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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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  • Confused about implementing Single Responsibility Principle

    - by HichemSeeSharp
    Please bear with me if the question looks not well structured. To put you in the context of my issue: I am building an application that invoices vehicles stay duration in a parking. In addition to the stay service there are some other services. Each service has its own calculation logic. Here is an illustration (please correct me if the design is wrong): public abstract class Service { public int Id { get; set; } public bool IsActivated { get; set; } public string Name { get; set } public decimal Price { get; set; } } public class VehicleService : Service { //MTM : many to many public virtual ICollection<MTMVehicleService> Vehicles { get; set; } } public class StayService : VehicleService { } public class Vehicle { public int Id { get; set; } public string ChassisNumber { get; set; } public DateTime? EntryDate { get; set; } public DateTime? DeliveryDate { get; set; } //... public virtual ICollection<MTMVehicleService> Services{ get; set; } } Now, I am focusing on the stay service as an example: I would like to know at invoicing time which class(es) would be responsible for generating the invoice item for the service and for each vehicle? This should calculate the duration cost knowing that the duration could be invoiced partially so the like is as follows: not yet invoiced stay days * stay price per day. At this moment I have InvoiceItemsGenerator do everything but I am aware that there is a better design.

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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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  • Application configuration save to file or object serialization

    - by Venno
    Hi, I am working on a project and I need to save user configuration. The configuration is a set of Jchechboxes I need to store their state (true, false). Which do you think is the better way of saving it, in a file and if yes in what (ini, cfg, txt), or it is better to serialize the object with the states?? Or if there is another way, please tell me :) Cheers

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  • Hibernate proxy serialization and receive on client side.

    - by Bubba88
    I lack understanding of how does the generated proxy class object (CGLib enhanced POJO) is transferred to the remote client and still pertains its ability to generate Lazy Init. Exceptions. Does that mean that there is some kind of contract that all the transferred objects of some kind of class (a proxy) will be reinstantiated as proxies again? Where does the client obtain those generated classes? Sorry, but I totally do not understand.

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  • Substitute for Iterator that is Serialization

    - by Mahmoud
    I'm working on a GWT project, and I have a bunch of Java classes that use Java Object Iterators on the server side. As I was reading through the internet...Iterators seem to not be serializable preventing me from sending them over to the client side from the server side. My question is is there an efficient way to serialize the iterator or use a substitute that might be serializable ? Many thanks!

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  • Java serialization testing

    - by Jeff Storey
    Does anyone know if there is a library that exists to help test if an object graph is fully serializable? It would probably be as simple as writing it out and reading it back in, but I figured someone must have abstracted this already - I just can't find it.

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  • Settings File as Dictionary with Serialization

    - by AKRamkumar
    This is a three part question. One: Would using a Dictionary<String,Object> be a good way of saving data where it would be Dictionary<Key,Value> as the basis? Two: What would be a better way without using app.settings or xml? Three: How would you serialize this(Or the better solution) into a binary format that is compact and serializes quickly?

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  • Java serialization problem

    - by stefan89
    I have two classes X and Y, like this: class X implements Serializable { int val1; Y val2; } class Y implements Serializable { int val; } I want to transmit an object of type X from a client to server but i can't because the class X has a field of type Y. I replaced the field of type Y with a field of type X in class X and it works.

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  • Control XML serialization of generic types

    - by Luca
    I'm investigating about XML serialization, and since I use lot of dictionary, I would like to serialize them as well. I found the following solution for that (I'm quite proud of it! :) ). [XmlInclude(typeof(Foo))] public class XmlDictionary<TKey, TValue> { /// <summary> /// Key/value pair. /// </summary> public struct DictionaryItem { /// <summary> /// Dictionary item key. /// </summary> public TKey Key; /// <summary> /// Dictionary item value. /// </summary> public TValue Value; } /// <summary> /// Dictionary items. /// </summary> public DictionaryItem[] Items { get { List<DictionaryItem> items = new List<DictionaryItem>(ItemsDictionary.Count); foreach (KeyValuePair<TKey, TValue> pair in ItemsDictionary) { DictionaryItem item; item.Key = pair.Key; item.Value = pair.Value; items.Add(item); } return (items.ToArray()); } set { ItemsDictionary = new Dictionary<TKey,TValue>(); foreach (DictionaryItem item in value) ItemsDictionary.Add(item.Key, item.Value); } } /// <summary> /// Indexer base on dictionary key. /// </summary> /// <param name="key"></param> /// <returns></returns> public TValue this[TKey key] { get { return (ItemsDictionary[key]); } set { Debug.Assert(value != null); ItemsDictionary[key] = value; } } /// <summary> /// Delegate for get key from a dictionary value. /// </summary> /// <param name="value"></param> /// <returns></returns> public delegate TKey GetItemKeyDelegate(TValue value); /// <summary> /// Add a range of values automatically determining the associated keys. /// </summary> /// <param name="values"></param> /// <param name="keygen"></param> public void AddRange(IEnumerable<TValue> values, GetItemKeyDelegate keygen) { foreach (TValue v in values) ItemsDictionary.Add(keygen(v), v); } /// <summary> /// Items dictionary. /// </summary> [XmlIgnore] public Dictionary<TKey, TValue> ItemsDictionary = new Dictionary<TKey,TValue>(); } The classes deriving from this class are serialized in the following way: <XmlProcessList xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <Items> <DictionaryItemOfInt32Foo> <Key/> <Value/> </DictionaryItemOfInt32XmlProcess> <Items> This give me a good solution, but: How can I control the name of the element DictionaryItemOfInt32Foo What happens if I define a Dictionary<FooInt32, Int32> and I have the classes Foo and FooInt32? Is it possible to optimize the class above? THank you very much!

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