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  • null pointers vs. Null Object Pattern

    - by GlenH7
    Attribution: This grew out of a related P.SE question My background is in C / C++, but I have worked a fair amount in Java and am currently coding C#. Because of my C background, checking passed and returned pointers is second-hand, but I acknowledge it biases my point of view. I recently saw mention of the Null Object Pattern where the idea is than an object is always returned. Normal case returns the expected, populated object and the error case returns empty object instead of a null pointer. The premise being that the calling function will always have some sort of object to access and therefore avoid null access memory violations. So what are the pros / cons of a null check versus using the Null Object Pattern? I can see cleaner calling code with the NOP, but I can also see where it would create hidden failures that don't otherwise get raised. I would rather have my application fail hard (aka an exception) while I'm developing it than have a silent mistake escape into the wild. Can't the Null Object Pattern have similar problems as not performing a null check? Many of the objects I have worked with hold objects or containers of their own. It seems like I would have to have a special case to guarantee all of the main object's containers had empty objects of their own. Seems like this could get ugly with multiple layers of nesting.

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  • What is the use of Association, Aggregation and Composition (Encapsulation) in Classes

    - by SahilMahajanMj
    I have gone through lots of theories about what is encapsulation and the three techniques of implementing it, which are Association, Aggregation and Composition. What i found is, Encapsulation Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding. Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface. The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code. Association Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take an example of Teacher and Student. Multiple students can associate with single teacher and single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. Both can create and delete independently. Aggregation Aggregation is a specialize form of Association where all object have their own lifecycle but there is ownership and child object can not belongs to another parent object. Let’s take an example of Department and teacher. A single teacher can not belongs to multiple departments, but if we delete the department teacher object will not destroy. We can think about “has-a” relationship. Composition Composition is again specialize form of Aggregation and we can call this as a “death” relationship. It is a strong type of Aggregation. Child object dose not have their lifecycle and if parent object deletes all child object will also be deleted. Let’s take again an example of relationship between House and rooms. House can contain multiple rooms there is no independent life of room and any room can not belongs to two different house if we delete the house room will automatically delete. The question is: Now these all are real world examples. I am looking for some description about how to use these techniques in actual class code. I mean what is the point for using three different techniques for encapsulation, How these techniques could be implemented and How to choose which technique is applicable at time.

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  • Validation and Error Generation when using the Data Mapper Pattern

    - by AndyPerlitch
    I am working on saving state of an object to a database using the data mapper pattern, but I am looking for suggestions/guidance on the validation and error message generation step (step 4 below). Here are the general steps as I see them for doing this: (1) The data mapper is used to get current info (assoc array) about the object in db: +=====================================================+ | person_id | name | favorite_color | age | +=====================================================+ | 1 | Andy | Green | 24 | +-----------------------------------------------------+ mapper returns associative array, eg. Person_Mapper::getPersonById($id) : $person_row = array( 'person_id' => 1, 'name' => 'Andy', 'favorite_color' => 'Green', 'age' => '24', ); (2) the Person object constructor takes this array as an argument, populating its fields. class Person { protected $person_id; protected $name; protected $favorite_color; protected $age; function __construct(array $person_row) { $this->person_id = $person_row['person_id']; $this->name = $person_row['name']; $this->favorite_color = $person_row['favorite_color']; $this->age = $person_row['age']; } // getters and setters... public function toArray() { return array( 'person_id' => $this->person_id, 'name' => $this->name, 'favorite_color' => $this->favorite_color, 'age' => $this->age, ); } } (3a) (GET request) Inputs of an HTML form that is used to change info about the person is populated using Person::getters <form> <input type="text" name="name" value="<?=$person->getName()?>" /> <input type="text" name="favorite_color" value="<?=$person->getFavColor()?>" /> <input type="text" name="age" value="<?=$person->getAge()?>" /> </form> (3b) (POST request) Person object is altered with the POST data using Person::setters $person->setName($_POST['name']); $person->setFavColor($_POST['favorite_color']); $person->setAge($_POST['age']); *(4) Validation and error message generation on a per-field basis - Should this take place in the person object or the person mapper object? - Should data be validated BEFORE being placed into fields of the person object? (5) Data mapper saves the person object (updates row in the database): $person_mapper->savePerson($person); // the savePerson method uses $person->toArray() // to get data in a more digestible format for the // db gateway used by person_mapper Any guidance, suggestions, criticism, or name-calling would be greatly appreciated.

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  • Object Oriented Design Questions

    - by Robert
    Hello there. I am going to develop a Tic-Tac-Toe game using Java(or maybe other OO Languages).Now I have a picture in my mind about the general design. Interface: Player ,then I will be able to implement a couple of Player classes,based on how I want the opponent to be,for example,random player,intelligent player. Classes: Board class,with a two-dimensional array of integers,0 indicates open,1 indicates me,-1 indicates opponent.The evaluation function will be in here as well,to return the next best move based on the current board arrangement and whose turn it is. Refree class,which will create instance of the Board and two player instances,then get the game begin. This is a rough idea of my OO design,could anybody give me any critiques please,I find this is really beneficial,thank you very much.

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  • Class Design - Returning a List<Object> From <Object>

    - by Mike
    Given a simple class: public class Person { public string FirstName; public string LastName; public string GetFullName() { return FirstName + LastName; } } The user of this class will populate a List<Person> object by reading an Xml file or some other data source. Should the logic for populating the List be in the Person class or should it just remain in the calling class? In other words, should there be a public List<Persons> GetPersons() method in the Person class or in the calling class? Or should the data accessor be in another class altogether? I know this is a rather simplistic question but I'm just curious how others typically do it.

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  • Object.Watch with disabled attribute

    - by Benjamin Fleming
    <html> <head> <script type="text/javascript"> window.onload = function() { var btn = document.getElementById("button"); var tog = document.getElementById("toggle"); tog.onclick = function() { if(btn.disabled) { btn.disabled = false; } else { btn.disabled = true; } }; //btn.watch("disabled", function(prop, val, newval) { }); }; </script> </head> <body> <input type="button" value="Button" id="button" /> <input type="button" value="Toggle" id="toggle" /> </body> </html> If you test this code, the Toggle button will successfully enable and disable the other button. However, un-commenting the btn.watch() line will somehow always set the disabled tag to true. Any ideas?

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  • WPF ComboBox Object Binding - not updating DataContext Object

    - by Jaysen
    Hello, I have the following scenario: 1 class called 'Widget' with the properties: ID, Code, Description 1 class called 'MyWidget' with a property: m_Widget As Widget 1 ComboBox The ComboBox has a List(Of Widget) set as the ItemSource. I create an instance of 'MyWidget' named MyWidget1 and I set the property values of the m_Widget to match one of the items in the 'ComboBox List(Of Widget)'. I then set the DataContext of the ComboBox to MyWidget1.Widget. When I change the ComboBox selected item, only the ID property of 'MyWidget1.Widget' gets updated... How do I get the object 'Widget' on 'MyWidget1' to be updated instead of just 'MyWidget1.Widget.ID'? Here is a link to a sample project demonstrating this scenario: http://www.webpersona.com/ObjectBinding.zip Thanks in advance for any help :)

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  • Object declaration in Objective-C

    - by Sahat
    Is there any difference in declaring objects in Objective-C between (1) and (2), besides the style and personal preference? (1) One-line declaration, allocation, initialization. Student *myStudent = [[Student alloc] init]; (2) Multi-line declaration, allocation, initialization. Student *myStudent; myStudent = [Student alloc]; myStudent = [myStudent init];

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  • copying the request header from request object to urlConnection object

    - by Bunny Rabbit
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // TODO Auto-generated method stub URL url = new URL("http://localhost:8080/testy/Out"); HttpURLConnection connection = (HttpURLConnection) url.openConnection(); connection.setDoOutput(true); connection.setRequestMethod("POST"); PrintWriter out=response.getWriter(); for(Enumeration e=request.getHeaderNames();e.hasMoreElements();){ Object o=e.nextElement(); String value=request.getHeader(o.toString()); out.println(o+"--is--"+value+"<br>"); connection.setRequestProperty((String) o, value); } connection.connect(); } i wrote the above code in a servlet to post form so some alternate locations than this servlet,but its not working.is it okay to use connection.setRequestProperty to set the header fields to what they are in the incoming request to servlet.

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  • Returning new object, overwrite the existing one in Java

    - by lupin
    Note: This is an assignment. Hi, Ok I have this method that will create a supposedly union of 2 sets. i mport java.io.*; class Set { public int numberOfElements; public String[] setElements; public int maxNumberOfElements; // constructor for our Set class public Set(int numberOfE, int setE, int maxNumberOfE) { this.numberOfElements = numberOfE; this.setElements = new String[setE]; this.maxNumberOfElements = maxNumberOfE; } // Helper method to shorten/remove element of array since we're using basic array instead of ArrayList or HashSet from collection interface :( static String[] removeAt(int k, String[] arr) { final int L = arr.length; String[] ret = new String[L - 1]; System.arraycopy(arr, 0, ret, 0, k); System.arraycopy(arr, k + 1, ret, k, L - k - 1); return ret; } int findElement(String element) { int retval = 0; for ( int i = 0; i < setElements.length; i++) { if ( setElements[i] != null && setElements[i].equals(element) ) { return retval = i; } retval = -1; } return retval; } void add(String newValue) { int elem = findElement(newValue); if( numberOfElements < maxNumberOfElements && elem == -1 ) { setElements[numberOfElements] = newValue; numberOfElements++; } } int getLength() { if ( setElements != null ) { return setElements.length; } else { return 0; } } String[] emptySet() { setElements = new String[0]; return setElements; } Boolean isFull() { Boolean True = new Boolean(true); Boolean False = new Boolean(false); if ( setElements.length == maxNumberOfElements ){ return True; } else { return False; } } Boolean isEmpty() { Boolean True = new Boolean(true); Boolean False = new Boolean(false); if ( setElements.length == 0 ) { return True; } else { return False; } } void remove(String newValue) { for ( int i = 0; i < setElements.length; i++) { if ( setElements[i] != null && setElements[i].equals(newValue) ) { setElements = removeAt(i,setElements); } } } int isAMember(String element) { int retval = -1; for ( int i = 0; i < setElements.length; i++ ) { if (setElements[i] != null && setElements[i].equals(element)) { return retval = i; } } return retval; } void printSet() { for ( int i = 0; i < setElements.length; i++) { if (setElements[i] != null) { System.out.println("Member elements on index: "+ i +" " + setElements[i]); } } } String[] getMember() { String[] tempArray = new String[setElements.length]; for ( int i = 0; i < setElements.length; i++) { if(setElements[i] != null) { tempArray[i] = setElements[i]; } } return tempArray; } Set union(Set x, Set y) { String[] newXtemparray = new String[x.getLength()]; String[] newYtemparray = new String[y.getLength()]; int len = newYtemparray.length + newXtemparray.length; Set temp = new Set(0,len,len); newXtemparray = x.getMember(); newYtemparray = x.getMember(); for(int i = 0; i < newYtemparray.length; i++) { temp.add(newYtemparray[i]); } for(int j = 0; j < newXtemparray.length; j++) { temp.add(newXtemparray[j]); } return temp; } Set difference(Set x, Set y) { String[] newXtemparray = new String[x.getLength()]; String[] newYtemparray = new String[y.getLength()]; int len = newYtemparray.length + newXtemparray.length; Set temp = new Set(0,len,len); newXtemparray = x.getMember(); newYtemparray = x.getMember(); for(int i = 0; i < newXtemparray.length; i++) { temp.add(newYtemparray[i]); } for(int j = 0; j < newYtemparray.length; j++) { int retval = temp.findElement(newYtemparray[j]); if( retval != -1 ) { temp.remove(newYtemparray[j]); } } return temp; } } // This is the SetDemo class that will make use of our Set class class SetDemo { public static void main(String[] args) { //get input from keyboard BufferedReader keyboard; InputStreamReader reader; String temp = ""; reader = new InputStreamReader(System.in); keyboard = new BufferedReader(reader); try { System.out.println("Enter string element to be added" ); temp = keyboard.readLine( ); System.out.println("You entered " + temp ); } catch (IOException IOerr) { System.out.println("There was an error during input"); } /* ************************************************************************** * Test cases for our new created Set class. * ************************************************************************** */ Set setA = new Set(0,10,10); setA.add(temp); setA.add("b"); setA.add("b"); setA.add("hello"); setA.add("world"); setA.add("six"); setA.add("seven"); setA.add("b"); int size = setA.getLength(); System.out.println("Set size is: " + size ); Boolean isempty = setA.isEmpty(); System.out.println("Set is empty? " + isempty ); int ismember = setA.isAMember("sixb"); System.out.println("Element sixb is member of setA? " + ismember ); Boolean output = setA.isFull(); System.out.println("Set is full? " + output ); //setA.printSet(); int index = setA.findElement("world"); System.out.println("Element b located on index: " + index ); setA.remove("b"); //setA.emptySet(); int resize = setA.getLength(); System.out.println("Set size is: " + resize ); //setA.printSet(); Set setB = new Set(0,10,10); setB.add("b"); setB.add("z"); setB.add("x"); setB.add("y"); Set setC = setA.union(setB,setA); System.out.println("Elements of setA"); setA.printSet(); System.out.println("Union of setA and setB"); setC.printSet(); } } The union method works a sense that somehow I can call another method on it but it doesn't do the job, i supposedly would create and union of all elements of setA and setB but it only return element of setB. Sample output follows: java SetDemo Enter string element to be added hello You entered hello Set size is: 10 Set is empty? false Element sixb is member of setA? -1 Set is full? true Element b located on index: 2 Set size is: 9 Elements of setA Member elements on index: 0 hello Member elements on index: 1 world Member elements on index: 2 six Member elements on index: 3 seven Union of setA and setB Member elements on index: 0 b Member elements on index: 1 z Member elements on index: 2 x Member elements on index: 3 y thanks, lupin

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  • getting an error on jslint while creating a new object using javascript

    - by user3712689
    For some reason this code is giving a lint. I can't really figure out why. It says: 'was expecting a assignment or function call, and instead saw an expression.' What does that mean? window.onload = function (){ function SuspectOne (naam, leeftijd, wie){ this.naam = Spencer Hawes; this.leeftijd = 22; this.wie = zoon van de man; } function SuspectTwo (naam, leeftijd, wie){ this.naam = Tyrone Biggums; this.leeftijd = 28; this.wie = lokale herionejunk; } function SuspectThree (naam, leeftijd, wie){ this.naam = Ellie Campbell Hawes; this.leeftijd = 40; this.wie = vrouw van de man; } var verdachten = new Array[]; verdachten[0] = new Verdachte("Spencer Hawes", 22, "zoon van de man"); verdachten[1] = new Verdachte("Tyrone Biggums", 28, "lokale herionejunk"); verdachten[2] = new Verdachte("Ellie Spencer Hawes", 40, "vrouw van de man"); for(x=0; x<verdachten.length; x++){ console.log("De verdachte is de " + verdachten[x].leeftijd + "jaar oud " + verdachten[x].naam + ", de " + verdachten[x].wie); } }; Can someone help me with this? I would really like a lint free code.

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  • Insert data in an object to a database.

    - by paul
    I am facing the following design/implementation dilemma. I have a class Customer which is below with getters and setters. I would like to insert the value of the Customer into a "Customer" table of a database. But Customer has an address which is of type "Address". How do I go about inserting this field into the database?(I am using sqlite3). I thought of writing a separate table "Address(customerId,doorNo,city,state,country,pinCode)". But I am having second thoughts about generating the primary key(customerId) which should be same for both the "customer" and "Address" table. Sqlite3 faq states that I can do "Integer Primary Key" to use the field to generate an auto number. But if I do that in customer table, I would have to retrieve the same Id to be used in Address table. This kinda looks wrong to me :-?. There should be an elegant method to solve this. Any ideas would be much appreciated. Thanks in advance. import java.io.*; import java.sql.*; class Customer { private String id; private String name; private Address address; private Connection connection; private ResultSet resultSet; private PreparedStatement preparedStatement; public void insertToDatabase(){ } } class Address{ private String doorNumber; private String streetName; private String cityName; private String districtName; private String stateName; private String countryName; private long pinCode; }

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  • Object Oriented Perl interface to read from and write to a socket

    - by user654967
    I need a perl client-server implementation as a wrapper for a server in C#. A perl script passes the server address and port number and an input string to a module, this module has to create the socket and send the input string to the server. The data sent has to follow ISO-8859-1 encoding. On receiving the information, the client has to first receive 3 byte, then the next 8 bytes, this has the length of the data that has to be received next.. so based on the length the client has to read the next data. each of the data that is read has to be stored in a variable and sent another module for further processing. Currently this is what my perl client looks like..which I'm sure isn't right..could someone tell me how to do this..and set me on the right direction.. sub WriteInfo { my ($addr, $port, $Input) = @_; $socket = IO::Socket::INET->new( Proto => "tcp", PeerAddr => $addr, PeerPort => $port, ); unless ($socket) { die "cannot connect to remote" } while (1) { $socket->send($Input); } } sub ReadData { while (1) { my $ExecutionResult = $socket->recv( $recv_data, 3); my $DataLength = $socket->recv( $recv_data, 8); $DataLength =~ s/^0+// ; my $decval = hex($DataLength); my $Data = $socket->recv( $recv_data, $decval); return($Data); } thanks a lot.. Archer

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  • Accessing variables of an object of a particular class through a different class within the construc

    - by Haxed
    class Student { private String name; public Student(String name){ this.name = name; } public String getName(){ return name; } } class StudentServer { public StudentServer(){ Student[] s = new Student[30]; s[0] = new Student("Nick"); System.out.println(s[0]); // LINE 01:But this compiles, although prints junk System.out.println(s[0].getName()); // LINE 02:I get a error called cannot find symbol } public static void main(){ new StudentServer(); } } Hey, there are two lines, I want the reader to focus on, the first line prints junk as usual, but suprizingly the second one gives me an error. Do you know why ? Many Thanks

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  • <object> for PDF is blocking drop-down menu

    - by Tumharyyaaden
    URL: http://hartford.uconn.edu/director/academic_plan.html It is an HTML page, and using to display PDF document. Which is blocking the jQuery drop down menu. I have tried using CSS z-index property with positioning specified. Also tried setting wmode="transparent" / wmode="opaque" / and other variations but nothing seems to work.

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  • Load random swf object with jquery

    - by Eggnog
    I'm working on a site that utilizes full screen background video. I have three flash videos I'd like to load into the page which would be displayed randomly on page refresh. I know you can achieve this through action script loading the movies into one main .swf, but I'm rubbish with Flash and am looking for an alternative solution. My search so far has uncovered a method using jquery. Unfortunately my attempts to implement it have failed. I'm hoping someone more knowledgeable than myself can tell me if this technique is valid or what I'm doing wrong. Here's my code to date: <script type="text/javascript" src="files/js/swfobject.js"></script> <script type="text/javascript"> $(document).ready(function() { var sPath; var i = Math.floor(Math.random()*2); switch(i) { case 0: sPath = "flash/homepage_video.swf"; break; case 1: sPath = "flash/homepage_video.swf"; break; } // load the flash version of the image var so = new SWFObject(sPath, "flash-background", "100%", "100%"); so.addParam("scale", "exactFit"); so.addParam("movie", sPath); // NOTE: the sPath variable is also used here so.addParam("quality", "high"); so.addParam("wmode", "transparent"); so.write("homeBackground"); // NOTE: The value in this call to write() MUST match the name of the // HTML element (<div>) you're expecting the swf to show up in }); </script> <body> <div id="homeBackground"> <p>This is alternative content</p> </div> </body> I'm open to other suggestions that don't involve jquery (php perhaps). Thanks.

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  • accessing php javascript object in asp.net user control

    - by Khushi
    Hi, I have a website in php which is using 4 frames(top, left, middle and right). The middle frame contains the web user control coded in asp.net. Now, in the right frame( which is coded in php ) some javascript contains the id of the items selected in right frame. I need to get those ids to the middle frame on asp.net user control. How can i do this?

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  • PHP Object Oriented Web Application

    - by Sev
    I have a class called "Layout" for the layout of the page, another class called "User" for the user. Every page I create, I instantiate a new Layout. When a user logs in, there is a new User instantiated. How do I get an instance of the layout class to know about the instantiated user? I could also save the entire instance of the User in a session variable. I assume that's a bad idea though. What are the best practices for this?

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