Object Oriented Design Principles 面向对象设计原则

原文地址：http://www.surfscranton.com/Architecture/ObjectOrientedDesignPrinciples.htm

 

Object Oriented Design Principles

Jim's Pages => Java Pages => Object Oriented Design Principles

The following content was extracted from a Blog Entry by Robert C. Martin. It provides an index to articles about eleven Object Oriented Design Principles. The  "PPP" book (Agile Software Development: Principles, Patterns and Practices) is highly recommended!

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In March of 1995, in comp.object, I wrote an article that was the first glimmer of a set of principles for OOD that I have written about many times since. You'll see them documented in my PPP book, and in many articles on the objectmentor website, including a well known summary.

These principles expose the dependency management aspects of OOD as opposed to the conceptualization and modeling aspects. This is not to say that OO is a poor tool for conceptualization of the problem space, or that it is not a good venue for creating models. Certainly many people get value out of these aspects of OO. The principles, however, focus very tightly on dependency management.

Dependency Management is an issue that most of us have faced. Whenever we bring up on our screens a nasty batch of tangled legacy code, we are experiencing the results of poor dependency management. Poor dependency management leads to code that is hard to change, fragile, and non-reusable. Indeed, I talk about several different design smells in the PPP book, all relating to dependency management. On the other hand, when dependencies are well managed, the code remains flexible, robust, and reusable. So dependency management, and therefore these principles, are at the foundation of the -ilities that software developers desire.

The first five principles are principles of class design. They are:

SRPThe Single Responsibility PrincipleA class should have one, and only one, reason to change.OCPThe Open Closed PrincipleYou should be able to extend a classes behavior, without modifying it.LSPThe Liskov Substitution PrincipleDerived classes must be substitutable for their base classes.DIPThe Dependency Inversion PrincipleDepend on abstractions, not on concretions.ISPThe Interface Segregation PrincipleMake fine grained interfaces that are client specific.

The next six principles are about packages. In this context a package is a binary deliverable like a .jar file, or a dll as opposed to a namespace like a java package or a C++ namespace.

The first three package principles are about package cohesion, they tell us what to put inside packages:

REPThe Release Reuse Equivalency PrincipleThe granule of reuse is the granule of release.CCPThe Common Closure PrincipleClasses that change together are packaged together.CRPThe Common Reuse PrincipleClasses that are used together are packaged together.

The last three principles are about the couplings between packages, and talk about metrics that evaluate the package structure of a system.

ADPThe Acyclic Dependencies PrincipleThe dependency graph of packages must have no cycles.SDPThe Stable Dependencies PrincipleDepend in the direction of stability.SAPThe Stable Abstractions PrincipleAbstractness increases with stability.