Design Patterns -Session II

8/8/2019 Design Patterns -Session II

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OO Design Patterns

Session 2

Observer, Strategy, Decorator



Observer Pattern

Or« How do I keep clients aware of

changes, when I¶m not sure, or don¶t

care, who they are?



Observer Pattern ± High Level

Category : Behavioral

The Observer Pattern allows one or

more objects (Observer) to monitor another object (Subject).

A subset of the Pub-Sub Pattern

When the state of one object changes,dependants are notified



Observer Pattern

OO Principals

Promotes loose couplings

The Subject does not need to be aware of

Observers except at runtime

Favors composition over inheritance

Program to an interface



Observer Pattern

Basic Structure

Observalable Subject

1 or more Observers



Observer Pattern ± Observer

The Observer can register and

unregister itself with the Subject

While registered any events the Subjectnotifies on are received by all Observers

The Observer can be programmed to an

interface that specifies a way for the

Subject to update it.



Observer Pattern ± Observer

The Observer is programmedto an interface that specifiesa way it to interact with theSubject

public interface Observer {

public void update(Subject o );

}

-OR-

public interface Observer {

public void update(Subject o, Data a );

}



Observer Pattern ± Subject

The Subject maintains a list

of Observers

The subject provides

access to Observers to

add or removethemselves from a

distribution list

When the Subject needs to

notify, it simply updates all

currently registeredObservers



Observer Pattern ± SUN Impl

Java has a number of Observer Pattern

implementation

Swing General Purpose Observer




General Purpose Observer

Java comes with an out of the box

implementation of the Observer Pattern

The subject extends the java.util.Obser vable

class

Observers implement the java.util.Obser ver

interface

void update(Observable o, Object arg)




Some Things to Consider

As mentioned this implementation forces us

to extend Observable for the subject

Remember favor composition over inheritance

This is implicitly single-threaded

Synchronous

As opposed to Asynch Pub/Sub

Push/Pull



Observer Pattern

Example



Observer Pattern

Q&A



Decorator Pattern

Or« How do I dynamically add

additional behaviors to an existing class?



Decorator Pattern ± High Level

Category : Structural

The Decorator Pattern allows

responsibility and functionality to beadded dynamically at run-time

Provides a flexible alternative to sub-

classing to extend functionality



Decorator Pattern

OO Principals

Encapsulate what varies

Favor composition over inheritance Program to interfaces

Strive for loosely coupled designs between

objects

Open/closed pr inciple



Decorator Pattern

Open/closed pr inciple

Objects should be open for extension, but

closed for modification

Allows an objects behavior to be modified

without altering its source



Decorator Pattern

Basic Structure

Abstract Component Class

Concrete Components Abstract Decorator

Concrete Decorator



Decorator Pattern

Basic Structure


A common ancestor that all involved objects

extend



Decorator Pattern

Basic Structure

Concrete Components

The object to add dynamic behaviors to



Decorator Pattern

Basic Structure

Abstract Decorator Enforces the contract of the Abstract Component

Class

Allows for more decorator specific behaviors to be

defined



Decorator Pattern

Basic Structure

Concrete Decorator

Implements the specific behavior



Decorator Pattern

Real World Examples

java.io


InputStream

Concrete Components

FileInputStream, StringBufferInputStream«

Abstract Decorator

FilterInputStream

Concrete Decorator

BufferedInputStream, DataInputStream«



Decorator Pattern

Example problem and Solution



Decorator Pattern

Cons

Can Cause ³Class Explosion´

Each Decorator is a class

Code can be hard to find/debug



Strategy Pattern

Or«How do I apply algorithms to data

dynamically?



Strategy Pattern ± High Level

Category : Behavioral

Allows algorithms vary independently

from clients that use them



Strategy Pattern

OO Principals

Encapsulate what varies

Favor composition over inheritance Program to interfaces

Strive for loosely coupled designs between

objects

Open/closed principle



Strategy Pattern

Basic structure

Context

Strategy Interface Concrete Strategies



Strategy Pattern

Context

Is Configured with aConcrete Strategy

Initiates the work of the Strategy via theStrategy Interface

May provide andinterface for theStrategy to accessit Data



Strategy Pattern

Strategy Interface

Defines a way for

the Context to

interact with eachalgorithm



Strategy Pattern

Concrete Strategy

Encapsulates the

algorithm

Performs the actualwork



Strategy Pattern

Real World Examples

Custom Sort options for lists



Strategy Pattern

Example problem and Solution



Strategy Pattern

Q&A



Strategy Pattern

DONE

Design Patterns -Session II

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