Design Patterns. Design Patterns Fundamental Design Patterns Creational Design Patterns Partitioning Patterns Behavioral Patterns Concurrency Patterns.

Design Patterns                

Design Patterns

Fundamental Design Patterns

Creational Design Patterns

Partitioning Patterns

Behavioral Patterns

Concurrency Patterns

Delegation **

Interface

Marker Interface

Immutable

Proxy

Fundamental Design Patterns (FDP)

FDP - Delegation

Synopsis:

Delegation is a fundamental method to extend and reuse a classes functionality ( behavior or methods)

Context:

Allows instances of a class to play multiple roles.

Solution:

Reuse and extend behavior using delegation

FDP - Delegation

class x

public void methodforx () { }

class y

x _x = new x();

……

_x.methodforx ( ) DELEGATION

FDP - Delegation - Example

Delegator Delegateeuses 1

1

InstanceClass or Instance

FDP - Delegation - Example

FlightSegment LuggageCompartment1.1 checkluggage() 1

1

1. checkluggage()

Instance

FDP - Delegation - Example

// Instance of this class represent a flight segment.

class FlightSegment {

LuggageCompartment luggage;

/**

* Check a piece of luggage

* @param piece The piece of luggage to be checked.

* @exception LuggageException if piece cannot be checked.

*/

void checkLuggage(Luggage piece) throws LuggageException {

luggage.checkLuggage(piece);

} // checkLuggage(Luggage)

} // class FlightSegment

DELEGATION

Delegation **

Interface - Allows classes with some similar data to use polymorphism to execute behavior.

Marker Interface - used for utilities – allows investigation into class information without knowing they are an instance of a particular class.

Immutable – forbids any of an object’s state information to change after the object is created.

Proxy - forces method calls to an object indirectly.

Fundamental Design Patterns (FDP)

Creational Design Patterns

Factory Method **

Abstract Factory

Builder

Prototype

Singleton

Object Pool **

CDP - Factory Method

Synopsis:

Need for a class to reuse with arbitrary data types. Reusable class remains independent of the classes it instantiates by delegating the choice of which class to instantiate to another object and referring to the newly created object through a common interface.

Context:

Creates a framework to support instantiations of various data types.

CDP - Factory Method

Solution:

Proxy object and the service providing object must either be instances of a common super class or implement a common interface.

CDP - Factory Method - Example

You have an application such as MS Office.

You want to perform some common functions with all the files.

Example of common functions might be open file, save file, etc.

The only difference is that the functions are done on different file types such as word doc files, excel xls files and PowerPoint ppt files.

You could write several independent functions for each of the different types but the code would be very similar with only the data type as a difference.

Factory method allows you to build a framework for common functions with only a few classes that reuse methods for each type.

Manage Files

file commands

ManageDoc Files

doc file commands

ManageXls Files

xls file commands

ManagePpt Files

ppt file commands

Make one Function

CDP - Factory Method - Example

CDP - Factory Method - Example

Document Applicationedits 1

*

MyDocument May be doc, xls, or ppt.

Write to Socket

stream

CDP - Factory Method - Example

Socket

But you also wish to have strings in which you encrypt the data.And you write an encrypted DataStream and read back an encrypted Data Stream decrypt it.

string

Suppose you have a process which reads and writes a DataStream to a socket.

Read from Socket

stream Socketstring

Write to Encrypted

Socket

encrypted stream

CDP - Factory Method - Example

EncryptedSocketencrypted string

Encrypt Data

stringWrite to

EncryptedSocket

encrypted stream

EncryptedSocketencrypted string

Read from Socket

encryptedstream

Socketencrypted stringDecrypt

Data

decrypted string

But now you realize that there are several different encryption algorithms and codes you wish to use.

CDP - Factory Method - Example

Encrypt Data

string Write to Encrypted

Socket

encrypted stream

EncryptedSocketencrypted String #1

algorithm # 1

The process to encrypt differs in using many different different algorithms (function/method) and type of string output must be written for each type.

Encrypt Data

string Write to Encrypted

Socket

encrypted stream

EncryptedSocketencrypted String #2

algorithm # 2

Encrypt Data

string Write to Encrypted

Socket

encrypted stream

EncryptedSocketencrypted String #n

algorithm # n

CDP - Factory Method - Example

Encrypt Data

string Write to Encrypted

Socket

encrypted stream

EncryptedSocketencrypted String

algorithm # 1

algorithm # 2

algorithm # n

The factory pattern allows you to have a framework that will handle any type of algorithm and encrypted data.

concrete product

CDP - Factory Method - Example

Encryption

Socket

encrypt, decrypt 1

*

DESEncryption

EncryptedSocket

Transcription

EncryptionFactory

EncryptionFactoryIF

creates

requestCreation

creates1

1

*

*

*

1

Concrete Product

Product

Factory

Socket

EncryptedSocket

Interface

Creation Requester

1

2

3

4

5

6

CDP - Object PoolSynopsis:

Manages reuse of objects when a type of object is expensive to create or only limited number needed.

Context:

You wish to limit access to a resource.

Solution:

Create a reusable class to collaborate with other objects for a limited amount of time.

Create a reusable pool to manage reusable objects for use by client objects.

Allow DatabaseAccess

database access

Suppose you have a database systems that need to allow only a limited number of accesses to the database at one time.

CDP - Object Pool

database access requested

You must write a counting semaphore to protect this resource from having more than the limited access.

CDP - Object Pool

Clientmanage objects

ReusablePool

uses

Reusable

Reusable Pool

Partitioning Patterns

Layered Initialization **

Filter

Composite

PP - Layered Initialization

Synopsis:

You need multiple implementations with common logic in super and specialized in subs.

However the common logic decides which specialized subclass to create.

Therefore layered initialization encapsulates common and specialized logic to create the multiple implementations.

PP - Layered Initialization

Context:

You have a piece of logic that requires partial execution prior to determining which subclass methods might be used.

You need to layer the initializations of the objects to process the complex logic or complex data.

PP - Layered Initialization

Forces:

A specialized class must be chosen to process complex data.

Constructor of the specialized classes and their sub classes are invoked after it has been decided which specialized class to instanciate.

Solution:

Essence of this pattern is to layer the initializations of the objects participating in the pattern.

PP - Layered Initialization

1. Objects that performs logic common to all cases is initialized

2. Initialization concludes by determining the class to instantiate

3. Specialized class constructor performs next layer of initialization logic.

4. After all initialization, one top-level object exist for logic

5. If method needs more specialized logic, it calls method one layer down

Consequences:

Complexity of initialization of objects using data requires analysis before initialization can proceed.

ResolveBusiness

Rules

query request

PP - Layered Initialization

trigger to resolve business rule

Suppose you have a business rule engine which must select a typeof database on which to query to resolve issues in the rule base.

SelectDatabase

Perform OracleQuery

Perform DB2

Query

Perform n

Query

Oraclequery

DB2query

nquery

You cannot perform query until you know what type of database

ResolveBusiness

Rules

query request

PP - Layered Initialization

trigger to resolve business rule

You need to initialize the database prior to query.

SelectDatabase

Perform OracleQuery

Perform DB2

Query

Perform n

Query

Oraclequery

DB2query

nquery

InitializeDatabase

PP - Layered Initialization

DataQueryFactoryIF DataQueryrequest creation

DataQueryImplFactory

OracleQuery DB2Query …..

DataQueryImplIF

creates

uses

Data Query factory method object appears like this.

ServiceImpFactoryIF

ServiceImpFactory

Service

ServiceImpIIF

Behavioral Patterns

Chain of Responsibility State

Command Null Object

Little Language Strategy **

Mediator Template Method

Snapshot Visitor

Observer **

Write to Socket

stream Socket

But you also wish to have strings in which you encrypt the data.And you write an encrypted DataStream and read back an encrypted Data Stream decrypt it.

string

REDO Suppose you have a process which reads and writes a DataStream to a socket.

Read from Socket

stream Socketstring

BP - Observer

Document Applicationedits 1

*

MyDocument May be doc, xls, or ppt.

BP - Observer

Concurrency Patterns

Single Threaded Execution

Guarded Suspension

Balking

Scheduler

Read/Write Lock

Producer-Consumer

Two-Phase Termination