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Gang of four design patterns in java pdf

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Gang of Four Design Patterns pdf - Download as PDF File .pdf), Text File .txt ) or read online. Programming In case you‟re familiar with Java Swing. Implementing the Abstract Factory Design Pattern. .. community that has grown over the decade-plus since the original GoF work is large and energetic. Gang of Four (GOF) Java Design Patterns Mock Exams Download PDF books with a selected sample of the JavaChamp question bank in SCJP, SCEA, EJB.


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Erich Gamma/Richard Helm/Ralph Johnson/John Vlissides, Design Patterns CD: Elements of Reusable Object-. Oriented Peter Haggar, Practical Java"" Programming Language Guide . It has seen four countries, three of its authors'. The GoF Design Patterns Reference. Version / / Generated Copyright © w3sDesign. All rights Creational Patterns. PDF | On Jun 1, , Benneth Christiansson and others published GoF Design Patterns with examples using Java and UML.

These incompatible classes may come from different libraries or frameworks. It starts with the basic definition of a design pattern. Builder is a creational pattern just like the Factory patterns. The abstract syntax tree is assembled from instances of the NonterminalExpression and TerminalExpression classes o invokes the Interpret operation Structural sample code The structural code demonstrates the Interpreter patterns.. Having this serialized representation as a prototype you can then use it to create copies of the original object.

Way cool! Somme very valid points! I appreciate you writing this article and the rest of the webgsite is als very good. Creational Design Patterns Pattern Name Tutorial Link Pattern Description Abstract Factory Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Factory Method read tutorial Click to Read Factory Method Design Pattern Tutorial Deals with the problem of creating related objects without specifying the exact class of object that will be created. Singleton This pattern ensures a class has only one instance and provides a global app-level point of access to it. Prototype read tutorial Click to Read Prototype Design Pattern Tutorial Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Builder read tutorial Read Builder Pattern Tutorial Separates the construction of a complex object from its representation, thus enabling the same construction process to create various representations. Bridge This pattern decouples an abstraction from its implementation so that they become loosely coupled. Composite read tutorial Link to Composite Design Pattern Tutorial This pattern allows aggregating objects such that individual objects and composition of objects can be treated uniformly.

Decorator This pattern attaches additional responsibilities to an object dynamically while keeping the interface same Facade read tutorial Link to Facade Design Pattern Tutorial This pattern provides a simpler interface to a larger and more complex system such as a class library or a complex API. Flyweight This pattern minimizes memory usage by sharing common data between objects.

Proxy read tutorial Click to Read to Proxy Design Pattern Tutorial Proxy is a surrogate or placeholder class for another class mostly done with an intention of intercepting access to the said class.

Behavioral Design Patterns Pattern Name Tutorial Link Pattern Description Chain of Responsibility read tutorial Read Chain of Responsibility Pattern Tutorial This pattern defines a chain of processing objects in a chain in such a way that the incoming request is processed by each processing objects in sequence. Command In this pattern an object is used to represent and encapsulate all the information needed to call a method at a later time.

Iterator read tutorial Read Iterator Pattern Tutorial This pattern provides a way to access the elements of an aggregate object sequentially without exposing its underlying representation. Mediator In this pattern communication between objects is encapsulated with a mediator object.

Objects no longer communicate directly with each other, but instead communicate through the mediator. Subject notifies the observers of any state changes. State read tutorial Click to Read State Design Pattern Tutorial State pattern allows an object to alter its behavior when its internal state changes.

Strategy read tutorial Strategy Design Pattern Tutorial This pattern defines a family of algorithms, encapsulate each one, and make them interchangeable. Template Method read tutorial Link to tutorial on Template Method Pattern Pattern defines steps of an algorithm as methods in an interface while allowing subclasses to override some steps in the overall algorithm.

Visitor read tutorial Link to Visitor Design Pattern Tutorial Pattern separates an algorithm from the object structure on which it operates, which provides the ability to add new operations to existing object structures without modifying those structures.

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Creational Design Patterns. Provides an interface for creating families of related or dependent objects without specifying their concrete classes. Deals with the problem of creating related objects without specifying the exact class of object that will be created. This pattern ensures a class has only one instance and provides a global app-level point of access to it.

A document management system. The example below will need adjustment.

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Page 15 of The Factory Method allows the client to use an interface for creating an object while still retaining control over which class to instantiate. 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 key objective of the Factory Method is extensibility. But they have in command that they have an author.

There are situations however. In your own programs you most likely have created methods that return new objects. ToBoolean myString.

Convert class which exposes many static methods that. Page 16 of An example is the System. The rules are: Likewise the Parse method on many built-in value types Int This is exactly where Abstract Factory and Factory Method differ: Abstract Factory methods are virtual or abstract and return abstract classes or interfaces..

In other words. All rights reserved ToBoolean myString In. Factory Methods are abstract and return class types. Open and Activator. Page 17 of Two static factory method examples are File. NET the Factory Method is typically implemented as a static method which creates an instance of a particular type determined at compile time. Prototype Definition Specify the kind of objects to create using a prototypical instance. Page 18 of NET prototype interface.

Structural Real-world sample code The real-world code demonstrates the Prototype pattern in which new Color objects are created by copying pre-existing. In this implementation the key is of type string i.

ICloneable is a built-in. Here the Serializable attribute is used to do just that as an aside: The colors collection in the ColorManager class is implemented with a type-safe generic Dictionary class.

ICloneable requires that the class hierarchy be serializable. The abstract classes have been replaced by interfaces because the abstract classes contain no implementation code.

Page 19 of RGB values range between The Prototype design pattern creates clones of pre-existing sample objects. Having this serialized representation as a prototype you can then use it to create copies of the original object.

Abstract Factory. You also need to watch out for circular references. The Prototype design pattern is not commonly used in the construction of business applications. Page 20 of NetOptimized Prototype: It is more often used in specific application types.

Deep copy copies the prototype object and all the objects it refers to. The ICloneable interface has a method called Clone that returns an object that is a copy. Shallow copy is easy to implement because the Object base class has a MemberwiseClone method that returns a shallow copy of the object.

Prototype in the. NET support for the Prototype pattern can be found in object serialization scenarios. Database connections. The best way to implement this in. NET is to use the built-in ICloneable interface on the objects that are used as prototypes. When implementing the Clone functionality you need to be aware of the two different types of cloning: Shallow copy is easier but only copies data fields in the object itself -. The copy strategy for deep copy may be more complicated -.

Instance is a class operation. Page 21 of Singleton Definition Ensure a class has only one instance and provide a global point of access to it. Structural sample code The structural code demonstrates the Singleton pattern which assures only a single instance the singleton of the class can be created. Only a single instance the singleton of the class should ever exist because servers may dynamically come on-line or off-line.

Here an elegant. The responsibilities of these objects often require that there be just one instance of the class. Server instances are created using object initialization. NET specific solution is offered. NetOptimized Singleton: In addition. Page 22 of Server lists are created using collection initialization. NET optimized code demonstrates the same code as above but uses more modern.

Thread safety is guaranteed by the compiler. Examples include cached database records see TopLink by Oracle. The Singleton pattern simply uses a private constructor and a static readonly instance variable that is lazily initialized.

The Server class has automatic properties. Using the Singleton pattern you centralize authority over a particular resource in a single object. Other reasons quoted for using Singletons are to improve performance. Global variables are frowned upon as a bad coding practice. A common scenario is when you have a stateless object that is created over and over again. An example follows: Using Singleton you can hold one or more global variables and this can be really handy.

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.

Page 23 of Other areas in the application rely on these special objects and they need a way to find them. Be careful though as the Singleton may not be the best solution in this scenario. This is where the Singleton design pattern comes in. A Singleton removes the need to constantly create and destroy objects. NET Framework An example where the. NET Remoting when launching server-activated objects.

One of the activation modes of server objects is called Singleton and their behavior is in line with the GoF pattern definition. NET Framework uses the Singleton pattern is with. Page 24 of 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.

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These incompatible classes may come from different libraries or frameworks. 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.

Bridge Definition Decouple an abstraction from its implementation so that the two can vary independently.

Page 29 of The DataObject abstract class has been replaced by an interface because DataObject contains no implementation code. Typically the Implementation interface provides only primitive operations.

Structural sample code The structural code demonstrates the Bridge pattern which separates decouples the interface from its implementation.

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. The implementation can evolve without changing clients which use the abstraction of the object.

This interface doesn't have to correspond exactly to Abstraction's interface. Page 30 of The DataObject implementations can evolve dynamically without changing any clients. NET libraries themselves. The overall system. 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.

A driver is an object that independently operates a computer system or external hardware device. The abstraction decouples the client.

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NetOptimized Bridge: The ODBC architecture decouples an abstraction from its implementation so that the two can vary independently — the Bridge pattern in action. If you build an application that uses a driver to communicate with a database. NET developers write better code. A classic example of the Bridge pattern is when coding against device drivers. It is important to realize that the client application is the abstraction. Applications that rely on these drivers are abstractions that work with any database SQL Server.

Bridge in the. Page 31 of NET Framework Bridge is a high-level architectural pattern and as such is not exposed by the. Most developers are not aware of this.

Interestingly enough. Bridge is a high-level architectural patterns and its main goal is through abstraction to help. 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.

A leaf has no children. 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.

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. Page 33 of Leaf PrimitiveElement o o represents leaf objects in the composition. The composite pattern is a great candidate for generics and you will find these used throughout this example. 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. NetOptimized Composite: Forms namespace and the other for ASP. Examples are the two Control classes one for Windows apps in the System. NET developers. The class named Shape does implement this generic interface so that comparisons can be made between shape objects.

Numerous interesting language features are used in this example. This code demonstrates much of the power that generics offer to. The nodes of the tree either contain an individual object leaf node or a group of objects a subtree of nodes. A tree control is a good example of a Composite pattern. Page 34 of It is used like any other collection. This is an open type which has the ability to accept any type parameter. This common interface greatly facilitates the design and construction of recursive algorithms that iterate over each object in the Composite collection.

This facilitates the process of adding and removing shapes from the list of tree nodes. NET apps in the System. WPF also has many built-in controls that are Composites. The built-in. NET framework. Page 35 of Decorator Definition Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

ConcreteComponent Book. Page 36 of A NET 3. Structural sample code The structural code demonstrates the Decorator pattern which dynamically adds extra functionality to an existing object. ConcreteDecorator Borrowable o adds responsibilities to the component. Decorator Decorator maintains a reference to a Component object and defines an interface that conforms to Component's interface.

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. NET optimized code demonstrates an example of the Decorator design pattern that uses generics.

Page 37 of NetOptimized Decorator: As an aside. It is polymorphic with the original class so that clients can invoke it just like the original class. Page 38 of Decorator classes usually have a constructor with an argument that represents the class they intent to decorate: The BufferedStream class is a Decorator that wraps the Stream class and reads and writes large chunks of bytes for better performance.

The Stream class is an abstract class that reads or writes a sequence of bytes from an IO device disk. In most cases. NET Framework include a set of classes that are designed around the Stream class.

Decoration is a flexible technique because it takes place at runtime. Flush and others. Decorator in the. Loan implement subsystem functionality. Facade Definition Provide a unified interface to a set of interfaces in a subsystem. Subsystem classes Bank. Page 39 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 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. NET optimized sample code This code is essentially the same as the real-world example.

The only difference is the use of. It exposes a simple API to the client.

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Page 41 of Complex operations. This service layer. Facades themselves are often implemented as singleton abstract factories. Calls into the business layer take place via a well defined service layer.

It is used in scenarios where there is a need to present a simplified view over more complex set of types. To discuss this properly we need to distinguish high-level architectural Facades from lower level component type facades. Facade in the. In a 3-tier application the presentation layer is the client. An example of an aggregate component is System.

NET Framework In the. NET developer can see. The objective of the. Page 42 of MessageQueue which provides access to a queue on a Message Queue server.

WebClient which provides a high-level interface for sending and retrieving data from a network resources identified by a general URI. SmtpMail to send mail messages. SerialPort which is a powerful serial port class. NET Framework libraries are no exception. Facades may reduce the expressiveness of the API. Flyweigth Definition Use sharing to support large numbers of fine-grained objects efficiently.

Page 43 of A ConcreteFlyweight object must be sharable.

Any state it stores must be intrinsic. ConcreteFlyweight CharacterA. CharacterZ o implements the Flyweight interface and adds storage for intrinsic state. 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. When a client requests a flyweight. Page 44 of 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.

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. The Flyweight interface enables sharing. NET Optimized code sample. NET uses Flyweights for strings that are declared at compile time and have the same sequence of characters.

Examples include. Shared flyweight objects are immutable. Flyweight in the. Page 45 of Flyweights are usually combined with the Factory pattern as demonstrated in the. NET Framework as a string management technique to minimize memory usage for immutable strings.

NET Framework As mentioned above. The generic collection increases the type-safety of the code. You will find flyweights mostly in utility type applications word processors. NetOptimized Flyweight: NET optimized code uses a generic Dictionary collection to hold and quickly access Flyweight Character objects in memory. The stateless flyweights refer to the same memory location that holds the immutable string.

They are rarely used in data-driven business type applications. Flyweights are used internally in the. 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. Structural Real-world sample code The real-world code demonstrates the Proxy pattern for a Math object represented by a MathProxy object. Page 47 of This code demonstrates the Remote Proxy pattern which provides a representative object i.

Structural sample code The structural code demonstrates the Proxy pattern which provides a representative object proxy that controls access to another similar object.

The proxy pattern is meant to provide a surrogate or placeholder for another object to control access to it. The Proxy forwards the request to a target object. Page 48 of A Smart Reference is a proxy for a pointer. 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.

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.

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

NetOptimized Proxy: There are 3 different types of proxies: As is common with proxies. Page 49 of NET the Proxy pattern manifests itself in the Remoting infrastructure. NET Framework In. Clients of WCF services also rely heavily on auto-generated proxy objects.

NET Remoting. Page 50 of President handles requests it is responsible for can access its successor if the ConcreteHandler can handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. 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. The Successor settings are simplified by using properties. 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.

The Purchase and other classes use. 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.

GOF / Gang of Four Design Patterns in Java

Page 51 of Each position has can have its own set of rules which orders they can approve. 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. NetOptimized Chain of Responsibility: 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. 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. Note that Chain-of-Responsibility is not commonly used in business application development. 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.

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.

Message sinks form a chain as each sink has a reference to the next sink in the chain. NET Remoting in which a message between a client and a server passes through one or more so-called message sinks. Page 52 of 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.

Chain of Responsibility in the. This search is usually modeled according to the specific needs of the application domain. Command Definition Encapsulate a request as an object. Page 53 of In this example the abstract Command class has been replaced by the ICommand interface because the abstract class had no implementation code at all.

Page 54 of 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. Prefixing it with ' ' allows using it as an identifier. Note that in C the word 'operator' is a keyword. Structural sample code The structural code demonstrates the Command pattern which stores requests as objects allowing clients to execute or playback the requests. The classic usage of this pattern is a menu system where each command object represents an action and an associated undo action.

Edit Copy. We would have expected that the Command pattern would be exposed in. File Save. NET as part of a unified WinForms command routing architecture. All Commands implement the same interface. Page 55 of Command in the. Typically their interface includes methods such as Do and Undo or Execute and Undo. Areas where you find Command patterns are: Menu actions include menu items such as File Open.

Page 56 of OneExpression o o implements an Interpret operation associated with terminal symbols in the grammar. Interpreter Definition Given a language. Rn in the grammar maintains instance variables of type AbstractExpression for each of the symbols R1 through Rn.

Structural Real-world sample code The real-world code demonstrates the Interpreter pattern which is used to convert a Roman numeral to a decimal. The abstract syntax tree is assembled from instances of the NonterminalExpression and TerminalExpression classes o invokes the Interpret operation Structural sample code The structural code demonstrates the Interpreter patterns..

Page 57 of Interpret typically calls itself recursively on the variables representing R1 through Rn. The truth is that if you really need this type of control it is probably easier and faster to use an existing command interpreter or expression evaluator tool out of the box. VBScript comes to mind. For an example visit: This type of problem. Grammars are usually hierarchical tree-like structures that step through multiple levels but end up with terminal nodes also called literals.

Here the abstract classes have been replaced by interfaces because the abstract classes have no implementation code. NET Dynamic Languages.

This language describes the problem domain which should be well-understood and welldefined. Page 58 of NetOptimized Interpreter: Certain types of problems lend themselves to be characterized by a language. The Interpreter design pattern solves this particular problem — that of creating a scripting language that allows the end user to customize their solution. The well-known Towers of Hanoi puzzle is an example of the type of problem that can be encoded by a simple grammar and implemented using the Interpreter design pattern.

It also has similarities with the Composite pattern. Interpreter in the. Just like State and Strategy. NET Framework libraries. Page 59 of