Design Patterns Introduction

What is a Design Pattern?

• A technique to repeat designer success.

​

• Borrowed from Civil and Electrical Engineering domains.

• A (Problem, Solution) pair.

How Patterns are used?

​

• Design Problem.
• Solution.
• Implementation details.

Reduce gap

Design

Implementation

Designer

Programmer

Gamma, E., Helm, R., Johnson, R., Vlissides, J.: Design patterns: elements of reusable object-oriented software. 1995.

​

Buschmann, F., Meunier, R., Rohnert, H., Sommerlad, P., Stal, M.: Pattern-oriented software architecture: a system of patterns. 2002.

Design patterns you have already seen

• Encapsulation (Data Hiding)
• Subclassing (Inheritance)
• Iteration
• Exceptions

​

Encapsulation pattern

• Problem: Exposed fields are directly manipulated from outside, leading to undesirable dependences that prevent changing the implementation.

​

• Solution: Hide some components, permitting only stylized access to the object.

Subclassing pattern

• Problem: Similar abstractions have similar members (fields and methods). Repeating these is tedious, error-prone, and a maintenance headache.

​

• Solution: Inherit default members from a superclass; select the correct implementation via run-time dispatching.

Iteration pattern

• Problem: Clients that wish to access all members of a collection must perform a specialized traversal for each data structure.

​

• Solution: Implementations perform traversals. The results are communicated to clients via a standard interface.

Exception pattern

• Problem: Code is cluttered with error-handling code.

​

• Solution: Errors occurring in one part of the code should often be handled elsewhere. Use language structures for throwing and catching exceptions.

Derived Conclusion

• Patterns are Programming language features.

• Programming languages are moving towards Design.

​

• Many patterns are being implemented in programming languages.

Pattern Categories

• Creational Patterns concern the process of object creation.

​

• Structural Patterns concern with integration and composition of classes and objects.

​

• Behavioral Patterns concern with class or object communication.

What is the addressing Quality Attribute?

• Modifiability, Exchangeability, Reusability, Extensibility, Maintainability.

What properties these patterns provide?

• More general code for better Reusability.

​

• Redundant code elimination for better Maintainability.

Design pattern example

Some examples of Design Patterns

Template for discussion

• Non-software example.
• Pattern intent.
• Software counterpart example.

Non-software examples are derived from paper

& PPT – “Non-software examples of software

design patterns” (OOPSLA 97).

Facade (Non software example)

Provide a unified

interface to a set

of interfaces in a

subsystem.

Facade (Software counterpart)

Flyweight (Non software example)

Use sharing to

support large

numbers of fine-

grained objects

efficiently

Flyweight (Software counterpart)

Memory

Chain of Responsibility (Non software example)

Chain the

receiving objects

and pass the

request along the

chain until an

object handles it.

Chain of Responsibility (Software counterpart)

Login page

Password checking

Balance statement

Internet

Memento (Non software example)

Externalize

object’s state so

that object can be

restored to this

state later.

Memento (Software counterpart)

Observer (Non software example)

When an object

changes its state,

all its dependants

are notified.

Observer (Software counterpart)

Internet

Cricinfo Server

Adapter (Non software example)

Convert the

interface of a

class into another

Interface clients

expect.

Adapter (Software counterpart)

Java module

C++ module

Java Native

Interface (JNI)

Builder (Non software example)

Separate the construction

process of a complex object

from its representation so

that the same construction

Process can create different

representations.

Builder (Software counterpart)

Lexical

analysis

Syntax

analysis

Semantic

analysis

Intermediate code

Interpretation

Compiler process

Java Lexer

Java Parser

Semantic

analysis

Java byte code

JVM

Java Compiler

Python Lexer

Python Parser

Semantic

analysis

Python byte code

PVM

Python Compiler

Iterator (Non software example)

Provide a way to

access the

elements of a set

sequentially.

Iterator (Software counterpart)

Interpreter (Non software example)

Interpreter interprets

the sentences in a

language based

on its grammar.

Interpreter (Software counterpart)

:-) is interpreted as ☺

​

:-( is interpreted as ☹

In Gtalk/Yahoo messengers

Broker (Non software example)

Broker component is

responsible for

coordinating

communication

between clients and

remote servers.

Broker (Software counterpart)

Publisher-Subscriber (Non software example)

Publishers register

themselves to a

broker and

subscribers discover

publisher from broker.

Publisher-Subscriber (Software counterpart)

Forums (e.g. movie forums)

Proxy (Non software example)

Provide a surrogate or

placeholder for

another object to

control access to it.

Proxy (Software counterpart)

Strategy (Non software example)

A Strategy defines a set of algorithms that can be

used interchangeably.

Strategy (Software counterpart)

Multiple

interchangeable

weapons available to

attack an enemy.

Mediator (Non software example)

Loose coupling

between colleague

objects is achieved by

having colleagues

communicate with the

Mediator, rather than

one another.

Mediator (Software counterpart)

Gtalk Server

Master-Slave (Non software example)

Master component

distributes work to

identical slave

components and

computes a final

result from the results

when the slaves

return.

Master-Slave (Software counterpart)

Movie players

High-resolution Game players

Graphics partitioning

Layers (Software example)

Layers structure applications whose dominant

characteristic is a mix of low- and high- level issues.

MVC (Software example)

MVC structures interactive applications.

Microkernel (Software example)

Microkernel separates the variant and non-variant

parts of a product-line application.

Reflection (Software example)

Reflection pattern provides a mechanism for changing

structure and behavior of a system dynamically.

State (Software example)

An object alters its

behavior when its

internal state

changes.

Template Method (Software example)

Define the skeleton of an

algorithm in an operation,

deferring some steps to

subclasses.

Event handling in Java GUI

components is unimplemented.

Observer example

*

52