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What I would like to see more in the book is a reference to typical, real life use cases of each pattern. For example, I have heard the command pattern can be used to radio groups or for writing wizards.

How is this done? Are there any examples of real life code using the visitor pattern? Are there any simple examples we can study? No need for extra book pages, just a link to a online repo with code examples probably would do. Maybe more examples would help the stick better to the memory.

Yes, it is up to me to come up with good use cases for each pattern but as a newbie I find it difficult to remember what each pattern does and what it is good for. Some, like the singleton, the facade, the observer etc. Also it would be great if there was an option for a dead tree version of the book. I want to keep it as reference. I will feed the PDF to my laser printer but I would like to have it printed and bound looking like a real book. I've read a considerable part of the book already.

I loved what I read! You explain the concepts in such an easy way. I wish I had the opportunity to read it when I was in college. I don't have, currently, any suggestions to improve it. But it would be great if it were available in more languages. Since I'm from Brazil, I'm not gonna lie, it would be great to have it in Portuguese so I could tell my non-English speaking friends to get it as well, 'cause they really need some of the knowledge in the book.

I stumbled across your website when searching "why refactoring is important" if I'm not mistaken. I immediately saved it to my bookmarks that day and shared it with some of my colleagues. I like your book, it is easy to understand even if I'm not good in English and really really beginner, of course the illustrate each pattern make me smile, real world example and class diagram help me better understanding, relation between pattern really help me to understand when and why pattern exist.

I must say you have a knack of telling the things in the right way. I am really enjoying the book and I am very sure even the refactoring book would be great as well. I like the way topics are presented and the examples. That helped be get the context and better understand them.

This way, I will not forget the concepts over a period of time. I wish the SOLID principles content should be expanded a little more by taking a real world complete use case and applying step by step like before and after. I know this kind of thing may be tricky to make, but, it would help. The design patterns then can augment the same.

I am glad to have purchased the books and they will definitely help me in understanding and applying them at my work. Keep up the good work. I will be watching out for any new content that you put out. I like the way you have explained each of design patterns although I have the original GoF books but it's really hard to understand.

The Design Patterns are something I was not very familiar with. I already learned a lot from your book and I hope I will be able soon to get a new role in my career and maybe to teach other people about the patterns. I would do more examples if I could. What I would put here is a brief of symptoms of a bad design like rigidity, fragility, imobility and viscosity. But for me, it's exactly what I was looking for - design patterns with explanations and concrete examples in one place.

And to be honest it was at the best price - personally I wouldn't do it for this amount. Thank you again and congratulations for your work. I would be glad to share you my pieces of code in the future if you want.

Good luck! This is a very good book you have. This is well explained at the level of the principles and through examples. My only concern is my level of English which is very low and it takes me a lot of time to understand the quite complex designs. I am a symfony developer and currently I am in the process of deciding the use of these patterns at the framwork level.

It's the others that I'm investing more. I found, for example, that symfony formlaries are rich enough to find use cases factoryMethod, builder, composite. This will interest me a lot if you can help me find the use of the symfony designs. I'm still reading the book, but so far I really like it! It has been a helpful refresher to me on OOP principles. I liked the explanation of UML diagram symbols.

I'm relatively new to design patterns, but you explain them in a very clear manner which is easy to understand. I enjoy the illustrations and the real-world examples. Other material I had previously read on design patterns was pretty abstract without saying how it would be helpful in real life. But your book makes it all very clear! The book is really awesome and explains the concepts in great detail. I read the book completely and I think its the fastest I completed any book. I am also considering to buy the other book on code refactoring.

The improvements I would like to see in the book is to add some more design patterns. I guess some design patterns from the Gang of Four missing and also some patterns outside GoF which we use most often. I would like to thank you for your excellent book. It's one of the best purchases I have recently made and has helped me a ton with work. I've read the book as soon as I got it after I had read through the examples available on your website, because it convinced me that the information in it would be very useful to me.

Since I have got your book, I have kept it open at work on one of the displays as a reference and guidelines for my designs and whenever I extend the functionality of my software I always follow the design patterns presented in the book. What I love about the book is that it is a recent and modern document about design patterns, which is in my experience the bread and butter of object oriented software design. It is very concise, provides many examples and very concrete applications, and actual implementations in all the programming languages I use at work.

While the information in the unavoidable "Gang of Four" design patterns book is very good on its own and a great text, I was looking for a book in the same scope, but a recent one to keep up with the new designs, technology and concepts that come and your book is a perfect candidate for that. The text itself is pretty, with very nice diagrams and images, great formatting and typesetting. While these are details, they make the information much more readable and enhances the whole experience.

And the cover art is very nice! I honestly have no idea on how to improve the book. It has been a pleasure to read, and has become a very important tool at work and one of my "definite references", so it's difficult to improve something you are already very satisfied with!

So, thank you again, props to you for making this great text. The first recommendation for books about design patterns and object oriented software design that I would give would be definitely yours, hands down. I liked everything, I read it only once for now. And I have not yet found anything that could be improved.

I keep it in my favorites as a reference book. I use your website, and bought the book to support the project! I like everything very much. Simple and user-friendly! Keep it up! To my surprise, the book is very well suited for different levels of specialists. I enjoyed the illustrations and the method of presenting the material. Thank you for popularizing useful educational materials. I believe the presentation is ideal: brief, without unnecessary discussion. A clear description of the problem, a way to solve it, a pseudocode: all you need is there.

Illustrations also help a lot. If I need to remember why one or another pattern is needed, I just look at the picture and I remember everything.

This is what could be improved, in my opinion. Maybe a few more examples of the practical application of a particular pattern. Without a code. Just a description of the situation and its solution using a pattern. I am, of course, more interested in Web development.

I'm reading the book right now. The website has almost everything that is written in the book, so technically I could have just read about it on the website. As to what could be improved, it is difficult to say.

Sometimes you write about the same thing over and over 10 times, but many of us including myself need it for initial understanding. I would love to see more examples, maybe even without code. For example - you can talk with developers from different industries and ask around, where and how they apply this or that pattern.

Maybe you should add some links to designs of other guys as an example. But overall the book is quite good. The pictures are cool. Sometimes it's hard to figure out what's what, but what can you do? We must persevere. Read again, try again. It would be great to see the version for Kotlin. I actually rewrite your examples in order to learn the language.

I started reading the book, and I'm very pleased with it. Great, high-quality examples and schemes. My first impression is a positive one, the author should continue writing. I'd like to buy the book for a friend. I understand that the only option is to register under a new account. I liked the simplicity of descriptions in your book, as well as the excellent usage examples. I use your book as a reference guide in order to refresh my knowledge about a particular template or to search for a template I may need at the moment.

I can't say anything about how the book can be improved, because I haven't even thought about it. Thanks for the great material! I'm reading the book in sections, I really like how the material is presented, user-friendly presentation on several levels, interesting examples, clear illustrations.

It is convenient not only to read the book in a sequential order, but to also use it as a reference guide. I plan to use the materials you have collected to put together training materials, reports inside and outside my company, in the context of front-end development. I think it would be cool for newbies if there were c examples as well, although everything is quite clear anyway. Since I have a stable Internet connection, I use the website as a reference guide instead of the book.

I'm reading the book, it's interesting, not boring, and it offers quite an exciting and fresh perspective. I would love to see it a paperback edition as well. Thank you for the book. The material is presented beautifully and in a user-friendly manner.

The examples are clear, the diagrams are accurate, the illustrations are interesting. It works as a reference book, as well as an introductory guide for the topic. I use it for both purposes. I work as a C developer. I had read an article online that included many references to patterns. I went to Google and found your website, where I read about the pattern that was of interest to me.

Your information is very well structured, but what I liked the most was the style of illustrations. There is something home-like about them As a beginner, I really liked the fact that the patterns are described from the point of view of the issues that they solve, and not from the standpoint of the description of the pattern itself. After reading the entire section on patterns on the website, I realized that I wanted to buy the book to thank you for the work you have done collecting and structuring the information.

Despite the fact that I read about all the patterns, I still don't understand what this section is about. Maybe you could add illustrations there, or even remove this section altogether and add a chapter about using combined patterns? The project as a whole, and the book in particular, make an excellent impression.

It would seem that there are quite a lot of books written on this topic, but you have structured everything and made it rather fun Wonderful illustrations that help readers memorize everything in a fun way. I started reading the book right after I purchased it. I've already learned how to apply a couple of the described patterns quite confidently in practice. The book can be improved by publishing it on paper. That way you can give it as a gift, for example. I am completely satisfied with the purchase, I have already read the book from cover to cover.

I think that this is the most user-friendly book on patterns I have ever read. I was really happy that I could read it on my iPhone. Thank you for the great book! Excellent book, everything is explained in detail and presented in a user-friendly manner.

Special thanks for supporting multiple formats and providing the readers with the ability to read the book on the go. I would like to note right away that the book is beautifully illustrated The information about SOLID, and in particular, about the principle of Barbara Liskov, is presented very well and in a user-friendly manner.

I found some new information for myself about the private members of the base classes and the fact that you explicitly separate the restrictions on preconditions by type and attribute values.

It is really easier to figure everything out this way. The book is great. Excellent illustrations and examples. I am now preparing a report on Design Patterns at work. Everything that is explained using cats is always clear, and the more allegories the better. I haven't finished the book yet, but the first impression is rather good, thank you! I confirm that I bought this book, and am satisfied with the purchase, otherwise I would not have bought it :.

I was looking to find some information on the most widely used patterns builder and factory , and I ended up finding the materials on the website. Your website is by far the BEST of all that I have come across on this topic, and that's why I bookmarked it and added the PDF version to the offline library on my tablet. I almost finished reading the book. I will say right away that I liked it; the book offers real-life examples, the descriptions and the pseudocode are good.

Another anti-example comes from the world of programming languages with dynamic typing: the base method returns a string, but the overridden method returns a number. In other words, types of exceptions should match or be subtypes of the ones that the base method is already able to throw. This rule comes from the fact that try-catch blocks in the client code target specific types of exceptions which the base method is likely to throw. Therefore, an unexpected exception might slip through the defensive lines of the client code and crash the entire application.

For example, the base method has a parameter with type int. If a subclass overrides this method and requires that the value of an argument passed to the method should be positive by throwing an exception if the value is negative , this strengthens the pre-conditions. The client code, which used to work fine when passing negative numbers into the method, now breaks if it starts working with an object of this subclass.

Say you have a class with a method that works with a database. A method of the class is supposed to always close all opened database connections upon returning a value. You created a subclass and changed it so that database connections remain open so you can reuse them. But the client might not know anything about your intentions. Because it expects the methods to close all the connections, it may simply terminate the program right after calling the method, polluting a system with ghost database connections.

This is probably the least formal rule of all. For example, invariants of a cat are having four legs, a tail, ability to meow, etc. The confusing part about invariants is that while they can be defined explicitly in the form of interface contracts or a set of assertions within methods, they could also be implied by certain unit tests and expectations of the client code.

The rule on invariants is the easiest to violate because you might misunderstand or not realize all of the invariants of a complex class. Therefore, the safest way to extend a class is to introduce new fields and methods, and not mess with any existing members of the superclass. It turns out some programming languages let you access private members of a class via reflection mechanisms.

The save method in the ReadOnlyDocuments subclass throws an exception if someone tries to call it. You can solve the problem by redesigning the class hierarchy: a subclass should extend the behavior of a superclass, therefore the read-only document becomes the base class of the hierarchy. The writable document is now a subclass which extends the base class and adds the saving behavior. Clients should implement only those methods that they really need.

Break it down into several more refined interfaces�you can implement them all in a single class if needed. However, some classes may be fine with implementing just one of them. Example Imagine that you created a library that makes it easy to integrate apps with various cloud computing providers.

At the time you assumed that all cloud providers have the same broad spectrum of features as Amazon. But when it came to implementing support for another provider, it turned out that most of the interfaces of the library are too wide.

Classes that are able to implement the original interface can now just implement several refined interfaces. Other classes can implement only those interfaces which have methods that make sense for them. AFTER: one bloated interface is broken down into a set of more granular interfaces. As with the other principles, you can go too far with this one. Remember that the more interfaces you create, the more complex your code becomes.

Keep the balance. Both should depend on abstractions. Details should depend on abstractions. Usually when designing software, you can make a distinction between two levels of classes. Sometimes people design low-level classes first and only then start working on high-level ones. With such an approach business logic classes tend to become dependent on primitive low-level classes.

The dependency inversion principle suggests changing the direction of this dependency. For starters, you need to describe interfaces for low-level operations that high-level classes rely on, preferably in business terms. For instance, business logic should call a method rather than a series of methods openReport file openFile x , readBytes n , closeFile x.

These interfaces count as high-level ones. Now you can make high-level classes dependent on those interfaces, instead of on concrete low-level classes. This dependency will be much softer than the original one.

Once low-level classes implement these interfaces, they become dependent on the business logic level, reversing the direction of the original dependency. Example In this example, the high-level budget reporting class uses a low-level database class for reading and persisting its data.

AFTER: low-level classes depend on a high-level abstraction. The first version of your app can only handle transportation by trucks, so the bulk of your code lives inside the Truck class. After a while, your app becomes pretty popular.

Each day you receive dozens of requests from sea transportation companies to incorporate sea logistics into the app. Great news, right? But how about the code? At present, most of your code is coupled to the Truck class. Adding Ships into the app would require making changes to the entire codebase. Moreover, if later you decide to add another type of transportation to the app, you will probably need to make all of these changes again.

At first glance, this change may look pointless: we just moved the constructor call from one part of the program to another.

However, consider this: now you can override the factory method in a subclass and change the class of products being created by the method. Also, the factory method in the base class should have its return type declared as this interface. All products must follow the same interface. For example, both Truck and Ship classes should implement the Transport interface, which declares a method called deliver.

Each class implements this method differently: trucks deliver cargo by land, ships deliver cargo by sea. The factory method in the RoadLogistics class returns truck objects, whereas the factory method in the SeaLogistics class returns ships. The client treats all the products as abstract Transport. The Product declares the interface, which is common to all objects that can be produced by the creator and its subclasses.

Concrete Products are different implementations of the product interface. The Creator class declares the factory method that returns new product objects. You can declare the factory method as abstract to force all subclasses to implement their own versions of the method. As an alternative, the base factory method can return some default product type. Note, despite its name, product creation is not the primary responsibility of the creator. Usually, the creator class already has some core business logic related to products.

The factory method helps to decouple this logic from the concrete product classes. Here is an analogy: a large software development company can have a training department for programmers.

However, the primary function of the company as a whole is still writing code, not producing programmers. Concrete Creators override the base factory method so it returns a different type of product. It can also return existing objects from a cache, an object pool, or another source.

The base dialog class uses different UI elements to render its window. Under various operating systems, these elements may look a little bit different, but they should still behave consistently. A button in Windows is still a button in Linux. The cross-platform dialog example. If we declare a factory method that produces buttons inside the base dialog class, we can later create a dialog subclass that returns Windows-styled buttons from the factory method.

For this pattern to work, the base dialog class must work with abstract buttons: a base class or an interface that all concrete buttons follow. Of course, you can apply this approach to other UI elements as well. However, with each new factory method you add to the dialog, you get closer to the Abstract Factory pattern.

Unknown operating system. But how would the framework recognize that your subclass should be used instead of a standard component? Imagine that you write an app using an open source UI framework. Your app should have round buttons, but the framework only provides square ones.

You extend the standard Button class with a glorious RoundButton subclass. But now you need to tell the main UIFramework class to use the new button subclass instead of a default one. To achieve this, you create a subclass UIWithRoundButtons from a base framework class and override its createButton method.

While this method returns Button objects in the base class, you make your subclass return RoundButton objects. First, you need to create some storage to keep track of all of the created objects. When someone requests an object, the program should look for a free object inside that pool. If there are no free objects, the program should create a new one and add it to the pool. However, a constructor must always return new objects by definition. Therefore, you need to have a regular method capable of creating new objects as well as reusing existing ones.

That sounds very much like a factory method. Make all products follow the same interface. This interface should declare methods that make sense in every product. Add an empty factory method inside the creator class. The return type of the method should match the common product interface. One by one, replace them with calls to the factory method, while extracting the product creation code into the factory method. You might need to add a temporary parameter to the factory method to control the type of returned product.

At this point, the code of the factory method may look pretty ugly. It may have a large switch operator that picks which product class to instantiate. Now, create a set of creator subclasses for each type of product listed in the factory method. Override the factory method in the subclasses and extract the appropriate bits of construction code from the base method. The client code can pass an argument to the factory method of the GroundMail class to control which product it wants to receive.

Truck 6. If, after all of the extractions, the base factory method has become empty, you can make it abstract. You can move the product creation code into one place in the program, making the code easier to support. You can introduce new types of products into the program without breaking existing client code. On the other hand, Prototype requires a complicated initialization of the cloned object.

At the same time, a Factory Method may serve as a step in a large Template Method. Your code consists of classes that represent: 1. A family of related products, say: CoffeeTable. Several variants of this family. Chair Product families and their variants. Customers get quite mad when they receive non-matching furniture.

Then you can make all variants of products follow those interfaces. For example, all chair variants can implement the Chair interface; all coffee table variants can implement the CoffeeTable interface, and so on. The next move is to declare the Abstract Factory�an interface with a list of creation methods for all products that are part of the product family for example, createChair , createSofa and createCoffeeTable.

These methods must return abstract product types represented by the interfaces we extracted previously: Chair , Sofa , CoffeeTable and so on. Now, how about the product variants? For each variant of a product family, we create a separate factory class based on the AbstractFactory interface. A factory is a class that returns products of a particular kind. The client code has to work with both factories and products via their respective abstract interfaces.

This lets you change the type of a factory that you pass to the client code, as well as the product variant that the client code receives, without breaking the actual client code. With this approach, the only thing that the client knows about the chair is that it implements the sitOn method in some way. One more thing left to clarify: if the client is only exposed to the abstract interfaces, what creates the actual factory objects? Usually, the application creates a concrete factory object at the initialization stage.

Just before that, the app must select the factory type depending on the configuration or the environment settings. Abstract Products declare interfaces for a set of distinct but related products which make up a product family.

Concrete Products are various implementations of abstract products, grouped by variants. The Abstract Factory interface declares a set of methods for creating each of the abstract products. Concrete Factories implement creation methods of the abstract factory. Each concrete factory corresponds to a specific variant of products and creates only those product variants.

Although concrete factories instantiate concrete products, signatures of their creation methods must return corresponding abstract products. The same UI elements in a cross-platform application are expected to behave similarly, but look a little bit different under different operating systems. The Abstract Factory interface declares a set of creation methods that the client code can use to produce different types of UI elements. Concrete factories correspond to specific operating systems and create the UI elements that match that particular OS.

It works like this: when an application launches, it checks the type of the current operating system. The rest of the code uses this factory to create UI elements. This prevents the wrong elements from being created. This also lets the client code support other factories or UI elements that you might add in the future. When a class deals with multiple product types, it may be worth extracting its factory methods into a standalone factory class or a full-blown Abstract Factory implementation.

Map out a matrix of distinct product types versus variants of these products. Declare abstract product interfaces for all product types. Then make all concrete product classes implement these interfaces. Declare the abstract factory interface with a set of creation methods for all abstract products. Implement a set of concrete factory classes, one for each product variant.

Create factory initialization code somewhere in the app. It should instantiate one of the concrete factory classes, depending on the application configuration or the current environment. Pass this factory object to all classes that construct products. Scan through the code and find all direct calls to product constructors. Replace them with calls to the appropriate creation method on the factory object. You can extract the product creation code into one place, making the code easier to support.

You can introduce new variants of products without breaking existing client code. Abstract Factory specializes in creating families of related objects. Abstract Factory returns the product immediately, whereas Builder lets you run some additional construction steps before fetching the product.

This pairing is useful when some abstractions defined by Bridge can only work with specific implementations.

In this case, Abstract Factory can encapsulate these relations and hide the complexity from the client code. The pattern allows you to produce different types and representations of an object using the same construction code. Such initialization code is usually buried inside a monstrous constructor with lots of parameters.

Or even worse: scattered all over the client code. You might make the program too complex by creating a subclass for every possible configuration of an object.

To build a simple house, you need to construct four walls and a floor, install a door, fit a pair of windows, and build a roof. But what if you want a bigger, brighter house, with a backyard and other goodies like a heating system, plumbing, and electrical wiring? Any new parameter, such as the porch style, will require growing this hierarchy even more.

You can create a giant constructor right in the base House class with all possible parameters that control the house object. While this approach indeed eliminates the need for subclasses, it creates another problem. The constructor with lots of parameters has its downside: not all the parameters are needed at all times.

In most cases most of the parameters will be unused, making the constructor calls pretty ugly. For instance, only a fraction of houses have swimming pools, so the parameters related to swimming pools will be useless nine times out of ten.

The Builder pattern lets you construct complex objects step by step. The pattern organizes object construction into a set of steps buildWalls , buildDoor , etc. To create an object, you execute a series of these steps on a builder object.

You can call only those steps that are necessary for producing a particular configuration of an object. For example, walls of a cabin may be built of wood, but the castle walls must be built with stone. In this case, you can create several different builder classes that implement the same set of building steps, but in a different manner. Then you can use these builders in the construction process i.

Different builders execute the same task in various ways. For example, imagine a builder that builds everything from wood and glass, a second one that builds everything with stone and iron and a third one that uses gold and diamonds. By calling the same set of steps, you get a regular house from the first builder, a small castle from the second and a palace from the third. Director You can go further and extract a series of calls to the builder steps you use to construct a product into a separate class called director.

The director class defines the order in which to execute the building steps, while the builder provides the implementation for those steps. The director knows which building steps to execute to get a working product. You can always call the building steps in a specific order directly from the client code. However, the director class might be a good place to put various construction routines so you can reuse them across your program. The client only needs to associate a builder with a director, launch the construction with the director, and get the result from the builder.

The Builder interface declares product construction steps that are common to all types of builders. Concrete Builders provide different implementations of the construction steps. Products are resulting objects. The Director class defines the order in which to call construction steps, so you can create and reuse specific configurations of products. The Client must associate one of the builder objects with the director.

Then the director uses that builder object for all further construction. In this case, you can use a different builder each time you produce something with the director. The example of step-by-step construction of cars and the user guides that fit those car models. A car is a complex object that can be constructed in a hundred different ways. This class has a set of methods for configuring various parts of a car.

If the client code needs to assemble a special, fine-tuned model of a car, it can work with the builder directly. On the other hand, the client can delegate the assembly to the director class, which knows how to use a builder to construct several of the most popular models of cars.

You might be shocked, but every car needs a manual seriously, who reads them? The manual describes every feature of the car, so the details in the manuals vary across the different models. This class implements the same building methods as its car-building sibling, but instead of crafting car parts, it describes them. By passing these builders to the same director object, we can construct either a car or a manual. The final part is fetching the resulting object.

A metal car and a paper manual, although related, are still very different things. Hence, we obtain the result of the construction from the builder which performed the job. Calling such a beast is very inconvenient; therefore, you overload the constructor and create several shorter versions with fewer parameters.

These constructors still refer to the main one, passing some default values into any omitted parameters. Creating such a monster is only possible in languages that support method overloading, such as C or Java. The Builder pattern lets you build objects step by step, using only those steps that you really need.

The base builder interface defines all possible construction steps, and concrete builders implement these steps to construct particular representations of the product. Meanwhile, the director class guides the order of construction.

You could defer execution of some steps without breaking the final product. You can even call steps recursively, which comes in handy when you need to build an object tree. This prevents the client code from fetching an incomplete result. Make sure that you can clearly define the common construction steps for building all available product representations.

Declare these steps in the base builder interface. Create a concrete builder class for each of the product representations and implement their construction steps.

Think about creating a director class. It may encapsulate various ways to construct a product using the same builder object. The client code creates both the builder and the director objects. Before construction starts, the client must pass a builder object to the director. The director uses the builder object in all further construction. The construction result can be obtained directly from the director only if all products follow the same interface.

Otherwise, the client should fetch the result from the builder. You can isolate complex construction code from the business logic of the product. How would you do it? First, you have to create a new object of the same class. Then you have to go through all the fields of the original object and copy their values over to the new object. The pattern declares a common interface for all objects that support cloning. This interface lets you clone an object without coupling your code to the class of that object.

Usually, such an interface contains just a single clone method. The implementation of the clone method is very similar in all classes. The method creates an object of the current class and carries over all of the field values of the old object into the new one. You can even copy private fields because most programming languages let objects access private fields of other objects that belong to the same class.

Pre-built prototypes can be an alternative to subclassing. When your objects have dozens of fields and hundreds of possible configurations, cloning them might serve as an alternative to subclassing. The division of a cell. After mitotic division, a pair of identical cells is formed. The original cell acts as a prototype and takes an active role in creating the copy.

The Prototype interface declares the cloning methods. The Concrete Prototype class implements the cloning method. The Client can produce a copy of any object that follows the prototype interface.

Prototype registry implementation 1. It stores a set of pre-built objects that are ready to be copied. However, if you need better search criteria than a simple name, you can build a much more robust version of the registry.

Cloning a set of objects that belong to a class hierarchy. All shape classes follow the same interface, which provides a cloning method. X 16 this. Y 17 this. The Prototype pattern provides the client code with a general interface for working with all objects that support cloning. This interface makes the client code independent from the concrete classes of objects that it clones. Somebody could have created these subclasses to be able to create objects with a specific configuration.

Instead of instantiating a subclass that matches some configuration, the client can simply look for an appropriate prototype and clone it. Create the prototype interface and declare the clone method in it.

Or just add the method to all classes of an existing class hierarchy, if you have one. A prototype class must define the alternative constructor that accepts an object of that class as an argument. The constructor must copy the values of all fields defined in the class from the passed object into the newly created instance.

The constructor is a more convenient place to do this because it delivers the resulting object right after you call the new operator. The cloning method usually consists of just one line: running a new operator with the prototypical version of the constructor. Note, that every class must explicitly override the cloning method and use its own class name along with the new operator. Otherwise, the cloning method may produce an object of a parent class.

Optionally, create a centralized prototype registry to store a catalog of frequently used prototypes. This method should search for a prototype based on search criteria that the client code passes to the method. The criteria might either be a simple string tag or a complex set of search parameters. After the appropriate prototype is found, the registry should clone it and return the copy to the client.

Applying the pattern lets you clone complex structures instead of re-constructing them from scratch. Ensure that a class has just a single instance.

Why would anyone want to control how many instances a class has? The most common reason for this is to control access to some shared resource�for example, a database or a file. Note that this behavior is impossible to implement with a regular constructor since a constructor call must always return a new object by design. Remember those global variables that you all right, me used to store some essential objects? Just like a global variable, the Singleton pattern lets you access some object from anywhere in the program.

However, it also protects that instance from being overwritten by other code. Nowadays, the Singleton pattern has become so popular that people may call something a singleton even if it solves just one of the listed problems. All following calls to this method return the cached object. So whenever that method is called, the same object is always returned. A country can have only one official government. Calling the getInstance method should be the only way of getting the Singleton object.

This method caches the first created object and returns it in all subsequent calls. This method either creates a new object or returns an existing one if it has already been created. Nothing, except for the Singleton class itself, can replace the cached instance. Note that you can always adjust this limitation and allow creating any number of Singleton instances.

The only piece of code that needs changing is the body of the getInstance method. Add a private static field to the class for storing the singleton instance.

Declare a public static creation method for getting the singleton instance. It should create a new object on its first call and put it into the static field.

The method should always return that instance on all subsequent calls. Make the constructor of the class private. The static method of the class will still be able to call the constructor, but not the other objects. The pattern solves two problems at the time. Since the constructor of the singleton class is private and overriding static methods is impossible in most languages, you will need to think of a creative way to mock the singleton.

But there are two fundamental differences between these patterns: 1. There should be only one Singleton instance, whereas a Flyweight class can have multiple instances with different intrinsic states. The Singleton object can be mutable. Flyweight objects are immutable. The app downloads the stock data from multiple sources in XML format and then displays nice-looking charts and diagrams for the user. At some point, you decide to improve the app by integrating a smart 3rd-party analytics library.

You could change the library to work with XML. However, this might break some existing code that relies on the library. This is a special object that converts the interface of one object so that another object can understand it. An adapter wraps one of the objects to hide the complexity of conversion happening behind the scenes. For example, you can wrap an object that operates in meters and kilometers with an adapter that converts all of the data to imperial units such as feet and miles.

Adapters can not only convert data into various formats but can also help objects with different interfaces collaborate. The adapter gets an interface, compatible with one of the existing objects.

Upon receiving a call, the adapter passes the request to the second object, but in a format and order that the second object expects. To solve the dilemma of incompatible formats, you can create XML-to-JSON adapters for every class of the analytics library that your code works with directly. Then you adjust your code to communicate with the library only via these adapters.

When an adapter receives a call, it translates the incoming XML data into a JSON structure and passes the call to the appropriate methods of a wrapped analytics object.

The power plug and sockets standards are different in different countries. The problem can be solved by using a power plug adapter that has the American-style socket and the European-style plug. It can be implemented in all popular programming languages. The Client is a class that contains the existing business logic of the program. The Client Interface describes a protocol that other classes must follow to be able to collaborate with the client code. The Service is some useful class usually 3rd-party or legacy.

The adapter receives calls from the client via the adapter interface and translates them into calls to the wrapped service object in a format it can understand. Thanks to this, you can introduce new types of adapters into the program without breaking the existing client code. This can be useful when the interface of the service class gets changed or replaced: you can just create a new adapter class without changing the client code.

Class adapter This implementation uses inheritance: the adapter inherits interfaces from both objects at the same time. The adaptation happens within the overridden methods. The resulting adapter can be used in place of an existing client class. Adapting square pegs to round holes. The much more elegant solution would be to put the missing functionality into an adapter class. Then you would wrap objects with missing features inside the adapter, gaining needed features dynamically.

This approach looks very similar to the Decorator pattern. Declare the client interface and describe how clients communicate with the service. Create the adapter class and make it follow the client interface. Leave all the methods empty for now. Add a field to the adapter class to store a reference to the service object. One by one, implement all methods of the client interface in the adapter class. The adapter should delegate most of the real work to the service object, handling only the interface or data format conversion.

Clients should use the adapter via the client interface. This will let you change or extend the adapters without affecting the client code. You can separate the interface or data conversion code from the primary business logic of the program. You can introduce new types of adapters into the program without breaking the existing client code, as long as they work with the adapters through the client interface.

On the other hand, Adapter is commonly used with an existing app to make some otherwise-incompatible classes work together nicely. Adapter usually wraps just one object, while Facade works with an entire subsystem of objects.

Indeed, all of these patterns are based on composition, which is delegating work to other objects. However, they all solve different problems. It can also communicate to other developers the problem the pattern solves. Sound scary? Say you have a geometric Shape class with a pair of subclasses: Circle and Square. You want to extend this class hierarchy to incorporate colors, so you plan to create Red and Blue shape subclasses.

Number of class combinations grows in geometric progression. Adding new shape types and colors to the hierarchy will grow it exponentially.

And after that, adding a new color would require creating three subclasses, one for each shape type. The further we go, the worse it becomes. The Bridge pattern attempts to solve this problem by switching from inheritance to composition. What this means is that you extract one of the dimensions into a separate class hierarchy, so that the original classes will reference an object of the new hierarchy, instead of having all of its state and behaviors within one class.

You can prevent the explosion of a class hierarchy by transforming it into several related hierarchies. The Shape class then gets a reference field pointing to one of the color objects. Now the shape can delegate any color-related work to the linked color object.

That reference will act as a bridge between the Shape and Color classes. In my opinion, the terms sound too academic and make the pattern seem more complicated than it really is. Abstraction also called interface is a high-level control layer for some entity. It should delegate the work to the implementation layer also called platform.

In a worst-case scenario, this app might look like a giant spaghetti bowl, where hundreds of conditionals connect different types of GUI with various APIs all over the code.

Making even a simple change to a monolithic codebase is pretty hard because you must understand the entire thing very well. Making changes to smaller, well-defined modules is much easier. The class hierarchy will grow exponentially because adding a new GUI or supporting a different API would require creating more and more classes.

One of the ways to structure a cross-platform application. The abstraction object controls the appearance of the app, delegating the actual work to the linked implementation object. Moreover, adding support for another operating system only requires creating a subclass in the implementation hierarchy.

The Abstraction provides high-level control logic. It relies on the implementation object to do the actual low-level work. The abstraction may list the same methods as the implementation, but usually the abstraction declares some complex behaviors that rely on a wide variety of primitive operations declared by the implementation. Concrete Implementations contain platform-specific code. Refined Abstractions provide variants of control logic.

Like their parent, they work with different implementations via the general implementation interface. Usually, the Client is only interested in working with the abstraction. The Device classes act as the implementation, whereas the Remote s act as the abstraction. The base remote control class declares a reference field that links it with a device object. All remotes work with the devices via the general device interface, which lets the same remote support multiple device types.

You can develop the remote control classes independently from the device classes. For example, a basic remote control might only have two buttons, but you could extend it with additional features, such as an extra battery or a touchscreen. The changes made to one of the variations of functionality may require making changes across the whole class, which often results in making errors or not addressing some critical side effects.

The Bridge pattern lets you split the monolithic class into several class hierarchies. After this, you can change the classes in each hierarchy independently of the classes in the others. This approach simplifies code maintenance and minimizes the risk of breaking existing code. The original class delegates the related work to the objects belonging to those hierarchies instead of doing everything on its own.

By the way, this last item is the main reason why so many people confuse the Bridge with the Strategy pattern. Remember that a pattern is more than just a certain way to structure your classes. It may also communicate intent and a problem being addressed. Identify the orthogonal dimensions in your classes. See what operations the client needs and define them in the base abstraction class. Determine the operations available on all platforms.

Declare the ones that the abstraction needs in the general implementation interface. For all platforms in your domain create concrete implementation classes, but make sure they all follow the implementation interface.

Inside the abstraction class, add a reference field for the implementation type. If you have several variants of high-level logic, create refined abstractions for each variant by extending the base abstraction class. After that, the client can forget about the implementation and work only with the abstraction object. You can introduce new abstractions and implementations independently from each other. You can focus on high-level logic in the abstraction and on platform details in the implementation.

For example, imagine that you have two types of objects: Products and Boxes. A Box can contain several Products as well as a number of smaller Boxes.

These little Boxes can also hold some Products or even smaller Boxes , and so on. An order might comprise various products, packaged in boxes, which are packaged in bigger boxes and so on. The whole structure looks like an upside down tree. Orders could contain simple products without any wrapping, as well as boxes stuffed with products How would you determine the total price of such an order? You could try the direct approach: unwrap all the boxes, go over all the products and then calculate the total.

All of this makes the direct approach either too awkward or even impossible. How would this method work? If one of these items were a smaller box, that box would also start going over its contents and so on, until the prices of all inner components were calculated. A box could even add some extra cost to the final price, such as packaging cost. You can treat them all the same via the common interface.

When you call a method, the objects themselves pass the request down the tree. An army consists of several divisions; a division is a set of brigades, and a brigade consists of platoons, which can be broken down into squads. Finally, a squad is a small group of real soldiers. Orders are given at the top of the hierarchy and passed down onto each level until every soldier knows what needs to be done. The Component interface describes operations that are common to both simple and complex elements of the tree.

The Container aka composite is an element that has sub-elements: leaves or other containers. It works with all sub-elements only via the component interface. Upon receiving a request, a container delegates the work to its sub-elements, processes intermediate results and then returns the final result to the client. The Client works with all elements through the component interface.

As a result, the client can work in the same way with both simple or complex elements of the tree. The CompoundGraphic class is a container that can comprise any number of sub-shapes, including other compound shapes. A compound shape has the same methods as a simple shape. The client code works with all shapes through the single interface common to all shape classes. The client can work with very complex object structures without being coupled to concrete classes that form that structure.

A container can be composed of both leaves and other containers. This lets you construct a nested recursive object structure that resembles a tree. Make sure that the core model of your app can be represented as a tree structure.

Try to break it down into simple elements and containers. Remember that containers must be able to contain both simple elements and other containers. Declare the component interface with a list of methods that make sense for both simple and complex components. Create a leaf class to represent simple elements. A program may have multiple different leaf classes. Create a container class to represent complex elements. In this class, provide an array field for storing references to subelements.

While implementing the methods of the component interface, remember that a container is supposed to be delegating most of the work to sub-elements. Finally, define the methods for adding and removal of child elements in the container. Keep in mind that these operations can be declared in the component interface.

This would violate the Interface Segregation Principle because the methods will be empty in the leaf class. However, the client will be able to treat all the elements equally, even when composing the tree. You can introduce new element types into the app without breaking the existing code, which now works with the object tree. In this case, when a leaf component gets a request, it may pass it through the chain of all of the parent components down to the root of the object tree.

A Decorator is like a Composite but only has one child component. The initial version of the library was based on the Notifier class that had only a few fields, a constructor and a single send method. The method could accept a message argument from a client and send the message to a list of emails that were passed to the notificator via its constructor. A third-party app which acted as a client was supposed to create and configure the notificator object once, and then use it each time something important happened.

A program could use the notifier class to send notifications about important events to a predefined set of emails. At some point, you realize that users of the library expect more than just email notifications.

Many of them would like to receive an SMS about critical issues. Others would like to be notified on Facebook and, of course, the corporate users would love to get Slack notifications. How hard can that be? You extended the Notifier class and put the additional notification methods into new subclasses. Now the client was supposed to instantiate the desired notification class and use it for all further notifications.

However, it quickly became apparent that this approach would bloat the code immensely, not only the library code but the client code as well. However, inheritance has several serious caveats that you need to be aware of. One of the ways to overcome these caveats is by using Composition instead of Inheritance. With composition one object has a reference to another and delegates it some work, whereas with inheritance, the object itself is able to do that work, inheriting the behavior from its superclass.

An object can use the behavior of various classes, having references to multiple objects and delegating them all kinds of work. Composition is the key principle behind many design patterns, including the Decorator. Inheritance vs. Composition Wrapper is the alternative nickname for the Decorator pattern that clearly expresses the main idea of the pattern. The wrapper contains the same set of methods as the target and delegates to it all requests it receives.

However, the wrapper may alter the result by doing something either before or after it passes the request to the target. When does a simple wrapper become the real decorator? As I mentioned, the wrapper implements the same interface as the wrapped object.

This will let you cover an object in multiple wrappers, adding the combined behavior of all the wrappers to it. Various notification methods become decorators.

The resulting objects will be structured as a stack. The last decorator in the stack would be the object that the client actually works with. Apps might configure complex stacks of notification decorators.

We could apply the same approach to other behaviors such as formatting messages or composing the recipient list. The client can decorate the object with any custom decorators, as long as they follow the same interface as the others.

The Component declares the common interface for both wrappers and wrapped objects. Concrete Component is a class of objects being wrapped. It defines the basic behavior, which can be altered by decorators. The Base Decorator class has a field for referencing a wrapped object. The base decorator delegates all operations to the wrapped object.

Concrete Decorators define extra behaviors that can be added to components dynamically. Concrete decorators override methods of the base decorator and execute their behavior either before or after calling the parent method. The Client can wrap components in multiple layers of decorators, as long as it works with all objects via the component interface. The application wraps the data source object with a pair of decorators.

The original class writes the encrypted and protected data to the file without knowing about the change. The decorators and the data source class implement the same interface, which makes them all interchangeable in the client code. Encrypt passed data. Get data from the wrappee's readData method. Try to decrypt it if it's encrypted. Return the result. Compress passed data. Try to decompress it if it's compressed. A simple example of a decorator assembly. Client code that uses an external data source.

Other useful methods The client code can treat all these objects in the same way, since they all follow a common interface. For a final class, the only way to reuse the existing behavior would be to wrap the class with your own wrapper, using the Decorator pattern. Make sure your business domain can be represented as a primary component with multiple optional layers over it.

Figure out what methods are common to both the primary component and the optional layers. Create a component interface and declare those methods there. Create a concrete component class and define the base behavior in it. Create a base decorator class. It should have a field for storing a reference to a wrapped object.

The field should be declared with the component interface type to allow linking to concrete components as well as decorators. The base decorator must delegate all work to the wrapped object.

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