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• Representing complex object dependencies

  - by max

  I have several classes with a reasonably complex (but acyclic) dependency graph. All the dependencies are of the form: class X instance contains an attribute of class Y. All such attributes are set during initialization and never changed again. Each class' constructor has just a couple parameters, and each object knows the proper parameters to pass to the constructors of the objects it contains. class Outer is at the top of the dependency hierarchy, i.e., no class depends on it. Currently, the UI layer only creates an Outer instance; the parameters for Outer constructor are derived from the user input. Of course, Outer in the process of initialization, creates the objects it needs, which in turn create the objects they need, and so on. The new development is that the a user who knows the dependency graph may want to reach deep into it, and set the values of some of the arguments passed to constructors of the inner classes (essentially overriding the values used currently). How should I change the design to support this? I could keep the current approach where all the inner classes are created by the classes that need them. In this case, the information about "user overrides" would need to be passed to Outer class' constructor in some complex user_overrides structure. Perhaps user_overrides could be the full logical representation of the dependency graph, with the overrides attached to the appropriate edges. Outer class would pass user_overrides to every object it creates, and they would do the same. Each object, before initializing lower level objects, will find its location in that graph and check if the user requested an override to any of the constructor arguments. Alternatively, I could rewrite all the objects' constructors to take as parameters the full objects they require. Thus, the creation of all the inner objects would be moved outside the whole hierarchy, into a new controller layer that lies between Outer and UI layer. The controller layer would essentially traverse the dependency graph from the bottom, creating all the objects as it goes. The controller layer would have to ask the higher-level objects for parameter values for the lower-level objects whenever the relevant parameter isn't provided by the user. Neither approach looks terribly simple. Is there any other approach? Has this problem come up enough in the past to have a pattern that I can read about? I'm using Python, but I don't think it matters much at the design level.

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• Design pattern to handle queries using multiple models

  - by coderkane

  I am presented with a dilemma while trying to re-designing the class structure for my PHP/MySQL application to make it more elegant and conform it to the SOLID principle. The problem goes like this: Let as assume, there is an abstract class called person which has certain properties to define a generic person, such as name, age, date of birth etc. There are two classes, student, and teacher, that implements this abstract class. They add their own unique properties to it. I have designed all the three classes to include all the operational logic (details of which are not relevant in context of the question). Now, I need to create views/reports/data grids which contain details from multiple classes, for example, say, a list of all students doing projects in Chemistry mentored by a teacher whose name is the parameter to the query. This is just one example of a view, there are many different views in the application, which uses data from 3-4 tables, and each of them have multiple input parameters to generate them. Considering this particular example, I have written the relevant query using JOIN and the results are as expected and proper, now here is the dilemma: Keeping in mind the single responsibility principle, where should I keep this query? It does not belong to either Student class, or Teacher class or any other classes currently present. a) Should I create a new class, say dataView class, and design it as a MVC pattern and keep the query there? What about the other views? how do they fit in this architecture? b) Should I not keep the query in code at all, and make it DB View ? c) Am I completely wrong in the approach? If so what is the right approach? My considerations are as follows: a) should be easy to add new views later on if requirement comes, without having to copy-paste-modify code b) would like to make it as loosely coupled as possible so that if minor db structure changes happen, it does not break I did google searches on report design and OOP report generators, but all the result seem to focus on the visual design of the report rather than fetching the data. I have already taken care of the visual aspect of the report using MVC with html templates. I am sure this is a very fundamental problem with known solution, but I am somehow not able to find it (maybe searching with wrong keyword). Edit1: Modified the title to make it more relevant Edit2: The accepted answer got me thinking in the right direction and identify my design flaws, which eventually led me to find this question and the solution in Stack Overflow which gave me the detailed answer to clear the confusion.

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• Is is common to use the command pattern for property get/sets?

  - by k rey

  Suppose I have a controller class with a bunch of properties. Each time a property is changed, I would like to update the model. Now also suppose that I use the command pattern to perform model updates. Is it common to use command classes within property get and sets of the controller class or is there a better way? Here is an example of what I am currently using: class MyController { private int _myInt; public int MyInt { get { return _myInt; } set { MyCommand cmd = new MyCommand(); cmd.Argument = _myInt; cmd.Execute(); // Command object updates the model } } }

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• Learning good OOP design & unlearning some bad habits

  - by Nick

  I have been mostly a C programmer so far in my career with knowledge of C++. I rely on C++ mostly for the convenience STL provides and I hardly ever focus on good design practices. As I have started to look for a new job position, this bad habit of mine has come back to haunt me. During the interviews, I have been asked to design a problem (like chess, or some other scenario) using OOP and I doing really badly at that (I came to know this through feedback from one interview). I tried to google stuff and came up with so many opinions and related books that I don't know where to begin. I need a good through introduction to OOP design with which I can learn practical design, not just theory. Can you point me to any book which meets my requirements ? I prefer C++, but any other language is fine as long as I can pick-up good practices. Also, I know that books can only go so far. I would also appreciate any good practice project ideas that helped you learn and improve your OOP concepts. Thanks.

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• How bad is it to have two methods with the same name but different signatures in two classes?

  - by Super User

  I have a design problem related to a public interface, the names of methods, and the understanding of my API and code. I have two classes like this: class A: ... function collision(self): .... ... class B: .... function _collision(self, another_object, l, r, t, b): .... The first class has one public method named collision, and the second has one private method called _collision. The two methods differs in argument type and number. As an example let's say that _collision checks if the object is colliding with another object with certain conditions l, r, t, b (collide on the left side, right side, etc) and returns true or false. The public collision method, on the other hand, resolves all the collisions of the object with other objects. The two methods have the same name because I think it's better to avoid overloading the design with different names for methods that do almost the same thing, but in distinct contexts and classes. Is this clear enough to the reader or I should change the method's name?

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• Philosophy behind the memento pattern

  - by TheSilverBullet

  I have been reading up on memento pattern from various sources of the internet. Differing information from different sources has left me in confusion regarding why this pattern is actually needed. The dofactory implementation says that the primary intention of this pattern is to restore the state of the system. Wiki says that the primary intention is to be able to restore the changes on the system. This gives a different impact - saying that it is possible for a system to have memento implementation with no need to restore. And that ability of restore is a feature of this. OODesign says that It is sometimes necessary to capture the internal state of an object at some point and have the ability to restore the object to that state later in time. Such a case is useful in case of error or failure. So, my question is why exactly do we use this one? Is it to save previous states - or to promote encapsulation between the Caretaker and the Memento? Why is this type of encapsulation so important? Edit: For those visiting, check out this Implementation!

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• What is the difference between all-static-methods and applying a singleton pattern?

  - by shahensha

  I am making a database to store information about the users of my website (I am using stuts2 and hence Java EE technology). For the database I'll be making a DBManager. Should I apply singleton pattern here or rather make all it's methods static? I will be using this DBManager for basic things like adding, deleting and updating User profiles. Along with it, I'll use for all other querying purposes, for instance to find out whether a username already exists and to get all users for administrative purposes and stuff like that. My questions What is the benefit of singleton pattern? Which thing is most apt here? All static methods or a singleton pattern? Please compare both of them. regards shahensha P.S. The database is bigger than this. Here I am talking only about the tables which I'll be using for storing User Information.

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• Techniques for separating game model from presentation

  - by liortal

  I am creating a simple 2D game using XNA. The elements that make up the game world are what i refer to as the "model". For instance, in a board game, i would have a GameBoard class that stores information about the board. This information could be things such as: Location Size Details about cells on the board (occupied/not occupied) etc This object should either know how to draw itself, or describe how to draw itself to some other entity (renderer) in order to be displayed. I believe that since the board only contains the data+logic for things regarding it or cells on it, it should not provide the logic of how to draw things (separation of concerns). How can i achieve a good partitioning and easily allow some other entity to draw it properly? My motivations for doing so are: Allow multiple "implementations" of presentation for a single game entity Easier porting to other environments where the presentation code is not available (for example - porting my code to Unity or other game technology that does not rely on XNA).

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• Is an event loop just a for/while loop with optimized polling?

  - by Alan

  I'm trying to understand what an event loop is. Often the explanation is that in the event loop, you do something until you're notified that an event occurred. You than handle the event and continue doing what you did before. To map the above definition with an example. I have a server which 'listens' in a event loop, and when a socket connection is detected, the data from it gets read and displayed, after which the server goes to the listening it did before. However, this event happening and us getting notified 'just like that' are to much for me to handle. You can say: "It's not 'just like that' you have to register an event listener". But what's an event listener but a function which for some reason isn't returning. Is it in it's own loop, waiting to be notified when an event happens? Should the event listener also register an event listener? Where does it end? Events are a nice abstraction to work with, however just an abstraction. I believe that in the end, polling is unavoidable. Perhaps we are not doing it in our code, but the lower levels (the programming language implementation or the OS) are doing it for us. It basically comes down to the following pseudo code which is running somewhere low enough so it doesn't result in busy waiting: while(True): do stuff check if event has happened (poll) do other stuff This is my understanding of the whole idea, and i would like to hear if this is correct. I am open in accepting that the whole idea is fundamentally wrong, in which case I would like the correct explanation. Best regards

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• What is the best approach to solve a factory method problem which has to be an instance?

  - by Iago

  I have to add new funcionality in a web service legacy project and I'm thinking what is the best approach for a concrete situation. The web service is simple: It receives a XML file, unmarshalling, generates response's objects, marshalling and finally it sends the response as a XML file. For every XML files received, the web service always responds with the same XML structure. What I have to do is to generate a different XML file according to the XML received. So I have a controller class which has all marshalling/unmarshalling operations, but this controller class has to be an instance. Depending on XML received I need some marshalling methods or others. Trying to make few changes on legacy source, what is the best approach? My first approach was to do a factory method pattern with the controller class, but this class has to be an instance. I want to keep, as far as it goes, this structure: classController.doMarshalling(); I think this one is a bit smelly: if(XMLReceived.isTypeOne()) classController.doMarshallingOne(); else if(XMLReceived.isTypeTwo()) classController.doMarshallingTwo(); else if(XMLReceived.isTypeThree()) classController.doMarshallingThree(); else if ... I hope my question is well understood

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• What's the best way to expose a Model object in a ViewModel?

  - by Angel

  In a WPF MVVM application, I exposed my model object into my viewModel by creating an instance of Model class (which cause dependency) into ViewModel. Instead of creating separate VM properties, I wrap the Model properties inside my ViewModel Property. My model is just an entity framework generated proxy class: public partial class TblProduct { public TblProduct() { this.TblPurchaseDetails = new HashSet<TblPurchaseDetail>(); this.TblPurchaseOrderDetails = new HashSet<TblPurchaseOrderDetail>(); this.TblSalesInvoiceDetails = new HashSet<TblSalesInvoiceDetail>(); this.TblSalesOrderDetails = new HashSet<TblSalesOrderDetail>(); } public int ProductId { get; set; } public string ProductCode { get; set; } public string ProductName { get; set; } public int CategoryId { get; set; } public string Color { get; set; } public Nullable<decimal> PurchaseRate { get; set; } public Nullable<decimal> SalesRate { get; set; } public string ImagePath { get; set; } public bool IsActive { get; set; } public virtual TblCompany TblCompany { get; set; } public virtual TblProductCategory TblProductCategory { get; set; } public virtual TblUser TblUser { get; set; } public virtual ICollection<TblPurchaseDetail> TblPurchaseDetails { get; set; } public virtual ICollection<TblPurchaseOrderDetail> TblPurchaseOrderDetails { get; set; } public virtual ICollection<TblSalesInvoiceDetail> TblSalesInvoiceDetails { get; set; } public virtual ICollection<TblSalesOrderDetail> TblSalesOrderDetails { get; set; } } Here is my ViewModel: public class ProductViewModel : WorkspaceViewModel { #region Constructor public ProductViewModel() { StartApp(); } #endregion //Constructor #region Properties private IProductDataService _dataService; public IProductDataService DataService { get { if (_dataService == null) { if (IsInDesignMode) { _dataService = new ProductDataServiceMock(); } else { _dataService = new ProductDataService(); } } return _dataService; } } //Get and set Model object private TblProduct _product; public TblProduct Product { get { return _product ?? (_product = new TblProduct()); } set { _product = value; } } #region Public Properties public int ProductId { get { return Product.ProductId; } set { if (Product.ProductId == value) { return; } Product.ProductId = value; RaisePropertyChanged("ProductId"); } } public string ProductName { get { return Product.ProductName; } set { if (Product.ProductName == value) { return; } Product.ProductName = value; RaisePropertyChanged(() => ProductName); } } private ObservableCollection<TblProduct> _productRecords; public ObservableCollection<TblProduct> ProductRecords { get { return _productRecords; } set { _productRecords = value; RaisePropertyChanged("ProductRecords"); } } //Selected Product private TblProduct _selectedProduct; public TblProduct SelectedProduct { get { return _selectedProduct; } set { _selectedProduct = value; if (_selectedProduct != null) { this.ProductId = _selectedProduct.ProductId; this.ProductCode = _selectedProduct.ProductCode; } RaisePropertyChanged("SelectedProduct"); } } #endregion //Public Properties #endregion // Properties #region Commands private ICommand _newCommand; public ICommand NewCommand { get { if (_newCommand == null) { _newCommand = new RelayCommand(() => ResetAll()); } return _newCommand; } } private ICommand _saveCommand; public ICommand SaveCommand { get { if (_saveCommand == null) { _saveCommand = new RelayCommand(() => Save()); } return _saveCommand; } } private ICommand _deleteCommand; public ICommand DeleteCommand { get { if (_deleteCommand == null) { _deleteCommand = new RelayCommand(() => Delete()); } return _deleteCommand; } } #endregion //Commands #region Methods private void StartApp() { LoadProductCollection(); } private void LoadProductCollection() { var q = DataService.GetAllProducts(); this.ProductRecords = new ObservableCollection<TblProduct>(q); } private void Save() { if (SelectedOperateMode == OperateModeEnum.OperateMode.New) { //Pass the Model object into Dataservice for save DataService.SaveProduct(this.Product); } else if (SelectedOperateMode == OperateModeEnum.OperateMode.Edit) { //Pass the Model object into Dataservice for Update DataService.UpdateProduct(this.Product); } ResetAll(); LoadProductCollection(); } #endregion //Methods } Here is my Service class: class ProductDataService:IProductDataService { /// <summary> /// Context object of Entity Framework model /// </summary> private MaizeEntities Context { get; set; } public ProductDataService() { Context = new MaizeEntities(); } public IEnumerable<TblProduct> GetAllProducts() { using(var context=new R_MaizeEntities()) { var q = from p in context.TblProducts where p.IsDel == false select p; return new ObservableCollection<TblProduct>(q); } } public void SaveProduct(TblProduct _product) { using(var context=new R_MaizeEntities()) { _product.LastModUserId = GlobalObjects.LoggedUserID; _product.LastModDttm = DateTime.Now; _product.CompanyId = GlobalObjects.CompanyID; context.TblProducts.Add(_product); context.SaveChanges(); } } public void UpdateProduct(TblProduct _product) { using (var context = new R_MaizeEntities()) { context.TblProducts.Attach(_product); context.Entry(_product).State = EntityState.Modified; _product.LastModUserId = GlobalObjects.LoggedUserID; _product.LastModDttm = DateTime.Now; _product.CompanyId = GlobalObjects.CompanyID; context.SaveChanges(); } } public void DeleteProduct(int _productId) { using (var context = new R_MaizeEntities()) { var product = (from c in context.TblProducts where c.ProductId == _productId select c).First(); product.LastModUserId = GlobalObjects.LoggedUserID; product.LastModDttm = DateTime.Now; product.IsDel = true; context.SaveChanges(); } } } I exposed my model object in my viewModel by creating an instance of it using new keyword, also I instantiated my DataService class in VM. I know this will cause a strong dependency. So: What's the best way to expose a Model object in a ViewModel? What's the best way to use DataService in VM?

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• Passing data between engine layers

  - by spaceOwl

  I am building a software system (game engine with networking support ) that is made up of (roughly) these layers: Game Layer Messaging Layer Networking Layer Game related data is passed to the messaging layer (this could be anything that is game specific), where they are to be converted to network specific messages (which are then serialized to byte arrays). I'm looking for a way to be able to convert "game" data into "network" data, such that no strong coupling between these layers will exist. As it looks now, the Messaging layer sits between both layers (game and network) and "knows" both of them (it contains Converter objects that know how to translate between data objects of both layers back and forth). I am not sure this is the best solution. Is there a good design for passing objects between layers? I'd like to learn more about the different options.

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• When to use functional programming approach and when not? (in Java)

  - by john smith optional

  let's assume I have a task to create a Set of class names. To remove duplication of .getName() method calls for each class, I used org.apache.commons.collections.CollectionUtils and org.apache.commons.collections.Transformer as follows: Snippet 1: Set<String> myNames = new HashSet<String>(); CollectionUtils.collect( Arrays.<Class<?>>asList(My1.class, My2.class, My3.class, My4.class, My5.class), new Transformer() { public Object transform(Object o) { return ((Class<?>) o).getName(); } }, myNames); An alternative would be this code: Snippet 2: Collections.addAll(myNames, My1.class.getName(), My2.class.getName(), My3.class.getName(), My4.class.getName(), My5.class.getName()); So, when using functional programming approach is overhead and when it's not and why? Isn't my usage of functional programming approach in snippet 1 is an overhead and why?

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• Need to re-build an application - how?

  - by Tom

  For our main system, we have a small monitor application that sits outside our network and periodically tries to log in to verify the system still works. We have a problem with the monitor though in that the communications component set (Asta 3 inside Delphi applications) doesn't always connect through. Overall, I'd say it's about 95% reliable, but that other 5% kills the monitor since it will try to log in and hang on the connection attempt (no timeout in the component). This really isn't an issue on the client side of the system since the clients don't disconnect and reconnect repeatedly on the same application instance, but I need a way to make sure the monitor stays up and continues working even when the component fails on a run. I have a few ideas as to which way to have the program run, the main idea being to put the communications inside a threaded data module so that if one thread crashes then another thread can test later and the program keep going. Does this sound like a valid way to go? Any other ideas how to ensure a reliable monitoring application with a less than 100% reliable component? Thanks. P.S. Not sure these tags are the most appropriate. Tried including "system-reliability" as one, but not high enough rep to create.

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• How essential is it to make a service layer?

  - by BornToCode

  I started building an app in 3 layers (DAL, BL, UI) [it mainly handles CRM, some sales reports and inventory]. A colleague told me that I must move to service layer pattern, that developers came to service pattern from their experience and it is the better approach to design most applications. He said it would be much easier to maintain the application in the future that way. Personally, I get the feeling that it's just making things more complex and I couldn't see much of a benefit from it that would justify that. This app does have an additional small partial ui that uses some (but only few) of the desktop application functions so I did find myself duplicating some code (but not much). Just because of some code duplication I wouldn't convert it to be service oriented, but he said I should use it anyway because in general it's a very good architecture, why programmers are so in love with services?? I tried to google on it but I'm still confused and can't decide what to do.

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• Absolute Top Programming Tips [closed]

  - by Eric

  I'm very intersted in the stuff that REALLY makes a critical difference to career in programming, other than intrinsic stuff like how smart your are, where you were born, etc... Some ideas: 1) Best approach to managing small, medium, and large teams. 2) Most important books to read. 3) Most important skills to know. 4) Correct balance of learning theory vs. just writing code. 5) A good approach to estimating time and cost of a project. 6) Etc... Please limit your answers. If you see somebody has already written your idea, please just vote for their response. I'd like to see what the community thinks are the true indicators of a successful career in our field.

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• Authorization design-pattern / practice?

  - by Lawtonfogle

  On one end, you have users. On the other end, you have activities. I was wondering if there is a best practice to relate the two. The simplest way I can think of is to have every activity have a role, and assign every user every role they need. The problem is that this gets really messy in practice as soon as you go beyond a trivial system. A way I recently designed was to have users who have roles, and roles have privileges, and activities require some combinations of privileges. For the trivial case, this is more complex, but I think it will scale better. But after I implemented it, I felt like it was overkill for the system I had. Another option would be to have users, who have roles, and activities require you to have a certain role to perform with many activities sharing roles. A more complex variant of this would given activities many possible roles, which you only needed one of. And an even more complex variant would be to allow logical statements of role ownership to use an activity (i.e. Must have A and (B exclusive or C) and must not have D). I could continue to list more, but I think this already gives a picture. And many of these have trade offs. But in software design, there are oftentimes solutions, while perhaps not perfect in every possible case, are clearly top of the pack to an extent it isn't even considered opinion based (i.e. how to store passwords, plain text is worse, hashing better, hashing and salt even better, despite the increased complexity of each level) (i.e. 2, Smart UI designs for applications are bad, even if it is subjective as to what the best design is). So, is there a best practice for authorization design that is not purely opinion based/subjective?

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• How should I architect a personal schedule manager that runs 24/7?

  - by Crawford Comeaux

  I've developed an ADHD management system for myself that's attempting to change multiple habits at once. I know this is counter to conventional wisdom, but I've tried the conventional for years & am now trying it my way. (just wanted to say that to try and prevent it from distracting people from the actual question) Anyway, I'd like to write something to run on a remote server that monitors me, helps me build/avoid certain habits, etc. What this amounts to is a system that: runs 24/7 may have multiple independent tasks to run at once may have tasks that require other tasks to run first lets tasks be scheduled by specific time, recurrence (ie. "run every 5 mins"), or interval (ie. "run from 2pm to 3pm") My first naive attempt at this was just a single PHP script scheduled to run every minute by cron (language was chosen in order to use a certain library, but no longer necessary). The logic behind when to run this or that portion of code got hairy pretty quick. So my question is how should I approach this from here? I'm not tied to any one language, though I'm partial to python/javascript. Thoughts: Could be done as a set of scripts that include a scheduling mechanism with one script per bit of logic...but the idea just feels wrong to me. Building it as a daemon could be helpful, but still unsure what to do about dozens of if-else statements for detecting the current time

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• Syncing client and server CRUD operations using json and php

  - by Justin

  I'm working on some code to sync the state of models between client (being a javascript application) and server. Often I end up writing redundant code to track the client and server objects so I can map the client supplied data to the server models. Below is some code I am thinking about implementing to help. What I don't like about the below code is that this method won't handle nested relationships very well, I would have to create multiple object trackers. One work around is for each server model after creating or loading, simply do $model->clientId = $clientId; IMO this is a nasty hack and I want to avoid it. Adding a setCientId method to all my model object would be another way to make it less hacky, but this seems like overkill to me. Really clientIds are only good for inserting/updating data in some scenarios. I could go with a decorator pattern but auto generating a proxy class seems a bit involved. I could use a generic proxy class that uses a __call function to allow for original object data to be accessed, but this seems wrong too. Any thoughts or comments? $clientData = '[{name: "Bob", action: "update", id: 1, clientId: 200}, {name:"Susan", action:"create", clientId: 131} ]'; $jsonObjs = json_decode($clientData); $objectTracker = new ObjectTracker(); $objectTracker->trackClientObjs($jsonObjs); $query = $this->em->createQuery("SELECT x FROM Application_Model_User x WHERE x.id IN (:ids)"); $query->setParameters("ids",$objectTracker->getClientSpecifiedServerIds()); $models = $query->getResults(); //Apply client data to server model foreach ($models as $model) { $clientModel = $objectTracker->getClientJsonObj($model->getId()); ... } //Create new models and persist foreach($objectTracker->getNewClientObjs() as $newClientObj) { $model = new Application_Model_User(); .... $em->persist($model); $objectTracker->trackServerObj($model); } $em->flush(); $resourceResponse = $objectTracker->createResourceResponse(); //Id mappings will be an associtave array representing server id resources with client side // id. //This method Dosen't seem to flexible if we want to return additional data with each resource... //Would have to modify the returned data structure, seems like tight coupling... //Ex return value: //[{clientId: 200, id:1} , {clientId: 131, id: 33}];

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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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• 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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• how should I design Objects around this business requirement?

  - by brainydexter

  This is the business requirement: " A Holiday Package (e.g. New York NY Holiday Package) can be offered in different ways based on the Origin city: From New Delhi to NY From Bombay to NY NY itself ( Land package ) (Bold implies default selection) a. and b. User can fly from either New Delhi or Bombay to NY. c. NY is a Land package, where a user can reach NY by himself and is a standalone holidayPackage. " Let's say I have a class that represents HolidayPackage, Destination (aka City). public class HolidayPackage{ Destination holidayCity; ArrayList<BaseHolidayPackageVariant> variants; BaseHolidayPackageVariant defaultVariant; } public abstract class BaseHolidayPackageVariant { private Integer variantId; private HolidayPackage holidayPackage; private String holidayPackageType; } public class LandHolidayPackageVariant extends BaseHolidayPackageVariant{ } public class FlightHolidayPackageVariant extends BaseHolidayPackageVariant{ private Destination originCity; } What data structure/objects should I design to support: options a default within those options Sidenote: A HolidayPackage can also be offered in different ways based on Hotel selections. I'd like to follow a design which I can leverage to support that use case in the future. This is the backend design I have in mind.

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• Query something and return the reason if nothing has been found

  - by Daniel Hilgarth

  Assume I have a Query - as in CQS that is supposed to return a single value. Let's assume that the case that no value is found is not exceptional, so no exception will be thrown in this case. Instead, null is returned. However, if no value has been found, I need to act according to the reason why no value has been found. Assuming that the Query knows the reason, how would I communicate it to the caller of the Query? A simple solution would be not return the value directly but a container object that contains the value and the reason: public class QueryResult { public TValue Value { get; private set; } public TReason ReasonForNoValue { get; private set; } } But that feels clumsy, because if a value is found, ReasonForNoValue makes no sense and if no value has been found, Value makes no sense. What other options do I have to communicate the reason? What do you think of one event per reason? For reference: This is going to be implemented in C#.

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• How to play many sounds at once in OpenAL

  - by Krom Stern

  I'm developing an RTS game and I would like to add sounds to it. My choice has landed on OpenAL. I have plenty of units which from time to time make sounds: fSound.Play(sfx_shoot, location). Sounds often repeat, e.g. when squad of archers shoots arrows, but they are not synced with each other. My questions are: What is the common design pattern to play multiple sounds in OpenAL, when some of them are duplicate? What are the hardware limitations on sounds count and tricks to overcome them?

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• Questioning one of the arguments for dependency injection: Why is creating an object graph hard?

  - by oberlies

  Dependency injection frameworks like Google Guice give the following motivation for their usage (source): To construct an object, you first build its dependencies. But to build each dependency, you need its dependencies, and so on. So when you build an object, you really need to build an object graph. Building object graphs by hand is labour intensive (...) and makes testing difficult. But I don't buy this argument: Even without dependency injection, I can write classes which are both easy to instantiate and convenient to test. E.g. the example from the Guice motivation page could be rewritten in the following way: class BillingService { private final CreditCardProcessor processor; private final TransactionLog transactionLog; // constructor for tests, taking all collaborators as parameters BillingService(CreditCardProcessor processor, TransactionLog transactionLog) { this.processor = processor; this.transactionLog = transactionLog; } // constructor for production, calling the (productive) constructors of the collaborators public BillingService() { this(new PaypalCreditCardProcessor(), new DatabaseTransactionLog()); } public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) { ... } } So there may be other arguments for dependency injection (which are out of scope for this question!), but easy creation of testable object graphs is not one of them, is it?

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