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Design patterns can be seen as a solution that has been tested for a problem. Design patterns provide solutions for problems which is one of the most important characteristic of design patterns. Thus a design pattern generally describes an instance of the solution or a solution that was used to solve a specific problem. The original idea of design patterns can be considered when a civil engineer "Christopher Alexander" proposed the creation of standards for architecture and found that the use of certain design constructs repeatedly heads to a desired result and explains design patterns as "Each pattern describes a problem which occurs over and over again in our environment and then describes the core of the solution to that problem in such a way that the solution can be used a million times over without ever doing it the same way twice"(Gamma, 1995, p2). Based on the work of Alexander, software professionals have incorporated these principles into the creation of the first documentation of design patterns as a guide for developers applied to OO design patterns. However, objects and interfaces are the solution for such patterns, contradictory to structures related to objects such as doors and walls. (Gamma, 1995)
The design patterns benefit the developers of a system in the following ways:
Helps to build a reliable software architectures tested and expertise accumulated by the industry.
Promoting design reuse in future systems.
Helps to identify common misconceptions and pitfalls that occur when you build systems.
Helps to design systems regardless of the language that they ultimately will be implemented.
Establishes a common vocabulary for the project among developers.
Shortens the design phase in the development process of software.
The concept of the use of design patterns to build software systems originated in the field of architecture. Architects used a number of established architectural elements such as arches and columns, when designing buildings. Designing with arches and columns is a proven strategy for built-perfect and these elements can be seen as design patterns architecture. In software, design patterns are not classes or objects. Instead, designers use design patterns to construct sets of classes and objects. To efficiently use design patterns, designers must meet the highest standards that are famous and effectively used in the industry and software engineering.
Design Patterns Structure:
The term Standards Project was originally quoted by the architect Christopher Alexander in A Pattern Language book published in 1977. Christopher Alexander, the renowned (building) architect widely acknowledged to be the originator of the pattern idea, explains that "each pattern is a three-part rule, which expresses a relation between a certain context, a problem, and a solution". He adds that "as an element in the world, each pattern is a relationship between a certain context, a certain system of forces which occurs repeatedly in that context, and a certain spatial configuration which allows these forces to resolve themselves" (Alexander, 1979). Jim Coplien, a key player in the pattern community, adds that "if we understand the forces in a pattern, then we understand the problem (because we understand the trade-offs) and the solution (because we know how it balances the forces), and we can intuit much of the rationale. For this reason, forces are the focus of a pattern"(Coplien, 1996).
In 1994, the first report was published on the Design Standards area of Software Engineering with the book Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson and John M. Vlissides, known as GOF (Group of Four). The authors define design patterns as solutions to simple and elegant to specific problems in the design of object-oriented software. These problems are very common in object-oriented projects were strictly logical experience which would require the developer to fix it as best even possible that these problems had already occurred and settled by other developers. Therefore, design patterns are generic solutions to problems specific that can be used in any language, object-oriented implemented in different ways.
Most patterns are written using predefined forms. Several pattern forms exist in the literature, each one differing from the other by the kind of categories they emphasize. Among others there exist the Alexandrian form (Alexander et. Al, 1977), the GOF (Gang of Four) form (Gamma et al., 1994), and the Coplien form (Coplien, 1995). See Coplien and Schmidt (1995) for more examples. All forms contain the basic categories: name, problem statement, context, description of forces, and solution. Sometimes pattern forms do not use all categories explicitly, but ask them to be discussed during the pattern presentation.
Each design pattern is divided by four terms or elements to simplify the understanding of the pattern and problem identification. These terms are commonly known as "Name Pattern", the "Problem", the "Solution", and finally "consequence", which describes the results found in using the default.
1- Naming a Pattern: Giving a good name to the pattern is an important and sometimes difficult task in developing work in this area. The name can be one word or more which is used to describe the problem, solution and consequences of the design. The name of design patterns helps us to communicate this element effectively with others while offering a simple approach.(Gamma, 1995)
2- The problem Applying the Pattern It is important to give an explanation about the problem as well as the context. This can be achieved by giving a description as to how to represent algorithms as an object.(Gamma, 1995)
3- The solution - Illustrating the Elements used in Design Creation In addition to the relationships and responsibilities of the design elements, this solution describes design problems and provides the general arrangements of elements to solve the problem.(Gamma, 1995)
4- Consequences: The consequences are the result of applying the pattern. These consequences are critical to evaluating design alternatives and help us to understand the advantages and disadvantages of applying patterns. This is clear when looking at the consequences of applying software patterns concerning space and time trade-offs. They also give a solution to language and implementation problems. (Gamma, 1995)
Benefícios de Design PatternsBenefits of Design Patterns
Design patterns possuem dois benefícios principais.Design patterns have two main benefits. Primeiro, eles fornecem a você First, they give youuma forma de resolver problemas relacionados com o desenvolvimento de software a way to solve problems related to software development utilizando soluções provadas. using proven solutions. A solução facilita o desenvolvimento de módulos The solution facilitates the development of modulesaltamente coesos com mínimo acoplamento. highly cohesive with minimal coupling. Eles isolam a variabilidade que pode They isolate the variability that can
existir nos requisitos do sistema, tornando todo o sistema mais fácil de se entender e exist in the system requirements, making the whole system easier to understand andmanter. maintain. Segundo, design patterns tornam a comunicação entre projetistas mais Second, design patterns make communication between designers more eficiente. efficient. Profissionais de software podem imediatemante visualizar o projeto de alto Software professionals can view the project imediatemante highnível em suas mentes quando eles sabem o nome do padrão utilizado para resolver um level in their minds when they know the name of the standard used to resolve a particular problem when discussing the design of the system. Design patterns are elegant, reusable solutions to recurring problemsque encontramos diariamente no processo de desenvolvimento de aplicativos para o we face daily in the process of developing applications for themundo real. real world. Eles tratam da concepção e da interação entre objetos, definindo ao They treat the design and interaction between objects, defining the mesmo tempo um padrão de comunicação que é compartilhado por toda a equipe de same time a communication pattern that is shared by all team desenvolvimento. development.Os padrões da gangue dos quatro (GOF - Gang of Four) são considerados as fundações Patterns of Gang of Four (GOF - Gang of Four) are considered the foundation para todos os outros padrões. for all other patterns. Eles são divididos em três categorias: Criacionais, They are divided into three categories: creational, Estruturais e Comportamentais. Structural and Behavioral.
Design Patterns & Modern Systems:
There are three main categories of design patterns which are: "Creational Patterns; Structural Patterns; and Behavioral Patterns" and their associated sub-patterns. (Gamma, 1995, p10)
Creational design patterns:
Creational design patterns examine issues related to creating objects, for example, preventing more than one system to create an object of a class (Singleton creational design pattern) or defers until runtime and decides what types of objects will be created.
Structural Design Standards
Structural design patterns describe common ways to organize classes and objects in a system.
Behavioral Design Patterns
The design patterns provide behavioral strategies tested to model how objects collaborate in the system and offer a special behavior appropriate for a wide variety of applications.
Builder Design Pattern
The Builder pattern allows a client object to construct a complex object by specifying only its type and content. The client is shielded from the details of the objects construction.
It is a pattern for step-by-step creation of a complex object so that the same construction process can create different representations is the routine in the builder pattern that also makes for finer control over the construction process. All the different builders generally inherit from an abstract builder class that declares the general functions to be used by the director to let the builder create the product in parts.
Builder has a similar motivation to the abstract factory but, whereas in that pattern, the client uses the abstract factory class methods to create its own object, in Builder the client instructs the builder class on how to create the object and then asks it for the result. How the class is put together is up to the Builder class. It's a subtle difference.
The Builder pattern is applicable when the algorithm for creating a complex object should be independent of the parts that make up the object and how they are assembled and the construction process must allow different representations for the object that is constructed
The form used in our example starts with its name and then with its general intent that says something about the solution. The intent section may also say something about important forces that are achieved in the solution (such as compact representation of the state of the system). The intent section is then followed by the context section which describes the current structure and behavior of the system in which the problem arises and the forces reside. Then the problem statement is given followed by the forces section. The forces section describes the tradeoffs and considerations of the designer. It may also include discussions of (obvious) solutions that a designer may think of but would not work within that particular context and set of forces. Then the solution is presented together with an explanation why the solution resolves the forces in a favorite manner. Finally, related patterns are discussed that the designer may find useful after having applied the solution.
DEFINITION OF DESIGN STANDARDS
The term Standards Project was originally quoted by the architect Christopher Alexander in A Pattern Language book published in 1977. Each pattern describes a problem Which Occurs over and over again in Our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution to times over million, without ever doing it the same way twice. In 1994 he published the first report on the Design Standards area of Software Engineering with the book Design Patterns: Elements of Reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson and John M. Vlissides, known as GOF (Group of Four). The authors define design patterns as solutions to simple and elegant to specific problems in the design of object-oriented software. These problems are very common in object-oriented projects were strictly logical experience which would require the developer to fix it as best even possible that these problems had already occurred and settled by other developers. Therefore, design patterns are generic solutions to problems specific that can be used in any language, object-oriented implemented in different ways. Each design pattern is divided by four terms or elements to simplify the understanding of the pattern and problem identification. These terms are commonly known as "Name Pattern", which describes the name of the solution, the "Problem", which describes when to use the default, "Solution", which describes how the problem will be solved, and finally "consequence", which describes the results found in using the default. Were also sorted into three categories, Standards Creation, which are responsible for the abstraction of object creation; Standards Structural and Behavioral Patterns.
Design Patterns Structure:
Documentation of design patterns is extremely structured. The patterns are authenticated from a template as a guide to identify the required information in order to understand the software problem as well as a solution to the problem through classes and objects essential to implement or enforce the solution. Describing a design pattern template; there has not been any general agreement within the design pattern community. Different pattern styles are adopted by different authors. Some of them try to be much more communicative and little structured, while others choose their patterns to be much more precise and structured. However there are some crucial elements that make design patterns reusable oriented. Given below is an example of design pattern template in order to illustrate an example:
Pattern Name: The name of the design pattern is very important as it facilitates effective communication to explain the problem, its solution as well as the consequences with others with the contribution of a simple approach. (Gamma, 1995)
Intent: This explains the purpose of the pattern.
Motivation: This section of the design pattern provides an example of same type and explains how the pattern solves the problem.
Applicability: Explains the applicability of the pattern in various situations.
Structure: Depicts the pattern with the help of classes and objects.
Participants: Explains the participating classes and objects of the design pattern as well as their responsibilities.
Collaborations: Explains how different participants collaborate in order to carry out their given responsibility.
Components of a design pattern:
Name - describes the essence of the pattern
Objective - describes how the standard operates.
Problem - Describe the problem
Solution - describes the solution
Consequences - describes the benefits of using the default.
In this item, describe the general design standards and details Singleton and Factory patterns.
Design pattern documentation is highly structured. The patterns are documented from a template that identifies the information needed to understand the software problem and the solution in terms of the relationships between the classes and objects necessary to implement the solution. There is no uniform agreement within the design pattern community on how to describe a pattern template. Different authors prefer different styles for their pattern templates. Some authors prefer to be more expressive and less structured, while others prefer their pattern templates to be more precise and high grain in structure. We will use the template first described by the authors of Design Patterns to illustrate a template.
The historical patterns of object-oriented design Between 1991 and 1994, Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, collectively known as the 'Gang of Four'- used his expertise to write the book Design Patterns: Elements of reusable object-oriented software. This book describes 23 patterns design, each providing a solution to a common problem in software design industry. The book brings together the patterns project into three categories-creational design patterns, design patterns, structural and behavioral design patterns. Creational design patterns describe the techniques to instantiate objects (or groups of objects). Structural Design Standards
enable designers to organize classes and objects into larger structures. Behavioral design patterns give responsibilities to classes and objects.
The book of the 'Gang of Four' has shown that the design standards have evolved naturally over the years of experience in industry. In his article Seven Habits of Successful Pattern Writers, 1 John Vlissides states that "the most important activity in the process writing standards is the reflection. " This statement implies that the creation of standards, developers should think about, and document, their successes (and mistakes). Developers use design patterns to capture and use that collective experience of industry, which ultimately helps them to avoid repeating the same mistakes. New design standards are created over time around quickly and presented to designers from around the world via the Internet. The design patterns are a more advanced topic that may not appear in the sequences of most introductory courses. As you advance in their studies about Java, it is certain that the design standards will have a higher value. If you are a student and his instructor does not plan to include this material in your course, we encourage the reading of this material on their own. Section M.8 presents a list of Web resources that relate to design patterns and their relevance to the Java programming. As you go through this appendix, you should consult the URLs provided to learn more about a particular design pattern introduced in the text or to read about new developments in community design standards.
Introducing design patterns, creational, structural and behavioral Section M.1, we mentioned that the 'Gang of Four' described 23 design patterns using three categories-creational, structural and behavior. In this and in other sections of this appendix, we discuss the design standards in each category and their importance and how each pattern is related to the material in this book about Java. For example, many Java Swing components that we present in Chapters 11:22 using the Composite design pattern. Figure M.1 identifies 18 design standards discussed in this Gang of Four Appendix. Many popular patterns were documented on the basis of the Gang of Four book-these include design standards competition, especially useful in designing systems for multiple threads. Section M.4 discusses some of these patterns used in industry. Architectural standards, as discussed in Section M.5, specify how subsystems interact with one another. Figure M.2 lists the standards of competition and the architectural standards that we address in this appendix.
"A pattern describes a problem that occurs over and over again in a given context, and also describes the core solution to the problem so that this solution can be systematically used in different situations."
The aim of this paper was to analyze the system of command and control
divisional under the focus of modeling systems. The introduction consisted of a
brief presentation of the theme and rationale for their study, some quotes from
other scientific studies highlighting the importance of the approach chosen and,
brief, the steps developed for the study. It was presented to
methodology, which included literature searches, comparisons
with the American military doctrine, development of a model for
command and control system and a divisional field research. The main
results gained from applying the methodology are presented,
emphasizing the partial reflections and conclusions derived from the modeling
systems. On the topic discussed some aspects that extend the concept of command
and control were placed on the lessons Americans were extracted to
application to the topic under study and a proposal on the employment of means of
communications was laid. Finally, based on a series of conclusions
Partial presented in this study, we concluded that the Brazilian military doctrine
in force is not adequate in some respects, the possibilities of the system
divisional command and control.
Design Patterns - Design Patterns
No article - Design Patterns: Model MVC - Model View Controller - after talking design patterns in Design Patterns and I got some emails asking about more details about Design Patterns. In this article I hope to be given to those requests ..
After all they came from and what they really are Design Patterns? (Hereafter, Design Patterns.)
Design patterns can be seen as a solution that has been tested for a problem. Thus, a design pattern generally describes an instance of the solution or a solution that was used to solve a specific problem. Standards projects are solutions to problems that someone once had and solved by applying a model that has been documented and that you can adapt in full or according to need your solution.
Note: In a more formal definition: "Patterns for software architecture efficiency solutions are proven and widely used to solve common problems in software design. These solutions are developed and known by specialists and become standards because they are reused several times on several projects and have proven effectiveness. "
We can consider that the original idea of Design Patterns with Christopher Alexander came when he proposed the creation of standards for architecture catalago. (He was Civil Engineer). His publications in this respect were:
[Alexander1978] Christopher Alexander: A Pattern Language, Oxford Press, Oxford, R. Kingdom, 1978
[Alexander1979] Christopher Alexander: The Timeless Way of Building, Oxford Press, Oxford, R. Kingdom, 1978.
Christopher Alexander with the word:
"A pattern describes a problem that occurs throughout \ par times in a given context, and also describes the solution to this problem so that this solution can be systematically used in different situations." [Alexander78]
Based on the work of Alexander software professionals have begun to incorporate these principles into the creation of the first documentation of design patterns as a guide for beginning developers. The practical result was the publication of - Design Patterns: Elements of Reusable Object-Oriented Software - in 1995 by Eric Gamma, Richard Helm, Ralph Johnson and John Vlissides - (gang of four). This book is the main reference on the subject for software community. Described therein are 23 standards that were based on the experience of the authors.
Note: Super CD.NET you find the electronic edition of the bestseller "Design Patterns" - a publication geared to developers using the methodology of object orientation.
Main properties of design patterns
Among the main properties of design patterns include:
1 - capture knowledge and experience of experts in software design.
2 - specify abstractions that are above the level of individual objects or classes or components of [Gamma et al 1995].
3 - define a common vocabulary for discussing problems and solutions design [Gamma et al 1995].
4 - to facilitate the documentation and maintenance of software architecture [Buschmann et al 1996].
4 - assist in the design of an architecture with certain properties [Buschmann et al 1996].
5 - aid the design of architectures compl more exasperating.
Among the main benefits that the use of design patterns is warranted are:
- Provide solutions that have been tested and approved.
- They make the system easier to understand and maintain
- Facilitate the development of cohesive modules.
- Communication between project participants is more efficient
When and how to use design patterns
The first thing you have to have is common sense. Do your part, implement your solution and see if it works. Then make sure it can be optimized, if necessary use the design pattern that fits your case to improve the deficiencies in their design.
Naturally this will be so much easier if you have an overview of your project and its operation.
Remember, design patterns are not a magic wand that will make your project without flaws. If poorly implemented they could even reduce the understanding of your project and increase the amount of code. So design patterns do not solve all the problems of design projects.
The components of a design pattern are:
1 - Name - describes the essence of the pattern
2 - Objective - describes how the standard operates.
3 - Problem - Describe the problem
4 - Solution - describes the solution
5 - Consequences - describes the benefits of using the default.
Requirements of a good system of standards:
The system must contain a good amount of patterns.
The description of the standards must follow a standard format.
The system should be structured, organized patterns follow well-defined criteria.
The system should show the relationship between the standards.
The system must be structured in order to evolve.
The types are divided by affinity. I am only defining the most used. The main types of design patterns are:
1 - Patterns of Creation (creational)
Abstract Factory - A Factory Method is a method that produces objects of a particular type; Factory An object is an object that encapsulates methods Factory.
Builder - Separate the construction of a complex object from its representation so that the same construction process can create different representations.
Factory Method - An interface for instantiation of objects that remains insulated concrete classes used to request the creation of these objects.
Prototype - Prototype pattern provides another way to construct objects of arbitrary types.
Singleton. - Ensures that for a particular class can exist only a single instance, which is accessible in a global and uniform.
2 - Patterns of Structure (Structural)
Adapter - allows two objects to communicate even if they have incompatible interfaces.
Bridge - Decouple interface from implementation; Hiding implementation details from clients.
Composite - deals with a structure of elements grouped hierarchically (not as mere collections).
Decorator - Attach additional responsibilities to an object dynamically. The Decorator provides a flexible alternative to subclassing for extending functionality.
Facade - unified interface to a subsystem; makes the subsystem easier to use.
Flyweight - Use sharing to support various objects efficiently.
Proxy - Provides an object representative or attorney of another object to control access to it.
3 - Patterns of Behavior (Behavioral)
Chain of Responsibility - Avoid dependency on the sender (client) of a request to its receiver by giving more opportunity to object to handle the request.
Command - Associates an action with different objects through a familiar interface.
Interpreter - Used to help an application to understand a natural language statement and execute the functionality of the declaration.
Iterator - Provide a way to loop through the elements of a collection without violating its encapsulation.
Mediator - Creates an object that acts as a mediator controlling the interaction between a set of objects.
Memento - Makes it possible to save the state of an object so that it can be restored.
Observer - Defines a dependency relationship 1: N so that when a certain object (subject) has changed its status others (observers) are notified; enables loose coupling between objects and observers subject.
State - Allows object to change its behavior when its internal state changes.
Strategy - Enables a family of algorithms to be used independently and selectively.
Template Method - Define the skeleton of an algorithm in an operation put off setting a few steps to the subclass.
Visitor - Defines independent operations to be performed on elements of a structure.
The following are two examples of use of the concepts related to design patterns:
1 -) encapsulation (data hiding)
Problem to solve: exposed fields can be manipulated directly from external code, leading to violations of the invariant representation or dependencies that prevent the undesirable change in the implementation.
Solution presented by default: hide some components, allowing access only to the stylized object. Cons: the interface can not (efficiently) provide all the desired operations.
Cons: the interface can not (efficiently) provide all the desired operations. Indirect access can reduce performance.
2 -) Derivation of classes (Inheritance)
Problem to solve: Similar abstraction members have similar (fields and methods). This repetition is tedious, error prone, and a headache during maintenance.
Solution presented by default: default members derive from a superclass, at runtime, select the correct implementation by resolutions regarding which implementation should be executed.
Disadvantages: the code of a class ends up being distributed in many, reducing the potential for understanding. The use of resolutions introduced at runtime overhead (extra processing).