LECTURE 26:ADAPTER PATTERN

Computer Science 313 – Advanced Programming Topics

Real-World Problems

Have one thing, but need something different Try & make it work despite not being

correct But this often leads to problems

Outlets Around the World

2 standards for electric transmission Japan & the Americas use 110 volt 220 volts used by Europe, (rest of) Asia,

& Africa Variety of incompatible plugs defined

by each

Play With Electricity for Fun! Do not want new appliance for each

country First thought: jam single plug into

wall Plug interface only difference between

them

Play With Electricity for Fun! Do not want new appliance for each

country First thought: jam single plug into

wall Plug interface only difference between

them

Play With Electricity for Fun! Do not want new appliance for each

country First thought: jam single plug into

wall Plug interface only difference between

them Generally speaking, this is not a good

idea Use plug adapter as an alternate

approach Plug then good to use in existing outlets

now

Adapter Pattern Workings

Like plug adapters, but used with existing code Starts with some existing plug (instance) Need it to match a preexisting outlet

(interface) Do not want big changes; functionality

stays same Change what is exposed by wrapping plug

Adapter Pattern Intent

Makes existing class use client's interface Otherwise compatible classes now work

together Work is invisible to client code

Goal of pattern is client code is not modified

No changes to functionality with this pattern If we want real changes use DECORATOR

PATTERN Does not need to expose full

functionality Have not discussed what plug attached

to!

Adapter Pattern Example

Already wrote class sorting an arraypublic interface Comparable<T> {public int compareTo(T other);

}

public class QuickSort {static void <T extends Comparable<T>> sort(T[] array) { // Sorts this array}

}

Points on a Plane

Must sort, but Dimensions not Comparable Rewrite QuickSort to use Dimensions Dimension closed to modification &

inheritance Do not want to rerun tests on Dimension,

anyway Adapter does this without violating laziness

principle Two ways to do this with ADAPTER

PATTERN Hide object, via Object adapter using

composition Class adapter uses inheritance, but reuse

limited

Object Adapter

class ComparableDimension implements Comparable<ComparableDimension> {private Dimension d; // Object being adapted

public ComparableDimension(Dimension newD) { d = newD;}

public int compareTo(ComparableDimension o) { Integer myArea = d.height * d.width; Integer oArea = o.d.height * o.d.width; return myArea.compareTo(oArea);}

}

Class Adapter

class ComparableDimension extends Dimensionimplements Comparable<ComparableDimension> {

public ComparableDimension(int h, int w) { super(h, w);}

public int compareTo(ComparableDimension o) { Integer myArea = d.height * d.width; Integer oArea = o.d.height * o.d.width; return myArea.compareTo(oArea);}

}

How Example Works

Implementing ComparableDimension either way Is-a or Has-a Dimension to compare Comparable interface is implemented Can be used by QuickSort class

Class Adapter UML Diagram

Adapter extends Target & is-a Adaptee Not very easy in Java

Client, Target & Adaptee already exist Only Adapter created or edited to do

this

Class Adapter UML Diagram

Adapter extends Target & is-a Adaptee Not very easy in Java (or most sane

languages) Client, Target & Adaptee already

exist Only Adapter created or edited to do

this

Object Adapter UML Diagram Adapter extends Target & has-a Adaptee No multiple inheritance, so legal in Java

Client, Target & Adaptee already exist Only Adapter created or edited to do this

Object Adapter UML Diagram Adapter extends Target & has-a Adaptee No multiple inheritance, so legal in Java

Client, Target & Adaptee already exist Only Adapter created or edited to do this

For Next Class

Lab #5 due on Friday before next lab starts Get it working while you can still earn

credit Will discuss Façade pattern on

Wednesday How does this differ from Adapter? What is important when discussing

patterns? What is the most “Hollywood” of

patterns?