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Design Patterns

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What is a Design Pattern?• "a general reusable solution to a commonly occurring problem in software design" (Wikipedia)

• "descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context" (Gamma et al.)

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Examples from Other Fields

• Architecture: You can build many houses from the same floor plan without having any that are identical.

• Creative Writing: – Macbeth and Hamlet are both variations of

the "tragically flawed hero" story– Genre fiction (e.g. romances) are based on

a common sequence of plot elements.

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Simple Programming Examples

• Console menu: programs which are based on a loop which waits for a user to select from a limited number of choices and acts on that selection

• Maintaining a collection:

• Two-person game:

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Elements of a design pattern• Pattern name• Problem for which the pattern provides

a solution• Solution - elements that make up the

design– relationships– responsibilities– collaborations

• Consequences

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Pattern Categories• Creational

– Factory Method– Singleton

• Structural– Composite– Decorator

• Behavioral– Iterator– Strategy– Observer

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Iterator

• Problem: a client class need access to individual elements of an aggregate object

• Solution: Define an iterator class that steps through the elements one at a time and remembers where it is in the sequence.

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Factory Method

• Allows instantiation of an object to be deferred to a subclass– iterator method of a collection class is a

factory method

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Components and Containers

• Components are grouped together into containers

• Containers can also be added to other containers– Containers need to be components as well

• Composite pattern provides a solution

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Composite

• Combines several objects into an object with the same behavior as the parts

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Layout Managers

• Consider the problem of organizing components within a container– Different layout managers use different

rules for how to organize the components

• This is an example of the strategy pattern

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Strategy

• Define a family of algorithms which are interchangeable

• Uses– Allows one class to have different

behaviors– Allow the use of variants of an algorithm– Allow algorithms to hide data that client

doesn't need to see

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Decorator• Enhances the behavior of a single

component– e.g. scroll bars

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Singleton

• Used to insure that a class only has one instance

• The one object should be globally accessible

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Command

• Encapsulate a request as an object

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Frameworks

• Framework is a set of cooperating classes that implement mechanisms essential to a particular problem domain

• Frameworks can be customized to a particular application within the domain

• Java has a collections framework

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Model-View-Controller Architecture

• Consider a program that shows you two different views of the same data

• MVC architecture has three parts– Model - the data, not visible– View - a visual representation of the data– Controller - Processes user interaction

• Observer pattern is used for the view(s)

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Observer Pattern

• Define a one-to-many dependency between objects

• Changes in the one object are reflected in all the dependent objects

• Used to maintain consistency between objects

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Visitor Design Pattern• This is a common way of handling the

process of performing an action on each element of a collection

• The Visitor interface has a single method– visit( E element)

• The collection class can have traversal methods that take a Visitor as a parameter

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Adaptor

• You have a class that has all the functionality required by a client but doesn't match the required interface

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Antipatterns

• Designs that should be avoided– Blob - a class with many unrelated

responsibilities– Poltergeist - creates short-lived objects

with no significant responsibilities

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References

• Design Patterns: Elements of reusable Object-Oriented Software by Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides

• Object-Oriented Design and Patterns by Cay Horstman

• http://hillside.net/patterns