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 D ECORATOR PATTERN  Does not need to expose full functionality  Have not discussed what plug attached to!

Adapter Pattern Example  Already wrote class sorting an array public interface Comparable { public int compareTo(T other); } public class QuickSort { static void > sort(T[] array) { // Sorts this array } }

Points on a Plane  Must sort, but Dimension s not Comparable  Rewrite QuickSort to use Dimension s  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 A DAPTER PATTERN  Hide object, via Object adapter using composition  Class adapter uses inheritance, but reuse limited

Object Adapter class ComparableDimension implements Comparable { 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 Dimension implements Comparable { 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 is-a  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 is-a  Adapter extends Target & is-a Adaptee  Not very easy in Java  Client, Target & Adaptee already exist  Only Adapter created or edited to do this

Object Adapter UML Diagram has-a  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 has-a  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 #6 available on Angel  Assignment due before we go on break  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?