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1 GRASP GRASP stands for General Responsibility Assignment Solution Patterns. There are nine pattern in the Layman text: Information Expert (Expert) Creator High Cohesion Low Coupling Controller Polymorphism Pure Fabrication Indirection Protected Variations Information Expert Name: Information Expert Problem: What is the general principle of assigning responsibilities to a class? Once the classes present in a problem description have been identified, how do we determine which class handles a responsibility. Solution: Assign a responsibility to the information expert, the class that has the information necessary to fulfill the responsibility. Discussion: Consider, 1 1 * * Course Student WorkIte which class should determine the final mark the student receives in a course. While the WorkItem class can determine the value of an individual items, they can not determine the final mark. The Student class should be assigned this responsibility since it knows about all of the work items. The Student class will rely on the WorkItem class to determine the individual marks. The Expert pattern has a real world analogy, who do you ask about X, you ask the expert who knows about X. Information Expert Examples In the Resistor design example, some of the responsibilities are: Who is responsible for maintaining the collection of resistors, and for accessing selected ranges of the resistor? Who is responsible for calculating the voltage divisor ratio? The Information expert design pattern can be used to assign the above responsibilities. Since the ResistorManager stores the list of resistors (i.e., it knows about the resistors), it should be responsible for selecting a range of resistors. The voltage divider ration must be calculated by the VoltageDivider since this class knows about the two resistors that form the voltage divider. More Information Expert Examples Consider the following responsibilities from paint estimator program: 1 of 22 09/12/ :23 AM

2 Who should calculate the amount of paint need to cover a given area? Who should be responsible calculating the amount of paint need for a wall? Since the Paint class knows about the coverage, it should be responsible for calculating the required paint for a given area. A wall knows about its paint, its area, and the list of doors and windows that a part of the wall, thus it has all the information necessary to calculate the amount of paint required. Marking System Example The marking system can be modeled with the following domain classes: WorkItem, MarkingScheme, Student, and Course. Consider the following responsibilities: the calculation of the final grade for a student, editing a working item, the report of all the grades in a class, a list of all the student name and numbers in the class, Using the Expert design pattern, decide which class if possible of the domain class should be assigned the given responsibility. If no domain class is possible, suggest a software class that should be responsible. Creator Name: Creator Problem: Which class should be responsible for creating a new instance of some class. Since creating objects is a common activity, having some patterns help the quality of the design. Solution: Assign class B the responsibility of creating and instance of class A if: B aggregates A objects, B contains A objects, B closely uses A objects, B has data needed to create an A object. Discussion: The last point is also an application of the Information Expert Design pattern, since an Expert knows about the information needed to create objects of class A. Low Coupling Name: Low Coupling Problem: How to support low dependency, low change impact, and increase reuse? Solution: Assign a responsibility so that coupling remains low. 2 of 22 09/12/ :23 AM

3 Discussion: A class with strong coupling relies on many other classes. This can result in: changes in the class propagate to changes elsewhere, a class that is hard to understand in isolation, a class that is hard to reuse. Low coupling in Java The low coupling pattern is an evaluation principle. It is applied so that all design decisions can be evaluated. In Java, common forms of coupling from type X to type Y include: Type X contains an attribute of type Y, Type X has a method that references a type Y object, Type X is a direct or indirect subclass of type Y, Type Y is an interface that type X implements. Low coupling Low coupling supports classes that are more independent, which reduces the impact of change. Subclasses should be created very carefully since the subclass is highly coupled to the super class. Extreme low coupling should be avoided since it can result in in-cohesive and bloated classes. The low coupling pattern should not be used as an excuse to avoid common utility classes that are found in java.util.*. When a class is stable (i.e., not likely to change) then the harm of coupling is greatly reduced. The real problem of coupling is not the linkage, but the fact that changes to couple classes affect each other. Low coupling Exercise Consider the method addtrack for an Album class, two possible methods are: addtrack( Track t ) and addtrack( int no, String title, double duration ) Which method reduces coupling? The second one does, since the class using the Album class does not have to know a Track class. In general, parameters to methods should use base types (int, char...) and classes from the java.* packages. Coupling between Wall and PaintAmountReporter Consider the reportpaint method. public void reportpaint( PaintAmountReporter reporter ) { reporter.addpaint( paint.getcolour(), paintamount() ); for( int i = 0 ; i < openings.size(); i++ ) { Opening o = openings.get( i ); reporter.addpaint( o.gettrimpaint().getcolour(), o.trimarea()); Is the coupling between PaintAmountReporter and Wall good or bad? How could it be removed? High Cohesion 3 of 22 09/12/ :23 AM

4 Name: High Cohesion Problem: How to keep objects focused, understandable, and manageable? Solution: Assign responsibilities so that cohesion remains high. Discussion: Cohesion measures the degree that the responsibilities in a class are related and if they are focused on the same goals. A software component with highly functionally related responsibilities is cohesive. This class does not attempt to do too much. A class with low cohesion does many unrelated things, or does too much work. A class with low cohesion suffers from the following problems: hard to comprehend, hard to reuse, hard to maintain, not robust. Class with low cohesion often represent a very large grain of abstraction. They have taken on too many responsibilities. Cohesion Examples Examples illustrating varying degrees of functional cohesion are: very low cohesion - A class is solely responsible for many things in very different functional areas. If most programs are implemented in one class, then that class would have very low cohesion. low cohesion - A class has sole responsibility for a complex task in one functional area. high cohesion - A class has moderate responsibilities in one functional area and collaborates with other classes to fulfill the task. High Cohesion Classes with high cohesion generally have a relatively small number of methods. The methods are related functionally. A real world analogy of low cohesion is a person that takes on too many unrelated responsibilities, especially ones that should properly be delegated to others. In any design decision both coupling and cohesion should be considered. Coupling and Cohesion Exercise Identify coupling and cohesion issues with the following code. import java.util.arraylist; import java.util.iterator; examples/numbersstatscommand.java public class NumbersStatsCommand { public static void main( String[] args ) { NumberSequence seq = new NumberSequence(); int i; for( i = 0 ; i < args.length; i++ ) { try { seq.appendnumber( Double.parseDouble( args[i] ) ); 4 of 22 09/12/ :23 AM

5 catch( NumberFormatException ex ) { break; for(; i < args.length; i++ ) { System.out.println( args[i] + " " + seq.calculate( args[i] ) ); class NumberSequence { private ArrayList<Double> list; public NumberSequence() { this.list = new ArrayList<Double>(); public void appendnumber( double d ) { list.add( new Double( d ) ); public double calculate( String cmd ) { if ( cmd.equals("sum") ) { double sum = 0.0; Iterator<Double> dit = list.iterator(); while ( dit.hasnext() ) { sum += dit.next().doublevalue(); return sum; else if ( cmd.equals("min") ) { Iterator<Double> dit = list.iterator(); double min = dit.next().doublevalue(); while ( dit.hasnext() ) { double d = dit.next().doublevalue(); if ( d < min ) { min = d; return min; return 0.0; Any bad practices? Coupling and Cohesion Improvements import java.util.arraylist; import java.util.iterator; examples/numbersstatscommandv2.java public class NumbersStatsCommandV2 { public static void main( String[] args ) { NumberSequence seq = new NumberSequence(); int i; for( i = 0 ; i < args.length; i++ ) { try { seq.appendnumber( Double.parseDouble( args[i] ) ); catch( NumberFormatException ex ) { break; for(; i < args.length; i++ ) { if ( args[i].equals("sum") ) { 5 of 22 09/12/ :23 AM

6 System.out.println( "sum: " + seq.sum() ); else if ( args[i].equals("min") ) { System.out.println( "min: " + seq.min() ); else { System.out.println( "unknown command: " + args[i] ); class NumberSequence { private ArrayList<Double> list; public NumberSequence() { this.list = new ArrayList<Double>(); public void appendnumber( double d ) { list.add( new Double( d ) ); public double sum() { double s = 0.0; Iterator<Double> dit = list.iterator(); while ( dit.hasnext() ) { s += dit.next().doublevalue(); return s; public double min() { Iterator<Double> dit = list.iterator(); double m = dit.next().doublevalue(); while ( dit.hasnext() ) { double d = dit.next().doublevalue(); if ( d < m ) { m = d; return m; Bad coupling and cohesion can result from assigning too many responsibilities to the same class or from assigning the same responsibility to more than one class. In NumbersStatsCommand, the responsibility to parsing the command line has been assigned to two classes. Implementation Improvements public class NumbersStatsCommandV3 { examples/numbersstatscommandv3.java public static void main( String[] args ) { NumberSequence seq = new NumberSequence(); int i; for( i = 0 ; i < args.length; i++ ) { try { seq.appendnumber( Double.parseDouble( args[i] ) ); catch( NumberFormatException ex ) { break; if ( i == 0 ) { System.out.println( "no numbers" ); System.exit( 1 ); 6 of 22 09/12/ :23 AM

7 for(; i < args.length; i++ ) { if ( args[i].equals("sum") ) { System.out.println( "sum: " + seq.sum() ); else if ( args[i].equals("min") ) { System.out.println( "min: " + seq.min() ); else { System.out.println( "unknown command: " + args[i] ); class NumberSequence { private double currentsum; private double currentmin; public NumberSequence() { this.currentsum = 0.0; // Double.MAX_VALUE is the maximum double value // what is the benefit of this assignment? this.currentmin = Double.MAX_VALUE; public void appendnumber( double d ) { currentsum += d; if ( d < currentmin ) { currentmin = d; public double sum() { return currentsum; public double min() { return currentmin; Reduced coupling, enables a simpler and more efficient implementation, The client classes are not affected by this change. Another cohesion and coupling exercise Identify the low cohesion and high coupling in the following program. import java.util.regex.pattern; import java.util.regex.matcher; import java.util.scanner; cc/generatereport.java public class GenerateReport { private static final Pattern pat = Pattern.compile( "<(-?\\d+),(-?\\d+)>" ); private double maxx = Double.MIN_VALUE; private double maxy = Double.MIN_VALUE; private double minx = Double.MAX_VALUE; private double miny = Double.MAX_VALUE; public void processdata() { Scanner sc = new Scanner(System.in); while( sc.hasnextline() ) { String line = sc.nextline(); Matcher matcher = pat.matcher( line ); double x, y; 7 of 22 09/12/ :23 AM

8 try { if ( matcher.matches() ) { x = Double.parseDouble( matcher.group(1)); y = Double.parseDouble( matcher.group(2)); if ( x < minx ) minx = x; if ( y < miny ) miny = y; if ( x > maxx ) maxx = x; if ( y > maxy ) maxy = y; catch( NumberFormatException ex ) { continue; // discard bad line public void genreport() { double largestmax = Math.max( maxx, maxy); double smallestmin = Math.min( minx, miny); double dist = Math.hypot(maxX-minX,maxY-minY); System.out.println("Largest maximum is " + largestmax ); System.out.println("Smallest minimum is " + smallestmin ); System.out.println("Largest distance is " + dist ); public static void main( String[] args ) { GenerateReport rep = new GenerateReport(); rep.processdata(); rep.genreport(); Improved version The class CoordinateData is only responsible for the coordinate information and any calculations on this information. It does not deal with handling input or producing the report. public class CoordinateData { private double maxx = Double.MIN_VALUE; private double maxy = Double.MIN_VALUE; private double minx = Double.MAX_VALUE; private double miny = Double.MAX_VALUE; cc/coordinatedata.java public void updatecoordinatedata( double x, double y ) { if ( x < minx ) minx = x; if ( y < miny ) miny = y; if ( x > maxx ) maxx = x; if ( y > maxy ) maxy = y; public double getlargestmax() { return Math.max( maxx, maxy); public double getsmallestmin() { return Math.min( minx, miny); public double getlargestdist() { return Math.hypot(maxX-minX,maxY-minY); // Anything missing from this class? Classes with high cohesion generally have a relatively small number of methods. The methods are related functionally. 8 of 22 09/12/ :23 AM

9 These conditions are true for the CoordinateData class. Parsing the coordinate data The responsibility of parsing the data is now handled by the ParseCoordinateStream class. It does not handle any calculation. import java.io.inputstream; import java.util.scanner; import java.util.regex.pattern; import java.util.regex.matcher; cc/parsecoordinatestream.java class ParseCoordinateStream { private static final Pattern pat = Pattern.compile( "<(-?\\d+),(-?\\d+)>" ); private InputStream instream; private Scanner scanner; private double x, y; public ParseCoordinateStream( InputStream instream ) { this.instream = instream; this.scanner = new Scanner( instream ); public boolean fetchnextpair() { while ( scanner.hasnextline() ) { String line = scanner.nextline(); Matcher matcher = pat.matcher( line ); try { if ( matcher.matches() ) { x = Double.parseDouble( matcher.group(1)); y = Double.parseDouble( matcher.group(2)); return true; catch( NumberFormatException ex ) { continue; // discard bad line return false; public double getx() { return x; public double gety() { return y; public static void main(string args[] ) { ParseCoordinateStream parser = new ParseCoordinateStream(System.in); CoordinateData cd = new CoordinateData(); while ( parser.fetchnextpair() ) { cd.updatecoordinatedata( parser.getx(), parser.gety() ); System.out.println("Largest maximum is " + cd.getlargestmax() ); System.out.println("Smallest minimum is " + cd.getsmallestmin() ); System.out.println("Largest distance is " + cd.getlargestdist() ); Again this class has a small number of functionally related methods, thus is cohesion is high. Push model 9 of 22 09/12/ :23 AM

10 Further reduction in coupling can occur with an implementation of a class the supports the push model of data processing. In the push model the data is feed to the consuming class. import java.io.inputstream; cc/pushcoordinates.java class PushCoordinates { private ParseCoordinateStream pullcoords; public PushCoordinates( InputStream instream ) { this.pullcoords = new ParseCoordinateStream( instream ); public interface Push { public void push( double x, double y ); public void pushcoordinates( Push p ) { while ( pullcoords.fetchnextpair() ) { p.push( pullcoords.getx(), pullcoords.gety() ); public static void main(string args[] ) { PushCoordinates pusher = new PushCoordinates(System.in); final CoordinateData cd = new CoordinateData(); pusher.pushcoordinates( new PushCoordinates.Push() { public void push( double x, double y ) { cd.updatecoordinatedata(x, y); ); System.out.println("Largest maximum is " + cd.getlargestmax() ); System.out.println("Smallest minimum is " + cd.getsmallestmin() ); System.out.println("Largest distance is " + cd.getlargestdist() ); Any class that needs data, implements the Push interface. Coupling can be easily reduce, how? Controller Name: Controller Problem: Which first object beyond the UI layer receives and coordinates a system operations? Solution: Assign responsibility for receiving events to either: a class the represents the system, or a class the represents the particular use case. Discussion: Controller classes provides the glue between the system events and software model. Since the type of use case tends to change, the classes can change without affecting the rest of the system (i.e., coupling is reduced). The UI classes only interact with the controller classes. The controller classes normally delegates most of the work to other classes in the system. 10 of 22 09/12/ :23 AM

11 Entity, Boundary, and Control Objects Entity objects are instances of domain classes. Boundary objects represent the interaction between actors and the system Control objects are in charge of realizing use cases. Actors interact with boundary objects. Boundary object communicate and are controlled by control objects (Controller design pattern). Control object interact with entity objects to accomplish the goals of the system. Grouping objects into entity, boundary, and control classes provides a structure that aids and improves the software design of a system. Boundary objects can represent UI components, control objects are responsible for managing use cases, and entity objects represent the real world. HiLo Game In the HiLo game, a player has to guess a randomly selected number between a low and a high number. If their guess is too high or too low or correct they are informed, The goal of the game is to guess the correct number in a limited number of trys. The games should track the number of player wins and the total number of games played. The player should also be given the opportunity to to choose the low and high numbers. PlayGame Use Case Use Case Name: PlayGame Actor: HiLo Player Precondition: A game has started with a selected range and a randomly chosen number to guess. The user has 10 guesses. Flow of events: Basic Path A hilo game has started with a selected range and a randomly chosen number to be guess. While the user still has guesses available: 1. The user is prompted to enter a guess. 2. If the user enters a number, then 1. if the guessed number is higher than the correct number, report that the guess is higher, and return to step if the guessed number is lower than the correct number, report that the guess is lower, and return to step if the guessed number matches the choose number, report the success and the use case ends. 3. If the user quits, the use case ends. The use case ends. Postcondition: A user has either won, lost or quit the game. For this use case: is the use case too complicated, is the use case too simple, does the post condition describe a real change, is the interaction too specific to an interface technology, are the any problems, is there anything missing, and can the description be improved (i.e., made to read better). 11 of 22 09/12/ :23 AM

12 StartGame Use Case Use Case Name: StartGame Actor: HiLo Player Precondition: At least one hilo game has occurred. Flow of events: Basic Path A user requests the statistics of the last set of games played. The system displays the number of wins and the number of games played. The use case ends. Postcondition: None For this use case: is the use case too complicated, is the use case too simple, does the post condition describe a real change, is the interaction too specific to an interface technology, are the any problems, is there anything missing, and can the description be improved (i.e., made to read better). StartGame Use Case Use Case Name: StartGame Actor: HiLo Player Precondition: None Flow of events: Basic Path The user is informed that a new games is starting and asked to select the low number and high number. While the user has not correctly entered the low and high numbers: 1. The user is prompted to enter the low number and high number. 2. If the user enters valid numbers, the use case ends. 3. If the user requests to quit, the game stops, and the use case ends. The use case ends. Postcondition: The range of numbers to guess has been selected. For this use case: is the use case too complicated, is the use case too simple, does the post condition describe a real change, is the interaction too specific to an interface technology, are the any problems, is there anything missing, and can the description be improved (i.e., made to read better). HiLoGame (entity) The state of a HiLo can be modeled by: package hilo; hilo/hilo/hilogame.java import java.util.random; public class HiLoGame { private static Random random = new Random(); 12 of 22 09/12/ :23 AM

13 private int high, low; private int range; private int guess; private int guesscount; private int nogames; private int nowins; public HiLoGame( int low, int high ) { this.low = low; this.high = high; this.range = (high-low) + 1; this.nogames = 0; this.nowins = 0; newgame(); public void newgame() { guesscount = 0; guess = random.nextint( range ) + low; nogames++; /* * Return 0 if the guess is correct, > 0 if too high, * and < 0 if too low. */ public int makeguess( int userguess ) { if ( userguess == guess ) { nowins++; return 0; guesscount++; return userguess - guess; public int numguesses() { return guesscount; public int numwins() { return nowins; public int numgames() { return nogames; Does this class do one thing, and is it done well? How many other classes does it depend on? StartHiLoGame (control class) This class implements the StartHiLoGame use case, and is an example of the Controller pattern. package text; hilo/text/starthilogame.java import hilo.hilogame; import java.util.scanner; public class StartHiLoGame { private int defaultlo = 0; private int defaulthi = 100; /* 13 of 22 09/12/ :23 AM

14 * Return a HiLoGame or null to indicate end of game. */ public HiLoGame start() { Scanner scan = new Scanner( System.in ); System.out.println("Welcome to HiLo"); do { System.out.println("Please pick the guessing range [0 100] or quit"); String line = scan.nextline(); Scanner sl = new Scanner( line ); int hi = defaulthi; int lo = defaultlo; if ( sl.hasnextint() ) { lo = sl.nextint(); if ( sl.hasnextint() ) { hi = sl.nextint(); System.out.println("Starting game with " + lo + " " + hi); return new HiLoGame( lo, hi ); else if ( sl.hasnext() ) { String cmd = sl.next(); if ( cmd.equals( "quit" ) ) { System.out.println("quitting"); return null; else { System.out.println("Starting game with " + lo + " " + hi); return new HiLoGame( lo, hi ); while( true ); Does this class do one thing, and is it done well? How many other classes does it depend on? PlayHiLoGame (control class) This class implements the PlayHiLoGame use case, and is an example of the Controller pattern. package text; hilo/text/playhilogame.java import hilo.hilogame; import java.util.scanner; public class PlayHiLoGame { private HiLoGame game; public PlayHiLoGame( HiLoGame game ) { this.game = game; public boolean play() { Scanner scan = new Scanner(System.in); while( game.numguesses() <= 10 ) { System.out.print( "Enter a guess: "); String line = scan.nextline(); Scanner sl = new Scanner( line ); if ( sl.hasnextint() ) { int guess = sl.nextint(); int result = game.makeguess( guess ); if ( result == 0 ) { System.out.println("Correct guess, you win"); break; 14 of 22 09/12/ :23 AM

15 else if ( result > 0 ) { System.out.println("too high"); else { System.out.println("too low"); int rem = 10 - game.numguesses(); System.out.println(rem + " trys left"); else if ( sl.hasnext() ) { String cmd = sl.next(); if ( cmd.equals("quit") ) { return false; System.out.println("Invalid command, only quit accepted"); else { System.out.println("empty line"); return true; Does this class do one thing, and is it done well? How many other classes does it depend on? ReportStatstHiLoGame (control class) This class implements the ReportStatstHiLoGame use case, and is an example of the Controller pattern. package text; hilo/text/reportstatsthilogame.java import hilo.hilogame; public class ReportStatstHiLoGame { private HiLoGame game; public ReportStatstHiLoGame( HiLoGame game ) { this.game = game; public void report() { int wins = game.numwins(); int games = game.numgames(); System.out.println( "won " + wins + " out of " + games ); Does this class do one thing, and is it done well? How many other classes does it depend on? HiLoGameSystem (control class) This class implements the system and is an example of the Controller pattern. package text; hilo/text/hilogamesystem.java import hilo.hilogame; public class HiLoGameSystem { public static void main( String[] args ) { StartHiLoGame startgame = new StartHiLoGame(); while ( true ) { HiLoGame game = startgame.start(); 15 of 22 09/12/ :23 AM

16 if ( game == null ) break; PlayHiLoGame playgame = new PlayHiLoGame( game ); ReportStatstHiLoGame statsgame = new ReportStatstHiLoGame(game); while( playgame.play() ) { statsgame.report(); game.newgame(); Does this class do one thing, and is it done well? How many other classes does it depend on? HiLo Game Class Diagram Comment on the coupling and cohesion of the HiLo game implementation. Swing version of HiLo A screen shot of the the HiLo game implemented with swing is: 16 of 22 09/12/ :23 AM

17 HiLoControl (control) package gui; hilo/gui/hilocontrol.java import hilo.hilogame; public class HiLoControl { public HiLoControl( HiLoApp gui ) { this.gui = gui; this.game = new HiLoGame(0, 100); private HiLoApp gui; private HiLoGame game; public void playgame( int guess ) { int result = game.makeguess( guess ); if ( result == 0 ) { gui.displayplayresult("you Win, New game"); int wins = game.numwins(); int games = game.numgames(); gui.appendscoreresults( "won " + wins + " out of " + games + "\n" ); game.newgame(); else { int remaining = 10 - game.numguesses(); if ( remaining <= 0 ) { gui.displayplayresult("too many trys, New game"); int wins = game.numwins(); int games = game.numgames(); gui.appendscoreresults( "won " + wins + " out of " + games + "\n" ); game.newgame(); else { if ( result < 0 ) { gui.displayplayresult("too low, trys " + remaining + " left"); else { gui.displayplayresult("too high, trys " + remaining + " left"); 17 of 22 09/12/ :23 AM

18 public void setrange( int hi, int low ) { game = new HiLoGame( low, hi ); gui.displayplayresult( "Guess a number between " + low + " " + hi ); public static void main( String args[] ) { final HiLoApp app = new HiLoApp(); HiLoControl control = new HiLoControl( app ); app.setcontrol( control ); java.awt.eventqueue.invokelater(new Runnable() { public void run() { app.setvisible(true); ); Does this class do one thing, and is it done well? How many other classes does it depend on? HiLoApp (boundary) /* * To change this template, choose Tools Templates * and open the template in the editor. */ hilo/gui/hiloapp.java /* * App.java * * Created on Oct 31, 2009, 6:50:26 PM */ package gui; /** * rod */ public class HiLoApp extends javax.swing.jframe { private HiLoControl control; public void setcontrol( HiLoControl control ) { this.control = control; public void displayplayresult( String msg ) { playresultfield.settext( msg ); public void appendscoreresults( String msg ) { scoretextarea.append( msg ); /** Creates new form App */ public HiLoApp() { initcomponents(); /** This method is called from within the constructor to * initialize the form. * WARNING: Do NOT modify this code. The content of this method is * always regenerated by the Form Editor. 18 of 22 09/12/ :23 AM

19 // <editor-fold defaultstate="collapsed" desc="generated Code">//GEN-BEGIN:initComponents private void initcomponents() { toppanel = new javax.swing.jpanel(); frametitle = new javax.swing.jlabel(); playform = new javax.swing.jpanel(); playinputform = new javax.swing.jpanel(); guesslabel = new javax.swing.jlabel(); guessfield = new javax.swing.jtextfield(); guessbutton = new javax.swing.jbutton(); playresultfield = new javax.swing.jlabel(); rangeform = new javax.swing.jpanel(); rangebutton = new javax.swing.jbutton(); highform = new javax.swing.jpanel(); highlabel = new javax.swing.jlabel(); highfield = new javax.swing.jtextfield(); lowform = new javax.swing.jpanel(); lowlabel = new javax.swing.jlabel(); lowfield = new javax.swing.jtextfield(); scoreform = new javax.swing.jpanel(); jscrollpane1 = new javax.swing.jscrollpane(); scoretextarea = new javax.swing.jtextarea(); setdefaultcloseoperation(javax.swing.windowconstants.exit_on_close); settitle("hilo"); // NOI18N setbackground(new java.awt.color(48, 219, 231)); setbounds(new java.awt.rectangle(0, 0, 100, 90)); setcursor(new java.awt.cursor(java.awt.cursor.default_cursor)); setname("top_frame"); // NOI18N getcontentpane().setlayout(new java.awt.gridlayout(1, 1)); toppanel.setbackground(new java.awt.color(173, 240, 203)); toppanel.setborder(javax.swing.borderfactory.createemptyborder(5, 5, 5, 5)); toppanel.setmaximumsize(new java.awt.dimension(500, 600)); toppanel.setminimumsize(new java.awt.dimension(300, 400)); toppanel.setpreferredsize(new java.awt.dimension(300, 400)); toppanel.setlayout(new javax.swing.boxlayout(toppanel, javax.swing.boxlayout.page_axis)); frametitle.sethorizontalalignment(javax.swing.swingconstants.center); frametitle.setlabelfor(this); frametitle.settext("hilo Game"); frametitle.setalignmentx(0.5f); frametitle.setborder(javax.swing.borderfactory.createemptyborder(1, 1, 1, 1)); frametitle.sethorizontaltextposition(javax.swing.swingconstants.leading); toppanel.add(frametitle); playform.setborder(javax.swing.borderfactory.createtitledborder("play")); playform.setopaque(false); playform.setlayout(new javax.swing.boxlayout(playform, javax.swing.boxlayout.page_axis)); playinputform.setopaque(false); guesslabel.sethorizontalalignment(javax.swing.swingconstants.left); guesslabel.setlabelfor(guessfield); guesslabel.settext("guess"); guesslabel.setalignmenty(0.0f); playinputform.add(guesslabel); guessfield.setcolumns(10); playinputform.add(guessfield); guessbutton.settext("play"); guessbutton.setminimumsize(new java.awt.dimension(70, 29)); guessbutton.setpreferredsize(new java.awt.dimension(100, 29)); guessbutton.setrequestfocusenabled(false); guessbutton.addactionlistener(new java.awt.event.actionlistener() { public void actionperformed(java.awt.event.actionevent evt) { 19 of 22 09/12/ :23 AM

20 guessbuttonactionperformed(evt); ); playinputform.add(guessbutton); playform.add(playinputform); playresultfield.settext("guess a number between 0 and 100"); playresultfield.setalignmentx(0.5f); playform.add(playresultfield); toppanel.add(playform); rangeform.setbackground(new java.awt.color(227, 231, 38)); rangeform.setborder(javax.swing.borderfactory.createtitledborder("range")); rangeform.setopaque(false); rangeform.setlayout(new javax.swing.boxlayout(rangeform, javax.swing.boxlayout.page_axis)); rangebutton.settext("set Guess Range"); rangebutton.addactionlistener(new java.awt.event.actionlistener() { public void actionperformed(java.awt.event.actionevent evt) { rangebuttonactionperformed(evt); ); rangeform.add(rangebutton); highform.setborder(javax.swing.borderfactory.createemptyborder(5, 1, 1, 1)); highform.setopaque(false); highlabel.settext("high Guess"); highlabel.sethorizontaltextposition(javax.swing.swingconstants.left); highform.add(highlabel); highfield.setcolumns(10); highfield.sethorizontalalignment(javax.swing.jtextfield.left); highfield.settext("100"); highform.add(highfield); rangeform.add(highform); lowform.setborder(javax.swing.borderfactory.createemptyborder(1, 1, 1, 1)); lowform.setopaque(false); lowlabel.settext("low Guess"); lowlabel.sethorizontaltextposition(javax.swing.swingconstants.left); lowform.add(lowlabel); lowfield.setcolumns(10); lowfield.settext("0"); lowform.add(lowfield); rangeform.add(lowform); toppanel.add(rangeform); scoreform.setborder(javax.swing.borderfactory.createtitledborder("scores")); scoreform.setopaque(false); scoreform.setlayout(new java.awt.gridlayout(1, 1)); scoretextarea.setcolumns(20); scoretextarea.setrows(10); jscrollpane1.setviewportview(scoretextarea); scoreform.add(jscrollpane1); toppanel.add(scoreform); getcontentpane().add(toppanel); 20 of 22 09/12/ :23 AM

21 pack(); // </editor-fold>//gen-end:initcomponents private void guessbuttonactionperformed(java.awt.event.actionevent evt) {//GEN-FIRST:event_guessButtonActionPerformed if ( control == null ) return; String guess = guessfield.gettext(); try { int g = Integer.parseInt( guess ); control.playgame( g ); catch( NumberFormatException ex ) { displayplayresult("not a number, try again"); //GEN-LAST:event_guessButtonActionPerformed private void rangebuttonactionperformed(java.awt.event.actionevent evt) {//GEN-FIRST:event_rangeButtonActionPerformed if ( control == null ) return; try { String lowtext = lowfield.gettext(); String hightext = highfield.gettext(); int hi = Integer.parseInt( hightext ); int lo = Integer.parseInt( lowtext ); control.setrange( hi, lo ); catch( NumberFormatException ex ) { displayplayresult("invalid range"); //GEN-LAST:event_rangeButtonActionPerformed /** args the command line arguments */ public static void main(string args[]) { java.awt.eventqueue.invokelater(new Runnable() { public void run() { new HiLoApp().setVisible(true); ); // Variables declaration - do not modify//gen-begin:variables private javax.swing.jlabel frametitle; private javax.swing.jbutton guessbutton; private javax.swing.jtextfield guessfield; private javax.swing.jlabel guesslabel; private javax.swing.jtextfield highfield; private javax.swing.jpanel highform; private javax.swing.jlabel highlabel; private javax.swing.jscrollpane jscrollpane1; private javax.swing.jtextfield lowfield; private javax.swing.jpanel lowform; private javax.swing.jlabel lowlabel; private javax.swing.jpanel playform; private javax.swing.jpanel playinputform; private javax.swing.jlabel playresultfield; private javax.swing.jbutton rangebutton; private javax.swing.jpanel rangeform; private javax.swing.jpanel scoreform; private javax.swing.jtextarea scoretextarea; private javax.swing.jpanel toppanel; // End of variables declaration//gen-end:variables Does this class do one thing, and is it done well? How many other classes does it depend on? HiLo Game Class Diagram (gui) 21 of 22 09/12/ :23 AM

22 GUI and Text Comment on the differences between the two implementations. 22 of 22 09/12/ :23 AM

23 2 Remaining GRASP The remaining 4 GRASP patterns will be covered. Polymorphism - behaviour depends on the type Pure Fabrication - class based in software world Indirection - avoid direct coupling with an intermediary Protected Variations - information hiding - open/close The first five GRASP patterns were: Expert - assign responsibilities to class with knowledge Creator - knows the necessary details to create High Cohesion - does related things Low Coupling - reduce connectivity Controller - use cases or system based classes Polymorphism Name: Polymorphism Problem: How to handle behaviour based on type (i.e., class), but not with an if or switch statement? Polymorphism is also used to create pluggable software components. When alternate behaviors are selected based on the type of an object, use polymorphic method call to select the behavior, rather than use if statements to test the type., Patterns in Java, Vol 2., p 69 Solution: In Java, polymorphism is realized by overriding a method from a super class or implementing an interface. The implemented overridden methods are polymorphic in that a client class uses the same method, but the behaviour depends on the type of the object being referenced. Discussion: The Polymorphism GRASP pattern deals with how a general responsibility gets distributed to a set of classes or interfaces. For example, the responsibility of calculating grades depends on the type of marking scheme for a particular student. If polymorphism is not used, and instead the code tests the type of the object, then that section of code will grow as more types are added to the system. This section of code becomes more coupled (i.e., it knows about more types) and less cohesive (i.e., it is doing too much). A software component is pluggable if it presents itself as the known class or interface. 1 of 20 09/12/ :24 AM

24 2 Responsibilities that can be polymorphic Consider the following. How creatures respond to being attacked in a DandD game. How a connection is made to a WIFI access point. How a ship in the game of Battleship is displayed on screen. How much time a route takes to travel depends on the type of segments in the route. How a wall is detected by a robot depends on the type of sensors used. How a player plays a game a RISK depends of if the player is a human or machine. In each of the above cases, the type of the object being asks results in different behaviour. The polymorphism pattern assigns the responsibility to a subclass or a class the implements a known interface. Anymore examples from the projects? Polymorphism in Java API Polymorphism is used throughout the Java API, consider: The hashcode(), tostring(), getclass(), equals(), and clone() methods of java.lang.object should or must be provided by all Java class (since all Java classes inherit java.lang.object). The drawing and event processing of Swing components is the responsibilities of each distinct Swing component. The Swing package provides many more examples of polymorphism (e.g., javax.swing.plaf.uiresource, javax.swing.border.abstractborder, javax.swing.text.abstractdocument,... ). Similar to the Swing package, the AWT package also has examples of polymorphism. The data structures and algorithms used by the classes that implement java.util.list depend on the implementing class (e.g., array for ArrayList, linked list for LinkedList). The java.util.bag, java.util.set, and java.util.map are interfaces for concrete classes that implement different data structures and algorithms for these interfaces. The java.io.reader, java.io.writer have extended class that specialize the basic IO operations. Other Java examples of polymorphism are: java.util.eventobject, java.util.concurrent.executorservice, javax.imageio.iioparam, java.sql.resultset, java.security.key,... See for class diagrams of the Java api. Polymorphism in java.io Identify the polymorphism is the code that copies files: import java.io.ioexception; import java.io.bufferedreader; import java.io.filereader; import java.io.bufferedwriter; import java.io.filewriter; poly/copyfile.java 2 of 20 09/12/ :24 AM

25 2 public class CopyFile { public static void main( String[] args ) throws IOException { if( args.length!= 2 ) { System.out.println("usage: java CopyFile infile outfile"); System.exit( 1 ); BufferedReader rd = new BufferedReader( new FileReader( args[0] )); BufferedWriter wt = new BufferedWriter( new FileWriter( args[1] )); String line = null; while( (line=rd.readline())!= null ) { wt.write( line ); wt.newline(); rd.close(); wt.close(); Polymorphism with streams The polymorphism is more explicit in this example: import java.io.ioexception; import java.io.inputstream; import java.io.outputstream; import java.io.fileinputstream; import java.io.bufferedinputstream; import java.io.bufferedoutputstream; import java.io.fileoutputstream; poly/copyfilestream.java public class CopyFileStream { public static void main( String[] args ) throws IOException { if( args.length!= 2 ) { System.out.println("usage: java CopyFileStream infile outfile"); System.exit( 1 ); InputStream ins = new BufferedInputStream( new FileInputStream( args[0] )); OutputStream os = new BufferedOutputStream( new FileOutputStream( args[1] )); byte[] buf = new byte[2048]; int amt; while( (amt=ins.read( buf )) > 0 ) { os.write( buf, 0, amt ); ins.close(); os.close(); Polymorphism example (interface) 3 of 20 09/12/ :24 AM

26 2 A possible set of two dimensional shapes include: circle, triangle, and square. Each of these shapes has an area and a perimeter. The calculation of area and perimeter depend on the type of shape. A possible shape interface is: public interface Shape2D { public double area(); public double perimeter(); shapes/shape2d.java Class implementations for circle, triangle, and square are: import static java.lang.math.pi; shapes/circle.java public class Circle implements Shape2D { private double radius; public Circle( double radius ) { this.radius = radius; public double area() { return PI * radius * radius; public double perimeter() { return PI * 2 * radius; import static java.lang.math.sqrt; shapes/triangle.java public class Triangle implements Shape2D { private double a, b, c; public Triangle( double a, double b, double c ) { this.a = a; this.b = b; this.c = c; public double area() { // Heron's formula return sqrt( (a+b-c)*(a-b+c)*(-a+b+c)*(a+b+c) ) / 4.0; public double perimeter() { return a+b+c; 4 of 20 09/12/ :24 AM

27 2 public class Square implements Shape2D { private double side; shapes/square.java public Square( double side ) { this.side = side; public double area() { return side * side; public double perimeter() { return 4 * side; The Information Expert pattern suggests that the area and perimeter calculations belong to each class, since the information need to calculate the area is present in each class. Given a shape, code that print its area is: Shape s = // a Triangle, Circle, or Square object System.out.println( s.area() ); Without polymorphism, the code would be: Shape s = // Triangle, Circle, or Square if ( s instanceof Triangle ) { Triangle t = (Triangle)s; System.out.println( t.area() ); else if ( s instanceof Circle ) { Circle c = (Triangle)c; System.out.println( c.area() ); else if ( s instanceof Square ) { Square sq = (Square)s; System.out.println( sq.area() ); Of course without polymorphism, interfaces would not make any sense. The second code fragment is not as cohesive as the first. It also has more coupling, since it knows about Triangle, Circle, and Square. Additionally, any new shapes require the modification of the above code. Polymorphism example (abstract class) Inheriting from a class can also be used to realize the polymorphism of the perimeter and 5 of 20 09/12/ :24 AM

28 2 area responsibilities. The class is abstract, since there are no meaningfully definition of area and perimeter without a real shape. import static java.lang.math.sqrt; import static java.lang.math.pi; shapes/abstractshape.java public abstract class AbstractShape { public abstract double area(); public abstract double perimeter(); class Triangle extends AbstractShape { private double a, b, c; public Triangle( double a, double b, double c ) { this.a = a; this.b = b; this.c = c; public double area() { // Heron's formula return sqrt( (a+b-c)*(a-b+c)*(-a+b+c)*(a+b+c) ) / 4.0; public double perimeter() { return a+b+c; class Square extends AbstractShape { private double side; public Square( double side ) { this.side = side; public double area() { return side * side; public double perimeter() { return 4 * side; class Circle extends AbstractShape { private double radius; public Circle( double radius ) { this.radius = radius; 6 of 20 09/12/ :24 AM

29 2 public double area() { return PI * radius * radius; public double perimeter() { return PI * 2 * radius; Larman suggest that interfaces should be used..., when you want to support polymorphism without being committed to a particular class hierarchy. Interfaces are also required when more then one set of methods must be polymorphic (i.e., multiple inheritance is required). Expression Polymorphism The expression, 3.0 * , can represented by an abstract syntax tree. + * Every element in the tree can be evaluated. A class that represents any expression is: public abstract class Expr { public abstract double evaluate(); AST/Expr.java The polymorphism pattern suggest that every distinct expression object should override the evaluate method. Constant, plus, and times expression can be implemented with: public class ConstantExpr extends Expr { private double value; AST/ConstantExpr.java public double evaluate() { return value; public ConstantExpr( double value ) { this.value = value; AST/PlusExpr.java 7 of 20 09/12/ :24 AM

30 2 public class PlusExpr extends Expr { private Expr left, right; public double evaluate() { return left.evaluate() + right.evaluate(); public PlusExpr( Expr left, Expr right ) { this.left = left; this.right = right; public class TimesExpr extends Expr { private Expr left, right; AST/TimesExpr.java public double evaluate() { return left.evaluate() * right.evaluate(); public TimesExpr( Expr left, Expr right ) { this.left = left; this.right = right; How would a subtraction expression be supported? Testing Expr import org.junit.*; import static org.junit.assert.*; AST/ExprTesting.java /* * javac -cp junit-4.7.jar:. ExprTesting.java * java -cp junit-4.7.jar:. org.junit.runner.junitcore ExprTesting */ public class ExprTesting { private static final Expr one = new ConstantExpr( 1.0 ); private static final Expr negone = new ConstantExpr( -1.0 ); private static final Expr two = new ConstantExpr( 2.0 ); private static final double EPSILON = public void singleexpressions() { // expr: 1.0 asserttrue( one.evaluate() == 1.0 ); // expr: -1.0 asserttrue( negone.evaluate() == -1.0 ); // expr: 2.0 asserttrue( two.evaluate() == 2.0 ); 8 of 20 09/12/ :24 AM

31 public void compoundexpressions() { // expr: PlusExpr pe = new PlusExpr( one, two ); assertequals( pe.evaluate(), 3.0, EPSILON ); // expr: 1.0 * 2.0 TimesExpr te = new TimesExpr( one, two ); assertequals( te.evaluate(), 2.0, EPSILON ); // expr: -1.0 * ( ) TimesExpr e1 = new TimesExpr( negone, pe ); assertequals( e1.evaluate(), -3.0, EPSILON ); // expr: (1.0 * 2.0) * ( ) TimesExpr e2 = new TimesExpr( te, pe ); assertequals( e2.evaluate(), 6.0, EPSILON ); Nonpolymophic Expression Implementation An implementation of a nonpolymophic expression evaluator is: public class NonPolyExpr { public static final int CONST_EXPR = 1; public static final int PLUS_EXPR = 2; public static final int TIMES_EXPR = 3; AST/NonPolyExpr.java private int type; private double value; private NonPolyExpr left=null, right=null; public NonPolyExpr( int type, double value ) { this.type = type; this.value = value; public NonPolyExpr( int type, NonPolyExpr left, NonPolyExpr right) { this.type = type; this.left = left; this.right = right; public double evaluate() { switch( type ) { case CONST_EXPR: return value; case PLUS_EXPR: return left.evaluate() + right.evaluate(); case TIMES_EXPR: return left.evaluate() * right.evaluate(); return java.lang.double.nan; // what is this? 9 of 20 09/12/ :24 AM

32 2 The nonpolymophic implementation suffers from the following design problems: The NonPolyExpr.evaluate method is not as cohesive as the polymorphic versions, since the NonPolyExpr class must handle all the cases, while the polymorphic class only deal with one operation. The NonPolyExpr cannot be easily extended, while the polymorphic version is extended by adding new class the extends Expr. This new expression operation can then be used by all other classes. Using NonPolyExpr.evaluate, requires knowledge of the expression types, and therefore is more coupled to the client classes. What is the result of: // a, b are NonPolyExpr objects NonPolyExpr e = new NonPolyExpr(NonPolyExpr.CONST_EXPR,a,b); System.out.print( e.evaluate() ); An implementation of polymorphism in C The following C-language program demonstrates the essentials of how polymorphism is implemented. In addition to the data parts of an object, the object contains a table of method pointers for each overridden method. The table is consulted to invoke the correct method. #include <stdio.h> c-poly/poly_in_c.c typedef void (*func_v_vp_ptr)(void *); typedef struct { int value; // data part func_v_vp_ptr print; // virtual method base_class; void base_print(void *p) { base_class *self = (base_class *)p; printf("base %d\n", self->value ); void base_init( base_class *self, int value) { self->value = value; self->print = base_print; void invoke_print( base_class *self ) { (*self->print)(self); 10 of 20 09/12/ :24 AM

33 2 void derived_print(void *p) { base_class *self = (base_class *)p; printf("derived %d\n", self->value ); void derived_init( base_class *self, int value) { self->value = value; self->print = derived_print; int main( int ac, char *av[] ) { base_class b1, d1; base_init( &b1, 1); derived_init( &d1, 2); invoke_print( &b1 ); invoke_print( &d1 ); Hopeful, this demonstrates why learning C is good for you (or perhaps not!). Pure Fabrication Name: Pure Fabrication Problem: What object should have the responsibility, when you do not want to violate High Cohesion and Low Coupling, or other goals, but solutions offered by Expert are not appropriate., Larman, p 421. You must assign a responsibility to a class, but assigning it to a class that represents a conceptual model entity would ruin its low coupling or high cohesion. You resolve this problem by fabricating a class that does not represent an entity in your conceptual model., Patterns in Java, Vol. 2, p 73. In other words, not all responsibilities can be assigned to domain classes, especially responsibilities that deal with implementation technologies (i.e., persistence storage, network communication,...). Solution: Assign a highly cohesive set of responsibilities to an artificial or convenience class that does 11 of 20 09/12/ :24 AM