In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes a group of objects, treated the same way as a single instance of the same type of object; the intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly; the Composite design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. What problems can the Composite design pattern solve? A part-whole hierarchy should be represented so that clients can treat part and whole objects uniformly. A part-whole hierarchy should be represented as tree structure; when defining Part objects and Whole objects that act as containers for Part objects, clients must treat them separately, which complicates client code.
What solution does the Composite design pattern describe? Define a unified Component interface for both part objects and whole objects. Individual Leaf objects implement the Component interface directly, Composite objects forward requests to their child components; this enables clients to work through the Component interface to treat Leaf and Composite objects uniformly: Leaf objects perform a request directly, Composite objects forward the request to their child components recursively downwards the tree structure. This makes client classes easier to implement, change and reuse. See the UML class and object diagram below; when dealing with Tree-structured data, programmers have to discriminate between a leaf-node and a branch. This makes code more complex, therefore, more error prone; the solution is an interface that allows treating primitive objects uniformly. In object-oriented programming, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality.
This is known as a "has-a" relationship between objects. The key concept is that you can manipulate a single instance of the object just as you would manipulate a group of them; the operations you can perform on all the composite objects have a least common denominator relationship. For example, if defining a system to portray grouped shapes on a screen, it would be useful to define resizing a group of shapes to have the same effect as resizing a single shape. Composite should be used when clients ignore the difference between compositions of objects and individual objects. If programmers find that they are using multiple objects in the same way, have nearly identical code to handle each of them composite is a good choice. In the above UML class diagram, the Client class doesn't refer to the Leaf and Composite classes directly. Instead, the Client refers to the common Component interface and can treat Leaf and Composite uniformly; the Leaf class implements the Component interface directly. The Composite class maintains a container of child Component objects and forwards requests to these children.
The object collaboration diagram shows the run-time interactions: In this example, the Client object sends a request to the top-level Composite object in the tree structure. The request is forwarded to all child. Defining Child-Related Operations There are two design variants for defining and implementing child-related operations like adding/removing a child component to/from the container and accessing a child component: Design for uniformity: Child-related operations are defined in the Component interface; this enables clients to treat Composite objects uniformly. But type safety is lost. Design for type safety: Child-related operations are defined only in the Composite class. Clients must treat Composite objects differently, but type safety is gained. The Composite design pattern emphasizes uniformity over type safety. Componentis the abstraction for all components, including composite ones declares the interface for objects in the composition defines an interface for accessing a component's parent in the recursive structure, implements it if that's appropriateLeafrepresents leaf objects in the composition implements all Component methodsCompositerepresents a composite Component implements methods to manipulate children implements all Component methods by delegating them to its children As it is described in Design Patterns, the pattern involves including the child-manipulation methods in the main Component interface, not just the Composite subclass.
More recent descriptions sometimes omit these methods. The following example, written in Java, implements a graphic class, which can be either an ellipse or a composition of several graphics; every graphic can be printed. In Backus-Naur form, Graphic::= ellipse | GraphicList GraphicList::= empty | Graphic GraphicList It could be extended to implement several other shapes and methods. Perl Design Patterns Book Mixin Law of Demeter Composite Pattern implementation in Java Composite pattern description from the Portland Pattern Repository Composite pattern in UML and in LePUS3, a formal modellin
In computer programming, the proxy pattern is a software design pattern. A proxy, in its most general form, is a class functioning as an interface to something else; the proxy could interface to anything: a network connection, a large object in memory, a file, or some other resource, expensive or impossible to duplicate. In short, a proxy is a wrapper or agent object, being called by the client to access the real serving object behind the scenes. Use of the proxy can be forwarding to the real object, or can provide additional logic. In the proxy, extra functionality can be provided, for example caching when operations on the real object are resource intensive, or checking preconditions before operations on the real object are invoked. For the client, usage of a proxy object is similar to using the real object, because both implement the same interface; the Proxy design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse.
The access to an object should be controlled. Additional functionality should be provided; when accessing sensitive objects, for example, it should be possible to check that clients have the needed access rights. Define a separate Proxy object that can be used as substitute for another object and implements additional functionality to control the access to this subject; this enables to work through a Proxy object to perform additional functionality when accessing a subject. For example, to check the access rights of clients accessing a sensitive object. To act as substitute for a subject, a proxy must implement the Subject interface. Clients can't tell whether they work with its proxy. See the UML class and sequence diagram below. In the above UML class diagram, the Proxy class implements the Subject interface so that it can act as substitute for Subject objects, it maintains a reference to the substituted object. The sequence diagram shows the run-time interactions: The Client object works through a Proxy object that controls the access to a RealSubject object.
In this example, the Proxy forwards the request to the RealSubject. In distributed object communication, a local object represents a remote object; the local object is a proxy for the remote object, method invocation on the local object results in remote method invocation on the remote object. An example would be an ATM implementation, where the ATM might hold proxy objects for bank information that exists in the remote server. In place of a complex or heavy object, a skeleton representation may be advantageous in some cases; when an underlying image is huge in size, it may be represented using a virtual proxy object, loading the real object on demand. A protection proxy might be used to control access to a resource based on access rights; the program's output is: Loading HiRes_10MB_Photo1 Displaying HiRes_10MB_Photo1 Displaying HiRes_10MB_Photo1 Loading HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo1 Output Sorry, the driver is too young to drive.
Car has been driven! Notes: A proxy may hide information about the real object to the client. A proxy may perform optimization like on demand loading. A proxy may do additional house-keeping job like audit tasks. Proxy design pattern is known as surrogate design pattern. Output Sorry, the driver is too young to drive. Car has been driven! Usage Output Sorry, the driver is too young to drive. Car has been driven! The following Java example illustrates the "virtual proxy" pattern; the ProxyImage class is used to access a remote method. The example creates first an interface against; this interface contains only one method to display the image, called displayImage, that has to be coded by all classes implementing it. The proxy class ProxyImage is running on another system than the real image class itself and can represent the real image RealImage over there; the image information is accessed from the disk. Using the proxy pattern, the code of the ProxyImage avoids multiple loading of the image, accessing it from the other system in a memory-saving manner.
The lazy loading demonstrated in this example is not part of the proxy pattern, but is an advantage made possible by the use of the proxy. The program's output is: Loading HiRes_10MB_Photo1 Displaying HiRes_10MB_Photo1 Displaying HiRes_10MB_Photo1 Loading HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo2 Displaying HiRes_10MB_Photo1 Composite pattern Decorator pattern Lazy initialization Proxy pattern in UML and in LePUS3 Take control with the Proxy design pattern by David Geary, JavaWorld.com PerfectJPattern Open Source Project, Provides componentized implementation of the Proxy Pattern in Java Adapter vs. Proxy vs. Facade Pattern Comparison Proxy Design Pattern Proxy pattern C++ implementation example Proxy pattern description from the Portland Pattern Repository
The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, cloned to produce new objects; this pattern is used to: avoid subclasses of an object creator in the client application, like the factory method pattern does. Avoid the inherent cost of creating a new object in the standard way when it is prohibitively expensive for a given application. To implement the pattern, declare an abstract base class that specifies a pure virtual clone method. Any class that needs a "polymorphic constructor" capability derives itself from the abstract base class, implements the clone operation; the client, instead of writing code that invokes the "new" operator on a hard-coded class name, calls the clone method on the prototype, calls a factory method with a parameter designating the particular concrete derived class desired, or invokes the clone method through some mechanism provided by another design pattern.
The mitotic division of a cell — resulting in two identical cells — is an example of a prototype that plays an active role in copying itself and thus, demonstrates the Prototype pattern. When a cell splits, two cells of identical genotype result. In other words, the cell clones itself; the Prototype design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. The Prototype design pattern solves problems like: How can objects be created so that which objects to create can be specified at run-time? How can dynamically loaded classes be instantiated? Creating objects directly within the class that requires the objects is inflexible because it commits the class to particular objects at compile-time and makes it impossible to specify which objects to create at run-time; the Prototype design pattern describes how to solve such problems: Define a Prototype object that returns a copy of itself.
Create new objects by copying a Prototype object. This enables configuration of a class with different Prototype objects, which are copied to create new objects, more, Prototype objects can be added and removed at run-time. See the UML class and sequence diagram below. In the above UML class diagram, the Client class that requires a Product object doesn't instantiate the Product1 class directly. Instead, the Client refers to the Prototype interface for cloning an object; the Product1 class implements the Prototype interface by creating a copy of itself. The UML sequence diagram shows the run-time interactions: The Client object calls clone on a prototype:Product1 object, which creates and returns a copy of itself. Sometimes creational patterns overlap — there are cases when either prototype or abstract factory would be appropriate. At other times they complement each other: abstract factory might store a set of prototypes from which to clone and return product objects. Abstract factory and prototype can use singleton in their implementations..
Abstract factory classes are implemented with factory methods, but they can be implemented using prototype. Designs start out using Factory Method and evolve toward abstract factory, prototype, or builder as the designer discovers where more flexibility is needed. Prototype does not require subclassing. Factory method does not require initialization. Designs that make heavy use of the composite and decorator patterns can benefit from Prototype as well; the rule of thumb could be that you would need to clone an Object when you want to create another Object at runtime, a true copy of the Object you are cloning. True copy means all the attributes of the newly created Object should be the same as the Object you are cloning. If you could have instantiated the class by using new instead, you would get an Object with all attributes as their initial values. For example, if you are designing a system for performing bank account transactions you would want to make a copy of the Object that holds your account information, perform transactions on it, replace the original Object with the modified one.
In such cases, you would want to use clone instead of new. Let's write an occurrence browser class for a text; this class lists the occurrences of a word in a text. Such an object is expensive to create as the locations of the occurrences need an expensive process to find. So, to duplicate such an object, we use the prototype pattern: class WordOccurrences is field occurrences is The list of the index of each occurrence of the word in the text. Constructor WordOccurrences is input: the text in which the occurrences have to be found input: the word that should appear in the text Empty the occurrences list for each textIndex in text isMatching:= true for each wordIndex in word if the current word character does not match the current text character isMatching:= false if isMatching is true Add the current textIndex into the occurrences list method getOneOccurrenceIndex is input: a number to point on the nth occurrence. Output: the index of the nth occurrence. Return the nth item of the occurrences field if any.
Method clone is output: a WordOccurrences object containing the same data. Call clone on the super class
In software engineering, the mediator pattern defines an object that encapsulates how a set of objects interact. This pattern is considered to be a behavioral pattern due to the way it can alter the program's running behavior. A program is made up of a large number of classes. Logic and computation are distributed among these classes. However, as more classes are added to a program during maintenance and/or refactoring, the problem of communication between these classes may become more complex; this makes the program harder to maintain. Furthermore, it can become difficult to change the program, since any change may affect code in several other classes. With the mediator pattern, communication between objects is encapsulated within a mediator object. Objects no longer communicate directly with each other, but instead communicate through the mediator; this reduces the dependencies between communicating objects, thereby reducing coupling. The Mediator design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse.
What problems can the Mediator design pattern solve? Tight coupling between a set of interacting objects should be avoided, it should be possible to change the interaction between a set of objects independently. Defining a set of interacting objects by accessing and updating each other directly is inflexible because it couples the objects to each other and makes it impossible to change the interaction independently from the objects, and it makes them hard to test. Coupled objects are hard to implement, change and reuse because they refer to and know about many different objects. What solution does the Mediator design pattern describe? Define a separate object that encapsulates the interaction between a set of objects. Objects delegate their interaction to a mediator object instead of interacting with each other directly; the objects interact with each other indirectly through a mediator object that controls and coordinates the interaction. This makes the objects loosely coupled, they only refer to and know about their mediator object and have no explicit knowledge of each other.
See the UML class and sequence diagram below. The essence of the Mediator Pattern is to "define an object that encapsulates how a set of objects interact", it promotes loose coupling by keeping objects from referring to each other explicitly, it allows their interaction to be varied independently. Client classes can use the mediator to send messages to other clients, can receive messages from other clients via an event on the mediator class. In the above UML class diagram, the Colleague1 and Colleague2 classes do not refer to each other directly. Instead, they refer to the common Mediator interface for controlling and coordinating interaction, which makes them independent of how the interaction is carried out; the Mediator1 class implements the interaction between Colleague1 and Colleague2. The UML sequence diagram shows the run-time interactions: In this example, a Mediator1 object mediates the interaction between Colleague1 and Colleague2 objects. Assuming that Colleague1 wants to interact with Colleague2, Colleague1 calls mediate on the Mediator1 object, which gets the changed data from Colleague1 and performs an action2 on Colleague2.
Thereafter, Colleague2 calls mediate on the Mediator1 object, which gets the changed data from Colleague2 and performs an action1 on Colleague1. ParticipantsMediator - defines the interface for communication between Colleague objects ConcreteMediator - implements the Mediator interface and coordinates communication between Colleague objects, it is aware of all of their purposes with regards to inter-communication. Colleague - defines the interface for communication with other Colleagues through its Mediator ConcreteColleague - implements the Colleague interface and communicates with other Colleagues through its Mediator The Mediator pattern ensures that components are loosely coupled, such that they don't call each other explicitly, but instead do so through calls to a mediator. In the following example, the Mediator registers all Components and calls their SetState methods. A chat room could use the Mediator pattern, or a system where many ‘clients’ each receive a message each time one of the other clients performs an action.
In reality using the Mediator pattern for a chat room would only be practical when used with remoting. Using raw sockets wouldn’t allow for the delegate callbacks. In the following example, a Mediator object controls the values of several Storage objects forcing the user code to access to the stored values through the mediator; when a storage object want to emit an event indicating that its value has changed, it go back to the mediator object that controls the list of the observers. Data mediation Design Patterns, the book which gave rise to the study of design patterns in computer science Design pattern, a standard solution to common problems in software design Mediator Design Pattern Is the use of the mediator pattern recommend? - Stack Overflow
In computer programming, the interpreter pattern is a design pattern that specifies how to evaluate sentences in a language. The basic idea is to have a class for each symbol in a specialized computer language; the syntax tree of a sentence in the language is an instance of the composite pattern and is used to evaluate the sentence for a client. See Composite pattern; the Interpreter design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. What problems can the Interpreter design pattern solve? A grammar for a simple language should be defined so that sentences in the language can be interpreted; when a problem occurs often, it could be considered to represent it as a sentence in a simple language so that an interpreter can solve the problem by interpreting the sentence. For example, when many different or complex search expressions must be specified.
Implementing them directly into a class is inflexible because it commits the class to particular expressions and makes it impossible to specify new expressions or change existing ones independently from the class. What solution does the Interpreter design pattern describe? Define a grammar for a simple language by defining an Expression class hierarchy and implementing an interpret operation. Represent a sentence in the language by an abstract syntax tree made up of Expression instances. Interpret a sentence by calling interpret on the AST; the expression objects are composed recursively into a composite/tree structure, called abstract syntax tree. The Interpreter pattern doesn't describe; this can be done either automatically by a parser. See the UML class and object diagram below. Specialized database query languages such as SQL. Specialized computer languages that are used to describe communication protocols. Most general-purpose computer languages incorporate several specialized languages. In the above UML class diagram, the Client class refers to the common AbstractExpression interface for interpreting an expression interpret.
The TerminalExpression class interprets an expression directly. The NonTerminalExpression class maintains a container of child expressions and forwards interpret requests to these expressions; the object collaboration diagram shows the run-time interactions: The Client object sends an interpret request to the abstract syntax tree. The request is forwarded to all objects downwards the tree structure; the NonTerminalExpression objects forward the request to their child expressions. The TerminalExpression objects perform the interpretation directly; the following Backus–Naur form example illustrates the interpreter pattern. The grammar defines a language that contains Reverse Polish Notation expressions like: a b + a b c + - a b + c a - - This structural code demonstrates the Interpreter patterns, which using a defined grammar, provides the interpreter that processes parsed statements. Following the interpreter pattern, we need to implement the interface Expr with a lambda for each grammar rule. While the interpreter pattern does not address parsing a parser is provided for completeness.
Evaluating the expression "w x z - +" with w = 5, x = 10, z = 42. Backus–Naur form Combinatory logic in computing Design Patterns Domain-specific language Interpreter Interpreter implementation in Ruby Interpreter implementation in C++ SourceMaking tutorial Interpreter pattern description from the Portland Pattern Repository
Factory method pattern
In class-based programming, the factory method pattern is a creational pattern that uses factory methods to deal with the problem of creating objects without having to specify the exact class of the object that will be created. This is done by creating objects by calling a factory method—either specified in an interface and implemented by child classes, or implemented in a base class and optionally overridden by derived classes—rather than by calling a constructor; the Factory Method design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change and reuse. The Factory Method design pattern solves problems like: How can an object be created so that subclasses can redefine which class to instantiate? How can a class defer instantiation to subclasses? Creating an object directly within the class that requires the object is inflexible because it commits the class to a particular object and makes it impossible to change the instantiation independently from the class.
The Factory Method design pattern describes how to solve such problems: Define a separate operation for creating an object. Create an object by calling a factory method; this enables writing of subclasses to change the way. See the UML class diagram below. "Define an interface for creating an object, but let subclasses decide which class to instantiate. The Factory method lets a class defer instantiation it uses to subclasses." Creating an object requires complex processes not appropriate to include within a composing object. The object's creation may lead to a significant duplication of code, may require information not accessible to the composing object, may not provide a sufficient level of abstraction, or may otherwise not be part of the composing object's concerns; the factory method design pattern handles these problems by defining a separate method for creating the objects, which subclasses can override to specify the derived type of product that will be created. The factory method pattern relies on inheritance, as object creation is delegated to subclasses that implement the factory method to create objects.
In the above UML class diagram, the Creator class that requires a Product object doesn't instantiate the Product1 class directly. Instead, the Creator refers to a separate factoryMethod to create a product object, which makes the Creator independent of which concrete class is instantiated. Subclasses of Creator can redefine. In this example, the Creator1 subclass implements the abstract factoryMethod by instantiating the Product1 class. A maze game may be played in two modes, one with regular rooms that are only connected with adjacent rooms, one with magic rooms that allow players to be transported at random. Room is the base class for a final product. MazeGame declares the abstract factory method to produce such a base product. MagicRoom and OrdinaryRoom are subclasses of the base product implementing the final product. MagicMazeGame and OrdinaryMazeGame are subclasses of MazeGame implementing the factory method producing the final products, thus factory methods decouple callers from the implementation of the concrete classes.
This makes the "new" Operator redundant, allows adherence to the Open/closed principle and makes the final product more flexible in the event of change. Same code for C# In the above code you can see the creation of one interface called IPerson and two implementations called Villager and CityPerson. Based on the type passed into the Factory object, we are returning the original concrete object as the interface IPerson. A factory method is just an addition to Factory class, it creates the object of the class through interfaces but on the other hand, it lets the subclass decide which class is instantiated. You can see; as a result, you can call MakeProduct from it to get the IProduct. You might write your custom logic after getting the object in the concrete Factory Method; the GetObject is made abstract in the Factory interface. This Java example is similar to one in the book Design Patterns; the MazeGame uses Rooms but it puts the responsibility of creating Rooms to its subclasses which create the concrete classes.
The regular game mode could use this template method: In the above snippet, the MazeGame constructor is a template method that makes some common logic. It refers to the makeRoom factory method that encapsulates the creation of rooms such that other rooms can be used in a subclass. To implement the other game mode that has magic rooms, it suffices to override the makeRoom method: Another example in PHP follows, this time using interface implementations as opposed to subclassing, it is important to note that the factory method can be defined as public and called directly by the client code. Same as Java example. Factory pattern deals with the instantiation of objects without exposing the instantiation logic. In other words, a Factory is a creator of objects which have a common interface. In ADO. NET, IDbCommand. CreateParameter is an example of the use of factory method to connect parallel class hierarchies. In Qt, QMainWindow::createPopupMenu is a factory method declared in a framework that can be overridden in application code.
In Java, several factories are used in the javax.xml.parsers package. E.g. javax.xml.parsers. DocumentBuilderFactory or javax.xml.parsers. SAXParserFactory. In the HTML5 DOM API, the Docu
A facade is one exterior side of a building the front. It is a foreign loan word from the French façade, which means "frontage" or "face". In architecture, the facade of a building is the most important aspect from a design standpoint, as it sets the tone for the rest of the building. From the engineering perspective of a building, the facade is of great importance due to its impact on energy efficiency. For historical facades, many local zoning regulations or other laws restrict or forbid their alteration; the word comes from the French foreign loan word façade, which in turn comes from the Italian facciata, from faccia meaning face from post-classical Latin facia. The earliest usage recorded by the Oxford English Dictionary is 1656, it was quite common in the Georgian period for existing houses in English towns to be given a fashionable new facade. For example, in the city of Bath, The Bunch of Grapes in Westgate Street appears to be a Georgian building, but the appearance is only skin deep and some of the interior rooms still have Jacobean plasterwork ceilings.
This new construction has happened in other places: in Santiago de Compostela the 3-metres-deep Casa do Cabido was built to match the architectural order of the square, the main Churrigueresque facade of the Santiago de Compostela Cathedral, facing the Praza do Obradoiro, is encasing and concealing the older Portico of Glory. In modern highrise building, the exterior walls are suspended from the concrete floor slabs. Examples include precast concrete walls; the facade can at times be required to have a fire-resistance rating, for instance, if two buildings are close together, to lower the likelihood of fire spreading from one building to another. In general, the facade systems that are suspended or attached to the precast concrete slabs will be made from aluminium or stainless steel. In recent years more lavish materials such as titanium have sometimes been used, but due to their cost and susceptibility to panel edge staining these have not been popular. Whether rated or not, fire protection is always a design consideration.
The melting point of aluminium, 660 °C, is reached within minutes of the start of a fire. Firestops for such building joints can be qualified, too. Putting fire sprinkler systems on each floor has a profoundly positive effect on the fire safety of buildings with curtain walls; some building codes limit the percentage of window area in exterior walls. When the exterior wall is not rated, the perimeter slab edge becomes a junction where rated slabs are abutting an unrated wall. For rated walls, one may choose rated windows and fire doors, to maintain that wall's rating. On a film set and within most themed attractions, many of the buildings are only facades, which are far cheaper than actual buildings, not subject to building codes. In film sets, they are held up with supports from behind, sometimes have boxes for actors to step in and out of from the front if necessary for a scene. Within theme parks, they are decoration for the interior ride or attraction, based on a simple building design. Façades: Principles of Construction.
By Ulrich Knaack, Tillmann Klein, Marcel Bilow and Thomas Auer. Boston/Basel/Berlin: Birkhaüser-Verlag, 2007. ISBN 978-3-7643-7961-2 ISBN 978-3-7643-7962-9 Giving buildings an illusion of grandeur Facades of Casas Chorizo in Buenos Aires, Argentina Poole, Thomas. "Façade". In Herbermann, Charles. Catholic Encyclopedia. 5. New York: Robert Appleton Company; the article outlines the development of the facade in ecclesiastical architecture from the early Christian period to the Renaissance