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  • Why should I declare a class as an abstract class?

    - by Pied Piper
    I know the syntax, rules applied to abstract class and I want know usage of an abstract class Abstract class can not be instantiated directly but can be extended by other class What is the advantage of doing so? How it is different from an Interface? I know that one class can implement multiple interfaces but can only extend one abstract class. Is that only difference between an interface and an abstract class? I am aware about usage of an Interface. I have learned that from Event delegation model of AWT in Java. In which situations I should declare class as an abstract class? What is benefits of that?

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  • design for supporting entities with images

    - by brainydexter
    I have multiple entities like Hotels, Destination Cities etc which can contain images. The way I have my system setup right now is, I think of all the images belonging to this universal set (a table in the DB contains filePaths to all the images). When I have to add an image to an entity, I see if the entity exists in this universal set of images. If it exists, attach the reference to this image, else create a new image. E.g.: class ImageEntityHibernateDAO { public void addImageToEntity(IContainImage entity, String filePath, String title, String altText) { ImageEntity image = this.getImage(filePath); if (image == null) image = new ImageEntity(filePath, title, altText); getSession().beginTransaction(); entity.getImages().add(image); getSession().getTransaction().commit(); } } My question is: Earlier I had to write this code for each entity (and each entity would have a Set collection). So, instead of re-writing the same code, I created the following interface: public interface IContainImage { Set<ImageEntity> getImages(); } Entities which have image collections also implements IContainImage interface. Now, for any entity that needs to support adding Image functionality, all I have to invoke from the DAO looks something like this: // in DestinationDAO::addImageToDestination { imageDao.addImageToEntity(destination, imageFileName, imageTitle, imageAltText); // in HotelDAO::addImageToHotel { imageDao.addImageToEntity(hotel, imageFileName, imageTitle, imageAltText); It'd be great help if someone can provide me some critique on this design ? Are there any serious flaws that I'm not seeing right away ?

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  • How to deal with data on the model specific to the technology being used?

    - by user1620696
    There are some cases where some of the data on a class of the domain model of an application seems to be dependent on the technology being used. One example of this is the following: suppose we are building one application in .NET such that there's the need of an Employee class. Suppose further that we are going to implement relational database, then the Employee has a primary key right? So that the classe would be something like public class Employee { public int EmployeeID { get; set; } public string Name { get; set; } ... } Now, that EmployeeID is dependent on the technology right? That's something that has to do with the way we've choose to persist our data. Should we write down a class independent of such things? If we do it this way, how should we work? I think I would need to map all the time between domain model and persistence specific types, but I'm not sure.

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  • Fetching database query through function

    - by Shubham Maurya
    I am sick of connecting database in each script i need a more OOP approach to fetching database results. ex like wordpress use wpdb class to fetch results. This what wordpress does to get data <?php $posts = $wpdb->get_results("SELECT ID, post_title FROM $wpdb->posts WHERE post_status = 'publish' AND post_type='post' ORDER BY comment_count DESC LIMIT 0,4") ?> How can i create the same feature too using any class or function and use it in my script Thank you

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  • How to verify the Liskov substitution principle in an inheritance hierarchy?

    - by Songo
    Inspired by this answer: Liskov Substitution Principle requires that Preconditions cannot be strengthened in a subtype. Postconditions cannot be weakened in a subtype. Invariants of the supertype must be preserved in a subtype. History constraint (the "history rule"). Objects are regarded as being modifiable only through their methods (encapsulation). Since subtypes may introduce methods that are not present in the supertype, the introduction of these methods may allow state changes in the subtype that are not permissible in the supertype. The history constraint prohibits this. I was hoping if someone would post a class hierarchy that violates these 4 points and how to solve them accordingly. I'm looking for an elaborate explanation for educational purposes on how to identify each of the 4 points in the hierarchy and the best way to fix it. Note: I was hoping to post a code sample for people to work on, but the question itself is about how to identify the faulty hierarchies :)

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  • should I extend or create instance of the class

    - by meWantToLearn
    I have two classes Class A and Class B in Class A, i have three methods that perform the save, delete and select operation based upon the object I pass them. in Class B I perform the logic operations, such as modification to the property of the object before being passed to the methods of Class A, My problem is in Class B, should it extend Class A, and call the methods of class A , by parent::methodName or create instance of class A and then call Class A does not includes any property just methods. class A{ public function save($obj){ //code here } public function delete($obj){ //code here } public function select($obj){ //code here } } //Should I extend class A, and call the method by parent::methodName($obj) or create an instance of class A, call the method $instanceOfA-methodName($obj); class B extends A{ public function checkIfHasSaved($obj){ if($obj->saved == 'Yes'){ parent::save($obj); //**should I call the method like this** $instanceOFA = new A(); //**or create instance of class A and call without extending class A** instanceOFA->save($obj); } //other logic operations here } }

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  • Instantiate proper class based on some input

    - by Adam Backstrom
    I'm attempting to understand how "switch as a code smell" applies when the proper code path is determined by some observable piece of data. My Webapp object sets an internal "host" object based on the hostname of the current request. Each Host subclass corresponds to one possible hostname and application configuration: WwwHost, ApiHost, etc. What is an OOP way for a host subclass to accept responsibility for a specific hostname, and for Webapp to get an instance of the appropriate subclass? Currently, the hostname check and Host instantiation exists within the Webapp object. I could move the test into a static method within the Host subclasses, but I would still need to explicitly list those subclasses in Webapp unless I restructure further. It seems like any solution will require new subclasses to be added to some centralized list.

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  • Setting up ASP.NET structure for code

    - by user1175327
    I've always coded in C# MVC3 when developing web applications. But now i wanted to learn a bit more about developing web sites with just ASP.NET. But now i'm wondering what a good setup for my code would be. For me, an MVC like pattern seems to be a good way to go. But obviously ASP.NET doesn't have any router and controller classes. So i guess people have a different way of setting up their code when they do ASP.NET. So i'm looking for more information on how to get started with this. So not really the basics of ASP.NET, but something that focuses on a good code setup. Any good tutorials/information about this/?

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  • Law of Demeter confusion [duplicate]

    - by user2158382
    This question already has an answer here: Rails: Law of Demeter Confusion 4 answers I am reading a book called Rails AntiPatterns and they talk about using delegation to to avoid breaking the Law of Demeter. Here is their prime example: They believe that calling something like this in the controller is bad (and I agree) @street = @invoice.customer.address.street Their proposed solution is to do the following: class Customer has_one :address belongs_to :invoice def street address.street end end class Invoice has_one :customer def customer_street customer.street end end @street = @invoice.customer_street They are stating that since you only use one dot, you are not breaking the Law of Demeter here. I think this is incorrect, because you are still going through customer to go through address to get the invoice's street. I primarily got this idea from a blog post I read: http://www.dan-manges.com/blog/37 In the blog post the prime example is class Wallet attr_accessor :cash end class Customer has_one :wallet # attribute delegation def cash @wallet.cash end end class Paperboy def collect_money(customer, due_amount) if customer.cash < due_ammount raise InsufficientFundsError else customer.cash -= due_amount @collected_amount += due_amount end end end The blog post states that although there is only one dot customer.cash instead of customer.wallet.cash, this code still violates the Law of Demeter. Now in the Paperboy collect_money method, we don't have two dots, we just have one in "customer.cash". Has this delegation solved our problem? Not at all. If we look at the behavior, a paperboy is still reaching directly into a customer's wallet to get cash out. Can somebody help me clear the confusion. I have been searching for the past 2 days trying to let this topic sink in, but it is still confusing.

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  • Service layer coupling

    - by Justin
    I am working on writing a service layer for an order system in php. It's the typical scenario, you have an Order that can have multiple Line Items. So lets say a request is received to store a line item with pictures and comments. I might receive a json request such as { 'type': 'Bike', 'color': 'Red', 'commentIds': [3193,3194] 'attachmentIds': [123,413] } My idea was to have a Service_LineItem_Bike class that knows how to take the json data and store an entity for a bike. My question is, the Service_LineItem class now needs to fetch comments and file attachments, and store the relationships. Service_LineItem seems like it should interact with a Service_Comment and a Service_FileUpload. Should instances of these two other services be instantiated and passed to the Service_LineItem constructor,or set by getters and setters? Dependency injection seems like the right solution, allowing a service access to a 'service fetching helper' seems wrong, and this should stay at the application level. I am using Doctrine 2 as a ORM, and I can technically write a dql query inside Service_LineItem to fetch the comments and file uploads necessary for the association, but this seems like it would have a tighter coupling, rather then leaving this up to the right service object.

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  • Should all, none, or some overriden methods call Super?

    - by JoJo
    When designing a class, how do you decide when all overridden methods should call super or when none of the overridden methods should call super? Also, is it considered bad practice if your code logic requires a mixture of supered and non-supered methods like the Javascript example below? ChildClass = new Class.create(ParentClass, { /** * @Override */ initialize: function($super) { $super(); this.foo = 99; }, /** * @Override */ methodOne: function($super) { $super(); this.foo++; }, /** * @Override */ methodTwo: function($super) { this.foo--; } }); After delving into the iPhone and Android SDKs, I noticed that super must be called on every overridden method, or else the program will crash because something wouldn't get initialized. When deriving from a template/delegate, none of the methods are supered (obviously). So what exactly are these "je ne sais quoi" qualities that determine whether a all, none, or some overriden methods should call super?

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  • Is having 'Util' classes a cause for concern? [closed]

    - by Matt Fenwick
    I sometimes create 'Util' classes which primarily serve to hold methods and values that don't really seem to belong elsewhere. But every time I create one of these classes, I think "uh-oh, I'm gonna regret this later ...", because I read somewhere that it's bad. But on the other hand, there seem to be two compelling (at least for me) cases for them: implementation secrets that are used in multiple classes within a package providing useful functionality to augment a class, without cluttering its interface Am I on the way to destruction? What you say !! Should I refactor?

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  • Do objects maintain identity under all non-cloning conditions in PHP?

    - by Buttle Butkus
    PHP 5.5 I'm doing a bunch of passing around of objects with the assumption that they will all maintain their identities - that any changes made to their states from inside other objects' methods will continue to hold true afterwards. Am I assuming correctly? I will give my basic structure here. class builder { protected $foo_ids = array(); // set in construct protected $foo_collection; protected $bar_ids = array(); // set in construct protected $bar_collection; protected function initFoos() { $this->foo_collection = new FooCollection(); foreach($this->food_ids as $id) { $this->foo_collection->addFoo(new foo($id)); } } protected function initBars() { // same idea as initFoos } protected function wireFoosAndBars(fooCollection $foos, barCollection $bars) { // arguments are passed in using $this->foo_collection and $this->bar_collection foreach($foos as $foo_obj) { // (foo_collection implements IteratorAggregate) $bar_ids = $foo_obj->getAssociatedBarIds(); if(!empty($bar_ids) ) { $bar_collection = new barCollection(); // sub-collection to be a component of each foo foreach($bar_ids as $bar_id) { $bar_collection->addBar(new bar($bar_id)); } $foo_obj->addBarCollection($bar_collection); // now each foo_obj has a collection of bar objects, each of which is also in the main collection. Are they the same objects? } } } } What has me worried is that foreach supposedly works on a copy of its arrays. I want all the $foo and $bar objects to maintain their identities no matter which $collection object they become of a part of. Does that make sense?

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  • Should all, none, or some overridden methods call Super?

    - by JoJo
    When designing a class, how do you decide when all overridden methods should call super or when none of the overridden methods should call super? Also, is it considered bad practice if your code logic requires a mixture of supered and non-supered methods like the Javascript example below? ChildClass = new Class.create(ParentClass, { /** * @Override */ initialize: function($super) { $super(); this.foo = 99; }, /** * @Override */ methodOne: function($super) { $super(); this.foo++; }, /** * @Override */ methodTwo: function($super) { this.foo--; } }); After delving into the iPhone and Android SDKs, I noticed that super must be called on every overridden method, or else the program will crash because something wouldn't get initialized. When deriving from a template/delegate, none of the methods are supered (obviously). So what exactly are these "je ne sais quoi" qualities that determine whether a all, none, or some overriden methods should call super?

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  • Easy to use JSON Web Service Hosts?

    - by Serguei Fedorov
    I saw this being used by someone in a college class once and cannot find anything that is analogous to it. I am not sure if this is the right place to ask about something like this, but hopefully I can get some direction. I want to write an app which uses web services that can obtain and push data back to the client apps. Right now I am gathering up the design and documentation of this app. Not having to code the web service myself would reduce development time by a lot; instead using an easy to setup web service that will be easy to setup and manage. Either XML based on JSON based is totally fine; though I would prefer JSON for its reduced overhead. Like I said I have seen this demonstrated before; you define the data structure to be stored and how it is treated. I cannot find the person who demonstrated this; hopefully maybe someone can suggest something? The service he used was free with a limited amount of requests allowed. EDIT: He was using an online service to do this not a script which is installed onto an existing web hosting account. Thank you!

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  • Using visitor pattern with large object hierarchy

    - by T. Fabre
    Context I've been using with a hierarchy of objects (an expression tree) a "pseudo" visitor pattern (pseudo, as in it does not use double dispatch) : public interface MyInterface { void Accept(SomeClass operationClass); } public class MyImpl : MyInterface { public void Accept(SomeClass operationClass) { operationClass.DoSomething(); operationClass.DoSomethingElse(); // ... and so on ... } } This design was, however questionnable, pretty comfortable since the number of implementations of MyInterface is significant (~50 or more) and I didn't need to add extra operations. Each implementation is unique (it's a different expression or operator), and some are composites (ie, operator nodes that will contain other operator/leaf nodes). Traversal is currently performed by calling the Accept operation on the root node of the tree, which in turns calls Accept on each of its child nodes, which in turn... and so on... But the time has come where I need to add a new operation, such as pretty printing : public class MyImpl : MyInterface { // Property does not come from MyInterface public string SomeProperty { get; set; } public void Accept(SomeClass operationClass) { operationClass.DoSomething(); operationClass.DoSomethingElse(); // ... and so on ... } public void Accept(SomePrettyPrinter printer) { printer.PrettyPrint(this.SomeProperty); } } I basically see two options : Keep the same design, adding a new method for my operation to each derived class, at the expense of maintainibility (not an option, IMHO) Use the "true" Visitor pattern, at the expense of extensibility (not an option, as I expect to have more implementations coming along the way...), with about 50+ overloads of the Visit method, each one matching a specific implementation ? Question Would you recommand using the Visitor pattern ? Is there any other pattern that could help solve this issue ?

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  • How to handle status columns in designing tables

    - by altsyset
    How to handle multiple statuses for a table entry, for example an item table may have an active, inactive, fast moving, and/or batch statuses. And I wanted to handle them in single column with VARCHAR type. Also I might set each of those attributes as a boolean with different columns. But I am not sure what consequences this might lead to. So if you have experienced such situations which one would be the best way to handle it?

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  • Defining formula through user interface in user form

    - by BriskLabs Pakistan
    I am a student and developing a simple assignment - windows form application in visual studio 2010. The application is suppose to construct formulas as per user requirement. The process: It has to pick data from columns of Microsoft Access database and the user should be able to pick the data by column name like we do in a drop down menu. and create reusable formulas in it ( configure it once and can change it again). followings are column titles from database that can be picked for example. e.g Col -1 : Marks in Maths Col -2 : Total Marks in Maths Col -3 : Marks in science Col -4 : Total marks in science Finally we should be able to construct any formula in the UI like (Col 1 + Col 3 ) / ( col 2 + col 4) = Formula 1 once this is formula is set saved and a name is assigned to it by user. he/she can use the formula and results shall appear in a window below. i.e He would be able to calculate his desired figures (formula) by only manipulating underlying data on the UI layer....choose the data for a period and apply the formula and get the answer Problem: It looks like I have to create an app where rules are set through UI....... this means no stored procedures are required in SQL.... please suggest the right approach.

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  • Ruby - when to use instance variables vs parameters between methods?

    - by Michael Durrant
    I'm writing several methods that call other methods. To pass the information I have a couple of choices: Pass the information as parameters Set instance variables so that other methods can access them When should I choose one option over the other? It seems that the first option is good as it is very specific about what is being passed. the downside seems to be that a lot of values are being passed around. The second method doesn't require passing all the values around but seems to lead to a lot of magic where methods set instance variables 'somewhere' Should I always be very explicit about gets passed to other methods in the class? Are there exceptions so this?

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  • To identify the classes for uml diagrams?

    - by user106535
    I want to implement a software engineering project based on "crime management system". The main modules are: visitors, users, administrator. The main events that are taking place are: registration, report complaint, report crime, report most wanted, view status of reported crime. So could you please help me to identify the classes that are to be used in this project and help me to draw the class diagram?

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  • Object construction design

    - by James
    I recently started to use c# to interface with a database, and there was one part of the process that appeared odd to me. When creating a SqlCommand, the method I was lead to took the form: SqlCommand myCommand = new SqlCommand("Command String", myConnection); Coming from a Java background, I was expecting something more similar to SqlCommand myCommand = myConnection.createCommand("Command String"); I am asking, in terms of design, what is the difference between the two? The phrase "single responsibility" has been used to suggest that a connection should not be responsible for creating SqlCommands, but I would also say that, in my mind, the difference between the two is partly a mental one of the difference between a connection executing a command and a command acting on a connection, the latter of which seems less like what I have been lead to believe OOP should be. There is also a part of me wondering if the two should be completely separate, and should only come together in some sort of connection.execute(command) method. Can anyone help clear up these differences? Are any of these methods "more correct" than the others from an OO point of view? (P.S. the fact that c# is used is completely irrelevant. It just highlighted to me that different approaches were used)

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  • Why setter method when getter method enough in PHP OOP

    - by phphunger
    I am practicing OOP with PHP, and I am struck at setter and getter methods. I can directly access the class properties and methods with getter method then what's the use of setter method? See my example. <?php class MyClass{ public $classVar = "Its a class variable"; public function Getter(){ return $this -> classVar; } } $obj = new MyClass; echo $obj -> Getter(); ?>

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  • Breaking up classes and methods into smaller units

    - by micahhoover
    During code reviews a couple devs have recommended I break up my methods into smaller methods. Their justification was (1) increased readability and (2) the back trace that comes back from production showing the method name is more specific to the line of code that failed. There may have also been some colorful words about functional programming. Additionally I think I may have failed an interview a while back because I didn't give an acceptable answer about when to break things up. My inclination is that when I see a bunch of methods in a class or across a bunch of files, it isn't clear to me how they flow together, and how many times each one gets called. I don't really have a good feel for the linearity of it as quickly just by eye-balling it. The other thing is a lot of people seem to place a premium of organization over content (e.g. 'Look at how organized my sock drawer is!' Me: 'Overall, I think I can get to my socks faster if you count the time it took to organize them'). Our business requirements are not very stable. I'm afraid that if the classes/methods are very granular it will take longer to refactor to requirement changes. I'm not sure how much of a factor this should be. Anyway, computer science is part art / part science, but I'm not sure how much this applies to this issue.

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  • Cookie access within a HTTP Class

    - by James Jeffery
    I have a HTTP class that has a Get, and Post, method. It's a simple class I created to encapsulate Post and Get requests so I don't have to repeat the get/post code throughout the application. In C#: class HTTP { private CookieContainer cookieJar; private String userAgent = "..."; public HTTP() { this.cookieJar = new CookieContainer(); } public String get(String url) { // Make get request. Return the JSON } public String post(String url, String postData) { // Make post request. Return the JSON } } I've made the CookieJar a property because I want to preserve the cookie values throughout the session. If the user is logged into Twitter with my application, each request I make (be it get or post) I want to use the cookies so they remain logged in. That's the basics of it anyway. But, I don't want to return a string in all instances. Sometimes I may want the cookie, or a header value, or something else from the request. Ideally I'd like to be able to do this in my code: Cookie cookie = http.get("http://google.com").cookie("g_user"); String g_user = cookie.value; or String source = http.get("http://google.com").body; My question - To do this, would I need to have a Get class, and a Post class, that are included within the HTTP class and are accessible via accessors? Within the Get and Post class I would then have the Cookie method, and the body property, and whatever else is needed. Should I also use an interface, or create a Request class and have Post and Get extend it so that common methods and properties are available to both classes? Or, am I thinking totally wrong?

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  • PHP OOP: Am i following right way?

    - by sineverba
    I'm learning OOP (PHP). I've realized my own CRUD Class, that performs some kind of queries SQL. Btw, a Gasoline asked us to realize a smart, simple web-app where he can update prices of his gasoline (gasoline, diesel, lpg) and via an API i could recall them and display in his site. So, I did create a new Class Gasoline but it perform some methods of CRUD Class public function getPrezzoBenzina($id) { $prezzo_benzina = $this->distributore->sql('SELECT prezzo_benzina FROM prezzi WHERE id = '.$id); return $prezzo_benzina } And so on (code is pseudocode, just to explain). I could perform all my code only with help of Crud Class... without necessity of Class Gasoline. So, what I'm missing about OOP? Where am I wrong?

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