Welcome to Design patterns

Welcome to Design PatternsWelcome to Design Patterns

2011.11.18

Design Patterns

Ngo Doan Lap

ContentsContentsI. Introduce to design pattern through example

1. Problem

2. Issues

3. Solutions

II. Design patterns

III. Design patterns classification

ProblemProblem1. Simulate Duck application

SimUDuck

1. show a large variety of duck species swimming

2. making quacking sounds

Initial designSupper class

(abstract class)

Inherited classes

(concrete classes)

1

2

3 4

ProblemProblem

1

2

1. Upgrade the system

Duck can fly

IssuesIssues

1

23

Solution is …inheritance?Solution is …inheritance?Override fly() method to do nothing

1

2

No fly, no quack

Disadvantage of inheritance?

3

1. Code is duplicated across subclasses

2. Changes can unintentionally affect others

3. Hard to gain knowledge of all object behaviors

4. Runtime behavior change are difficult

Difficult to maintain

Difficult to maintain

Solution is …interface?Solution is …interface?1. Flyable() interface.

2. Spec will keep changing

3. Override fly() and quack()

What happen if you need to make a little change to the flying

Behavior….in all of 100 the flying Duck subclasses?

1

2

31. Interface solves part of the problem

2. Completely destroys code reuse for those behaviors

Solution is …CHANGESolution is …CHANGENo matter how well you design an application, over time an

application must grow and change or it will die.

StableChange

1

2

3

4

5

SolutionSolution1 fly() and quack() are the parts of the

Duck class that vary across ducks

Separate change and stable

2

How to design Duck behaviors?

3

What we want:1. Instantiate a particular object and initialize it with specific type of flying behavior2. Can change flying behavior at runtime.

1

2

~ API

use an interface to represent each behavior

3

4

Any behaviors changed will not affected to Duck class and it’s child classes

5

SolutionSolution

Program to an interface vs Program to an implementationProgram to an interface vs Program to an implementation

Program to an implementation

Program to an interface

assign the concrete implementation object at runtime

Have demo as quick as possibleDon’t care about extend/upgrade/maintain

Take time to design but it is flexible

Implementing the Duck BehaviorsImplementing the Duck Behaviorstwo interfaces: FlyBehavior and QuackBehavior

1 2

Implementing the Duck BehaviorsImplementing the Duck Behaviors

1

2

Easy to reuse for other objects (not duck)

Easy to extend/modify that make no side effect

Integrating the Duck BehaviorIntegrating the Duck BehaviorAdd two instance variables to the Duck class1

2 Implement performQuack()

don’t care what kind of object it is.Care abouthow to quack()!

flyBehavior and quackBehavior instance variables are set

Integrating the Duck BehaviorIntegrating the Duck Behavior3

3.1

3.2

3.3

TestingTesting

TestingTesting

TestingTesting

TestingTesting

Setting behavior dynamicallySetting behavior dynamically

TestingTesting

The Big PictureThe Big Picture

1

2

3

4

Strategy for upgrading

has a

has a

is a

(*)Change behavior at runtime

5

“better than”(*)

(*)

Strategy PatternStrategy Pattern

Example 2Example 2

Example 3Example 3

Design patternsDesign patterns

1. General reusable solution to a commonly occurring problem

within a given context in software design

2. Type of patterns

Creational patterns

Structural patterns

Behavioral patterns

Design patterns classificationDesign patterns classification

Creational design patterns

This design patterns is all about class instantiation

Design patterns classificationDesign patterns classification

Structural design patterns

This design patterns is all about Class and Object composition

Design patterns classificationDesign patterns classificationBehavioral

design patterns:

Class's objects

communication

How to use design patternsHow to use design patterns

Load to your brain

1

2

Recognize places to apply

QAQA

Investigate more about design patterns?

Thank you for your attention!