Design Patterns

Design Patterns
An Introduction for Beginning Programmers

Prof. David Bernstein
James Madison University

Computer Science Department

Software Design Patterns

Defined:
- "Descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context." (Gamma, Helm, Johnson and Vlissides)
Level of Abstraction:
- Design patterns are at a higher level of abstraction than data structures and algorithms

Software Design Patterns (cont.)

Types:
- Creational (i.e. concerned with object creation)
- Structural (i.e., concerned with the composition of objects)
- Behavioral (i.e., concerned with interactions and responsibilities)
Benefits:
- Simplifies the development of designs
- Promotes better communication
- May increase re-use and quality

The Purpose(s) of this Presentation

Convince you that you have something to learn
Convince you that design patterns are useful
Discuss some design patterns (that even beginning programmers can understand and use)

Singleton Pattern - Motivation

Some applications require exactly one instance of a class:
- Windowing systems with one event queue
- Word processors with one menu bar (for all documents)
Unfortunately, constructors can be called repeatedly:
- In one method
- In one class
- Across multiple classes

Singleton Pattern - The Traditional Approach

Singleton Pattern - An Example

A FileViewer javaexamples/singleton/v1/FileViewer.java (Fragment: skeleton)

javaexamples/singleton/v1/FileViewer.java (Fragment: createInstance)

Singleton Pattern - An Example (cont.)

Using a FileViewer javaexamples/singleton/v1/FileChooser.java (Fragment: valueChanged)

Iterator - Motivation

Computers vs. Calculators:
- Computers can perform the same operation many times
Programming Languages:
- Harness this power using loops
A Problem:
- Traditional looping requires an understanding of the structure of the "aggregate"

Iterator - Motivation (cont.)

Looping Over an Array

javaexamples/iterator/ArrayExample.java (Fragment: 1)

Looping Over a Vector

javaexamples/iterator/VectorExample.java (Fragment: 1)

Iterator - The Traditional Approach

In UML:

Iterator - Examples in Java

The Iterator:
- Enumeration
Some Aggregates:
- Vector
- Hashtable

Model-View-Controller - Motivation

Many Objects:
- Manage information/content
- Present/display that information
- React to events
Examples:
- Components/widgets in a GUI
- Database management systems

M-V-C - The Traditional Approach

M-V-C - An Example

M-V-C - An Example: (cont.)

Handling an arrowkeyPressed Event

M-V-C - An Example: (cont.)

Handling a mouseUp Event

M-V-C - An Example: (cont.)

Handling a characterKeyPressed Event

images/MVCTextField_characterkeyPressed.gif

M-V-C - Concrete Examples

Creating an Integer Entry Field

javaexamples/mvc/IntegerDocument.java

javaexamples/mvc/IntegerEntryField.java

Creating a Password Entry Field

What would you change and why?

Skins

What would you change and why?

Other Software Design Patterns

Some Examples:
- Command Pattern
- Composite Pattern
- Decorator Pattern
- Facade Pattern
- Observer Pattern
- Proxy Pattern
Prerequesites - You Must Understand:
- Specialization and Inheritance
- Interfaces
- Abstract Classes

Decorator - Motivation

Adding Capabilities to a Class:
- Specialization
Adding Capabilities to an Object:
- Needs to be done "dynamically" (i.e., at run time)

Decorator - The Traditional Approach

Decorator - A Generic Application

images/decorator_pattern_sequence-diagram.gif

Decorator - An Example

images/decorator_pattern_printer-example.gif

Decorator - An Example (cont.)

javaexamples/decorator/Printer.java

javaexamples/decorator/ConsolePrinter.java

javaexamples/decorator/PrinterDecorator.java

javaexamples/decorator/UppercasePrinter.java

javaexamples/decorator/WrappingPrinter.java

javaexamples/decorator/Driver.java

There's Always More to Learn