在网上看了一个技术博主的一篇文章，讨论的的是代码质量。博主指出，目前国内普遍，都缺乏方法论和抽象能力的积累。有所感，于是回头看，也重新整理面向对象设计的六大原则。

1 单一职责（Single Responsibility Principle）

这个原则顾名就可以思义，就是一个类应该只负责一个职责，术语叫：仅有一个引起其变化的原因。简单点说：一个类中应该是一组相关性很高的函数及数据的封装，个中含义请自行意会。看起来简单，但是做起来就难了，这可能是六大原则中最难以熟练掌握的一个原则了，它高度依赖程序员的自身素质及业务场景。

The single-responsibility principle (SRP) is a computer-programming principle that states that every module, class or function in a computer program should have responsibility over a single part of that program’s functionality, and it should encapsulate that part. All of that module, class or function’s services should be narrowly aligned with that responsibility.
Robert C. Martin, the originator of the term, expresses the principle as, “A class should have only one reason to change,”although, because of confusion around the word “reason” he also stated “This principle is about people.”.In some of his talks, he also argues that the principle is, in particular, about roles or actors. For example, while they might be the same person, the role of an accountant is different from a database administrator. Hence, each module should be responsible for each role

2 开闭原则（Open Close Principle）

开闭原则(Open-Closed Principle, OCP)：一个软件实体应当对扩展开放，对修改关闭。即软件实体应尽量在不修改原有代码的情况下进行扩展。

它是面向对象最重要的设计原则，由Bertrand Meyer（勃兰特.梅耶）在1988年出版的《面向对象软件构造》。中提出的。

In object-oriented programming, the open–closed principle states “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”; that is, such an entity can allow its behaviour to be extended without modifying its source code.
The name open–closed principle has been used in two ways. Both ways use generalizations (for instance, inheritance or delegate functions) to resolve the apparent dilemma, but the goals, techniques, and results are different.
Open–closed principle is one of the five SOLID principles of object-oriented design.

3 里氏替换原则（Liskov Substitution Principle）

里氏替换原则（Liskov Substitution Principle，LSP）由麻省理工学院计算机科学实验室的里斯科夫（Liskov）女士在 1987 年的“面向对象技术的高峰会议”（OOPSLA）上发表的一篇文章《数据抽象和层次》（Data Abstraction and Hierarchy）里提出来的，她提出：继承必须确保超类所拥有的性质在子类中仍然成立（Inheritance should ensure that any property proved about supertype objects also holds for subtype objects）。

里氏替换原则主要阐述了有关继承的一些原则，也就是什么时候应该使用继承，什么时候不应该使用继承，以及其中蕴含的原理。里氏替换原是继承复用的基础，它反映了基类与子类之间的关系，是对开闭原则的补充，是对实现抽象化的具体步骤的规范。

Subtype Requirement: Let {\displaystyle \phi (x)}\phi (x) be a property provable about objects {\displaystyle x}x of type T. Then {\displaystyle \phi (y)}{\displaystyle \phi (y)} should be true for objects {\displaystyle y}y of type S where S is a subtype of T.

Substitutability is a principle in object-oriented programming stating that, in a computer program, if S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., an object of type T may be substituted with any object of a subtype S) without altering any of the desirable properties of the program (correctness, task performed, etc.). More formally, the Liskov substitution principle (LSP) is a particular definition of a subtyping relation, called (strong) behavioral subtyping, that was initially introduced by Barbara Liskov in a 1988 conference keynote address titled Data abstraction and hierarchy. It is a semantic rather than merely syntactic relation, because it intends to guarantee semantic interoperability of types in a hierarchy, object types in particular. Barbara Liskov and Jeannette Wing described the principle succinctly in a 1994 paper as follows:

Subtype Requirement: Let {\displaystyle \phi (x)}\phi (x) be a property provable about objects {\displaystyle x}x of type T. Then {\displaystyle \phi (y)}{\displaystyle \phi (y)} should be true for objects {\displaystyle y}y of type S where S is a subtype of T.

In the same paper, Liskov and Wing detailed their notion of behavioral subtyping in an extension of Hoare logic, which bears a certain resemblance to Bertrand Meyer’s design by contract in that it considers the interaction of subtyping with preconditions, postconditions and invariants.

4.依赖倒置原则（Dependence Inversion Principle）

依赖倒转原则(Dependency Inversion Principle, DIP)：抽象不应该依赖于细节，细节应当依赖于抽象。换言之，要针对接口编程，而不是针对实现编程。

关键点：
1. 高层模块不应该依赖低层模块，两者都应该依赖其抽象
2. 抽象不应该依赖细节
3. 细节应该依赖抽象

High-level modules should not import anything from low-level modules. Both should depend on abstractions (e.g., interfaces).
Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.

5.接口隔离原则（Interface Segregation Principle）

目的就是控制接口的粒度。
接口隔离原则(Interface Segregation Principle, ISP)：使用多个专门的接口，而不使用单一的总接口，即客户端不应该依赖那些它不需要的接口。其实这个原则是很容易理解的，就是让调用者依赖的接口尽可能的小。

In the field of software engineering, the interface-segregation principle (ISP) states that no client should be forced to depend on methods it does not use. ISP splits interfaces that are very large into smaller and more specific ones so that clients will only have to know about the methods that are of interest to them. Such shrunken interfaces are also called role interfaces. ISP is intended to keep a system decoupled and thus easier to refactor, change, and redeploy. ISP is one of the five SOLID principles of object-oriented design, similar to the High Cohesion Principle of GRASP.

6. 迪米特原则（Law of Demeter 又名Least Knowledge Principle)

迪米特法则来自于1987年美国东北大学(Northeastern University)一个名为“Demeter”的研究项目，又称最少知识原则(LeastKnowledge Principle, LKP)，其定义为：一个软件实体应当尽可能少地与其他实体发生相互作用。

一个类应该对自己需要调用的类知道得最少，类的内部如何实现、如何复杂都与调用者或者依赖者没关系，调用者或者依赖者只需要知道他需要的方法即可，其他的我一概不关心。

The Law of Demeter (LoD) or principle of least knowledge is a design guideline for developing software, particularly object-oriented programs. In its general form, the LoD is a specific case of loose coupling. The guideline was proposed by Ian Holland at Northeastern University towards the end of 1987, and can be succinctly summarized in each of the following ways:

Each unit should have only limited knowledge about other units: only units “closely” related to the current unit.
Each unit should only talk to its friends; don’t talk to strangers.
Only talk to your immediate friends.
The fundamental notion is that a given object should assume as little as possible about the structure or properties of anything else (including its subcomponents), in accordance with the principle of “information hiding”. It may be viewed as a corollary to the principle of least privilege, which dictates that a module possess only the information and resources necessary for its legitimate purpose.

It is so named for its origin in the Demeter Project, an adaptive programming and aspect-oriented programming effort. The project was named in honor of Demeter, “distribution-mother” and the Greek goddess of agriculture, to signify a bottom-up philosophy of programming which is also embodied in the law itself.

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