Overview
Design patterns represent the best practices used by experienced object-oriented software developers. Design patterns are solutions to general problems that software developers faced during software development. These solutions were obtained by trial and error by numerous software developers over quite a substantial period of time.
What is Gang of Four (GOF)?
In 1994, four authors Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides published a book titled Design Patterns - Elements of Reusable Object-Oriented Software which initiated the concept of Design Pattern in Software development.
These authors are collectively known as Gang of Four (GOF). According to these authors design patterns are primarily based on the following principles of object orientated design.
Program to an interface not an implementation
Favor object composition over inheritance
Usage of Design Pattern
Design Patterns have two main usages in software development.
Common platform for developers
Design patterns provide a standard terminology and are specific to particular scenario. For example, a singleton design pattern signifies use of single object so all developers familiar with single design pattern will make use of single object and they can tell each other that program is following a singleton pattern.
Best Practices
Design patterns have been evolved over a long period of time and they provide best solutions to certain problems faced during software development. Learning these patterns helps unexperienced developers to learn software design in an easy and faster way.
Types of Design Patterns
As per the design pattern reference book Design Patterns - Elements of Reusable Object-Oriented Software , there are 23 design patterns which can be classified in three categories: Creational, Structural and Behavioral patterns. We'll also discuss another category of design pattern: J2EE design patterns.
| S.N. | Pattern & Description |
|---|---|
| 1 | Creational Patterns These design patterns provide a way to create objects while hiding the creation logic, rather than instantiating objects directly using new opreator. This gives program more flexibility in deciding which objects need to be created for a given use case. |
| 2 | Structural Patterns These design patterns concern class and object composition. Concept of inheritance is used to compose interfaces and define ways to compose objects to obtain new functionalities. |
| 3 | Behavioral Patterns These design patterns are specifically concerned with communication between objects. |
| 4 | J2EE Patterns</br> These design patterns are specifically concerned with the presentation tier. These patterns are identified by Sun Java Center. |
设计模式的分类
总体来说设计模式分为三大类：
创建对象型的模式，共五种：工厂方法模式、抽象工厂模式、单例模式、建造者模式、原型模式。
结构型模式，共七种：适配器模式、装饰器模式、代理模式、外观模式、桥接模式、组合模式、享元模式。
行为型模式，共十一种：策略模式、方法模式、观察者模式、迭代子模式、责任链模式、命令模式、备忘录模式、状态模式、访问者模式、中介者模式、解释器模式。
其实还有两类：并发型模式和线程池模式。用一个图片来整体描述一下：
根据上图的关系，我们可以看出以下几个模式是跟其他的模式关系相当密切的，也就是我们需要重点掌握的几种模式：
-组合（Composite）模式
-迭代器（Iterator）模式
-抽象工厂（Abstract Factory）模式
-享元（Flyweight）模式
-访问者（Visitor）模式
设计模式的六大原则
1、开闭原则（Open Close Principle）
开闭原则就是说对扩展开放，对修改关闭。在程序需要进行拓展的时候，不能去修改原有的代码，实现一个热插拔的效果。所以一句话概括就是：为了使程序的扩展性好，易于维护和升级。想要达到这样的效果，我们需要使用接口和抽象类，后面的具体设计中我们会提到这点。
2、里氏代换原则（Liskov Substitution Principle）
里氏代换原则(Liskov Substitution Principle LSP)面向对象设计的基本原则之一。 里氏代换原则中说，任何父类可以出现的地方，子类一定可以出现。 LSP是继承复用的基石，只有当子类可以替换掉父类，软件单位的功能不受到影响时，子类才能真正被复用，而父类也能够在子类的基础上增加新的行为。里氏代换原则是对“开-闭”原则的补充。实现“开-闭”原则的关键步骤就是抽象化。而父类与子类的继承关系就是抽象化的具体实现，所以里氏代换原则是对实现抽象化的具体步骤的规范。
3、依赖倒转原则（Dependence Inversion Principle）
这个是开闭原则的基础，具体内容：真对接口编程，依赖于抽象而不依赖于具体。
4、接口隔离原则（Interface Segregation Principle）
这个原则的意思是：使用多个隔离的接口，比使用单个接口要好。还是一个降低类之间的耦合度的意思，从这儿我们看出，其实设计模式就是一个软件的设计思想，从大型软件架构出发，为了升级和维护方便。所以上文中多次出现：降低依赖，降低耦合。
5、迪米特法则（最少知道原则）（Demeter Principle）
为什么叫最少知道原则，就是说：一个实体应当尽量少的与其他实体之间发生相互作用，使得系统功能模块相对独立。
6、合成复用原则（Composite Reuse Principle）
原则是尽量使用合成/聚合的方式，而不是使用继承。