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School of. Computer Science. 23 Patterns in 80 Minutes: a Whirlwind Java- centric Tour of the Gang-of-Four Design Patterns. Josh Bloch. Charlie Garrod. COMPUTER SCIENCE. Gang of Four (GoF). OO Design Patterns. CS / ECE May 11th, IMPORTANT NOTICE TO STUDENTS. These slides are. public class Singleton { private static final Singleton INSTANCE = new Singleton ();. // Private constructor prevents. // instantiation from other classes.

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[email protected] the subject "subscribe".) This list has quite . With this book, the Gang of Four have made a seminalcontribution to software. Design Pattern Framework™ 4. Design patterns are recurring solutions to software design problems you find again and again in real-world application development. The Gang of Four (GoF) patterns are generally considered the foundation for all other patterns. Automatically exported from jinzihao.info - dieforfree/ edsebooks.

GangOfFour solution open. NET Remoting when launching server-activated objects. Page 34 of Page 24 of State Definition Allow an object to alter its behavior when its internal state changes.

Design patterns and anti-patterns Singleton pattern Prototype pattern Factory pattern Builder pattern Adapter pattern Proxy pattern Decorator pattern Command pattern Observer pattern Strategy pattern. Skill Level Intermediate. Show More Show Less. Related Courses. Preview course. Databases for Node. Testing and Code Quality with Jon Peck. Search This Course Clear Search. Welcome 1m 7s. What you should know 1m 43s. Using the exercise files 1m 13s. What are design patterns?

Gang of Four Design Patterns 4.0.pdf

Design patterns 2m 26s. Anti-patterns 1m 16s. Creational Patterns. The singleton problem 3m 30s. The Singletons pattern 2m 25s. Singletons in Node 2m 6s. The Prototype pattern 3m 11s. The prototype clone method 3m 3s. The Factory pattern 4m 16s. The Builder pattern 4m 38s. Implementing a builder 5m 23s. Structural Patterns. The Adapter pattern 2m 20s. Implementing an adapter 6m 24s. The Proxy pattern 2m 11s. Implementing a proxy 5m 48s.

The Composite pattern 3m 12s. Implementing composites 5m 59s. The Decorator pattern 4m 3s. Implementing decorators 4m 24s. Behavioral Patterns. Chain of responsibility 4m 42s. Implementing the storage class 6m 22s. The Command pattern 4m 47s. Implementing commands 4m 59s. Improving the conductor 6m 38s. The Iterator pattern 4m 5s. Implementing iterators 4m 59s. The Observer pattern 4m 19s.

Implementing observers 2m 48s. The Strategy pattern 4m 25s. Builder gives you more control in that each step in the construction process can be customized. Delete are similar. Page 12 of Builder in the. A scenario where the Builder can be helpful is when building a code generator. Builder is a creational pattern just like the Factory patterns. Visual Studio. The CreateGenerator methods return an ICodeGenerator interface through which the generation of source code can be controlled.

Page 13 of Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object. ExperiencePage implements the Product interface Creator Document declares the factory method. Factory Method Definition Define an interface for creating an object. Factory Method lets a class defer instantiation to subclasses.

Page 14 of Structural Real-world sample code The real-world code demonstrates the Factory method offering flexibility in creating different documents. Structural sample code The structural code demonstrates the Factory method offering great flexibility in creating different objects. The derived Document classes Report and Resume instantiate extended versions of the Document class. The Abstract class may provide a default object. NET 3. Factory Methods are frequently used in applications that manage.

The key objective of the Factory Method is extensibility. Page 15 of To support this new document type the Factory Method may or may not have to be adjusted depending on how it was implemented. Visio diagrams. When a new document type is introduced it simply implements the IDocument interface and it will fit in with the rest of the documents.

Word documents. The Factory Method allows the client to use an interface for creating an object while still retaining control over which class to instantiate. There are situations however. The example below will need adjustment. A document management system. These documents may be Text files. But they have in command that they have an author.

In your own programs you most likely have created methods that return new objects. Page 16 of An example is the System. The rules are: Likewise the Parse method on many built-in value types Int Convert class which exposes many static methods that. ToBoolean myString. Page 17 of Factory Methods are abstract and return class types. Two static factory method examples are File.

Open and Activator.

ToBoolean myString In This is exactly where Abstract Factory and Factory Method differ: Abstract Factory methods are virtual or abstract and return abstract classes or interfaces. NET the Factory Method is typically implemented as a static method which creates an instance of a particular type determined at compile time..

In other words Page 18 of Prototype Definition Specify the kind of objects to create using a prototypical instance. In this implementation the key is of type string i. Structural Real-world sample code The real-world code demonstrates the Prototype pattern in which new Color objects are created by copying pre-existing.

The colors collection in the ColorManager class is implemented with a type-safe generic Dictionary class. RGB values range between ICloneable is a built-in. ICloneable requires that the class hierarchy be serializable. The abstract classes have been replaced by interfaces because the abstract classes contain no implementation code. NET prototype interface. Here the Serializable attribute is used to do just that as an aside: Page 19 of Deep copy copies the prototype object and all the objects it refers to.

NET is to use the built-in ICloneable interface on the objects that are used as prototypes. Prototype in the. Having this serialized representation as a prototype you can then use it to create copies of the original object. The ICloneable interface has a method called Clone that returns an object that is a copy.

It is more often used in specific application types. The best way to implement this in. When implementing the Clone functionality you need to be aware of the two different types of cloning: The copy strategy for deep copy may be more complicated -.

NetOptimized Prototype: The Prototype design pattern creates clones of pre-existing sample objects. Page 20 of You also need to watch out for circular references. Database connections. The Prototype design pattern is not commonly used in the construction of business applications. Shallow copy is easier but only copies data fields in the object itself -. Shallow copy is easy to implement because the Object base class has a MemberwiseClone method that returns a shallow copy of the object.

NET support for the Prototype pattern can be found in object serialization scenarios. Abstract Factory. Instance is a class operation.

Structural sample code The structural code demonstrates the Singleton pattern which assures only a single instance the singleton of the class can be created. Singleton Definition Ensure a class has only one instance and provide a global point of access to it. Page 21 of NetOptimized Singleton: NET specific solution is offered.

The Server class has automatic properties. Server lists are created using collection initialization. The responsibilities of these objects often require that there be just one instance of the class.

Only a single instance the singleton of the class should ever exist because servers may dynamically come on-line or off-line. Examples include cached database records see TopLink by Oracle.

NET optimized code demonstrates the same code as above but uses more modern. In addition. Server instances are created using object initialization. Here an elegant. Page 22 of The Singleton pattern simply uses a private constructor and a static readonly instance variable that is lazily initialized. Thread safety is guaranteed by the compiler.

The intent of the Singleton pattern is to ensure that a class has only one instance and to provide a global point of access to this instance.

Using the Singleton pattern you centralize authority over a particular resource in a single object. Using Singleton you can hold one or more global variables and this can be really handy. Other reasons quoted for using Singletons are to improve performance. A common scenario is when you have a stateless object that is created over and over again. This is where the Singleton design pattern comes in.

Other areas in the application rely on these special objects and they need a way to find them. Page 23 of Global variables are frowned upon as a bad coding practice. A Singleton removes the need to constantly create and destroy objects. An example follows: Be careful though as the Singleton may not be the best solution in this scenario. NET Framework An example where the. NET Framework uses the Singleton pattern is with.

Page 24 of One of the activation modes of server objects is called Singleton and their behavior is in line with the GoF pattern definition. NET Remoting when launching server-activated objects. Definition Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces. Structural sample code The structural code demonstrates the Adapter pattern which maps the interface of one class onto another so that they can work together.

These incompatible classes may come from different libraries or frameworks.

Gang of Four Design Patterns pdf | Class (Computer Programming) | Method (Computer Programming)

Real-world sample code The real-world code demonstrates the use of a legacy chemical databank. Chemical compound objects access the databank through an Adapter interface. NET optimized code demonstrates the same code as above but uses more modern, built-in.

This will allow the derived class to access these variables via properties rather than directly. Finally, two enumerations Chemical and State were added for increased type safety. NET developers write classes that expose methods that are called by clients.

Most of the time they will be able to control the interfaces, but there are situations, for example, when using 3rd party libraries, where they may not be able to do so. The 3 rd party library performs the desired services but the interface methods and property names are different from what the client expects. This is a scenario where you would use the Adapter pattern.

The Adapter provides an interface the client expects using the services of a class with a different interface. Adapters are commonly used in programming environments where new components or new applications need to be integrated and work together with existing programming components.

Adapters are also useful in refactoring scenarios. Say, you have two classes that perform similar functions but have different interfaces. The client uses both classes, but the code would be far cleaner and simpler to understand if they would share the same interface. You cannot alter the interface, but you can shield the differences by using an Adapter which allows the client to communicate via a common interface.

The Adapter handles the mapping between the shared interface and the original interfaces. Adapter in the. NET Framework The. NET Framework uses the Adapter pattern extensively by providing the ability for.

As you know, there are significant differences between COM and. NET expects an Exception to be thrown in case of an error. The Adapter adapts the COM interface to what.

NET clients expect. Page 29 of Bridge Definition Decouple an abstraction from its implementation so that the two can vary independently. This interface doesn't have to correspond exactly to Abstraction's interface. Structural Real-world sample code The real-world code demonstrates the Bridge pattern in which a BusinessObject abstraction is decoupled from the implementation in DataObject.

Page 30 of Structural sample code The structural code demonstrates the Bridge pattern which separates decouples the interface from its implementation. Typically the Implementation interface provides only primitive operations. The implementation can evolve without changing clients which use the abstraction of the object.

The DataObject abstract class has been replaced by an interface because DataObject contains no implementation code. The DataObject implementations can evolve dynamically without changing any clients. If you build an application that uses a driver to communicate with a database. A Bridge pattern is created by moving a set of abstract operations to an interface so that both the client and the service can vary independently.

NetOptimized Bridge: Bridge is a high-level architectural patterns and its main goal is through abstraction to help. The ODBC architecture decouples an abstraction from its implementation so that the two can vary independently — the Bridge pattern in action. The overall system. A classic example of the Bridge pattern is when coding against device drivers.

Bridge in the. Interestingly enough. NET Framework Bridge is a high-level architectural pattern and as such is not exposed by the. The abstraction decouples the client. It is important to realize that the client application is the abstraction. NET libraries themselves. Page 31 of Applications that rely on these drivers are abstractions that work with any database SQL Server. Most developers are not aware of this. A driver is an object that independently operates a computer system or external hardware device.

NET developers write better code. Page 32 of Composite lets clients treat individual objects and compositions of objects uniformly. Composite Definition Compose objects into tree structures to represent part-whole hierarchies. Structural sample code The structural code demonstrates the Composite pattern which allows the creation of a tree structure in which individual nodes are accessed uniformly whether they are leaf nodes or branch composite nodes.

The composite pattern is a great candidate for generics and you will find these used throughout this example. Page 33 of Leaf PrimitiveElement o o represents leaf objects in the composition. A leaf has no children. Structural Real-world sample code The real-world code demonstrates the Composite pattern used in building a graphical tree structure made up of primitive nodes lines. Forms namespace and the other for ASP. This facilitates the process of adding and removing shapes from the list of tree nodes.

This common interface greatly facilitates the design and construction of recursive algorithms that iterate over each object in the Composite collection. This code demonstrates much of the power that generics offer to. Numerous interesting language features are used in this example. NetOptimized Composite: NET developers. NET apps in the System. The class named Shape does implement this generic interface so that comparisons can be made between shape objects.

It is used like any other collection. Page 34 of A tree control is a good example of a Composite pattern. The nodes of the tree either contain an individual object leaf node or a group of objects a subtree of nodes. Examples are the two Control classes one for Windows apps in the System.

All nodes in the Composite pattern share a common interface which supports individual items as well as groups of items. UI namespace. Composite in the. This is an open type which has the ability to accept any type parameter.

NET framework. WPF also has many built-in controls that are Composites. Page 35 of The built-in. Page 36 of ConcreteComponent Book. Decorator Definition Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

NetOptimized Decorator: A NET 3. Decorator Decorator maintains a reference to a Component object and defines an interface that conforms to Component's interface. Structural sample code The structural code demonstrates the Decorator pattern which dynamically adds extra functionality to an existing object. Page 37 of ConcreteDecorator Borrowable o adds responsibilities to the component. NET optimized code demonstrates an example of the Decorator design pattern that uses generics.

Structural Real-world sample code The real-world code demonstrates the Decorator pattern in which 'borrowable' functionality is added to existing library items books and videos. Flush and others. Page 38 of Decorator in the. As an aside. In most cases. The BufferedStream class is a Decorator that wraps the Stream class and reads and writes large chunks of bytes for better performance. Decoration is a flexible technique because it takes place at runtime.

The Stream class is an abstract class that reads or writes a sequence of bytes from an IO device disk. It is polymorphic with the original class so that clients can invoke it just like the original class.

Decorator classes usually have a constructor with an argument that represents the class they intent to decorate: NET Framework include a set of classes that are designed around the Stream class. Loan implement subsystem functionality. Facade Definition Provide a unified interface to a set of interfaces in a subsystem. Subsystem classes Bank.

Page 39 of The only difference is the use of. Structural sample code The structural code demonstrates the Facade pattern which provides a simplified and uniform interface to a large subsystem of classes. Page 40 of NET optimized sample code This code is essentially the same as the real-world example. Structural Real-world sample code The real-world code demonstrates the Facade pattern as a MortgageApplication object which provides a simplified interface to a large subsystem of classes measuring the creditworthiness of an applicant.

Calls into the business layer take place via a well defined service layer. NET Framework In the. An example of an aggregate component is System. Page 41 of In a 3-tier application the presentation layer is the client. It is used in scenarios where there is a need to present a simplified view over more complex set of types.

It exposes a simple API to the client. Complex operations. Facade in the. To discuss this properly we need to distinguish high-level architectural Facades from lower level component type facades.

This service layer. Facades themselves are often implemented as singleton abstract factories. The objective of the. Page 42 of SerialPort which is a powerful serial port class. WebClient which provides a high-level interface for sending and retrieving data from a network resources identified by a general URI. NET Framework libraries are no exception. Facades may reduce the expressiveness of the API.

MessageQueue which provides access to a queue on a Message Queue server. NET developer can see. SmtpMail to send mail messages.

CharacterZ o implements the Flyweight interface and adds storage for intrinsic state. A ConcreteFlyweight object must be sharable. Flyweigth Definition Use sharing to support large numbers of fine-grained objects efficiently. ConcreteFlyweight CharacterA.. Page 43 of Any state it stores must be intrinsic.. It is common for UnsharedConcreteFlyweight objects to have ConcreteFlyweight objects as children at some level in the flyweight object structure as the Row and Column classes have.

When a client requests a flyweight. Structural Real-world sample code The real-world code demonstrates the Flyweight pattern in which a relatively small number of Character objects is shared many times by a document that has potentially many characters.

Structural sample code The structural code demonstrates the Flyweight pattern in which a relatively small number of objects is shared many times by different clients. Page 44 of The Flyweight interface enables sharing. NET uses Flyweights for strings that are declared at compile time and have the same sequence of characters. Shared flyweight objects are immutable. The stateless flyweights refer to the same memory location that holds the immutable string. Flyweight in the. Flyweights are usually combined with the Factory pattern as demonstrated in the.

They are rarely used in data-driven business type applications. Examples include. Page 45 of NetOptimized Flyweight: NET Framework As mentioned above.

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Flyweights are used internally in the. The generic collection increases the type-safety of the code. NET optimized code uses a generic Dictionary collection to hold and quickly access Flyweight Character objects in memory.

You will find flyweights mostly in utility type applications word processors. NET Optimized code sample. NET Framework as a string management technique to minimize memory usage for immutable strings. Proxy Definition Provide a surrogate or placeholder for another object to control access to it. Page 46 of Proxy may refer to a Subject if the RealSubject and Subject interfaces are the same. NET optimized sample code The..

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Structural sample code The structural code demonstrates the Proxy pattern which provides a representative object proxy that controls access to another similar object. Structural Real-world sample code The real-world code demonstrates the Proxy pattern for a Math object represented by a MathProxy object. This code demonstrates the Remote Proxy pattern which provides a representative object i.

Page 47 of There are 3 different types of proxies: NetOptimized Proxy: The proxy pattern is meant to provide a surrogate or placeholder for another object to control access to it. A Smart Reference is a proxy for a pointer. Page 48 of The interface of the Proxy object is the same as the original object and clients may not even be aware they are dealing with a proxy rather than the real object.

The Proxy forwards the request to a target object. Clients of these objects expect this work to be done quickly and efficiently. Typically this occurs when there is a dependency on a remote resource a call to another computer for example or when an object takes a long time to load. Clients of WCF services also rely heavily on auto-generated proxy objects. NET Framework In. As is common with proxies. Page 49 of NET the Proxy pattern manifests itself in the Remoting infrastructure. NET Remoting.

Chain or Responsibility Definition Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. President handles requests it is responsible for can access its successor if the ConcreteHandler can handle the request. Page 50 of Structural Real-world sample code The real-world code demonstrates the Chain of Responsibility pattern in which several linked managers and executives can respond to a purchase request or hand it off to a superior.

The delegates are implemented using generics in which event handlers are type safe and not restricted to senders of type object but rather to types that are appropriate for the event — in the sample code type Approver see.

Each position has can have its own set of rules which orders they can approve. The Purchase and other classes use. The Successor settings are simplified by using properties. Page 51 of The chain of responsibility pattern is frequently used in the Windows event model in which a UI control can either handle an event for example a mouse click or let it fall through to the next control in the event chain.

Page 52 of NET you can identify a Chain of Responsibility in the Windows event model where each UI control can decide to process an event or let it fall through to the next control in the event chain. This is slightly different from the GoF definition in which just one object in a chain decides to handle the request.

This pattern requires a way to order the search for an object that can handle the request. Chain of Responsibility in the. The Chain of Responsibility design pattern offers an opportunity to build a collection of loosely coupled objects by relieving a client from having to know which objects in a collection can satisfy a request by arranging these objects in a chain.

Message sinks form a chain as each sink has a reference to the next sink in the chain. Loosely coupled objects have the advantage that they are easier to maintain and easier to change compared to systems where there is tight coupling between objects i.

NetOptimized Chain of Responsibility: Occasionally you may run into a Chain of Responsibility implementation in which a chain of objects process a message between a sender and a receiver. Note that Chain-of-Responsibility is not commonly used in business application development. This search is usually modeled according to the specific needs of the application domain.

Gang of Four: Design patterns

NET Remoting in which a message between a client and a server passes through one or more so-called message sinks. Command Definition Encapsulate a request as an object. Page 53 of Page 54 of Prefixing it with ' ' allows using it as an identifier. Structural Real-world sample code The real-world code demonstrates the Command pattern used in a simple calculator with unlimited number of undo's and redo's.

Structural sample code The structural code demonstrates the Command pattern which stores requests as objects allowing clients to execute or playback the requests.

Note that in C the word 'operator' is a keyword. In this example the abstract Command class has been replaced by the ICommand interface because the abstract class had no implementation code at all. All Commands implement the same interface.

NET as part of a unified WinForms command routing architecture. File Save. We would have expected that the Command pattern would be exposed in. Areas where you find Command patterns are: Page 55 of Edit Copy. The classic usage of this pattern is a menu system where each command object represents an action and an associated undo action. Typically their interface includes methods such as Do and Undo or Execute and Undo.

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Menu actions include menu items such as File Open. Command in the. Interpreter Definition Given a language. Page 56 of OneExpression o o implements an Interpret operation associated with terminal symbols in the grammar.

Structural Real-world sample code The real-world code demonstrates the Interpreter pattern which is used to convert a Roman numeral to a decimal.