Design Patterns: Singleton, Structural (Adapter), and Their Implementations, Study notes of Software Engineering

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Singleton, Structural, and

Adapter Patterns

By Anton Roukhline, John Setzer, and

Jingfeng Yu

Information taken from Design Patterns

Overview

n Singleton

n Summary of Structural Patterns

n Adapter

Singleton Creational Pattern

n Intent: To ensure that a class has

exactly one instance and provide global

access to that class.

n Pages 127-134 in Design Patterns

Printer Example

Printer 1 Printer 2 Printer 3 Printer 4

Print Spooler

A system may have many printers, but

it should should only have one printer

spooler.

Singleton’s Code

public class Singleton { private static Singleton instance; private Singleton() {} public static Singleton getInstance() { if (instance ==null) { instance = new Singleton(); } return instance; }

Singleton Benefits

1. Controlled access to sole instance

2. Reduced name space

3. Easy refinement of operations and

representation

4. Allows a variable number of instances

5. More flexible that static methods that

do the same thing

Discuss with a Partner

n Try to name as many programs that

you have used or written that could

benefit from implementing a Singleton

structure.

n For Example: “It might be good for a

program X to use the Singleton pattern

because…”

Structural Patterns

n “Structural patterns are concerned with

how classes and objects are composed

to form larger structures.”

n “Structural class patterns use

inheritance to compose interfaces or

implementations.”

Solution

Adaptee Adapter Target

Adapter

n Converts the interface of one class into

a form that a second class understands.

n Lets two classes with previously

incompatible interfaces work together.

n Pages 139-150 in Design Patterns

Discuss with a Partner

n Why would a developer want to be

able to make two interfaces

compatible?

Adapter That Uses Object

Composition

Code

class Adapter extends Quadrilateral {

public Adapter() {} // draw is an abstract method in class Quadrilateral public draw ( point1, point2, point3, point4) { bottomLeftCorner = point1; sideLength = point2.x – point1.x; Square.draw(bottomLeftCorner, sideLength); }

Example: Make SQL Easier to

Use

public query(table, column, target) { … // initialize table connection here or preferably in a constructor … resultSet = Statement.executeQuery( “SELECT ” + target + “ FROM ” + table+ “ WHERE “ + column+ ” = “ +target+ ”;” ); … list = resultSet.toList(); return list; }

Benefits

n Makes old code more reusable.

n Lets you override adaptee’s behavior.

Consequences

n Adapter will have to be changed when

adaptee’s interface is changed.

n Makes it harder to override adaptee.