SE450: Principles and Patterns [2/16] Previous pageContentsNext page

Read the first/second link for next week.


Summary of the Principles of OOD

(By Uncle Bob)

How I explained OOD

(Someone attempts to explain these principles. Please ignore the sexism!)

Book Length version

(Buy here) (Available free on Safari)

Individual article-length versions

(Partially copied below)

How I explained Design Patterns

(Please ignore the sexism!)

Holub Design Patterns Summary

(A great two page summary of each pattern)


(A short summary of each pattern, with non-code examples)

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 managment 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 foudation of the -ilities that software developers desire.

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

SRP The Single Responsibility Principle A class should have one, and only one, reason to change.
OCP The Open Closed Principle You should be able to extend a classes behavior, without modifying it.
LSP The Liskov Substitution Principle Derived classes must be substitutable for their base classes.
ISP The Interface Segregation Principle Make fine grained interfaces that are client specific.
DIP The Dependency Inversion Principle Depend on abstractions, not on concretions.

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:

REP The Release Reuse Equivalency Principle The granule of reuse is the granule of release.
CCP The Common Closure Principle Classes that change together are packaged together.
CRP The Common Reuse Principle Classes 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.

ADP The Acyclic Dependencies Principle The dependency graph of packages must have no cycles.
SDP The Stable Dependencies Principle Depend in the direction of stability.
SAP The Stable Abstractions Principle Abstractness increases with stability.

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