setUpListeners() { this.dep1.addSomeEventListener(.... some listener code ...); } } Typically, you want to test what when the dependency fires the event, the class under tests reacts appropriately (in some situation, the only purpose of such classes is to wire lots of other components, that can be independently tested. So far, to test this, I always end up doing something like : creating a 'stub' that implements both a addXXXXListener, that simply stores the callback, and a fireXXXX, that simply calls any registered listener. This is a bit tedious since you have to create the mock with the right interface, but that can do use an introspective framework that can 'spy' on a method, and inject the real dependency in tests Is there a cleaner way to do this kind of things ?

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  • Alternative way of developing for ASP.NET to WebForms - Any problems with this?

    - by John
    So I have been developing in ASP.NET WebForms for some time now but often get annoyed with all the overhead (like ViewState and all the JavaScript it generates), and the way WebForms takes over a lot of the HTML generation. Sometimes I just want full control over the markup and produce efficient HTML of my own so I have been experimenting with what I like to call HtmlForms. Essentially this is using ASP.NET WebForms but without the form runat="server" tag. Without this tag, ASP.NET does not seem to add anything to the page at all. From some basic tests it seems that it runs well and you still have the ability to use code-behind pages, and many ASP.NET controls such as repeaters. Of course without the form runat="server" many controls won't work. A post at Enterprise Software Development lists the controls that do require the tag. From that list you will see that all of the form elements like TextBoxes, DropDownLists, RadioButtons, etc cannot be used. Instead you use normal HTML form controls. But how do you access these HTML controls from the code behind? Retrieving values on post back is easy, you just use Request.QueryString or Request.Form. But passing data to the control could be a little messy. Do you use a ASP.NET Literal control in the value field or do you use <%= value % in the markup page? I found it best to add runat="server" to my HTML controls and then you can access the control in your code-behind like this: ((HtmlInputText)txtName).Value = "blah"; Here's a example that shows what you can do with a textbox and drop down list: Default.aspx <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="NoForm.Default" %> <%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs" Inherits="NoForm.Default" %> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN""http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head runat="server"> <title></title> </head> <body> <form action="" method="post"> <label for="txtName">Name:</label> <input id="txtName" name="txtName" runat="server" /><br /> <label for="ddlState">State:</label> <select id="ddlState" name="ddlState" runat="server"> <option value=""></option> </select><br /> <input type="submit" value="Submit" /> </form> </body> </html> Default.aspx.cs using System; using System.Web.UI.HtmlControls; using System.Web.UI.WebControls; namespace NoForm { public partial class Default : System.Web.UI.Page { protected void Page_Load(object sender, EventArgs e) { //Default values string name = string.Empty; string state = string.Empty; if (Request.RequestType == "POST") { //If form submitted (post back) name = Request.Form["txtName"]; state = Request.Form["ddlState"]; //Server side form validation would go here //and actions to process form and redirect } ((HtmlInputText)txtName).Value = name; ((HtmlSelect)ddlState).Items.Add(new ListItem("ACT")); ((HtmlSelect)ddlState).Items.Add(new ListItem("NSW")); ((HtmlSelect)ddlState).Items.Add(new ListItem("NT")); ((HtmlSelect)ddlState).Items.Add(new ListItem("QLD")); ((HtmlSelect)ddlState).Items.Add(new ListItem("SA")); ((HtmlSelect)ddlState).Items.Add(new ListItem("TAS")); ((HtmlSelect)ddlState).Items.Add(new ListItem("VIC")); ((HtmlSelect)ddlState).Items.Add(new ListItem("WA")); if (((HtmlSelect)ddlState).Items.FindByValue(state) != null) ((HtmlSelect)ddlState).Value = state; } } } As you can see, you have similar functionality to ASP.NET server controls but more control over the final markup, and less overhead like ViewState and all the JavaScript ASP.NET adds. Interestingly you can also use HttpPostedFile to handle file uploads using your own input type="file" control (and necessary form enctype="multipart/form-data"). So my question is can you see any problems with this method, and any thoughts on it's usefulness? I have further details and tests on my blog.

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  • Custom extensible file format for 2d tiled maps

    - by Christian Ivicevic
    I have implemented much of my game logic right now, but still create my maps with nasty for-loops on-the-fly to be able to work with something. Now I wanted to move on and to do some research on how to (un)serialize this data. (I do not search for a map editor - I am speaking of the map file itself) For now I am looking for suggestions and resources, how to implement a custom file format for my maps which should provide the following functionality (based on MoSCoW method): Must have Extensibility and backward compatibility Handling of different layers Metadata on whether a tile is solid or can be passed through Special serialization of entities/triggers with associated properties/metadata Could have Some kind of inclusion of the tileset to prevent having scattered files/tilesets I am developing with C++ (using SDL) and targetting only Windows. Any useful help, tips, suggestions, ... would be appreciated!

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  • Creating widgets using GtkBuilder

    - by user72943
    I am using Glade to design a Box with widgets and then load these widgets into my UI at run-time. To create each Box with widgets at run-time, I create a new GtkBuilder, call add_from_string passing in the text from the .ui file Glade creates, and then use the object returned from get_object("box1") in the UI. I know I could create the widgets with code, but for now, I'd like to use the .ui files Glade creates. It seems inefficient though to instantiate a new GtkBuilder object and the wasted Window object for every Box I want to create. Is there a more efficient method to load .ui files without creating a new GtkWidget object and wasted Window object? Thanks, Vance

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  • What are the basic skills a beginner JavaScript programmer should have?

    - by Sanford
    In NYC, we are working on creating a collaborative community programming environment and trying to segment out software engineers into differing buckets. At present, we are trying to define: Beginners Intermediates Advanced Experts (and/or Masters) Similar to an apprenticeship, you would need to demonstrate specific skills to achieve different levels. Right now, we have identified beginner programming skills as: Object - method, attributes, inheritance Variable - math, string, array, boolean - all are objects Basic arithmetic functions - precedence of functions String manipulation Looping - flow control Conditionals - boolean algebra This is a first attempt, and it is a challenge since we know the natural tension between programming and software engineering. How would you create such a skills-based ranking for JavaScript in this manner? For example, what would be the beginner JavaScript skills that you would need to have to advance to the intermediate training? And so on.

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  • How is the terrain generated in Commandos and Commandos game clones/look-alikes?

    - by teodron
    The Commandos series of games and its similar western counterpart, Desperados, use a mix of 2D and 3D elements to achieve a very pleasing and immersive atmosphere. Apart from the concept that alone made the series a best-seller, the graphics eye-candy was also a much appreciated asset of that game. I was very curious on what was the technique used to model and adorn the realistic terrains in those titles? Below are some screenshots that could be relevant as a reference for whomever has a candidate answer: The tiny details and patternless distribution of ornamental textures make me think that these terrains were not generated using a standard heightmap-blendmap method.

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  • Using another domain with Google App Engine

    - by gsingh2011
    I'm trying to change my google app engine domain (domain.appspot.com) to the domain I bought from 1&1.com (mydomain.com). I went into the google app engine settings and added the domain. After making a Google Apps account, I was asked to verify my domain. The directions say that 1&1 doesn't allow me to create TXT records, so I can't use that method for verification. Their alternative is to upload an HTML file to my server, but I didn't buy hosting with my domain, I just bought the domain. My files are on domain.appspot.com. How can I make mydomain.com point to domain.appspot.com? I've added the ns1.googleghs.com as my nameservers in my 1&1 DNS settings, but I still can't verify my domain with Google Apps.

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  • Designing an API on top with Java RMI and Rest APIs

    - by user1303881
    I'm working on the backend of a java web application. We have a document repository (Fedora Commons specifically) where we house xml files. I want to abstract the API of the repository internally so that we aren't tightly coupled to one product. I'd also like to give the flexibility of connecting to to a repository via Java RMI or REST APIs. I was hoping to get advice or resources on how to implement something like this. My thought it that I'd have some abstract repository class that had methods like getRecord, updateRecord, and deleteRecord. In the constructor I would pass the URI for the repository and the API method and port. This would allow some flexibility in the future so that if the REST api became more practical, but allow the flexibility or using RMI which could (should?) have better performance. Am I over thinking this or am I on the right path?

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  • Area of testing

    - by ?????? ??????????
    I'm trying to understand which part of my code I should to test. I have some code. Below is example of this code, just to understand the idea. Depends of some parametrs I put one or another currency to "Event" and return his serialization in the controller. Which part of code I should to test? Just the final serialization, or only "Event" or every method: getJson, getRows, fillCurrency, setCurrency? class Controller { public function getJson() { $rows = $eventManager->getRows(); return new JsonResponse($rows); } } class EventManager { public function getRows() { //some code here if ($parameter == true) { $this->fillCurrency($event, $currency); } } public function fillCurrency($event, $currency) { //some code here if ($parameters == true) { $event->setCurrency($currency); } } } class Event { public function setCurrency($currency) { $this->updatedAt = new Datetime(); $this->currency = $currency; } }

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  • WWW.yoursite.com or HTTP://yoursite.com which one is futureproof?

    - by Sam
    http://yoursite.com www.yoursite.com http://www.yoursite.com yoursite.com Which of these would you choose as your favourite to work with, if you were to make a site for 2011 and beyond, which domainname would you provide to clients, websites linking to you, your letterhead, contact cards. Why one OR other? Which to avoid? Thinking of the following aspects: validity, correctly loading URL audience, most geeks know http://, most seniors/clients don't easiest to remember / URL as a brand misspellings by user input (in mobile phone or desktop browser) browsers not understanding protocol-less links total length of chars for easy user input method of peferance by major search engines/social media sites consistency sothat links dont fragment but all point to the same

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  • setPosition of Sprite onUpdate in AndEngine

    - by SSH This
    I am trying to get a "highlighter" circle to follow around a sprite, but I am having trouble, I thought I could use the onUpdate method that's available to me in SequenceEntityModifier but it's not working for me. Here is my code: // make sequence mod with move modifier SequenceEntityModifier modifier = new SequenceEntityModifier(myMovemod) { @Override protected void onModifierFinished(IEntity pItem) { // animation finished super.onModifierFinished(pItem); } public float onUpdate(float pSecondsElapsed, IEntity pItem) { highlighter.setPosition(player2.getX() - highlighterOffset, player2.getY() - highlighterOffset); return pSecondsElapsed; } }; When onUpdate is completely commented out, the sprite moves like I want it to, everything is ok. When I put the onUpdate in, the sprite doesn't move at all. I have a feeling that I am overriding the original onUpdate's actions? Am I going about this the wrong way? I am new to Java, so please feel free to advise if this isn't going to work. UPDATE: The player2 is the sprite that I'm trying to get the highlighter to follow.

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  • EJB Lifecycle and Relation to WARs

    - by Adam Tannon
    I've been reading up on EJBs (3.x) and believe I understand the basics. This question is a "call for confirmation" that I have interpreted the Java EE docs correctly and that I understand these fundamental concepts: An EJB is to an App Container as a Web App (WAR) is to a Web Container Just like you deploy a WAR to a Web Container, and that container manages your WAR's life cycle, you deploy an EJB to an App Container, and the container manages your EJB's life cycle When the App Container fires up and deploys an EJB, it is given a unique "identifier" and URL that can be used by JNDI to look up the EJB from another tier (like the web tier) So, when your web app wants to invoke one of your EJB's methods, it looks the EJB up using some kind of service locator (JNDI) and invoke the method that way Am I on-track or way off-base here? Please correct me & clarify for me if any of these are incorrect. Thanks in advance!

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  • Community to discuss project ideas

    - by Auxiliary
    Although I already predict the down votes but the question has stuck in my throat for a while now. I think this has happened to many of us. Sometimes we find a great idea for a project and obviously think this is THE GREATEST idea ever but then one of the following things will happen: The project is a small one, so you might actually give it a try and see how it goes. The project is a big one, even a risk, and you just need a good programmer's community that you could just discuss your idea with them and see what they say and even get some help to make it happen. And there's always the possibility of others stealing your idea which is really bad. So could anyone suggest an online community or place or even method of talking about ideas and the ways of developing them? and do you think it's a good thing to tell others about your idea?

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  • constructor should not call methods

    - by Stefano Borini
    I described to a colleague why a constructor calling a method is an antipattern. example (in my rusty C++) class C { public : C(int foo); void setFoo(int foo); private: int foo; } C::C(int foo) { setFoo(foo); } void C::setFoo(int foo) { this->foo = foo } I would like to motivate better this fact through your additional contribute. If you have examples, book references, blog pages, or names of principles, they would be very welcome. Edit: I'm talking in general, but we are coding in python.

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  • C# replacing out parameters with struct

    - by Jonathan
    I'm encountering a lot of methods in my project that have a bunch of out parameters embedded in them and its making it cumbersome to call the methods as I have to start declaring the variables before calling the methods. As such, I would like to refactor the code to return a struct instead and was wondering if this is a good idea. One of the examples from an interface: void CalculateFinancialReturnTotals(FinancialReturn fr, out decimal expenses, out decimal revenue, out decimal levyA, out decimal levyB, out decimal profit, out decimal turnover, out string message) and if I was to refactor that, i would be putting all the out parameters in the struct such that the method signature is much simpler as below. [structName] CalculateFinancialReturnTotals(FinancialReturn fr); Please advise.

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  • Cant install wine1.5 13.04

    - by Drew S
    So I tried to install wine 1.5 a few times, I got 1.4 installed, and 1.6 installed oddly enough, I completely removed and purged all wine and still nothing. I installed ia32-lib and still nothing, tried installing from synaptic, ubuntu software center, and apt-get method. I get this error from apt-get in the terminal Reading package lists... Done Some packages could not be installed. This may mean that you have requested an impossible situation or if you are using the unstable distribution that some required packages have not yet been created or been moved out of Incoming. The following information may help resolve the situation: The following packages have unmet dependencies: wine1.5 : Depends: wine1.6 but it is not going to be installed E: Unable to correct problems, you have held broken packages. The particular windows program I want to install is confirmed working in 1.5(have it working on laptop)

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  • Clarification of "avoid if-else" advice [duplicate]

    - by deviDave
    This question already has an answer here: Elegant ways to handle if(if else) else 21 answers The experts in clean code advise not to use if/else since it's creating an unreadable code. They suggest rather using IF and not to wait till the end of a method without real need. Now, this if/else advice confuses me. Do they say that I should not use if/else at all (!) or to avoid if/else nesting? Also, if they refer to the nesting of if/else, should I not do even a single nesting or I should limit it to max 2 nestings (as some recommends)? When I say single nesting, I mean this: if (...) { if (...) { } else { } } else { } EDIT Also tools like Resharper will suggest reformatting if/else statements. It usually concerts them to if stand-alone statement, and sometimes even to ternary expression.

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  • How do I draw video frames onto the screen permanently using XNA?

    - by izb
    I have an app that plays back a video and draws the video onto the screen at a moving position. When I run the app, the video moves around the screen as it plays. Here is my Draw method... protected override void Draw(GameTime gameTime) { Texture2D videoTexture = null; if (player.State != MediaState.Stopped) videoTexture = player.GetTexture(); if (videoTexture != null) { spriteBatch.Begin(); spriteBatch.Draw( videoTexture, new Rectangle(x++, 0, 400, 300), /* Where X is a class member */ Color.White); spriteBatch.End(); } base.Draw(gameTime); } The video moves horizontally acros the screen. This is not exactly as I expected since I have no lines of code that clear the screen. My question is why does it not leave a trail behind? Also, how would I make it leave a trail behind?

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  • Any enlightenment for understanding Object Oriented Programming? [closed]

    - by ????
    I studied computer science near the end of 1980s, and wasn't taught OOP that formally. With Pascal or C, when I understand the top-down design of functions, and the idea of black box, then everything just seem to make sense, as if there is a "oh I get it!" -- some kind of totally getting it and enlightenment feeling. But with OOP, all I know was the mechanics: the class, instance, method, inheritance, polymorphism, encapsulation. It was like, I knew all the "this is how it is", but never had the feeling of "I totally get it", the enlightened feeling. Would somebody be able to describe it, or point to a chapter in some book or paper which talks about OOP so that the reader can feel: "I totally get it!" on OOP?

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  • Computer Networks UNISA - Chap 8 &ndash; Wireless Networking

    - by MarkPearl
    After reading this section you should be able to Explain how nodes exchange wireless signals Identify potential obstacles to successful transmission and their repercussions, such as interference and reflection Understand WLAN architecture Specify the characteristics of popular WLAN transmission methods including 802.11 a/b/g/n Install and configure wireless access points and their clients Describe wireless MAN and WAN technologies, including 802.16 and satellite communications The Wireless Spectrum All wireless signals are carried through the air by electromagnetic waves. The wireless spectrum is a continuum of the electromagnetic waves used for data and voice communication. The wireless spectrum falls between 9KHZ and 300 GHZ. Characteristics of Wireless Transmission Antennas Each type of wireless service requires an antenna specifically designed for that service. The service’s specification determine the antenna’s power output, frequency, and radiation pattern. A directional antenna issues wireless signals along a single direction. An omnidirectional antenna issues and receives wireless signals with equal strength and clarity in all directions The geographical area that an antenna or wireless system can reach is known as its range Signal Propagation LOS (line of sight) uses the least amount of energy and results in the reception of the clearest possible signal. When there is an obstacle in the way, the signal may… pass through the object or be obsrobed by the object or may be subject to reflection, diffraction or scattering. Reflection – waves encounter an object and bounces off it. Diffraction – signal splits into secondary waves when it encounters an obstruction Scattering – is the diffusion or the reflection in multiple different directions of a signal Signal Degradation Fading occurs as a signal hits various objects. Because of fading, the strength of the signal that reaches the receiver is lower than the transmitted signal strength. The further a signal moves from its source, the weaker it gets (this is called attenuation) Signals are also affected by noise – the electromagnetic interference) Interference can distort and weaken a wireless signal in the same way that noise distorts and weakens a wired signal. Frequency Ranges Older wireless devices used the 2.4 GHZ band to send and receive signals. This had 11 communication channels that are unlicensed. Newer wireless devices can also use the 5 GHZ band which has 24 unlicensed bands Narrowband, Broadband, and Spread Spectrum Signals Narrowband – a transmitter concentrates the signal energy at a single frequency or in a very small range of frequencies Broadband – uses a relatively wide band of the wireless spectrum and offers higher throughputs than narrowband technologies The use of multiple frequencies to transmit a signal is known as spread-spectrum technology. In other words a signal never stays continuously within one frequency range during its transmission. One specific implementation of spread spectrum is FHSS (frequency hoping spread spectrum). Another type is known as DSS (direct sequence spread spectrum) Fixed vs. Mobile Each type of wireless communication falls into one of two categories Fixed – the location of the transmitted and receiver do not move (results in energy saved because weaker signal strength is possible with directional antennas) Mobile – the location can change WLAN (Wireless LAN) Architecture There are two main types of arrangements Adhoc – data is sent directly between devices – good for small local devices Infrastructure mode – a wireless access point is placed centrally, that all devices connect with 802.11 WLANs The most popular wireless standards used on contemporary LANs are those developed by IEEE’s 802.11 committee. Over the years several distinct standards related to wireless networking have been released. Four of the best known standards are also referred to as Wi-Fi. They are…. 802.11b 802.11a 802.11g 802.11n These four standards share many characteristics. i.e. All 4 use half duplex signalling Follow the same access method Access Method 802.11 standards specify the use of CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to access a shared medium. Using CSMA/CA before a station begins to send data on an 802.11 network, it checks for existing wireless transmissions. If the source node detects no transmission activity on the network, it waits a brief period of time and then sends its transmission. If the source does detect activity, it waits a brief period of time before checking again. The destination node receives the transmission and, after verifying its accuracy, issues an acknowledgement (ACT) packet to the source. If the source receives the ACK it assumes the transmission was successful, – if it does not receive an ACK it assumes the transmission failed and sends it again. Association Two types of scanning… Active – station transmits a special frame, known as a prove, on all available channels within its frequency range. When an access point finds the probe frame, it issues a probe response. Passive – wireless station listens on all channels within its frequency range for a special signal, known as a beacon frame, issued from an access point – the beacon frame contains information necessary to connect to the point. Re-association occurs when a mobile user moves out of one access point’s range and into the range of another. Frames Read page 378 – 381 about frames and specific 802.11 protocols Bluetooth Networks Sony Ericson originally invented the Bluetooth technology in the early 1990s. In 1998 other manufacturers joined Ericsson in the Special Interest Group (SIG) whose aim was to refine and standardize the technology. Bluetooth was designed to be used on small networks composed of personal communications devices. It has become popular wireless technology for communicating among cellular telephones, phone headsets, etc. Wireless WANs and Internet Access Refer to pages 396 – 402 of the textbook for details.

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