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• OOD: All classes at bottom of hierarchy contain the same field

  - by My Head Hurts

  I am creating a class diagram for what I thought was a fairly simple problem. However, when I get to the bottom of the hierarchy, all of the classes only contain one field and it is the same one. This to me looks very wrong, but this field does not belong in any of the parent classes. I was wondering if there are any suggested design patterns in a situation like this? A simplified version of the class diagram can be found below. Note, fields named differently cannot belong to any other class +------------------+ | ObjectA | |------------------| | String one | | String two | | | +---------+--------+ | +---------------+----------------+ | | +--------|--------+ +--------|--------+ | ObjectAA | | ObjectAB | |-----------------| |-----------------| | String three | | String four | | | | | +--------+--------+ +--------+--------+ | | | | +--------|--------+ +--------|--------+ | ObjectAAA | | ObjectABA | |-----------------| |-----------------| | String five | | String five | | | | | +-----------------+ +-----------------+ ASCII tables drawn using http://www.asciiflow.com/

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• Liskov substitution and abstract classes / strategy pattern

  - by Kolyunya

  I'm trying to follow LSP in practical programming. And I wonder if different constructors of subclasses violate it. It would be great to hear an explanation instead of just yes/no. Thanks much! P.S. If the answer is no, how do I make different strategies with different input without violating LSP? class IStrategy { public: virtual void use() = 0; }; class FooStrategy : public IStrategy { public: FooStrategy(A a, B b) { c = /* some operations with a, b */ } virtual void use() { std::cout << c; } private: C c; }; class BarStrategy : public IStrategy { public: BarStrategy(D d, E e) { f = /* some operations with d, e */ } virtual void use() { std::cout << f; } private: F f; };

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• Help migrating from VB style programming to OO programming [closed]

  - by Agent47DarkSoul

  Being a hobbyist Java developer, I quickly took on with OO programming and understood its advantages over procedural code from C, that I did in college. But I couldn't grasp VB event based code (weird, right?). Bottom-line is OOP came natural to me. Curently I work in a small development firm developing C# applications. My peers here are a bit attached to VB style programming. Most of the C# code written is VB6 event handling code in C#'s skin. I tried explaining to them OOP with its advantages but it wasn't clear to them, maybe because I have never been much of a VB programmer. So can anybody provide any resources: books, web articles on how to migrate from VB style to OO style programming ?

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• Using dot To Access Object Attribute and Proper abstraction

  - by cobie

  I have been programming in python and java for quite a number of years and one thing i find myself doing is using the setters and getters from java in python but a number of blogs seem to think using the dot notation for access is the pythonic way. What I would like to know is if using dot to access methods does not violate abstraction principle. If for example I implement an attribute as a single object and use dot notation to access, if I wanted to change the code later so that the attribute is represented by a list of objects, that would require quite some heavy lifting which violates abstraction principle.

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• Child object free movement on Parent object

  - by The415

  Just to be straightforward, I am completely new to many aspects of coding and am searching for different specs and guidelines to aid me on my journey to crafting a wonderful game in Epic Games' Unreal Engine 4. Okay, I know upon viewing this, some may have little to no clue what I mean, so I'll put it like this to explain what I mean : Imagine a third person game with a simple model of a character. Now, say I have an object as a torso of a character in a game. Now Say I have an object as a head of the character. How could I keep the head as a child of the torso, but at the same time, allow it to move with the camera angle.

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• would a composite design pattern be useful for group membership?

  - by changokun

  I'm trying to think about the best way to handle group memberships on a website. People sign up and select checkboxes in a list of interests. Every week we send out interest-themed emails to those members that indicated that interest. however i store the information in the database, while i am working with the lists and generating lists of email addresses or manipulating group memberships, the composite design pattern looked interesting. it would be easy to populate the group, then do some aggregating functions that say... generate the list of email addresses based on the interests. but i'm not sure i'm seeing any other advantages. i do need something scalable, and flexible. thoughts?

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• Liskov substitution principle with abstract parent class

  - by Songo

  Does Liskov substitution principle apply to inheritance hierarchies where the parent is an abstract class the same way if the parent is a concrete class? The Wikipedia page list several conditions that have to be met before a hierarchy is deemed to be correct. However, I have read in a blog post that one way to make things easier to conform to LSP is to use abstract parent instead of a concrete class. How does the choice of the parent type (abstract vs concrete) impacts the LSP? Is it better to have an abstract base class whenever possible?

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• Handling array passed to object at creation

  - by cecilli0n

  When creating my object I pass it an array of a row from my database. (everything in the array we will need, disregarding unnecessary elements at sql query level) When I need to access certain array elements from within my class, I do so like $this->row['element'] However, As I continue development, I sometimes forget what exactly is in this passed array.(this itself doesn't seem good) I am wondering if their is a professional approach to dealing with this, Or am I the only one who has these "I wonder whats in the array" thoughts. One approach to tackling this could be that when we originally pass the array, in the constructor, we assign each element of the array to its own variable, but is this considered professional practice? Additionally by doing this, we could make those variables constants, in a attempt at immutability. Overall I am trying to adhere to good software craftsmanship. Regards.

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• "Best fit" to avoid reuse of object instances in a collection

  - by Simon

  Imagine I have a collection of object instances which represent activities for a user to undertake. Dependent on user attributes, I have to randomly select instances to present activities to the user. For some users, I need to present more activities to them than there are available activities in which case, I want to use the following algorithm. If all available activities have already been presented to the user, then re-select a "used" activity, selecting the earliest presented activity ordered by frequency of use. In other words, try to reduce repetition and where repetition is unavoidable, use the instances which have been repeated less often and were presented furthest back in time. Before I go on to code that algorithm, I wondered if there is some existing pattern I can re-use? [EDIT] "Furthest back in time" is not relevant as I will pass the algorithm an ordered collection of used instances where the first entry is the first presented.

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• How to change the state of a singleton in runtime

  - by user34401

  Consider I am going to write a simple file based logger AppLogger to be used in my apps, ideally it should be a singleton so I can call it via public class AppLogger { public static String file = ".."; public void logToFile() { // Write to file } public static log(String s) { AppLogger.getInstance().logToFile(s); } } And to use it AppLogger::log("This is a log statement"); The problem is, what is the best time I should provide the value of file since it is a just a singleton? Or how to refactor the above code (or skip using singleton) so I can customize the log file path? (Assume I don't need to write to multiple at the same time) p.s. I know I can use library e.g. log4j, but consider it is just a design question, how to refactor the code above?

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• Interfaces on an abstract class

  - by insta

  My coworker and I have different opinions on the relationship between base classes and interfaces. I'm of the belief that a class should not implement an interface unless that class can be used when an implementation of the interface is required. In other words, I like to see code like this: interface IFooWorker { void Work(); } abstract class BaseWorker { ... base class behaviors ... public abstract void Work() { } protected string CleanData(string data) { ... } } class DbWorker : BaseWorker, IFooWorker { public void Work() { Repository.AddCleanData(base.CleanData(UI.GetDirtyData())); } } The DbWorker is what gets the IFooWorker interface, because it is an instantiatable implementation of the interface. It completely fulfills the contract. My coworker prefers the nearly identical: interface IFooWorker { void Work(); } abstract class BaseWorker : IFooWorker { ... base class behaviors ... public abstract void Work() { } protected string CleanData(string data) { ... } } class DbWorker : BaseWorker { public void Work() { Repository.AddCleanData(base.CleanData(UI.GetDirtyData())); } } Where the base class gets the interface, and by virtue of this all inheritors of the base class are of that interface as well. This bugs me but I can't come up with concrete reasons why, outside of "the base class cannot stand on its own as an implementation of the interface". What are the pros & cons of his method vs. mine, and why should one be used over another?

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• How do I decide to which class a method should belong

  - by Eleeist

  I have TopicBusiness.class and PostBusiness.class. I have no problem with deciding into which class methods such as addPostToDatabase() or getAllPostsFromDatabase() should go. But what about getAllPostsFromTopic(TopicEntity topic) or getNumberOfPostsInTopic(TopicEntity topic)? Should the parameter be the deciding factor? So when the method takes TopicEntity as parameter it should belong to TopicBusiness.class? I am quite puzzled by this. EDIT: Some more info as requested. TopicBusiness.class and PostBusiness.class are classes holding all the business logic of the application concerning topics and posts respectively - that is fetching the data from database and/or performing some operations on them. TopicEntity is data (in this case representing single topic) fetched from database. getAllPostFromTopic(TopicEntity topic) gets all posts from database that belong to particular topic, while getNumberOfPostsInTopic(TopicEntity topic) performs database query and returns the number of posts that topic passed as parameter consists of.

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• Law of Demeter in MVC regarding Controller-View communication

  - by Antonio MG

  The scenario: Having a Controller that controls a view composed of complex subviews. Each one of those subviews is a separated class in a separate file. For example, one of those subviews is called ButtonsView, and has a bunch of buttons. The Controller has to access those buttons. Would accessing those buttons like this: controllerMainView.buttonsView.firstButton.state(); be a violation of the LOD? On one hand, it could be yes because the controller is accessing the inner hierarchy of the view. On the other, a Controller should be aware of what happens inside the view and how is composed. Any thoughts?

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• Using Bullet physics engine to find the moment of object contact before penetration

  - by MooMoo

  I would like to use Bullet Physics engine to simulate the objects in 3D world. One of the objects in the world will move using the position from 3D mouse control. I will call it "Mouse Object" and any object in the world as "Object A" I define the time before "mouse object" and "Object A" collide as t-1 The time "mouse object" penetrate "Object A" as t Now there is a problem about rendering the scene because when I move the mouse very fast, "Mouse object" will reside in "Object A" before "Object A" start to move. I would like the "Mouse Object" to stop right away attach to the "Object A". Also If the "Object A" move, the "Mouse object" should move following (attach) the "Object A" without stop at the first collision take place. This is what i did I find the position of the "Mouse Object" at time t-1 and time t. I will name it as pos(t-1) and pos(t) The contact time will be sometime between t-1 to t, which the time of contact I name it as t_contact, therefore the contact position (without penetration) between "Mouse object" and "Object A" will be pos(t_contact) then I create multiple "Mouse object"s using this equation pos[n] = pos(t-1) * C * ( pos(t) - pos(t-1) ) where 0 <= C <= 1 if I choose C = 0.1, 0.2, 0.3,0.4..... 1.0, I will get pos[n] for 10 values Then I test collision for all of these 10 "Mouse Objects" and choose the one that seperate between "no collision" and "collision". I feel this method is super non-efficient. I am not sure the way other people find the time-of-contact or the position-of-contact when "Object A" can move.

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• Why am I seeing so many instantiable classes without state?

  - by futlib

  I'm seeing a lot of instantiable classes in the C++ and Java world that don't have any state. I really can't figure out why people do that, they could just use a namespace with free functions in C++, or a class with a private constructor and only static methods in Java. The only benefit I can think of is that you don't have to change most of your code if you later decide that you want a different implementation in certain situations. But isn't that a case of premature design? It could be turned into a class later, when/if it becomes appropriate. Am I getting this wrong? Is it not OOP if I don't put everything into objects (i.e. instantiated classes)? Then why are there so many utility namespaces and classes in the standard libraries of C++ and Java? Update: I've certainly seen a lot examples of this in my previous jobs, but I'm struggling to find open source examples, so maybe it's not that common after all. Still, I'm wondering why people do it, and how common it is.

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• How to decide to which class does a method belong

  - by Eleeist

  I have TopicBusiness.class and PostBusiness.class. I have no problem with deciding into which class methods such as addPostToDatabase() or getAllPostsFromDatabase() should go. But what about getAllPostsFromTopic(TopicEntity topic) or getNumberOfPostsInTopic(TopicEntity topic)? Should the parameter be the deciding factor? So when the method takes TopicEntity as parameter it should belong to TopicBusiness.class? I am quite puzzled by this.

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• is it valid that a state machine can have more than one possible state for some transition?

  - by shankbond

  I have a requirement for a workflow which I am trying to model as a state machine, I see that there is more than one outcome of a given transition(or activity). Is it valid for a state machine to have more than one possible states, but only one state will be true at a given time? Note: This is my first attempt to model a state machine. Eg. might be: s1-t1-s2 s1-t1-s3 s1-t1-s4 where s1, s2, s3, s4 are states and t1 is transition/activity. A fictitious real world example might be: For a human, there can be two states: hungry, not hungry A basket can have only one item from: apple, orange. So, to model it we will have: hungry-pick from basket-apple found hungry-pick from basket-orange found apple found-eat-not hungry orange found-take juice out of it and then drink- not hungry

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• Passing context between templatetags, django

  - by Kasper Gadensgaard

  Hi overflowers, I am using django to create a web-application. I have created a template in where I load a templatetag. In this templatetag i load another templatetag. From the template I pass context to the first templatetag, but the context is not available from the second templatetag (inside the first templatetag) - see below. I hope this makes sense, and that one of you have the answer. Thanks in advance Regards Kasper Gadensgaard Template snippit: {% load templatetags %} {% some_tag argument %} some_tag Templatetag: {% load templatetags %} {% some_other_tag another_argument %} some_other_tag Templatetag: In this templatetag i am trying to access context to get user info i.e. using request = context['request'] request.user

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• Global Entity Framework Context in WPF Application

  - by OffApps Cory

  Good day, I am in the middle of development of a WPF application that is using Entity Framework (.NET 3.5). It accesses the entities in several places throughout. I am worried about consistency throughout the application in regard to the entities. Should I be instancing separate contexts in my different views, or should I (and is a a good way to do this) instance a single context that can be accessed globally? For instance, my entity model has three sections, Shipments (with child packages and further child contents), Companies/Contacts (with child addresses and telephones), and disk specs. The Shipments and EditShipment views access the DiskSpecs, and the OptionsView manages the DiskSpecs (Create, Edit, Delete). If I edit a DiskSpec, I have to have something in the ShipmentsView to retrieve the latest specs if I have separate contexts right? If it is safe to have one overall context from which the rest of the app retrieves it's objects, then I imagine that is the way to go. If so, where would that instance be put? I am using VB.NET, but I can translate from C# pretty good. Any help would be appreciated. I just don't want one of those applications where the user has to hit reload a dozen times in different parts of the app to get the new data. Update: OK so I have changed my app as follows: All contexts are created in Using Blocks to dispose of them after they are no longer needed. When loaded, all entities are detatched from context before it is disposed. A new property in the MainViewModel (ContextUpdated) raises an event that all of the other ViewModels subscribe to which runs that ViewModels RefreshEntities method. After implementing this, I started getting errors saying that an entity can only be referenced by one ChangeTracker at a time. Since I could not figure out which context was still referencing the entity (shouldn't be any context right?) I cast the object as IEntityWithChangeTracker, and set SetChangeTracker to nothing (Null). This has let to the current problem: When I Null the changeTracker on the Entity, and then attach it to a context, it loses it's changed state and does not get updated to the database. However if I do not null the change tracker, I can't attach. I have my own change tracking code, so that is not a problem. My new question is, how are you supposed to do this. A good example Entity query and entity save code snipped would go a long way, cause I am beating my head in trying to get what I once thought was a simple transaction to work. Any help would elevate you to near god-hood.

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• Monitoring pthread context switching

  - by simon

  I would like to monitor the the context switching behavior in a multi-threaded pthread application. In other RTOSes(Micro C OS) I have been able to register a context switch callback for each thread in the application, and then log (or toggle a gpio) and watch the thread context switching in real time. This was a valuable tool for debugging the real time behavior and interaction of the multiple threads. My current environment is embedded linux utilizing the pthread api. Is there a way to montior each of the context switches?

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• Object oriented EDI handling in PHP

  - by Robert van der Linde

  I'm currently starting a new sub project where I will: Retrieve the order information from our mainframe Save the order information to our web-apps' database Send the order as EDI (either D01b or D93a) Receive the order response, despatch advice and invoice messages Do all kinds of fun things with the resulting datasets. However I am struggling with my initial class designs. The order information will be retrieved from the mainframe which will result in a "AOrder" class, this isn't a problem, I am not sure about how to mold this local object into an EDI string. Should I create EDIOrder/EDIOrderResponse/etc classes with matching decorators (EDIOrderD01BDecorator, EDIOrderD93ADecorator)? Do I need builder objects or can I do: // $myOrder is instance of AOrder $myOrder->toEDIOrder(); $decorator = new EDIOrderD01BDecorator($myOrder); $edi = $decorator->getEDIString(); And it'll have to work the other way around as well. Is the following code a good way of handling this problem or should I go about this differently? $ediString = $myEDIMessageBroker->fetch(); $ediOrderResponse = EDIOrderResponse::fromString($ediString); I'm just not so sure about how I should go about designing the classes and interactions between them. Thanks for reading and helping.

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• Object oriented wrapper around a dll

  - by Tom Davies

  So, I'm writing a C# managed wrapper around a native dll. The dll contains several hundred functions. In most cases, the first argument to each function is an opaque handle to a type internal to the dll. So, an obvious starting point for defining some classes in the wrapper would be to define classes corresponding to each of these opaque types, with each instance holding and managing the opaque handle (passed to its constructor) Things are a little awkward when dealing with callbacks from the dll. Naturally, the callback handlers in my wrapper have to be static, but the callbacks arguments invariable contain an opaque handle. In order to get from the static callback back to an object instance, I've created a static dictionary in each class, associating handles with class instances. In the constructor of each class, an entry is put into the dictionary, and this entry is then removed in the Destructors. When I receive a callback, I can then consult the dictionary to retrieve the class instance corresponding to the opaque reference. Are there any obvious flaws to this? Something that seems to be a problem is that the existence static dictionary means that the garbage collector will not act on my class instances that are otherwise unreachable. As they are never garbage collected, they never get removed from the dictionary, so the dictionary grows. It seems I might have to manually dispose of my objects, which is something absolutely would like to avoid. Can anyone suggest a good design that allows me to avoid having to do this?

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• Problem with Closure properties of context free languages

  - by altius

  hello i have the following sentence a language L1={a^n * b^n : n=0} and L2={b^n * a^n : n=0} are context free languages so they are close a=under the L1L2 so L={a^n * b^2n A^n : n=0} must be context free too because it is generated by a close property I have to prove if this sentence is true or not so i check the L language and i do not think that it is context free then i also saw that L2 is L1 reversed do i have to check if L1, L2 are deterministic ? please help because i am in a dead end

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• REST API wrapper - class design for 'lite' object responses

  - by sasfrog

  I am writing a class library to serve as a managed .NET wrapper over a REST API. I'm very new to OOP, and this task is an ideal opportunity for me to learn some OOP concepts in a real-life situation that makes sense to me. Some of the key resources/objects that the API returns are returned with different levels of detail depending on whether the request is for a single instance, a list, or part of a "search all resources" response. This is obviously a good design for the REST API itself, so that full objects aren't returned (thus increasing the size of the response and therefore the time taken to respond) unless they're needed. So, to be clear: .../car/1234.json returns the full Car object for 1234, all its properties like colour, make, model, year, engine_size, etc. Let's call this full. .../cars.json returns a list of Car objects, but only with a subset of the properties returned by .../car/1234.json. Let's call this lite. ...search.json returns, among other things, a list of car objects, but with minimal properties (only ID, make and model). Let's call this lite-lite. I want to know what the pros and cons of each of the following possible designs are, and whether there is a better design that I haven't covered: Create a Car class that models the lite-lite properties, and then have each of the more detailed responses inherit and extend this class. Create separate CarFull, CarLite and CarLiteLite classes corresponding to each of the responses. Create a single Car class that contains (nullable?) properties for the full response, and create constructors for each of the responses which populate it to the extent possible (and maybe include a property that returns the response type from which the instance was created). I expect among other things there will be use cases for consumers of the wrapper where they will want to iterate through lists of Cars, regardless of which response type they were created from, such that the three response types can contribute to the same list. Happy to be pointed to good resources on this sort of thing, and/or even told the name of the concept I'm describing so I can better target my research.

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• Counting context switches per thread

  - by Sarmun

  Is there a way to see how many context switches each thread generates? (both in and out if possible) Either in X/s, or to let it run and give aggregated data after some time. (either on linux or on windows) I have found only tools that give aggregated context-switching number for whole os or per process. My program makes many context switches (50k/s), probably a lot not necessary, but I am not sure where to start optimizing, where do most of those happen.

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