Bruce Eckel, Thinking in Patterns with Java, cf. José Valente de Oliveira 10-1

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1 The desig patter Template Method Erich Gamma, Richard Helm, Ralph Johso, Joh Vlissides, Desig Patters Elemets of Reusable Object-Orieted Software, Addiso-Wesley, 1995, AKA GoF Bruce Eckel, Thikig i Patters with Java, cf. jvo@ualg.pt José Valete de Oliveira 10-1 Overview The otio of Patter The desig patter TemplateMethod A case study: the multiple versios of quicksort jvo@ualg.pt José Valete de Oliveira

2 A case study We wat to implemet several versios of quicksort reusig code as much as possible. José Valete de Oliveira 10-3 Recallig quicksort José Valete de Oliveira

3 Prelimiary questios: How should our cliet code looks like? Is this a commo type of problem? jvo@ualg.pt José Valete de Oliveira 10-5 Re: How should our cliet code looks like? Commad lie > Java Mai <Eter> 1 public class Mai { 3 public static void mai(strig[] args) { 8 Iteger[] a = ew Iteger[args.legth]; for (it i=0; i<args.legth; i++) a[i] = Iteger.parseIt(args[i]); ProbQuickSort pqs = ew ProbQuickSort(); pqs.quicksort(a); for(it i=0; i< a.legth; i++ ) System.out.pritl (a[i]); jvo@ualg.pt José Valete de Oliveira

4 Re: Is this a commo type of problem? Quicksort basics: 1. Let a elemet be the pivot 2. Rearrage the remaiig elemets i two groups: The group with elemets (whose keys are) less or equal to the pivot (key) The group with elemets (whose keys are) great or equal to the pivot (key) void quicksort(it left, it right) { 3. Apply the above method to both groups if (right <= left) retur; it i = partitio(left, right); quicksort(left, i-1); quicksort(i+1, right); jvo@ualg.pt 10-7 Is this a commo type of problem? Patters: a first otio Erich Gamma, Richard Helm, Ralph Johso, Joh Vlissides, Desig Patter Elemets of Reusable Object-Orieted Software, Addiso-Wesley, 1995 jvo@ualg.pt José Valete de Oliveira 3-8 4

5 Patter Each patter describes a problem which occurs over ad over agai i our eviromet, ad the describes the core of the solutio to that problem, i such a way that you ca use this solutio a millio times over, without ever doig it the same way twice, (Christopher Alexader et al, A Patter Laguage: Tows, Buildigs, Costructio, 1977) jvo@ualg.pt José Valete de Oliveira 3-9 Patter Patter is a amed ad well-kow problem/solutio pair that ca be applied i ew cotexts, with advice o how to apply it i ovel situatios ad discussio of its tradeoffs, implemetatios, variatios, ad so forth. (Craig Larma, Applyig UML ad Patters, 1995) jvo@ualg.pt José Valete de Oliveira

6 The Expert patter (padrão Especialista) Name: Problem: Solutio: (advice) Expert What is a basic priciple by which to assig resposibilities to objects? Assig a resposibility to the class that has the iformatio eeded to fulfill it. jvo@ualg.pt José Valete de Oliveira 3-11 The Expert patter (padrão Especialista) Discussão: O Expert leva-os habitualmete a programas ode o objeto o programa tem o comportameto que é ormalmete ecotrado o objeto do mudo real. O Expert promove o ecapsulameto, uma vez que o objeto usa os seus próprios dados para realizar as suas tarefas. jvo@ualg.pt José Valete de Oliveira

7 Desig patters A desig patter captures desig expertise abstracted from existig desig examples Desig patters allow to reuse desig expertise Desig patters allow oe to study how experts do desig jvo@ualg.pt José Valete de Oliveira 3-13 Re: Is this a commo type of problem? Remember: Patter is a amed ad well-kow problem/solutio pair that ca be applied i ew cotexts, with advice o how to apply it i ovel situatios ad discussio of its trade-offs, implemetatios, variatios, ad so forth. (Craig Larma, 1995) jvo@ualg.pt José Valete de Oliveira

8 From a patter catalogue: Template Method Itet q To defie a skeleto algorithm by deferrig some steps to subclasses q To allow the subclasses to redefie certai steps jvo@ualg.pt José Valete de Oliveira Template Method is used: To implemet the ivariat parts of a algorithm oce ad let the subclasses to implemet the behavior that ca vary. Whe commo behavior amog subclasses should be factored ad localized i a commo class to avoid code duplicatio. To cotrol subclasses extesios. That is, to defie a specific poits where extesios are permited. jvo@ualg.pt José Valete de Oliveira

9 Template Method - Structure jvo@ualg.pt José Valete de Oliveira A Quicksort class as the Template Method abstract uchecked ) abstract class Quicksort { private Comparable[] data; public fial void quicksort(comparable[] d) { data = d; quicksort(0, data.legth-1); /** quicksort method as the template method */ private fial void quicksort(it left, it right) { if (right <= left) retur; it i = partitio(left, right); quicksort(left, i-1); quicksort(i+1, right); jvo@ualg.pt José Valete de Oliveira

10 Method to be implemet by subclasses protected abstract it partitio (it i, it j); José Valete de Oliveira Methods for subclasses to use /* Call this i a subclass for swapig elemets i ad j */ protected fial void exchage(it i, it j) { Comparable tmp = data[i]; data[i] = data[j]; data[j] = tmp; jvo@ualg.pt José Valete de Oliveira

11 Methods for subclasses to use /* Call this i a subclass for swapig elemets i ad j */ protected fial void exchage(it i, it j) { Comparable tmp = data[i]; data[i] = data[j]; data[j] = tmp; /** Use this i subclasses to compare elemets x.compareto(y) aswer 0 if x ==y; -1 if x < y; 1 se x>y */ protected fial boolea less(it i, it j) { retur (data[i].compareto(data[j]) < 0); jvo@ualg.pt José Valete de Oliveira Methods for subclasses to use /* Call this i a subclass for swapig elemets i ad j */ protected fial void exchage(it i, it j) { Comparable tmp = data[i]; data[i] = data[j]; data[j] = tmp; /** Use this i subclasses to compare elemets x.compareto(y) aswer 0 if x ==y; -1 if x < y; 1 se x>y */ protected fial boolea less(it i, it j) { retur (data[i].compareto(data[j]) < 0); jvo@ualg.pt José Valete de Oliveira

12 ProbQuickSort as a cocrete class class ProbQuickSort exteds Quicksort protected it partitio(it left, it right) { it last=left; it pp = left + (it) (Math.radom() * (right-left)); // pivot positio exchage(left, pp); for(it i=left+1; i <= right; i++) if (less(i, left)) exchage(i, ++last); exchage(left, last); retur last; jvo@ualg.pt José Valete de Oliveira Example of aother cocrete class class ClassicalQuickSort exteds Quicksort protected it partitio(it left, it right) { it l2r=left-1; // left to right idex it pp=right-1; // pivot positio it r2l=pp; // right to left idex for(;;) { do ++l2r; while(less(l2r, pp)) ; do if(--r2l < 0) break; while(less(pp, r2l)); if(l2r >= r2l) break; exchage(l2r, r2l); exchage(pp, l2r); retur l2r; jvo@ualg.pt José Valete de Oliveira

13 Recallig cliet code public class Mai { public static void mai(strig[] args) { Iteger[] a = ew Iteger[args.legth]; for (it i=0; i<args.legth; i++) a[i] = Iteger.parseIt(args[i]); public fial void quicksort(comparable[] d) { ClassicalQuickSort qs = ew data ClassicalQuickSort(); = d; qs.quicksort(a); quicksort(0, data.legth-1); for(it i=0; i< a.legth; i++ ) System.out.pritl private (a[i]); fial void quicksort(it left, it right) { if (right <= left) retur; it i = partitio(left, right); quicksort(left, i-1); quicksort(i+1, right); jvo@ualg.pt José Valete de Oliveira Template Method brief discussio Template Method is a fudametal desig patter for code reuse. It s particularly importat i class libraries, as it is itet for factorig out commo behavior i the class hierarchy. It leads to a iverted cotrol structure kow as the Hollywood Priciple ("Do't call us, we'll call you ). This patter is so fudametal that it ca be foud i almost every abstract class. jvo@ualg.pt José Valete de Oliveira

14 How-to check list Examie the algorithm, ad decide which steps are stadard ad which steps are peculiar Defie a ew abstract base class to host the do t call us, we ll call you framework. Move the shell of the algorithm (ow called the template method ) ad the defiitio of all stadard steps to the ew base class. Defie a placeholder or hook method i the base class for each step that requires may differet implemetatios. This method ca host a default implemetatio or it ca be defied as abstract Ivoke the hook method(s) from the template method. Each of the existig classes declares a is-a relatioship to the ew abstract base class. Remove from the existig classes all the implemetatio details that have bee moved to the base class. The oly details that will remai i the existig classes will be the implemetatio details peculiar to each derived class. From: Take home today A case study: the multiple versios of quicksort The otio of Patter The desig patter Template Method jvo@ualg.pt José Valete de Oliveira