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• How do we keep dependent data structures up to date?

  - by Geo

  Suppose you have a parse tree, an abstract syntax tree, and a control flow graph, each one logically derived from the one before. In principle it is easy to construct each graph given the parse tree, but how can we manage the complexity of updating the graphs when the parse tree is modified? We know exactly how the tree has been modified, but how can the change be propagated to the other trees in a way that doesn't become difficult to manage? Naturally the dependent graph can be updated by simply reconstructing it from scratch every time the first graph changes, but then there would be no way of knowing the details of the changes in the dependent graph. I currently have four ways to attempt to solve this problem, but each one has difficulties. Nodes of the dependent tree each observe the relevant nodes of the original tree, updating themselves and the observer lists of original tree nodes as necessary. The conceptual complexity of this can become daunting. Each node of the original tree has a list of the dependent tree nodes that specifically depend upon it, and when the node changes it sets a flag on the dependent nodes to mark them as dirty, including the parents of the dependent nodes all the way down to the root. After each change we run an algorithm that is much like the algorithm for constructing the dependent graph from scratch, but it skips over any clean node and reconstructs each dirty node, keeping track of whether the reconstructed node is actually different from the dirty node. This can also get tricky. We can represent the logical connection between the original graph and the dependent graph as a data structure, like a list of constraints, perhaps designed using a declarative language. When the original graph changes we need only scan the list to discover which constraints are violated and how the dependent tree needs to change to correct the violation, all encoded as data. We can reconstruct the dependent graph from scratch as though there were no existing dependent graph, and then compare the existing graph and the new graph to discover how it has changed. I'm sure this is the easiest way because I know there are algorithms available for detecting differences, but they are all quite computationally expensive and in principle it seems unnecessary so I'm deliberately avoiding this option. What is the right way to deal with these sorts of problems? Surely there must be a design pattern that makes this whole thing almost easy. It would be nice to have a good solution for every problem of this general description. Does this class of problem have a name?

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• Open source textbook writing experiment in italian school

  <b>Stop:</b> "In order to solve this problem, Education Minister Mariastella Gelmini proposed to deduce cost (and weight!) of textbooks by encouraging schools to adopt digital textbooks starting from 2011."

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• whats the name of this pattern?

  - by Wes

  I see this a lot in frameworks. You have a master class which other classes register with. The master class then decides which of the registered classes to delegate the request to. An example based passed in class may be something this. public interface Processor { public boolean canHandle(Object objectToHandle); public void handle(Object objectToHandle); } public class EvenNumberProcessor extends Processor { public boolean canHandle(Object objectToHandle) { if (!isNumeric(objectToHandle)){ return false } return isEven(objectToHandle); } public void handle(objectToHandle) { //Optionally call canHandleAgain to ensure the calling class is fufilling its contract doSomething(); } } public class OddNumberProcessor extends Processor { public boolean canHandle(Object objectToHandle) { if (!isNumeric(objectToHandle)){ return false } return isOdd(objectToHandle); } public void handle(objectToHandle) { //Optionally call canHandleAgain to ensure the calling class is fufilling its contract doSomething(); } } //Can optionally implement processor interface public class processorDelegator { private List processors; public void addProcessor(Processor processor) { processors.add(processor); } public void process(Object objectToProcess) { //Lookup relevant processor either by keeping a list of what they can process //Or query each one to see if it can process the object. chosenProcessor=chooseProcessor(objectToProcess); chosenProcessor.handle(objectToProcess); } } Note there are a few variations I see on this. In one variation the sub classes provide a list of things they can process which the ProcessorDelegator understands. The other variation which is listed above in fake code is where each is queried in turn. This is similar to chain of command but I don't think its the same as chain of command means that the processor needs to pass to other processors. The other variation is where the ProcessorDelegator itself implements the interface which means you can get trees of ProcessorDelegators which specialise further. In the above example you could have a numeric processor delegator which delegates to an even/odd processor and a string processordelegator which delegates to different strings. My question is does this pattern have a name.

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• MVP Pattern Philsophical Question - Security Checking in UI

  - by Brian

  Hello, I have a philosophical question about the MVP pattern: I have a component that checks whether a user has access to a certain privilege. This privilege turns on or off certain UI features. For instance, suppose you have a UI grid, and for each row that gets bound, I do a security check to see if certain features in the grid should be enabled or disabled. There are two ways to do this: have the UI/view call the component's method, determine if it has access, and enable/disable or show/hide. The other is have the view fire an event to the presenter, have the presenter do the check and return the access back down to the view through the model or through the event arg. As per the MVP pattern, which component should security checks fit into, the presenter or the view? Since the view is using it to determine its accessibility, it seems more fitting in the view, but it is doing database checks and all inside this business component, and there is business logic there, so I can see the reverse argument too. Thoughts? Thanks.

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• Reusable skill class structure

  - by Martino Wullems

  Hello, Pretty new to the whole game development scene, but I have experience in other branches of programming. Anyway, I was wondering what methods are used to implement a skill structure. I imagine a skill in itself would a class. I'm using actionscript 3 for this project btw. public class Skill { public var power:int; public var delay:int; public var cooldown:int; public function Attack(user:Mob, target:Mob) { } } } Each skill would extend the Skill class and add it's own functionality. public class Tackle extends Skill { public function Tackle(user:Mob, target:Mob) { super(user, target); executeAttack(); } private function executeAttack():void { //multiply user.strength with power etc //play attack animation } } } This where I get stuck. How do I termine which mobs has which skills? And which skill will they later be able to retrieve (by reaching a certain level etc). How does the player actually execute the skill and how is it determine if it hits. It's all very new to me so I have no idea where to begin. Any links would also be appreciated. Thanks in advance.

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• Implementing the transport layer for a SIP UAC

  - by Jonathan Henson

  I have a somewhat simple, but specific, question about implementing the transport layer for a SIP UAC. Do I expect the response to a request on the same socket that I sent the request on, or do I let the UDP or TCP listener pick up the response and then route it to the correct transaction from there? The RFC does not seem to say anything on the matter. It seems that especially using UDP, which is connection-less, that I should just let the listeners pick up the response, but that seems sort of counter intuitive. Particularly, I have seen plenty of UAC implementations which do not depend on having a Listener in the transport layer. Also, most implementations I have looked at do not have the UAS receiving loop responding on the socket at all. This would tend to indicate that the client should not be expecting a reply on the socket that it sent the request on. For clarification: Suppose my transport layer consists of the following elements: TCPClient (Sends Requests for a UAC via TCP) UDPClient (Sends Requests for a UAC vid UDP) TCPSever (Loop receiving Requests and dispatching to transaction layer via TCP) UDPServer (Loop receiving Requests and dispatching to transaction layer via UDP) Obviously, the *Client sends my Requests. The question is, what receives the Response? The *Client waiting on a recv or recvfrom call on the socket it used to send the request, or the *Server? Conversely, the *Server receives my requests, What sends the Response? The *Client? doesn't this break the roles of each member a bit?

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• Access functions from user control without events?

  - by BornToCode

  I have an application made with usercontrols and a function on main form that removes the previous user controls and shows the desired usercontrol centered and tweaked: public void DisplayControl(UserControl uControl) I find it much easier to make this function static or access this function by reference from the user control, like this: MainForm mainform_functions = (MainForm)Parent; mainform_functions.DisplayControl(uc_a); You probably think it's a sin to access a function in mainform, from the usercontrol, however, raising an event seems much more complex in such case - I'll give a simple example - let's say I raise an event from usercontrol_A to show usercontrol_B on mainform, so I write this: uc_a.show_uc_b+= (s,e) => { usercontrol_B uc_b = new usercontrol_B(); DisplayControl(uc_b); }; Now what if I want usercontrol_B to also have an event to show usercontrol_C? now it would look like this: uc_a.show_uc_b+= (s,e) => { usercontrol_B uc_b = new usercontrol_B(); DisplayControl(uc_b); uc_b.show_uc_c += (s2,e2) => {usercontrol_C uc_c = new usercontrol_C(); DisplayControl(uc_c);} }; THIS LOOKS AWFUL! The code is much simpler and readable when you actually access the function from the usercontrol itself, therefore I came to the conclusion that in such case it's not so terrible if I break the rules and not use events for such general function, I also think that a readable usercontrol that you need to make small adjustments for another app is preferable than a 100% 'generic' one which makes my code look like a pile of mud. What is your opinion? Am I mistaken?

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• Liskov substitution and abstract classes / strategy pattern

  - by Kolyunya

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

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• Rails: Law of Demeter Confusion

  - by user2158382

  I am reading a book called Rails AntiPatterns and they talk about using delegation to to avoid breaking the Law of Demeter. Here is their prime example: They believe that calling something like this in the controller is bad (and I agree) @street = @invoice.customer.address.street Their proposed solution is to do the following: class Customer has_one :address belongs_to :invoice def street address.street end end class Invoice has_one :customer def customer_street customer.street end end @street = @invoice.customer_street They are stating that since you only use one dot, you are not breaking the Law of Demeter here. I think this is incorrect, because you are still going through customer to go through address to get the invoice's street. I primarily got this idea from a blog post I read: http://www.dan-manges.com/blog/37 In the blog post the prime example is class Wallet attr_accessor :cash end class Customer has_one :wallet # attribute delegation def cash @wallet.cash end end class Paperboy def collect_money(customer, due_amount) if customer.cash < due_ammount raise InsufficientFundsError else customer.cash -= due_amount @collected_amount += due_amount end end end The blog post states that although there is only one dot customer.cash instead of customer.wallet.cash, this code still violates the Law of Demeter. Now in the Paperboy collect_money method, we don't have two dots, we just have one in "customer.cash". Has this delegation solved our problem? Not at all. If we look at the behavior, a paperboy is still reaching directly into a customer's wallet to get cash out. EDIT I completely understand and agree that this is still a violation and I need to create a method in Wallet called withdraw that handles the payment for me and that I should call that method inside the Customer class. What I don't get is that according to this process, my first example still violates the Law of Demeter because Invoice is still reaching directly into Customer to get the street. Can somebody help me clear the confusion. I have been searching for the past 2 days trying to let this topic sink in, but it is still confusing.

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• Advice: How to convince my newly annointed team lead against writing the code base from scratch

  - by shan23

  I work in a pretty reknowned MNC, and the module that I work in has been assigned to a new "lead". The code base is pretty huge (~130K or more, with inter dependencies on other modules) , but stable - some parts have grown ugly over the years, but its provably in working state. (Our products are running for years on them, even new ones). The problem is, our lead wants to rewrite the code from scratch, to encompass "finer granularity and a proactive design". I know in my guts thats not a very good idea, but how do I convince him/the rest of the team(who are pretty much more senior than me in terms of years of exp), without sounding too pedantic myself (Thou shalt not rewrite , as Joel et al have clear articles prohibiting it)? I have a good working relation with the person concerned, and don't want to ruin it, but neither do I want to be party to a decision which would surely plague us for years to come !! Any suggestions for a milder,yet effective approach ? Even accounts of how you have tackled such a situation to your liking would help me a lot! EDIT: The code base I'm talking about is not a product/GUI, but at kernel level with all the critical functionalities for our product. I hope now you know why i sound so apprehensive !!

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• MVC + 3 tier; where ViewModels come into play?

  - by mikhairu

  I'm designing a 3-tiered application using ASP.NET MVC 4. I used the following resources as a reference. CodeProject: MVC + N-tier + Entity Framework Separating data access in ASP.NET MVC I have the following desingn so far. Presentation Layer (PL) (main MVC project, where M of MVC was moved to Data Access Layer): MyProjectName.Main Views/ Controllers/ ... Business Logic Layer (BLL): MyProjectName.BLL ViewModels/ ProjectServices/ ... Data Access Layer (DAL): MyProjectName.DAL Models/ Repositories.EF/ Repositories.Dapper/ ... Now, PL references BLL and BLL references DAL. This way lower layer does not depend on the one above it. In this design PL invokes a service of the BLL. PL can pass a View Model to BLL and BLL can pass a View Model back to PL. Also, BLL invokes DAL layer and DAL layer can return a Model back to BLL. BLL can in turn build a View Model and return it to PL. Up to now this pattern was working for me. However, I've ran into a problem where some of my ViewModels require joins on several entities. In the plain MVC approach, in the controller I used a LINQ query to do joins and then select new MyViewModel(){ ... }. But now, in the DAL I do not have access to where ViewModels are defined (in the BLL). This means I cannot do joins in DAL and return it to BLL. It seems I have to do separate queries in DAL (instead of joins in one query) and BLL would then use the result of these to build a ViewModel. This is very inconvenient, but I don't think I should be exposing DAL to ViewModels. Any ideas how I can solve this dilemma? Thanks.

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• Dependency injection and IOC containers in a closed project

  - by Puckl

  Does it make sense to assemble my project with dependency injection containers if I am the only one who will use the code of that project? The question came up when I read this IOC Article http://martinfowler.com/articles/injection.html The justification for using dependency injection in this article is that friends can reuse a class, and replace depending classes with their own classes because they get injected and not instantiated in the class. I would only use it to inject objects where they are needed instead of passing them through layers to their target. (Which is not so bad I learned here: Is it bad practice to pass instances through several layers?) (Maybe I will reuse parts of the project, who knows, but I don´t know if that is a good justification)

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• Clean MVC design when there is viewer latency

  - by Tony Suffolk 66

  It isn't clear if this question has already been answered, so apologies in advance if this is a duplicate : I am implementing a game and trying to design around a clean MVC pattern - so my Control plane will implement the rules of the game (but not how the game is displayed), and the View plane implements how the game is displayed, and user iteraction - i.e. what game items or controls the user has activated. The challenge that I have is this : In my game the Control Plane can move game items more or less instaneously (The decision about what item to place where - and some of the initial consequences of that placement are reasonably trivial to calculate), but I want to design the Control Plane so that the View plane can display these movements either instaneously or using movement animations. The other complication is that player interaction must be locked out while those game items are moving (similar to chess - you can't attack an opposing piece as it moves past one of your pieces) So do I : Implement all the logic in the Control Plane asynchronously - and separate the descision making from the actions - so the Control plane decides piece 'A' needs to move to a given place - tells the view plane, and but does not implement the move in data until the view plane informs the control plane that the move/animation is complete. A lot of interlock points between the two layers. Implement all the control plane logic in one place - decisions and movement (keeping track of what moved where), and pass all the movements in one go to the View plane to do with what it will. Control Plane is almost fire and forget here. A hybrid of 1 & 2 - The control plane implements all the moves in a temporary data store - but maintains a second store which reflects what is actually visible to the viewer, based on calls and feedback from the View plane. All 3 are relatively easy to implement (target language is python), but having never done a clean MVC pattern with view latency before - I am not sure which design is best

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• How do I know when should I package my classes in Ruby?

  - by Omega

  In Ruby, I'm creating a small game development framework. Just some personal project - a very small group of friends helping. Now I am in need of handling geometric concepts. Rectangles, Circles, Polygons, Vectors, Lines, etc. So I made a class for each of these. I'm stuck deciding whether I should package such classes in a module, such as Geometry. So I'd access them like Geometry::Rectangle, or just Rectangle if I include the module. Now then, my question isn't about this specific scenario. I'd like to know, when is it suitable to package similar classes into one module in Ruby? What factors should I consider? Amount of classes? Usage frequency? Complexity?

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• Designing a Business Rule Engine

  - by Nisha_Roy

  I have a requirement where there are 10 Rules to be applied on data in excel. If Rule 1 and Rule 2 fails rest of the rules are not checked. But if Rule 1 and Rule 2 passes the rest of all the Rules should be verified and if any errors found- they should be logged. Is there any design pattern which I can use to keep this Rule Engine flexible for adding these 10 rules and Closed for any additional chains in the Current Rule. I was thinking of something like a Decorator Pattern. Will this help me achieve that?

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• What is the simplest human readable configuration file format?

  - by Juha

  Current configuration file is as follows: mainwindow.title = 'test' mainwindow.position.x = 100 mainwindow.position.y = 200 mainwindow.button.label = 'apply' mainwindow.button.size.x = 100 mainwindow.button.size.y = 30 logger.datarate = 100 logger.enable = True logger.filename = './test.log' This is read with python to a nested dictionary: { 'mainwindow':{ 'button':{ 'label': {'value':'apply'}, ... }, 'logger':{ datarate: {'value': 100}, enable: {'value': True}, filename: {'value': './test.log'} }, ... } Is there a better way of doing this? The idea is to get XML type of behavior and avoid XML as long as possible. The end user is assumed almost totally computer illiterate and basically uses notepad and copy-paste. Thus the python standard "header + variables" type is considered too difficult. The dummy user edits the config file, able programmers handle the dictionaries. Nested dictionary is chosen for easy splitting (logger does not need or even cannot have/edit mainwindow parameters).

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• Is there a Design Pattern for preventing dangling references?

  - by iFreilicht

  I was thinking about a design for custom handles. The thought is to prevent clients from copying around large objects. Now a regular handle class would probably suffice for that, but it doesn't solve the "dangling reference problem"; If a client has multiple handles of the same object and deletes the object via one of them, all the others would be invalid, but not know it, so the client could write or read parts of the memory he shouldn't have access to. Is there a design pattern to prevent this from happening? Two ideas: An observer-like pattern where the destructor of an object would notify all handles. "Handle handles" (does such a thing even exist?). All the handles don't really point to the object, but to another handle. When the object gets destroyed, this "master-handle" invalidates itself and therefore all that point to it.

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• How to manage long running background threads and report progress with DDD

  - by Mr Happy

  Title says most of it. I have found surprising little information about this. I have a long running operation of which the user wants to see the progress (as in, item x of y processed). I also need to be able to pause and stop the operation. (Stopping doesn't rollback the items already processed.) The thing is, it's not that each item takes a long time to get processed, it's that that there are usually a lot of items. And what I've read about so far is that it's somewhat of an anti-pattern to put something like a queue in the DB. I currently don't have any messaging system in place, and I've never worked with one either. Another thing I read somewhere is that progress reporting is something that belongs in the application layer, but it didn't go into the details. So having said all this, what I have in mind is the following. User request with list of items enters the application layer. Application layer gets some information from the domain needed to process the items. Application layer passes the items and the information off to some domain service (should the implementation of this service belong in the infrastructure layer?) This service spins up a worker thread with callbacks for both progress reporting and pausing/stopping it. This worker thread will process each item in it's own UoW. This means the domain information from earlier needs to be stored in some DTO. Since nothing is really persisted, the service should be singleton and thread safe Whenever a user requests a progress report or wants to pause/stop the operation, the application layer will ask the service. Would this be a correct solution? Or am I at least on the right track with this? Especially the singleton and thread safe part makes the whole thing feel icky.

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• Relationship DAO, Servlet, JSP and POJO

  - by John Hendrik

  I want to implement a JSP, POJO, DAO and Servlet in my J2EE program. However, I don't fully understand how the relationship between these elements should be. Is the following (MVC) setup the right way to do it? Main class creates servlet(controller) Servlet has a DAO defined in its class DAO has a POJO defined in its class Servlet communicates with the view (JSP page) Please give your feedback.

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• Algorithm to Solve Most of a Problem

  - by Mike G

  I need an Algorithm/Design Pattern that allows me to try to get the maximum number of rules followed. So I have a couple teams and I need to pair them with a referee and against each other into a round robin. There a rules on who can compete with who and who can judge who so I need to find the configuration that satisfies the most of these. Some rules are more important than others and are "worth more" when evaluating "what satisfies the most of them" There probably isn't a algorithm for this, but is there a design pattern that could help me maximize my chances of finding this configuration?

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• How to have operations with character/items in binary with concrete operations?

  - by Piperoman

  I have the next problem. A item can have a lot of states: NORMAL = 0000000 DRY = 0000001 HOT = 0000010 BURNING = 0000100 WET = 0001000 COLD = 0010000 FROZEN = 0100000 POISONED= 1000000 A item can have some states at same time but not all of them Is impossible to be dry and wet at same time. If you COLD a WET item, it turns into FROZEN. If you HOT a WET item, it turns into NORMAL A item can be BURNING and POISON Etc. I have tried to set binary flags to states, and use AND to combine different states, checking before if it is possible or not to do it, or change to another status. Does there exist a concrete approach to solve this problem efficiently without having an interminable switch that checks every state with every new state? It is relatively easy to check 2 different states, but if there exists a third state it is not trivial to do.

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• How can I selectively update XNA GameComponents?

  - by Bill

  I have a small 2D game I'm working on in XNA. So far, I have a player-controlled ship that operates on vector thrust and is terribly fun to spin around in circles. I've implemented this as a DrawableGameComponent and registered it with the game using game.Components.Add(this) in the Ship object constructor. How can I implement features like pausing and a menu system with my current implementation? Is it possible to set certain GameComponents to not update? Is this something for which I should even be using a DrawableGameComponent? If not, what are more appropriate uses for this?

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• OIM 11g : Multi-thread approach for writing custom scheduled job

  - by Saravanan V S

  In this post I have shared my experience of designing and developing an OIM schedule job that uses multi threaded approach for updating data in OIM using APIs.  I have used thread pool (in particular fixed thread pool) pattern in developing the OIM schedule job. The thread pooling pattern has noted advantages compared to thread per task approach. I have listed few of the advantage here ·         Threads are reused ·         Creation and tear-down cost of thread is reduced ·         Task execution latency is reduced ·         Improved performance ·         Controlled and efficient management of memory and resources used by threads More about java thread pool http://docs.oracle.com/javase/tutorial/essential/concurrency/pools.html The following diagram depicts the high-level architectural diagram of the schedule job that process input from a flat file to update OIM process form data using fixed thread pool approach    The custom scheduled job shared in this post is developed to meet following requirement 1)      Need to process a CSV extract that contains identity, account identifying key and list of data to be updated on an existing OIM resource account. 2)      CSV file can contain data for multiple resources configured in OIM 3)      List of attribute to update and mapping between CSV column to OIM fields may vary between resources The following are three Java class developed for this requirement (I have given only prototype of the code that explains how to use thread pools in schedule task) CustomScheduler.java - Implementation of TaskSupport class that reads and passes the parameters configured on the schedule job to Thread Executor class. package com.oracle.oim.scheduler; import java.util.HashMap; import com.oracle.oim.bo.MultiThreadDataRecon; import oracle.iam.scheduler.vo.TaskSupport; public class CustomScheduler extends TaskSupport {      public void execute(HashMap options) throws Exception {             /*  Read Schedule Job Parameters */             String param1 = (String) options.get(“Parameter1”);             .             int noOfThread = (int) options.get(“No of Threads”);             .             String paramn = (int) options.get(“ParamterN”); /* Provide all the required input configured on schedule job to Thread Pool Executor implementation class like 1) Name of the file, 2) Delimiter 3) Header Row Numer 4) Line Escape character 5) Config and resource map lookup 6) No the thread to create */ new MultiThreadDataRecon(all_required_parameters, noOfThreads).reconcile();       }       public HashMap getAttributes() { return null; }       public void setAttributes() {       } } MultiThreadDataRecon.java – Helper class that reads data from input file, initialize the thread executor and builds the task queue. package com.oracle.oim.bo; import <required file IO classes>; import  <required java.util classes>; import  <required OIM API classes>; import <csv reader api>; public class MultiThreadDataRecon {  private int noOfThreads;  private ExecutorService threadExecutor = null;  public MetaDataRecon(<required params>, int noOfThreads)  {       //Store parameters locally       .       .       this.noOfThread = noOfThread;  }        /**        *  Initialize         */  private void init() throws Exception {       try {             // Initialize CSV file reader API objects             // Initialize OIM API objects             /* Initialize Fixed Thread Pool Executor class if no of threads                 configured is more than 1 */             if (noOfThreads > 1) {                   threadExecutor = Executors.newFixedThreadPool(noOfThreads);             } else {                   threadExecutor = Executors.newSingleThreadExecutor();             }             /* Initialize TaskProcess clas s which will be executing task                 from the Queue */                TaskProcessor.initializeConfig(params);       } catch (***Exception e) {                   // TO DO       }  }       /**        *  Method to reconcile data from CSV to OIM        */ public void reconcile() throws Exception {        try {             init();             while(<csv file has line>){                   processRow(line);             }             /* Initiate thread shutdown */             threadExecutor.shutdown();             while (!threadExecutor.isTerminated()) {                 // Wait for all task to complete.             }            } catch (Exception e) {                   // TO DO            } finally {                   try {                         //Close all the file handles                   } catch (IOException e) {                         //TO DO                   }             }       }       /**        * Method to process         */       private void processRow(String row) {             // Create task processor instance with the row data              // Following code push the task to work queue and wait for next                available thread to execute             threadExecutor.execute(new TaskProcessor(rowData));       } } TaskProcessor.java – Implementation of “Runnable” interface that executes the required business logic to update data in OIM. package com.oracle.oim.bo; import <required APIs> class TaskProcessor implements Runnable {       //Initialize required member variables       /**        * Constructor        */       public TaskProcessor(<row data>) {             // Initialize and parse csv row       }       /*       *  Method to initialize required object for task execution       */       public static void initializeConfig(<params>) {             // Process param and initialize the required configs and object       }           /*        * (non-Javadoc)        *         * @see java.lang.Runnable#run()        */            public void run() {             if (<is csv data valid>){                   processData();             }       }  /**   * Process the the received CSV input   */  private void processData() {     try{       //Find the user in OIM using the identity matching key value from CSV       // Find the account to be update from user’s account based on account identifying key on CSV       // Update the account with data from CSV       }catch(***Exception e){           //TO DO       }   } }

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• Pattern for a class that does only one thing

  - by Heinzi

  Let's say I have a procedure that does stuff: void doStuff(initalParams) { ... } Now I discover that "doing stuff" is quite a compex operation. The procedure becomes large, I split it up into multiple smaller procedures and soon I realize that having some kind of state would be useful while doing stuff, so that I need to pass less parameters between the small procedures. So, I factor it out into its own class: class StuffDoer { private someInternalState; public Start(initalParams) { ... } // some private helper procedures here ... } And then I call it like this: new StuffDoer().Start(initialParams); or like this: new StuffDoer(initialParams).Start(); And this is what feels wrong. When using the .NET or Java API, I always never call new SomeApiClass().Start(...);, which makes me suspect that I'm doing it wrong. Sure, I could make StuffDoer's constructor private and add a static helper method: public static DoStuff(initalParams) { new StuffDoer().Start(initialParams); } But then I'd have a class whose external interface consists of only one static method, which also feels weird. Hence my question: Is there a well-established pattern for this type of classes that have only one entry point and have no "externally recognizable" state, i.e., instance state is only required during execution of that one entry point?

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• Can you point me to a nontrivial strategy pattern implementation?

  - by Eugen Martynov

  We are faced implementing a registration workflow with many branches. There are three main flows which in some conditions lead to one another. Each flow has at least four different steps; some steps interact with the server, and every step adds more information to the state. Also the requirement is to have it persistent between sessions, so if the user closes the app (this is a mobile app), it will restore the process from the last completed step with the state from the previous session. I think this could benefit from the use of the strategy pattern, but I've never had to implement it for such a complex case. Does anyone know of any examples in open source or articles from which I could find inspiration? Preferably the examples would be from a live/working/stable application. I'm interested in Java implementation mostly; we are developing for Java mobile phones: android, blackberry and J2ME. We have an SDK which is quite well separated from platform specific implementations, but examples in C++, C#, Objective-C or Python would be acceptable.

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