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• Relationships in a Chen ERD

  - by Nibroc A Rehpotsirhc

  I am working on a Chen ERD to model our organizations merchandise. Our central entity is a Style. We have supplemental entities of Color and Season. I am defining our assortment as the relationship between these three entities, and this relationship itself will have attributes and is defined by the three entities which participate in the mandatory relationship. The rules are; Many Styles can be offered in a Season, and a Style can be offered in many Seasons. Within a Season, a Style can be offered in Many Colors. I then have 2 other entities, one of which I believe is a weak entity, Climate, and the other may be weak, but I am not sure, this being Transaction Channel. I am thinking of these as relationships off of a relationship? Meaning, for a given Style/Color combination offered in a Season, it can be available through 1 or more Transaction Channels. Additionally, within a season, a given Style/Color combination can be intended for 1 or more Climates. Is it valid to have relationships off of relationships? Or does this requirement dictate that I should think of this Style/Color/Season relationship as an entity itself, and define the relationships to Climate and Transaction Channel off of this entity?

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• Examples of Liskov Substitution

  - by james lewis

  I'm facilitating a session next week on the Liskov Substitution Principle and I was wondering if anyone had any examples of violations 'from the trenches'? I'm looking for something other than uncle Bob's rectangle - square problem and the persistent set problem he talks about in A-PPP (although that is a great example). So far I'm using the example of a (very simple) List and an IndexedList as the 'correct' use of inheritance. And the addition of a Set to this hierarchy as a violation (as a Set is distinct; strengthening the pre condition of the Add method). I've also taken this great example and it's solution from this question Both those examples are great but I'm looking for something more subtle and harder to spot. So far I've come up with nothing so if you've got a great, subtle example post it up. Also, any metaphors you've come across that helped you understand LSP would be really useful too.

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• Liskov principle: violation by type-hinting

  - by Elias Van Ootegem

  According to the Liskov principle, a construction like the one below is invalid, as it strengthens a pre-condition. I know the example is pointless/nonsense, but when I last asked a question like this, and used a more elaborate code sample, it seemed to distract people too much from the actual question. //Data models abstract class Argument { protected $value = null; public function getValue() { return $this->value; } abstract public function setValue($val); } class Numeric extends Argument { public function setValue($val) { $this->value = $val + 0;//coerce to number return $this; } } //used here: abstract class Output { public function printValue(Argument $arg) { echo $this->format($arg); return $this; } abstract public function format(Argument $arg); } class OutputNumeric extends Output { public function format(Numeric $arg)//<-- VIOLATION! { $format = is_float($arg->getValue()) ? '%.3f' : '%d'; return sprintf($format, $arg->getValue()); } } My question is this: Why would this kind of "violation" be considered harmful? So much so that some languages, like the one I used in this example (PHP), don't even allow this? I'm not allowed to strengthen the type-hint of an abstract method but, by overriding the printValue method, I am allowed to write: class OutputNumeric extends Output { final public function printValue(Numeric $arg) { echo $this->format($arg); } public function format(Argument $arg) { $format = is_float($arg->getValue()) ? '%.3f' : '%d'; return sprintf($format, $arg->getValue()); } } But this would imply repeating myself for each and every child of Output, and makes my objects harder to reuse. I understand why the Liskov principle exists, don't get me wrong, but I find it somewhat difficult to fathom why the signature of an abstract method in an abstract class has to be adhered to so much stricter than a non-abstract method. Could someone explain to me why I'm not allowed to hind at a child class, in a child class? The way I see it, the child class OutputNumeric is a specific use-case of Output, and thus might need a specific instance of Argument, namely Numeric. Is it really so wrong of me to write code like this?

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• Better solution then simple factory method when concrete implementations have different attributes

  - by danip

  abstract class Animal { function eat() {..} function sleep() {..} function isSmart() } class Dog extends Animal { public $blnCanBark; function isSmart() { return $this->blnCanBark; } } class Cat extends Animal { public $blnCanJumpHigh; function isSmart() { return $this->blnCanJumpHigh; } } .. and so on up to 10-20 animals. Now I created a factory using simple factory method and try to create instances like this: class AnimalFactory { public static function create($strName) { switch($strName) { case 'Dog': return new Dog(); case 'Cat': return new Cat(); default: break; } } } The problem is I can't set the specific attributes like blnCanBark, blnCanJumpHigh in an efficient way. I can send all of them as extra params to create but this will not scale to more then a few classes. Also I can't break the inheritance because a lot of the basic functionality is the same. Is there a better pattern to solve this?

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• C# vector class - Interpolation design decision

  - by Benjamin

  Currently I'm working on a vector class in C# and now I'm coming to the point, where I've to figure out, how i want to implement the functions for interpolation between two vectors. At first I came up with implementing the functions directly into the vector class... public class Vector3D { public static Vector3D LinearInterpolate(Vector3D vector1, Vector3D vector2, double factor) { ... } public Vector3D LinearInterpolate(Vector3D other, double factor { ... } } (I always offer both: a static method with two vectors as parameters and one non-static, with only one vector as parameter) ...but then I got the idea to use extension methods (defined in a seperate class called "Interpolation" for example), since interpolation isn't really a thing only available for vectors. So this could be another solution: public class Vector3D { ... } public static class Interpolation { public static Vector3D LinearInterpolate(this Vector3D vector, Vector3D other, double factor) { ... } } So here an example how you'd use the different possibilities: { var vec1 = new Vector3D(5, 3, 1); var vec2 = new Vector3D(4, 2, 0); Vector3D vec3; vec3 = vec1.LinearInterpolate(vec2, 0.5); //1 vec3 = Vector3D.LinearInterpolate(vec1, vec2, 0.5); //2 //or with extension-methods vec3 = vec1.LinearInterpolate(vec2, 0.5); //3 (same as 1) vec3 = Interpolation.LinearInterpolation(vec1, vec2, 0.5); //4 } So I really don't know which design is better. Also I don't know if there's an ultimate rule for things like this or if it's just about what someone personally prefers. But I really would like to hear your opinions, what's better (and if possible why ).

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• How did you get good practices for your OOP designs?

  - by Darf Zon

  I realized I have a difficulty creating OOP designs. I spent many time deciding if this property is correctly set it to X class. For example, this is a post which has a few days: http://codereview.stackexchange.com/questions/8041/how-to-improve-my-factory-design I'm not convinced of my code. So I want to improve my designs, take less time creating it. How did you learn creating good designs? Some books that you can recommend me?

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• Dependency Injection: Only for single-instance objects?

  - by HappyDeveloper

  What if I want to also decouple my application, from classes like Product or User? (which usually have more than one instance) Take a look at this example: class Controller { public function someAction() { $product_1 = new Product(); $product_2 = new Product(); // do something with the products } } Is it right to say that Controller now depends on Product? I was thinking that we could decouple them too (as we would with single-instance objects like Database) In this example, however ugly, they are decoupled: class Controller { public function someAction(ProductInterface $new_product) { $product_1 = clone $new_product; $product_2 = clone $new_product; // do something with the products } } Has anyone done something like this before? Is it excessive?

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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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• Make methods that do not depend on instance fields, static?

  - by m3th0dman

  Recently I started programming in Groovy for a integration testing framework, for a Java project. I use Intellij IDEA with Groovy plug-in and I am surprised to see as a warning for all the methods that are non-static and do not depend on any instance fields. In Java, however, this is not an issue (at least from IDE's point of view). Should all methods that do not depend onto any instance fields be transformed into static functions? If true, is this specific to Groovy or it is available for OOP in general? And why?

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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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• Is loose coupling w/o use cases an anti-pattern?

  - by dsimcha

  Loose coupling is, to some developers, the holy grail of well-engineered software. It's certainly a good thing when it makes code more flexible in the face of changes that are likely to occur in the foreseeable future, or avoids code duplication. On the other hand, efforts to loosely couple components increase the amount of indirection in a program, thus increasing its complexity, often making it more difficult to understand and often making it less efficient. Do you consider a focus on loose coupling without any use cases for the loose coupling (such as avoiding code duplication or planning for changes that are likely to occur in the foreseeable future) to be an anti-pattern? Can loose coupling fall under the umbrella of YAGNI?

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• Is ORM an Anti-Pattern?

  - by derphil

  I had a very stimulating and interessting discussion with a colleague about ORM and it's Pros and Cons. In my opinion, an ORM is useful only in the rarest cases. At least in my experience. But I don't want to list my own arguments at this time. So I ask you, what do you think about ORM? What are the Pros and the Cons? P.S. I've posted this "question" yesterday on Stackoverflow, but some of the user think, that this should better posted here.

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• A programming language that does not allow IO. Haskell is not a pure language

  - by TheIronKnuckle

  (I asked this on Stack Overflow and it got closed as off-topic, I was a bit confused until I read the FAQ, which discouraged subjective theoratical debate style questions. The FAQ here doesn't seem to have a problem with it and it sounds like this is a more appropriate place to post. If this gets closed again, forgive me, I'm not trying to troll) Are there any 100% pure languages (as I describe in the Stack Overflow post) out there already and if so, could they feasibly be used to actually do stuff? i.e. do they have an implementation? I'm not looking for raw maths on paper/Pure lambda calculus. However Pure lambda calculus with a compiler or a runtime system attached is something I'd be interested in hearing about.

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• When to decide to introduce interfaces (pure abstract base classes) in C++?

  - by Honza Brabec

  Assume that you are developing a functionality and are 90% sure that the implementation class will stay alone. If I was in this position in Java I would probably not use the interface right now to keep the things simple. In Java it is easy to refactor the code and extract the interface later. In C++ the refactoring is not always so easy. It may require replacing values with smart pointers (because of the introduction of polymorphism) and other non-trivial tasks. On the other hand I don't much like the idea of introducing virtual calls when I am 90% sure they won't be needed. After all speed is one of the reasons to prefer C++ over simpler languages.

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• Learning PHP OOP

  - by Ryan Murphy

  I have been coding PHP for about 2 years now and I THINK that I have a very good grasps of the fundamental parts of PHP, i.e. Functions foreach/IF statements sessions/cookies POST/GET Amongst a few others. I want to move on to learning OOP PHP now, so learning how to use classes and making it a really valuable skill. I have 1 requirement, the source must be a respected source that doesn't teach developers bad habits. I have the book: PHP and MySQL Web Development However, as useful as that is I would like an online source. I would like to know from people with experience in OOP PHP, how and where did they learn OOP PHP. Obviously by doing, but I would really appreciate some great resources which help me along the way.

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• What is considered third party code?

  - by Songo

  Inspired by this question Using third-party libraries - always use a wrapper? I wanted to know what people actually consider as third-party libraries. Example from PHP: If I'm building an application using Zend framework, should I treat Zend framework libraries as third party code? Example from C#: If I'm building a desktop application, should I treat all .Net classes as third party code? Example from Java: Should I treat all libraries in the JDK as third party libraries? Some people say that if a library is stable and won't change often then one doesn't need to wrap it. However I fail to see how one would test a class that depends on a third party code without wrapping it.

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• Metaobject protocol:Why is it known as an important concept

  - by sushant

  Metaobject protocol is protocol for metaobjects in a programming languages. Although I understand it on simple terms, I want to know the reason and a summary of real world usage patterns of this protocol. So, why exactly is metaobject and more importantly metaobject protocol is such a good idea. I want to know the problem which led to its evolution and also, its high power usage. Opinions as well as general overview/description/alternate explanations are also welcome.

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• Configuration Data in a Custom Timer job in Sharepoint 2010 : The Hierarchical Object Store

  - by Gino Abraham

  I was planning for a custom timer job for which i wanted to store some configuration data. Was looking for some best practices, found a useful links on The Hierarchical Object store Store http://www.chaholl.com/archive/2011/01/30/the-skinny-on-sppersistedobject-and-the-hierarchical-object-store-in.aspxInitially was planning for a custom list, but this would make us run a cross site query and the list name and the url should again be kept in some configuration which is an headache to maintain. Hierarchical object store was zeroed in and thanks to google for the same :)

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• Should I use JavaFx properties?

  - by Mike G

  I'm usually very careful to keep my Model, View, and Controller code separate. The thing is JavaFx properties are so convenient to bind them all together. The issue is that it makes my entire code design dependent on JavaFx, which I feel I should not being doing. I should be able to change the view without changing too much of the model and controller. So should I ignore the convenience of JavaFx properties, or should I embrace them and the fact that it reduces my codes flexibility.

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• Recommened design pattern to handle multiple compression algorithms for a class hierarchy

  - by sgorozco

  For all you OOD experts. What would be the recommended way to model the following scenario? I have a certain class hierarchy similar to the following one: class Base { ... } class Derived1 : Base { ... } class Derived2 : Base { ... } ... Next, I would like to implement different compression/decompression engines for this hierarchy. (I already have code for several strategies that best handle different cases, like file compression, network stream compression, legacy system compression, etc.) I would like the compression strategy to be pluggable and chosen at runtime, however I'm not sure how to handle the class hierarchy. Currently I have a tighly-coupled design that looks like this: interface ICompressor { byte[] Compress(Base instance); } class Strategy1Compressor : ICompressor { byte[] Compress(Base instance) { // Common compression guts for Base class ... // if( instance is Derived1 ) { // Compression guts for Derived1 class } if( instance is Derived2 ) { // Compression guts for Derived2 class } // Additional compression logic to handle other class derivations ... } } As it is, whenever I add a new derived class inheriting from Base, I would have to modify all compression strategies to take into account this new class. Is there a design pattern that allows me to decouple this, and allow me to easily introduce more classes to the Base hierarchy and/or additional compression strategies?

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• OOP private method parameters coding style

  - by Jake

  After coding for many years as a solo programmer, I have come to feel that most of the time there are many benefits to write private member functions with all of the used member variables included in the parameter list, especially development stage. This allow me to check at one look what member variables are used and also allow me to supply other values for tests and debugging. Also, a change in code by removing a particular member variable can break many functions. In this case however, the private function remains isolated am I can still call it using other values without fixing the function. Is this a bad idea afterall, especially in a team environment? Is it like redundant or confusing, or are there better ways?

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• As a tooling/automation developer, can I be making better use of OOP?

  - by Tom Pickles

  My time as a developer (~8 yrs) has been spent creating tooling/automation of one sort or another. The tools I develop usually interface with one or more API's. These API's could be win32, WMI, VMWare, a help-desk application, LDAP, you get the picture. The apps I develop could be just to pull back data and store/report. It could be to provision groups of VM's to create live like mock environments, update a trouble ticket etc. I've been developing in .Net and I'm currently reading into design patterns and trying to think about how I can improve my skills to make better use of and increase my understanding of OOP. For example, I've never used an interface of my own making in anger (which is probably not a good thing), because I honestly cannot identify where using one would benefit later on when modifying my code. My classes are usually very specific and I don't create similar classes with similar properties/methods which could use a common interface (like perhaps a car dealership or shop application might). I generally use an n-tier approach to my apps, having a presentation layer, a business logic/manager layer which interfaces with layer(s) that make calls to the API's I'm working with. My business entities are always just method-less container objects, which I populate with data and pass back and forth between my API interfacing layer using static methods to proxy/validate between the front and the back end. My code by nature of my work, has few common components, at least from what I can see. So I'm struggling to see how I can better make use of OOP design and perhaps reusable patterns. Am I right to be concerned that I could be being smarter about how I work, or is what I'm doing now right for my line of work? Or, am I missing something fundamental in OOP? EDIT: Here is some basic code to show how my mgr and api facing layers work. I use static classes as they do not persist any data, only facilitate moving it between layers. public static class MgrClass { public static bool PowerOnVM(string VMName) { // Perform logic to validate or apply biz logic // call APIClass to do the work return APIClass.PowerOnVM(VMName); } } public static class APIClass { public static bool PowerOnVM(string VMName) { // Calls to 3rd party API to power on a virtual machine // returns true or false if was successful for example } }

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• Trying to find a recent - PHP book - that utilizes SOLID principles! [closed]

  - by darga33

  Pulling my hair out! I have heard of Martin Fowler's book PoEAA and the other book Head First OOA OOD but those are not in PHP. I desperately want to read them, but ONLY in PHP utilizing the - SOLID acronym - principles! Does anyone know of the absolute best, most recent PHP book that utilizes the SOLID principles and GRASP, and all the other best practices? I want to learn from the best possible source! Not beginner books! I already understand OOP. This seems like an almost impossible question to find the answer to and so I thought, hey, might as well post on stackexchange!! Surely someone out there must know!!!!!!!!!! Or if noone happens to know, Maybe they know of an open source application that utilizes these principles that is relatively small that is not a framework. Something that I can go through every single class, and spend time understanding the insides and outs of how the program was developed. Thanks so much in advance! I really really really really appreciate it! Well it looks like we aren't supposed to ask about best books, so nevermind this question! Sorry about that!

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• Where to put business logic in MVC design?

  - by BriskLabs Pakistan

  I have created a simple MVC java application that adds records through data forms to a database. my app collects data, it also validates it and stores it. This is because the data is being sourced online from different users. the data is mostly numeric in nature. now on the numeric data being stored into database (SQL server) , i wish that my app should be able to perform computations... and display it. the user is not interested in how computations are done so they must be encapsulated. the user must only be able to view the simple computed data which for example A column data - B Column data / C column data etc... and just display it to the user... i know how to write stored procedures for same but i want a 3 tier app I want the data, that I put into the database as a record, worked upon by performing calculations on it. However, the original data should remain unaffected, while the new data, post-calculations, must be stored as a new entity record into the database. Where should I write the code for this background calculation? As it is the rules and business logic... in a new java beans files ?

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• Is there a good design pattern for this messaging class?

  - by salonMonsters

  Is there a good design pattern for this? I want to create a messaging class. The class will be passed: the type of message (eg. signup, signup confirmation, password reminder etc) the client's id The class needs to then look up the client's messaging preferences in the db (whether they want communication by email, sms or both) Then depending on the client's preference it will format the message for the medium (short version for sms, long form for email) and send it through our mail or sms provider's API. Because the fact that we want to be able to change out email and sms providers if need be I wondered if the Command Pattern would be a good choice.

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