Software Engineering for Students A Programming Approach

162 Chapter 12 ■ Design patterns
Some people like to distinguish types of patterns:
■ architectural patterns – the highest level structures, applicable to large numbers of
classes
■ design patterns – patterns applicable at the level of small groups of classes
■ coding patterns – applicable to detailed algorithms within methods
■ anti-patterns – the opposites of patterns.
You know you understand patterns when you can see how to use two or more in
your project. Many software systems use two or sometimes more patterns.
Summary
Patterns embody the experience of developers. They document useful architectural
structures (Some patterns also record techniques for activities such as testing.) A
pattern has a name, an application domain and a suggestion for a solution. Patterns
enable novices to use the knowledge of experts. They provide a body of knowledge
and a language for talking about structure. Patterns are documented in catalogs
(see the reading list below) for browsing prior to and during architectural software
design.
This chapter explains how to use some useful design patterns (and some antipatterns):
■ Inheritance – extending an existing class
■ Delegation – using other classes to perform useful tasks
■ Singleton – a class with only one instance
■ Factory Method – creating the appropriate class at run time
■ Façade – creating a front end to make using a collection of classes easier
■ Immutable – an object that does not change
■ Model-View-Controller – separating input, output and logic
■ Mediator – encapsulating the interaction between classes
■ Pipe and Filter – components that process a serial stream of data, passing it
from one to another
■ Proxy – a component that accepts requests on behalf of other components
■ Layers – separating large scale architecture into levels
■ Blob – an anti-pattern, a single complex class.