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• Correct Interactive Website System Design Concepts / Methods?

  - by Xandel

  Hi all, I hope this question isn't too open ended, but a nudge in the right direction is all I need! I am currently building an online accounting system - the idea is that users can register, log in, and then create customers, generate invoices and other documents and eventually print / email those documents out. I am a Java programmer but unfortunately haven't had too much experience in web projects and their design concepts... This is what I have got thus far - A Tomcat web server which loads Spring. Spring handles my DAO's and required classes for the business logic. Tomcat serves JSP's containing the pages which make up the website. To make it interactive I have used JavaScript in the pages (jQuery and its AJAX calls) to send and receive JSON data (this is done by posting to a page which calls a handleAction() method in one of my classes). My question is, am I tackling this project in the right way? Am I using the right tools and methods? I understand there are literally countless ways of tackling any project but I would really love to get feedback with regards to tried and tested methods, general practices etc. Thanks in advance! Xandel

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• Design Practice Retrieve Multiple Users - PHP

  - by pws5068

  Greetings all, I'm looking for a more efficient way to grab multiple users without too much code redundancy. I have a Users class, (here's a highly condensed representation) Class Users { function __construct() { ... ... } private static function getNew($id) { // This is all only example code $sql = "SELECT * FROM Users WHERE User_ID=?"; $user = new User(); $user->setID($id); .... return $user; } public static function getNewestUsers($count) { $sql = "SELECT * FROM Users ORDER BY Join_Date LIMIT ?"; $users = array(); // Prepared Statement Data Here while($stmt->fetch()) { $users[] = Users::getNew($id); ... } return $users; } // These have more sanity/security checks, demonstration only. function setID($id) { $this->id = $id; } function setName($name) { $this->name = $name; } ... } So clearly calling getNew() in a loop is inefficient, because it makes $count number of calls to the database each time. I would much rather select all users in a single SQL statement. The problem is that I have probably a dozen or so methods for getting arrays of users, so all of these would now need to know the names of the database columns which is repeating a lot of structure that could always change later. What are some other ways that I could approach this problem? My goals include efficiency, flexibility, scalability, and good design practice.

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• I like the way they Design/Architecture it but how do I implement this

  - by Rachel

  Summary: I have different components on homepage and each components shows some promotion to the user. I have Cart as one Component and depending upon content of the cart promotion are show. I have to track user online activities and send that information to Omniture for Report Generation. Now my components are loaded asynchronously basically are loaded when AjaxRequest is fired up and so there is not fix pattern or rather information on when components will appear on the webpages. Now in order to pass information to Omniture I need to call track function on $(document).(ready) and append information for each components(7 parameters are required by Omniture for each component). So in the init:config function of each component am calling Omniture and passing paramters but now no. of Omniture calls is directly proportional to no. of Components on the webpage but this is not acceptable as each call to Omniture is very expensive. Now I am looking for a way where in I can club the information about 7 parameters and than make one Call to Omniture wherein I pass those information. Points to note is that I do not know when the components are loaded and so there is no pre-defined time or no. of components that would be loaded. The thing is am calling track function when document is ready but components are loaded after call to Omniture has been made and so my question is Q: How can I collect the information for all the components and than just make one call to Omniture to send those information ? As mentioned, I do not know when the components are loaded as they are done on the Ajax Request. Hope I am able to explain my challenge and would appreciate if some one can provide from Design/Architect Solutions for the Challenge.

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• Design problem with callback functions in android

  - by Franz Xaver

  Hi folks! I'm currently developing an app in android that is accessing wifi values, that is, the application needs to scan for all access point and their specific signal strengths. I know that I have to extend the class BroadcastReceiver overwriting the method BroadcastReceiver.onReceive(Context context, Intent intent) which is called when the values are ready. Perhaps there exist solutions provided by the android system itself but I'm relatively new to android so I could need some help. The problem I encountered is that I got one class (an activity, thus controlled by the user) that needs this scan results for two different things (first to save the values in a database or second, to use them for further calculations but not both at one moment!) So how to design the callback system in order to "transport" the scan results from onReceive(Context context, Intent intent) to the operation intended by the user? My first solution was to define enums for each use case (save or use for calculations) which wlan-interested classes have to submit when querying for the values. But that would force the BroadcastReceiverextending class to save the current enum and use it as a parameter in the callback function of the querying class (this querying class needs to know what it asked for when getting backcalled) But that seems to me kind of dirty ;) So anyone a good idea for this?

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• MySQL database design question

  - by Greelmo

  I'm trying to weigh the pros and cons of a database design, and would like to get some feedback as to the best approach. Here is the situation: I have users of my system that have only a few required items (username, password). They can then supply a lot of optional information. This optional information continues to grow as the system grows, so I want to do it in such a way that adding new optional information is easy. Currently, I have a separate table for each piece of information. For example, there's a table called 'names' that holds 'user_id', 'first_name', and 'last_name'. There's 'address', 'occupation', etc. You get the drift. In most cases, when I talk to my database, I'm looking only for users with one particular qualifier (name, address, etc.). However, there are instances when I want to see what information a user has set. The 'edit account' page, for example, must run queries for each piece of information it wants. Is this wasteful? Is there a way I can structure my queries or my database to make it so I never have to do one query for each piece of information like that without getting my tables to huge? If i want to add 'marital status', how hard will that be if I don't have a one-table-per-attribute system? Thanks in advance.

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• Java application design question

  - by ring bearer

  I have a hobby project, which is basically to maintain 'todo' tasks in the way I like. One task can be described as: public class TodoItem { private String subject; private Date dueBy; private Date startBy; private Priority priority; private String category; private Status status; private String notes; } As you can imagine I would have 1000s of todo items at a given time. What is the best strategy to store a todo item? (currently on an XML file) such that all the items are loaded quickly up on app start up(the application shows kind of a dashboard of all the items at start up)? What is the best way to design its back-end so that it can be ported to Android/or a J2ME based phone? Currently this is done using Java Swing. What should I concentrate on so that it works efficiently on a device where memory is limited? The application throws open a form to enter new todo task. For now, I would like to save the newly added task to my-todos.xml once the user presses "save" button. What are the common ways to append such a change to an existing XML file?(note that I don't want to read the whole file again and then persist)

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• Design issue in Iphone Dev - Generic implementation for Game Bonuses

  - by Idan

  So, I thought consulting you guys about my design, cause I sense there might be a better way of doing it. I need to implement game bonuses mechanism in my app. Currently there are 9 bonuses available, each one is based of different param of the MainGame Object. What I had in mind was at app startup to initialize 9 objects of GameBonus while each one will have different SEL (shouldBonus) which will be responsible for checking if the bonus is valid. So, every end of game I will just run over the bonuses array and call the isBonusValid() function with the MainGame object(which is different after every game). How's that sound ? The only issue I have currently, is that I need to make sure that if some bonuses are accepted some other won't (inner stuff)... any advice how to do that and still maintain generic implementation ? @interface GameBonus : NSObject { int bonusId; NSString* name; NSString* description; UIImage* img; SEL shouldBonus; } @implementation GameBonus -(BOOL) isBonusValid(MainGame*)mainGame { [self shouldBonus:mainGame]; } @end

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• Programming methods design phase assignment

  - by Shakir

  Hey, i have an assignment (NCC) which deals with the design phase. The Scenario is that you have four soccer divisions (divisions 1,2,3 and 4) which consist of 22 teams each and hence each team plays 42 games (home and away). The concept is similar to the barclays premier league whereby ranking is based on points or else goal difference or else goals scored by team. The difference is that the top 2 teams are promoted and the bottom 2 are relegated and this includes Div 1 and Div 4 as the top 2 of Div1 are promoted to the national league which is above division 1 regional league and bottom 2 of Div4 are relegated to the local league below Division 4 regional league. Hence there are 3 total leagues and 4 divisions in the regional league (which has 22 teams each). Now the referee has to add the result of the match and thus automatic tables have to be generated now, There are two reports League Tables for 4 divisions List of all results for any chosen team during the season by date it was played on There are a couple of things to be done... i know its gonna be terrible to make everything but atleast explain to me how i should go about drawing these and what things i should include (generally) Logical Data Structure Diagram (DSD) for each report Preliminary Program Structure (PSD) for each report Detailed Program Specification for each report Flowchart for each report There are other things but i think our teacher will give us clear "clues" for it Thanks a lot Update - Project so far: Data Structure Diagram Preliminary Program Structure

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• Properly design a code editor application

  - by Hemaulo

  I'm working on personal project which is basically a code editor. Imagine standard File menu with menu items New, Open, Save, Save As, Save All, Close, Close All. I'm stuck with proper design. Currently I have: A Document class which represents a document - code editing control, respective tab in tab bar and various properties such as Caption, Filename, IsModified etc. A Documents class which represents all open documents. Contains methods like New, Open(FileName), ... The problem is that I can't figure out which class / menu command is responsible for which tasks. For example, with File-New menu command is simple - call Documents.New and that's it. But what for File-Open? The Documents.Open method expects filename as a parameter. So before calling this method I need to open an file selection dialog, let user select files and for each file call Documents.Open(FileName). Where is best place for this supporting code, in menu command, rewrite Documents.Open and put it there? The same with Save actions. Which is responsible for saving? Is it Documents class which uses Document.Editor.SaveToFile(FileName) or better create Save method in Document class? Somewhere in the middle also need to ask user if he wants to save current document... I'm stuck. Any ideas? Edited: The programming language is Delphi.

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• Application design question, best approach?

  - by Jamie Keeling

  Hello, I am in the process of designing an application that will allow you to find pictures (screen shots) made from certain programs. I will provide the locations of a few of the program in the application itself to get the user started. I was wondering how I should go about adding new locations as the time goes on, my first thought was simply hard coding it into the application but this will mean the user has to reinstall it to make the changes take affect. My second idea was to use an XML file to contain all the locations as well as other data, such as the name of the application. This also means the user can add their own locations if they wish as well as sharing them over the internet. The second option seemed the best approach but then I had to think how would it be managed on the users computer. Ideally I'd like just a single .exe without the reliance on any external files such as the XML but this would bring me back to point one. Would it be best to simply use the ClickOnce deployment to create an entry in the start menu and create a folder containing the .exe and the file names? Thanks for the feedback, I don't want to start implementing the application until the design is nailed.

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• Including partial views when applying the Mode-View-ViewModel design pattern

  - by Filip Ekberg

  Consider that I have an application that just handles Messages and Users I want my Window to have a common Menu and an area where the current View is displayed. I can only work with either Messages or Users so I cannot work simultaniously with both Views. Therefore I have the following Controls MessageView.xaml UserView.xaml Just to make it a bit easier, both the Message Model and the User Model looks like this: Name Description Now, I have the following three ViewModels: MainWindowViewModel UsersViewModel MessagesViewModel The UsersViewModel and the MessagesViewModel both just fetch an ObserverableCollection<T> of its regarding Model which is bound in the corresponding View like this: <DataGrid ItemSource="{Binding ModelCollection}" /> The MainWindowViewModel hooks up two different Commands that have implemented ICommand that looks something like the following: public class ShowMessagesCommand : ICommand { private ViewModelBase ViewModel { get; set; } public ShowMessagesCommand (ViewModelBase viewModel) { ViewModel = viewModel; } public void Execute(object parameter) { var viewModel = new ProductsViewModel(); ViewModel.PartialViewModel = new MessageView { DataContext = viewModel }; } public bool CanExecute(object parameter) { return true; } public event EventHandler CanExecuteChanged; } And there is another one a like it that will show Users. Now this introduced ViewModelBase which only holds the following: public UIElement PartialViewModel { get { return (UIElement)GetValue(PartialViewModelProperty); } set { SetValue(PartialViewModelProperty, value); } } public static readonly DependencyProperty PartialViewModelProperty = DependencyProperty.Register("PartialViewModel", typeof(UIElement), typeof(ViewModelBase), new UIPropertyMetadata(null)); This dependency property is used in the MainWindow.xaml to display the User Control dynamicly like this: <UserControl Content="{Binding PartialViewModel}" /> There are also two buttons on this Window that fires the Commands: ShowMessagesCommand ShowUsersCommand And when these are fired, the UserControl changes because PartialViewModel is a dependency property. I want to know if this is bad practice? Should I not inject the User Control like this? Is there another "better" alternative that corresponds better with the design pattern? Or is this a nice way of including partial views?

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• Design Philosophy Question - When to create new functions

  - by Eclyps19

  This is a general design question not relating to any language. I'm a bit torn between going for minimum code or optimum organization. I'll use my current project as an example. I have a bunch of tabs on a form that perform different functions. Lets say Tab 1 reads in a file with a specific layout, tab 2 exports a file to a specific location, etc. The problem I'm running into now is that I need these tabs to do something slightly different based on the contents of a variable. If it contains a 1 I may need to use Layout A and perform some extra concatenation, if it contains a 2 I may need to use Layout B and do no concatenation but add two integer fields, etc. There could be 10+ codes that I will be looking at. Is it more preferable to create an individual path for each code early on, or attempt to create a single path that branches out only when absolutely required. Creating an individual path for each code would allow my code to be extremely easy to follow at a glance, which in turn will help me out later on down the road when debugging or making changes. The downside to this is that I will increase the amount of code written by calling some of the same functions in multiple places (for example, steps 3, 5, and 9 for every single code may be exactly the same. Creating a single path that would branch out only when required will be a bit messier and more difficult to follow at a glance, but I would create less code by placing conditionals only at steps that are unique. I realize that this may be a case-by-case decision, but in general, if you were handed a previously built program to work on, which would you prefer?

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• Class design when working with dataset

  - by MC

  If you have to retrieve data from a database and bring this dataset to the client, and then allow the user to manipulate the data in various ways before updating the database again, what is a good class design for this if the data tables will not have a 1:1 relationship with the class objects? Here are some I came up with: Just manipulate the DataSet itself on the client and then send it back to the database as is. This will work though obviously the code will be very dirty and not well-structured. Same as #1, but wrap the dataset code around classes. What I mean is that you may have a class that takes a dataset or a datatable in its constructor, and then provides public methods and properties to simplify the code. Inside these methods and properties it will be reading or manipulating the dataset. To update the database afterwards will be easy because you already have the updated dataset. Get rid of the dataset entirely on the client, convert to objects, then convert back to a dataset when needing to update the database. Is there any good resources where I can find information on this?

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• Object Design: How to Organize/Structure a "Collection Class"

  - by CrimsonX

  I'm currently struggling to understand how I should organize/structure a class which I have already created. The class does the following: As its input in the constructor, it takes a collection of logs In the constructor it validates and filters the logs through a series of algorithms implementing my business logic After all filtering and validation is complete, it returns a collection (a List) of the valid and filtered logs which can be presented to the user graphically in a UI. Here is some simplified code describing what I'm doing: class FilteredCollection { public FilteredCollection( SpecialArray<MyLog> myLog) { // validate inputs // filter and validate logs in collection // in end, FilteredLogs is ready for access } Public List<MyLog> FilteredLogs{ get; private set;} } However, in order to access this collection, I have to do the following: var filteredCollection = new FilteredCollection( secialArrayInput ); //Example of accessing data filteredCollection.FilteredLogs[5].MyLogData; Other key pieces of input: I foresee only one of these filtered collections existing in the application (therefore should I make it a static class? Or perhaps a singleton?) Testability and flexibility in creation of the object is important (Perhaps therefore I should keep this an instanced class for testability?) I'd prefer to simplify the dereferencing of the logs if at all possible, as the actual variable names are quite long and it takes some 60-80 characters to just get to the actual data. My attempt in keeping this class simple is that the only purpose of the class is to create this collection of validated data. I know that there may be no "perfect" solution here, but I'm really trying to improve my skills with this design and I would greatly appreciate advice to do that. Thanks in advance.

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• Database design - table relationship question

  - by iama

  I am designing schema for a simple quiz application. It has 2 tables - "Question" and "Answer Choices". Question table has 'question ID', 'question text' and 'answer id' columns. "Answer Choices" table has 'question ID', 'answer ID' and 'answer text' columns. With this simple schema it is obvious that a question can have multiple answer choices & hence the need for the answer choices table. However, a question can have only one correct answer and hence the need for the 'answer ID' in the question table. However, this 'answer ID' column in the question table provides a illusion as though there can be multiple questions for a single answer which is not correct. The other alternative to eliminate this illusion is to have another table just for correct answer that will have just 2 columns namely the question ID and the answer ID with a 1-1 relationship between the two tables. However, I think this is redundant. Any recommendation on how best to design this thereby enforcing the rules that a question can have multiple answer choices but only one correct answer? Many Thanks.

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• Design question for windows application, best approach?

  - by Jamie Keeling

  Hello, I am in the process of designing an application that will allow you to find pictures (screen shots) made from certain programs. I will provide the locations of a few of the program in the application itself to get the user started. I was wondering how I should go about adding new locations as the time goes on, my first thought was simply hard coding it into the application but this will mean the user has to reinstall it to make the changes take affect. My second idea was to use an XML file to contain all the locations as well as other data, such as the name of the application. This also means the user can add their own locations if they wish as well as sharing them over the internet. The second option seemed the best approach but then I had to think how would it be managed on the users computer. Ideally I'd like just a single .exe without the reliance on any external files such as the XML but this would bring me back to point one. Would it be best to simply use the ClickOnce deployment to create an entry in the start menu and create a folder containing the .exe and the file names? Thanks for the feedback, I don't want to start implementing the application until the design is nailed.

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• Database design: Calculating the Account Balance

  - by 001

  How do I design the database to calculate the account balance? 1) Currently I calculate the account balance from the transaction table In my transaction table I have "description" and "amount" etc.. I would then add up all "amount" values and that would work out the user's account balance. I showed this to my friend and he said that is not a good solution, when my database grows its going to slow down???? He said I should create separate table to store the calculated account balance. If did this, I will have to maintain two tables, and its risky, the account balance table could go out of sync. Any suggestion? EDIT: OPTION 2: should I add an extra column to my transaction tables "Balance". now I do not need to go through many rows of data to perform my calculation. Example John buys $100 credit, he debt $60, he then adds $200 credit. Amount $100, Balance $100. Amount -$60, Balance $40. Amount $200, Balance $240.

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• Java: design for using many executors services and only few threads

  - by Guillaume

  I need to run in parallel multiple threads to perform some tests. My 'test engine' will have n tests to perform, each one doing k sub-tests. Each test result is stored for a later usage. So I have n*k processes that can be ran concurrently. I'm trying to figure how to use the java concurrent tools efficiently. Right now I have an executor service at test level and n executor service at sub test level. I create my list of Callables for the test level. Each test callable will then create another list of callables for the subtest level. When invoked a test callable will subsequently invoke all subtest callables test 1 subtest a1 subtest ...1 subtest k1 test n subtest a2 subtest ...2 subtest k2 call sequence: test manager create test 1 callable test1 callable create subtest a1 to k1 testn callable create subtest an to kn test manager invoke all test callables test1 callable invoke all subtest a1 to k1 testn callable invoke all subtest an to kn This is working fine, but I have a lot of new treads that are created. I can not share executor service since I need to call 'shutdown' on the executors. My idea to fix this problem is to provide the same fixed size thread pool to each executor service. Do you think it is a good design ? Do I miss something more appropriate/simple for doing this ?

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• SQL Databases and table design/organization

  - by John McMullen

  (NOOB disclaimer) I'm working on a system (a type of map), that is accessed mostly via 3 fields: ID (auto incremented), X coordinate, and Y coordinate. As it is right now, i have all data on the map, stored in 1 table. Whenever the map display is loaded it simply queries the database for contents in x and y, and the DB gives the data (other fields in the same entry). If an item on the map is doing something, it has a flag saying its doing something, and then has an ID of the action in another table holding that type of 'actions'. Essentially, for all map data, its stored in 1 table. All actions of a certain type are stored in their own table. I'm a noob, and i'm wondering what the most effective/efficient structure for such a design? (a map that has items, and each item has stats/actions). I'm using PHP atm, using standard SQL queries to get my data. Should i split up the tables so that there are only x number of entries on a table? (coord range limits)? Should it just keep growing and growing? There's a lot of queries to the table... so just tryin to see what is best :/

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• C++ design related question

  - by Kotti

  Hi! Here is the question's plot: suppose I have some abstract classes for objects, let's call it Object. It's definition would include 2D position and dimensions. Let it also have some virtual void Render(Backend& backend) const = 0 method used for rendering. Now I specialize my inheritance tree and add Rectangle and Ellipse class. Guess they won't have their own properties, but they will have their own virtual void Render method. Let's say I implemented these methods, so that Render for Rectangle actually draws some rectangle, and the same for ellipse. Now, I add some object called Plane, which is defined as class Plane : public Rectangle and has a private member of std::vector<Object*> plane_objects; Right after that I add a method to add some object to my plane. And here comes the question. If I design this method as void AddObject(Object& object) I would face trouble like I won't be able to call virtual functions, because I would have to do something like plane_objects.push_back(new Object(object)); and this should be push_back(new Rectangle(object)) for rectangles and new Circle(...) for circles. If I implement this method as void AddObject(Object* object), it looks good, but then somewhere else this means making call like plane.AddObject(new Rectangle(params)); and this is generally a mess because then it's not clear which part of my program should free the allocated memory. ["when destroying the plane? why? are we sure that calls to AddObject were only done as AddObject(new something).] I guess the problems caused by using the second approach could be solved using smart pointers, but I am sure there have to be something better. Any ideas?

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• What design pattern to use for one big method calling many private methods

  - by Jeune

  I have a class that has a big method that calls on a lot of private methods. I think I want to extract those private methods into their own classes for one because they contain business logic and I think they should be public so they can be unit tested. Here's a sample of the code: public void handleRow(Object arg0) { if (continueRunning){ hashData=(HashMap<String, Object>)arg0; Long stdReportId = null; Date effDate=null; if (stdReportIds!=null){ stdReportId = stdReportIds[index]; } if (effDates!=null){ effDate = effDates[index]; } initAndPutPriceBrackets(hashData, stdReportId, effDate); putBrand(hashData,stdReportId,formHandlerFor==0?true:useLiveForFirst); putMultiLangDescriptions(hashData,stdReportId); index++; if (stdReportIds!=null && stdReportIds[0].equals(stdReportIds[1])){ continueRunning=false; } if (formHandlerFor==REPORTS){ putBeginDate(hashData,effDate,custId); } //handle logic that is related to pricemaps. lstOfData.add(hashData); } } What design pattern should I apply to this problem?

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• Design by contracts and constructors

  - by devoured elysium

  I am implementing my own ArrayList for school purposes, but to spice up things a bit I'm trying to use C# 4.0 Code Contracts. All was fine until I needed to add Contracts to the constructors. Should I add Contract.Ensures() in the empty parameter constructor? public ArrayList(int capacity) { Contract.Requires(capacity > 0); Contract.Ensures(Size == capacity); _array = new T[capacity]; } public ArrayList() : this(32) { Contract.Ensures(Size == 32); } I'd say yes, each method should have a well defined contract. On the other hand, why put it if it's just delegating work to the "main" constructor? Logicwise, I wouldn't need to. The only point I see where it'd be useful to explicitly define the contract in both constructors is if in the future we have Intelisense support for contracts. Would that happen, it'd be useful to be explicit about which contracts each method has, as that'd appear in Intelisense. Also, are there any books around that go a bit deeper on the principles and usage of Design by Contracts? One thing is having knowledge of the syntax of how to use Contracts in a language (C#, in this case), other is knowing how and when to use it. I read several tutorials and Jon Skeet's C# in Depth article about it, but I'd like to go a bit deeper if possible. Thanks

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• Design an Application That Stores and Processes Files

  - by phasetwenty

  I'm tasked with writing an application that acts as a central storage point for files (usually document formats) as provided by other applications. It also needs to take commands like "file 395 needs a copy in X format", at which point some work is offloaded to a 3rd party application. I'm having trouble coming up with a strategy for this. I'd like to keep the design as simple as possible, so I'd like to avoid big extra frameworks or techniques like threads for as long as it makes sense. The clients are expected to be web applications (for example, one is a django application that receives files from our customers; the others are not yet implemented). The platform it will be running on is likely going to be Python on Linux, unless I have a strong argument to use something else. In the beginning I thought I could fit the information I wanted to communicate in the filenames, and let my application parse the filename to figure out what it needed to do, but this is proving too inflexible with the amount of information I'm realizing I need to make available. Another idea is to pair FTP with a database used as a communication medium (client uploads a file and updates the database with a command as a row in a table) but I don't like this idea because adding commands (a known change) looks like it will require adding code as well as changing database schemas. It will also muddy up the interface my clients will have to use. I looked into Pyro to let applications communicate more directly but I don't like the idea of running an extra nameserver for this one purpose. I also don't see a good way to do file transfer within this framework. What I'm looking for is techniques and/or technologies applicable to my problem. At the simplest level, I need the ability to accept files and messages with them.

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• is this a design pattern?

  - by Michel

  Hi all, i have to build some financial data report, and for making the calculation, there are a lot of 'if then' situations: if it's a large client, subtract 10%, if it's postal code equals '10101', add 10%, if the day is on a saturday, make a difficult calculation etc. so i once read about this kind of example, and what they did was (hope i remember well) create a class with some base info and make it possible to add all kinds of calculationobjects to it. So to put what i remembered in pseudo code Basecalc bc = new baseCalc(); //put the info in the bc so other objects can do their if bc.Add(new Largecustomercalc()); bc.Add(new PostalcodeCalc()); bc.add(new WeekdayCalc()); the the bc would run the Calc() methods of all of the added Calc objects. As i type this i think all the Calc objects must be able to see the Basecalc properties to correctly perform their calculation logic. So all the if's are in the different Calc objects and not ALL in the Basecalc. does this make sense? I was wondering if this is some kind of design pattern?

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• java : how to handle the design when template methods throw exception when overrided method not throw

  - by jiafu

  when coding. try to solve the puzzle: how to design the class/methods when InputStreamDigestComputor throw IOException? It seems we can't use this degisn structure due to the template method throw exception but overrided method not throw it. but if change the overrided method to throw it, will cause other subclass both throw it. So can any good suggestion for this case? abstract class DigestComputor{ String compute(DigestAlgorithm algorithm){ MessageDigest instance; try { instance = MessageDigest.getInstance(algorithm.toString()); updateMessageDigest(instance); return hex(instance.digest()); } catch (NoSuchAlgorithmException e) { LOG.error(e.getMessage(), e); throw new UnsupportedOperationException(e.getMessage(), e); } } abstract void updateMessageDigest(MessageDigest instance); } class ByteBufferDigestComputor extends DigestComputor{ private final ByteBuffer byteBuffer; public ByteBufferDigestComputor(ByteBuffer byteBuffer) { super(); this.byteBuffer = byteBuffer; } @Override void updateMessageDigest(MessageDigest instance) { instance.update(byteBuffer); } } class InputStreamDigestComputor extends DigestComputor{ // this place has error. due to exception. if I change the overrided method to throw it. evey caller will handle the exception. but @Override void updateMessageDigest(MessageDigest instance) { throw new IOException(); } }

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