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  • Pattern for version-specific implementations of a Java class

    - by Mike Monkiewicz
    So here's my conundrum. I am programming a tool that needs to work on old versions of our application. I have the code to the application, but can not alter any of the classes. To pull information out of our database, I have a DTO of sorts that is populated by Hibernate. It consumes a data object for version 1.0 of our app, cleverly named DataObject. Below is the DTO class. public class MyDTO { private MyWrapperClass wrapper; public MyDTO(DataObject data) { wrapper = new MyWrapperClass(data); } } The DTO is instantiated through a Hibernate query as follows: select new com.foo.bar.MyDTO(t1.data) from mytable t1 Now, a little logic is needed on top of the data object, so I made a wrapper class for it. Note the DTO stores an instance of the wrapper class, not the original data object. public class MyWrapperClass { private DataObject data; public MyWrapperClass(DataObject data) { this.data = data; } public String doSomethingImportant() { ... version-specific logic ... } } This works well until I need to work on version 2.0 of our application. Now DataObject in the two versions are very similar, but not the same. This resulted in different sub classes of MyWrapperClass, which implement their own version-specific doSomethingImportant(). Still doing okay. But how does myDTO instantiate the appropriate version-specific MyWrapperClass? Hibernate is in turn instantiating MyDTO, so it's not like I can @Autowire a dependency in Spring. I would love to reuse MyDTO (and my dozens of other DTOs) for both versions of the tool, without having to duplicate the class. Don't repeat yourself, and all that. I'm sure there's a very simple pattern I'm missing that would help this. Any suggestions?

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  • Single-Page Web Apps: Client-side datastores & server persistence

    - by fig-gnuton
    How should client-side datastores & persistence be handled in a single-page web application? Global vars vs. DI/IoC: Should datastores be assigned to global variables so any part of the application can access them? Or should they be dependency injected where required? Server persistence: Assuming a datastore's data needn't always be persisted to the server immediately, should the datastore itself handle persistence? If not, then what class should handle persistence and how should the persistence class fit into the client-side architecture overall? Is the datastore considered the model in MVC, or is it something else since it just stores raw data?

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  • pattern to transfer search model to dao

    - by zeroed
    We have a dao as a project (jar file). Clients use its interfaces and factories to operate with database. Using standard CRUD operations, dao allows you to search an entity by some search criteria. What is the best way to represent this criteria? Is transfer object appropriate pattern in this situation? How should client create SearchModel instance? Please, share. Regards.

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  • Multiple leaf methods problem in composite pattern

    - by Ondrej Slinták
    At work, we are developing an PHP application that would be later re-programmed into Java. With some basic knowledge of Java, we are trying to design everything to be easily re-written, without any headaches. Interesting problem came out when we tried to implement composite pattern with huge number of methods in leafs. What are we trying to achieve (not using interfaces, it's just an example): class Composite { ... } class LeafOne { public function Foo( ); public function Moo( ); } class LeafTwo { public function Bar( ); public function Baz( ); } $c = new Composite( Array( new LeafOne( ), new LeafTwo( ) ) ); // will call method Foo in all classes in composite that contain this method $c->Foo( ); It seems like pretty much classic Composite pattern, but problem is that we will have quite many leaf classes and each of them might have ~5 methods (of which few might be different than others). One of our solutions, which seems to be the best one so far and might actually work, is using __call magic method to call methods in leafs. Unfortunately, we don't know if there is an equivalent of it in Java. So the actual question is: Is there a better solution for this, using code that would be eventually easily re-coded into Java? Or do you recommend any other solution? Perhaps there's some different, better pattern I could use here. In case there's something unclear, just ask and I'll edit this post.

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  • What types of objects should the ViewModel reference in the MVVM pattern?

    - by Blanthor
    I've seen quite a few examples of MVVM. I can see that the View should reference the ViewModel. I've seen recently an example of a ViewModel referencing a View, which seems wrong to me, as it would result in tighter coupling. Given that ViewModel is often described as an intermediary between the View and the Model, is there more to the ViewModel than a facade to domain objects? I hope I used the term "facade" correctly here.

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  • How should I handle expected errors? eg. "username already exists"

    - by Pheter
    I am struggling to understand how I should design the error handling parts of my code. I recently asked a similar question about how I should go about returning server error codes to the user, eg. 404 errors. I learnt that I should handle the error from within the current part of the application; seem's simple enough. However, what should I do when I can't handle the error from the current link in the chain? For example, I may have a class that is used to manage authentication. One of it's methods could be createUser($username, $password). Within that function, I need to determine if the username already exists. If this is true, how should I alert the calling code about this? Returning null instead of a user object is one way. But how do I then know what caused the error? How should I handle errors in such a way that calling code can easily find out what caused the error? Is there a design pattern commonly used for this kind of situation?

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  • Approaching Java from a Ruby perspective

    - by Travis
    There are plenty of resources available to a Java developer for getting a jump-start into Ruby/Rails development. The reverse doesn't appear to be true. What resources would you suggest for getting up-to-date on the current state of java technologies? How about learning how to approach DRY (don't repeat yourself) without the use of metaprogramming? Or how to approach various scenarios where a ruby developer is used to passing in a function (proc/lambda/block) as an argument (callbacks, etc)?

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  • Why represent shopping carts and order invoices differently in a domain model?

    - by Todd
    I've built some shopping cart systems in the past, but I always designed them such that the final order invoice is just a shopping cart that has been marked as "purchased". All the logic for adding/removing/changing items in a cart is also the logic for the order. All data is stored in the same tables in the database. But it seems this is not the proper way to design an e-commerce site.. Can someone explain the benefit of separating the shopping cart from invoices in the domain model? It seems to me this would lead to a lot of duplicated code, an extra set of tables in the database, and make it harder to maintain in the event the system need to start accommodating more complicated orders (like specifying selected options for an item which may or may not change the price/availability/shipping time of the order). I'm assuming I just haven't seen the light, as every book and other example I see seems to separate these two seemingly similar concerns -- but I can't find any explanation as to the benefit of doing such! It's also the case in the systems that I design that changes are often made after the initial order is confirmed. It's not uncommon for items to be removed, replaced, or added afterwards (but prior to fulfillment).

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  • How to face observable object containing an observable field

    - by iseek
    Hello, I need a hint concerning MVC and Observer-Pattern. For example a model contains the classes "Address" and "Person". The Address class contains the fields street:String, zipcode:String, location:String. Whereas the Person class contains the fields name:String, firstName:String, address:Address. My approach so far looks something like this: Both, Address and Person are observable. If one of their setters is being called, I validate whether the current value and new value differ. Only in this case an update event is fired. The event contains the source, the name of the changed field, the old and the new value. The class for the view contains text fields to display and edit the information of a person: name, firstname, street, zipcode, location. It knows the Person model and is an subscribed observer for the person. So it gets the update events from the person object. My questions concerns the address field from type Address in the person class, since an address is observable on its own. If the view gets an update event from person when a new address has been set, I can update all of the address related fields in the view. But what if a field of the address changes? Should the view also register for update events from the address? Any hints about common design approaches would be appreciated. Greetings.

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  • Where to put a recursive function when following MVC?

    - by Glibly
    Hello, I have a recursive function being used to generate a menu on my site. The function is calling a database for each level of children in the menu, and generating html for them. I've currently put this function in a Model part of the code, however, I feel that generating html in the model goes against the MVC. I didn't put it in a Controller because I didn't want to have database calls or HTML generation there. I didn't put it in a View because I didn't want database calls there either. Is the 'correct' way of tackling this problem to have a Controller call a recursive function in a Model that returns a 2d array representing the menu. Then pass the array to a view which has it's own recursive function for generating html from the array?

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  • Name the pattern - Create, Set, Execute, Destroy?

    - by Seb Nilsson
    I somewhere heard that the .NET Framework was built around specific pattern, which they tried to uphold as much as possible. var rsa = new RSACryptoServiceProvider(); // Create rsa.ImportParameters(GetParameters()); // Set byte[] encrypted = rsa.Encrypt(data, true); // Execute // Destroyed by garbage-collector Are there any variants of this? What are the general pros and cons?

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  • What is the MVC version of this code?

    - by Ian Boyd
    i'm trying to wrap my head around how to enterprise up my code: taking a simple routine and splitting it up into 5 or 6 methods in 3 or 4 classes. i quickly came up three simple examples of code how i currently write it. Could someone please convert these into an MVC/MVP obfuscated version? Example 1: The last name is mandatory. Color the text box red if nothing is entered. Color it green if stuff is entered: private void txtLastname_TextChanged(object sender, EventArgs e) { //Lastname mandatory. //Color pinkish if nothing entered. Greenish if entered. if (txtLastname.Text.Trim() == "") { //Lastname is required, color pinkish txtLastname.BackColor = ControlBad; } else { //Lastname entered, remove the coloring txtLastname.BackColor = ControlGood; } } Example 2: The first name is optional, but try to get it. We'll add a bluish tint to this "try to get" field: private void txtFirstname_TextChanged(object sender, EventArgs e) { //Firstname can be blank. //Hint them that they should *try* to get it with a bluish color. //If they do enter stuff: it better be not all spaces. if (txtFirstname.Text == "") { //Nothing there, hint it blue txtFirstname.BackColor = ControlRequired; } else if (txtFirstname.Text.Trim() == "") { //They entered spaces - bad user! txtFirstname.BackColor = ControlBad; } else { //Entered stuff, remove coloring txtFirstname.BackColor = SystemColors.Window; } } Example 3 The age is totally optional. If an age is entered, it better be valid: private void txtAge_TextChanged(object sender, EventArgs e) { //Age is optional, but if entered it better be valid int nAge = 0; if (Int32.TryParse(txtAge.Text, out nAge)) { //Valid integer entered if (nAge < 0) { //Negative age? i don't think so txtAge.BackColor = ControlBad; } else { //Valid age entered, remove coloring txtAge.BackColor = SystemColors.Window; } } else { //Whatever is in there: it's *not* a valid integer, if (txtAge.Text == "") { //Blank is okay txtAge.BackColor = SystemColors.Window; } else { //Not a valid age, bad user txtAge.BackColor = ControlBad; } } } Every time i see MVC code, it looks almost like random splitting of code into different methods, classes, and files. i've not been able to determine a reason or pattern to their madness. Without any understanding of they why it's being one some way, it makes no sense. And using the words model, view, controller and presenter, like i'm supposed to know what that means, doesn't help. The model is your data. The view shows data on screen. The controller is used to carry out the users actions And oranges taste orangy. Here's my attempt at splitting things up in order to make the code more difficult to follow. Is this anywhere close to MVC? private void txtFirstname_TextChanged(object sender, EventArgs e) { FirstnameTextChangedHandler(sender, e); } private void FirstnameTextChangedHandler(sender, e) { string firstname = GetFirstname(); Color firstnameTextBoxColor = GetFirstnameTextBoxColor(firstname); SetFirstNameTextBoxColor(firstnameTextBoxColor); } private string GetFirstname() { return txtFirstname.Text; } private Color GetFirstnameTextBoxColor(string firstname) { //Firstname can be blank. //Hint them that they should *try* to get it with a bluish color. //If they do enter stuff: it better be not all spaces. if (firstname == "") { //Nothing there, hint it blue return GetControlRequiredColor(); } else if (firstname.Trim() == "") { //They entered spaces - bad user! return GetControlBadColor(); } else { //Entered stuff, remove coloring return GetControlDefaultColor(); } } private Color GetControlRequiredColor() { return ControlRequired; } private Color GetControlBadColor() { return ControlBad; } private Color GetControlGoodColor() { return ControlGood; } //am i doin it rite i've obfuscated the code, but it's still altogether. The next step in the MVC obfuscation, i gather, is to hide the code in 3 or 4 different files. It's that next step that i don't understand. What is the logical separation of which functions are moved into what other classes? Can someone translate my 3 simple examples above into full fledged MVC obfuscation? Edit: Not ASP/ASP.NET/Online. Pretend it's on a desktop, handheld, surface, kiosk. And pretend it's language agnostic.

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  • Should I be using the command pattern? Seems like a lot of work...

    - by Fedor
    My Room class has a lot of methods I used before I decided to use the command pattern. Previously, I was invoking a lot of commands and now it seems I have to make a method in my roomParser class for every method. If I wanted to invoke say, setHotelCode I would have to create a method in roomParser that iterates through and invokes the method. Is this the way I should be using the command pattern? <?php interface Parseable { public function parse( $arr, $dept ); } class Room implements Parseable { protected $_adults; protected $_kids; protected $_startDate; protected $_endDate; protected $_hotelCode; protected $_sessionNs; protected $_minRate; protected $_maxRate; protected $_groupCode; protected $_rateCode; protected $_promoCode; protected $_confCode; protected $_currency = 'USD'; protected $_soapAction; protected $_soapHeaders; protected $_soapServer; protected $_responseXml; protected $_requestXml; public function __construct( $startdate,$enddate,$rooms=1,$adults=2,$kids=0 ) { $this->setNamespace(SESSION_NAME); $this->verifyDates( $startdate, $enddate ); $this->_rooms= $rooms; $this->_adults= $adults; $this->_kids= $kids; $this->setSoapAction(); $this->setRates(); } public function parse( $arr, $dept ) { $this->_price = $arr * $dept * rand(); return $this; } public function setNamespace( $namespace ) { $this->_sessionNs = $namespace; } private function verifyDates( $startdate, $enddate ) {} public function setSoapAction( $str= 'CheckAvailability' ) { $this->_soapAction = $str; } public function setRates( $rates='' ) { } public function setHotelCode($code ) { $this->_hotelCode = $code; } private function getSoapHeader() { return '<?xml version="1.0" encoding="utf-8"?> <soap:Header> </soap:Header>'; } private function getSoapFooter() { return '</soap:Envelope>'; } private function getSource() { return '<POS> <Source><RequestorId ID="" ID_Context="" /></Source> </POS>'; } function requestXml() { $this->_requestXml = $this->getSoapHeader(); $this->_requestXml .='<soap:Body></soap:Body>'; return $this->_requestXml; } private function setSoapHeaders ($contentLength) { $this->_soapHeaders = array('POST /url HTTP/1.1', 'Host: '.SOAP_HOST, 'Content-Type: text/xml; charset=utf-8', 'Content-Length: '.$contentLength); } } class RoomParser extends SplObjectStorage { public function attach( Parseable $obj ) { parent::attach( $obj ); } public function parseRooms( $arr, $dept ) { for ( $this->rewind(); $this->valid(); $this->next() ) { $ret = $this->current()->parse( $arr, $dept ); echo $ret->getPrice(), PHP_EOL; } } } $arrive = '12/28/2010'; $depart = '01/02/2011'; $rooms = new RoomParser( $arrive, $depart); $rooms->attach( new Room( '12/28/2010', '01/02/2011') ); $rooms->attach( new Room( '12/29/2010', '01/04/2011') ); echo $rooms->count(), ' Rooms', PHP_EOL; Edit: I'm thinking it may be easier if I made the RoomParser less generic by storing properties that all the objects will share. Though I'll probably have to make methods if I want to override for a certain object.

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  • Stored Procedure or calculations via IQueryable?

    - by Shawn Mclean
    This is a question that is based on choosing performance over design practices. If I have a method that will be executed many times a second; public static IQueryable<IPerson> InRadius(this IQueryable<IPerson> query, Coordinate center, double radius) { return (from u in query where CallHeavyMathFormula(u, center, radius) select u); } This extension method for IQueryable generates a SQL that does some heavy maths calculation (Cosine, Sine, etc). This would mean the application sends 1-2KB of sql to the server per call. I've heard of placing all application logic, in your application. I also would like to change to a database such as azure or one of those scalable databases in the future. How do I handle something like this? Should I leave it as it is now or write stored procedures? How do applications like twitter or facebook do it?

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  • Buffering db inserts in multithreaded program

    - by Winter
    I have a system which breaks a large taks into small tasks using about 30 threads as a time. As each individual thread finishes it persists its calculated results to the database. What I want to achieve is to have each thread pass its results to a new persisance class that will perform a type of double buffering and data persistance while running in its own thread. For example, after 100 threads have moved their data to the buffer the persistance class then the persistance class swaps the buffers and persists all 100 entries to the database. This would allow utilization of prepared statements and thus cut way down on the I/O between the program and the database. Is there a pattern or good example of this type of multithreading double buffering?

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  • What is MVC and what are the advantages of it?

    - by Teifion
    I found What are mvp and mvc and what is the difference but it didn't really answer this question. I've recently started using MVC because it's part of the framework that myself and my work-partner are going to use. We chose it because it looked easy and separated process from display, are there advantages besides this that we don't know about and could be missing out on? Pros Display and Processing are seperated Cons None so far

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  • How to "wrap" implementation in C#

    - by igor
    Hello, I have these classes in C# (.NET Framework 3.5) described below: public class Base { public int State {get; set;} public virtual int Method1(){} public virtual string Method2(){} ... public virtual void Method10(){} } public class B: Base { // some implementation } public class Proxy: Base { private B _b; public class Proxy(B b) { _b = b; } public override int Method1() { if (State == Running) return _b.Method1(); else return base.Method1(); } public override string Method2() { if (State == Running) return _b.Method2(); else return base.Method2(); } public override void Method10() { if (State == Running) _b.Method10(); else base.Method10(); } } I want to get something this: public Base GetStateDependentImplementation() { if (State == Running) // may be some other rule return _b; else return base; // compile error } and my Proxy's implementation will be: public class Proxy: Base { ... public override int Method1() { return GetStateDependentImplementation().Method1(); } public override string Method2() { return GetStateDependentImplementation().Method2(); } ... } Of course, I can do this (aggregation of base implementation): public RepeaterOfBase: Base // no any overrides, just inheritance { } public class Proxy: Base { private B _b; private RepeaterOfBase _Base; public class Proxy(B b, RepeaterOfBase aBase) { _b = b; _base = aBase; } } ... public Base GetStateDependentImplementation() { if (State == Running) return _b; else return _Base; } ... But instance of Base class is very huge and I have to avoid to have other additional copy in memory. So I have to simplify my code have to "wrap" implementation have to avoid a code duplication have to avoid aggregation of any additional instance of Base class (duplication) Is it possible to reach these goals?

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  • ASP.NET MVC Filters: How to set Viewdata for Dropdown based on action parameter

    - by CRice
    Hi, Im loading an entity 'Member' from its id in route data. [ListItemsForMembershipType(true)] public ActionResult Edit(Member someMember) {...} The attribute on the action loads the membership type list items for a dropdown box and sticks it in viewdata. This is fine for add forms, and search forms (it gets all active items) but I need the attribute to execute BASED ON THE VALUE someMember.MembershipTypeId, because its current value must always be present when loading the item (i.e. all active items, plus the one from the loaded record). So the question is, what is the standard pattern for this? How can my attribute accept the value or should I be loading the viewdata for the drop down in a controller supertype or during model binding or something else? It is in an attribute now because the code to set the viewdata would otherwise be duplicated in each usage in each action.

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  • Should I dive into ASP.NET MVC or start with ASP.NET Webforms?

    - by Sahat
    I plan to pick up Silverlight in the future. Possibility of going into Microsoft WPF. Currently learning Objective-C 2.0 w/ Cocoa. I already know Pros and Cons of ASP.NET MVC vs ASP.NET Webforms. What I want to know is what would be more "efficient" for me to learn given the circumstances above? By efficient I mean learning one design pattern once and then re-using it. Objective-C I believe uses MVC approach? What about Silverlight? WPF? So what do you think? Also as a side question is it true that ASP.NET Webforms is often used by freelancers/small companies and ASP.NET MVC in large enterprises?

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  • What's the proper approach for writing multi-path "story" flows?

    - by Basiclife
    Hi, I wonder if you can help me. I'm writing a game (2d) which allows players to take multiple routes, some of which branch/merge - perhaps even loop. Each section of the game will decide which section is loaded next. I'm calling each section an IStoryElement - And I'm wondering how best to link these elements up in a way that is easily changed/configured and at the same time, graphable I'm going to have an engine/factory assembly which will load the appropriate StoryElement(s) based on various config options. I initially planned to give each StoryElement a NextElement() As IStoryElement property and a Completed() event. When the vent fires, the engine reads the NextElement property to find the next StoryElement. The downside to this is that if I ever wanted to graph all the routes through the game, I would be unable to - I couldn't determine all possible targets for each StoryElement. I considered a couple of other solutions but they all feel a little clunky - eg Do I need an additional layer of abstraction? ie StoryElementPlayers or similar - Each one would be responsible for stringing together multiple StoryElement perhaps a Series and a ChoicePlayer with each responsible for graphing its own StoryElement - But this will just move the problem up a layer. In short, I need some way of emulating a simple but dynamic workflow (but I'd rather not actually use WWF). Is there a pattern for something this simple? All the ones I've managed to find relate to more advanced control flow (parallel processing, etc.)

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  • command design pattern usage

    - by sagie
    Hi. I've read 3 descriptions of the command design pattern: wikipedia, dofactory and source making. In all of them, the UML shows a relation between the client to the receiver & the concrete command, but no relation to the invoker. But in all 3 examples the client is the one that initiates the invoker and call its Execute method. I think that should be a relation to the invoker as well. Am I missing somthing in here? Maybe even a basic UML knowladge?

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  • Implementing a live voting system

    - by Will Eddins
    I'm looking at implementing a live voting system on my website. The website provides a live stream, and I'd like to be able to prompt viewers to select an answer during a vote initiated by the caster. I can understand how to store the data in a mySQL database, and how to process the answers. However: How would I initially start the vote on the client-side and display it? Should a script be running every few seconds on the page, checking another page to see if a question is available for the user? Are there any existing examples of a real-time polling system such as what I'm looking at implementing?

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  • Command Query Separation validating for retries

    - by Neil Barnwell
    So I'm comfortable with the basic concept of CQS, where you might have a command that writes to one database, and that updates the query database that you read from. However, consider the scenario where you are entering data, and want to prevent duplicates. Using new employee data entry an employee register as an example, working through a pile of application forms to key in the new employees' details: Take top sheet. Key in employee name and unique payroll number to UI. Submit. Put paper in "completed pile". Repeat. How would you now prevent the user from keying in the same payroll number again, say for instance if they get distracted and can't remember whether they've keyed one in already and the "message" hasn't got all the way back to the query db for the user to search?

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  • Is there a design pattern to cut down on code duplication when subclassing Activities in Android?

    - by Daniel Lew
    I've got a common task that I do with some Activities - downloading data then displaying it. I've got the downloading part down pat; it is, of course, a little tricky due to the possibility of the user changing the orientation or cancelling the Activity before the download is complete, but the code is there. There is enough code handling these cases such that I don't want to have to copy/paste it to each Activity I have, so I thought to create an abstract subclass Activity itself such that it handles a single background download which then launches a method which fills the page with data. This all works. The issue is that, due to single inheritance, I am forced to recreate the exact same class for any other type of Activity - for example, I use Activity, ListActivity and MapActivity. To use the same technique for all three requires three duplicate classes, except each extends a different Activity. Is there a design pattern that can cut down on the code duplication? As it stands, I have saved much duplication already, but it pains me to see the exact same code in three classes just so that they each subclass a different type of Activity.

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