1. Java the UML Way http://www.tisip.no/JavaTheUmlWay/ version 2000-04-17 Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7 Collaboration Between Objects Collaboration between objectspage 2 Collaboration in the renovation casepage 3-4 The getNoOfMeters() method with sequence diagrampage 5-6 The getTotalPrice() methodpage 7 Collaboration beween objects of the same classpage 8-9 The renovation case, a menu-driven programpage 10-13 Several references to the same objectpage 14-18 Passing argumentspage 19-20

2. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 2 Collaboration Between Objects In reality objects often collaborate to solve tasks. Examples: –The teacher sends the message “Complete this task” to a student. The student searches for help from other students, and searches on the Internet or in books to complete the task. –Elisabeth turns her CD player on. It has to collaborate with the CD to be able to make music. In programs, we get an object to collaborate with another object by sending messages to it. Example: We send the message “Play this CD!” to the myCDplayer object in this way: –myCDplayer.play(“Four Seasons”); For an object to send messages to other objects, it must have access to the other objects.

3. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 3 Class Diagram, the Renovation Case An instance of the Flooring class has to collaborate with an instance of the Surface class to calculate the number of meters required to cover the surface. The Paint and Wallpaper objects have to collaborate with the Surface object to calculate the number of liters and rolls, respectively. Sorce code for the classes: Surface, page 86, Flooring and Wallpaper, page 182-184. (Paint, solution to problem 2 p. 189.)

4. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 4 Message to the flooring: What will it cost to cover the surface with you? Collaboration, Examples class FlooringClient { public static void main(String[] args) { Surface theSurface = new Surface("Margaret's Floor", 5, 6); Flooring theFlooring = new Flooring("Fitted carpet", 24.50, 5); double noOfMeters = theFlooring.getNoOfMeters(theSurface); double price = theFlooring.getTotalPrice(theSurface); System.out.println("You need " + noOfMeters + " meters, price $" + price); } The client instantiates two objects Message to the flooring: How many meters do we need of you to cover the surface? Printout: You need 6.0 meters, price $147.0

5. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 5 The getNoOfMeters() Method public double getNoOfMeters(Surface aSurface) { double lengthSurface = aSurface.getLength(); double widthSurface = aSurface.getWidth(); int noOfWidths = (int)(lengthSurface / widthOfFlooring); double rest = lengthSurface % widthOfFlooring; if (rest >= limit) noOfWidths++; return noOfWidths * widthSurface; } The method call: double noOfMeters = theFlooring.getNoOfMeters(theSurface); A reference to the surface object is sent along with the message. (As an argument to the method.) In this way the method gets access to the client's surface object. And, because of that, the method may send messages to the surface object.

6. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 6 The Collaboration is Shown by This Sequence Diagram Solve problem 1, page 189. time

7. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 7 The getTotalPrice() Method public double getTotalPrice(Surface aSurface) { return getNoOfMeters(aSurface) * price; } The method call: double price = theFlooring.getTotalPrice(theSurface);

8. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 8 Two Objects of the Same Class Collaborate Example: Comparing areas Surface surface1 = new Surface("A", 5, 4); Surface surface2 = new Surface("B", 4, 4); int result = surface1.compareAreas(surface2); if (result < 0) System.out.println(surface1.getName() + " is the smaller one."); else if (result > 0) System.out.println(surface2.getName() + " is the smaller one."); else System.out.println("The surfaces have the same area."); Here is the compareAreas() method in the Surface Class: public int compareAreas(Surface theOtherSurface) { final double precision = 0.00001; double area1 = getArea(); double area2 = theOtherSurface.getArea(); if (Math.abs(area2 - area1) < precision) return 0; else if (area1 < area2) return -1; else return 1; }

9. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 9 Two Surface-Objects Collaborate to Find the Larger Solve problem 3, page 189-190.

10. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 10 A Menu-Driven Program We'll calculate the materials required and the price for renovating a surface (wall or floor). public static String[] alternatives = {"Wallpaper", "Paint", " Flooring ", "Exit"}; JOptionPane.showOptionDialog(null, "What do you choose:", "Renovation", JOptionPane.DEFAULT_OPTION, JOptionPane.PLAIN_MESSAGE, null, alternatives, alternatives[0]); [option != exit] carry out the chosen task let the user do a menu choice [option == exit] let the user do a menu choice show instructions to the user

11. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 11 Which Classes Do We Need? Of course, the Surface, Flooring, Paint and Wallpaper classes. We need an object that can be responsible for carrying out the tasks in the activity diagram: –Display a short instruction to the user. –Input a menu choice from the user. –Carry out the chosen task We call this class ProjectChap7GUI. Do we need even more classes? Let us look at, for example, how the paint requirement has to be calculated: 1.Input data about the surface that will be painted (name, length, width). 2.Create an instance of the Surface class. 3.Input data about the paint that will be used (name, price, number of coats, number of square meters per liter). 4.Create an instance of the Paint class. 5.Ask the paint object how much paint is needed to cover the given surface and what it will cost. 6.Output the answers.

12. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 12 The ReaderRenovationCase Class Points 1 and 3 generate much use of JOptionPane to input data  many code lines  loosing overview Points 1 and 2, and points 3 and 4, are general tasks that may be useful in other situations. We compose a new class with methods for inputting information and instantiating objects: ReaderRenovationCase ReaderRenovationCase readAndInstantiateSurface readAndInstantiateWallpaper readAndInstantiatePaint readAndInstantiateFlooring ProjectChap7 details:: ReaderRenovationCase alternativs[1..* ]: String showInstructions() doAChoice(): int carryOutTheRightThing( option: int)

13. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 13 Show the Source Code, Program Listings 7.2 and 7.3 (pages 194-197) Solve problem 2, pp. 193-194.

14. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 14 Several References to the Same Object Collaboration between objects usually means that we have several references to the same object. It can be difficult to keep track of this if the references are in different methods or perhaps even in different classes. Which kinds of problems may arise? We’ll look at two concrete examples: –An access method gains access, via the parameter list, to an object that “belongs” to the client. –An instance variable refers to an object that it has received via the parameter list for a constructor or a mutation method. The object is created by the client.

15. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 15 A Reference Type as a Parameter for an Access Method public double getNoOfMeters(Surface aSurface) { double lengthSurface = aSurface.getLength(); double widthSurface = aSurface.getWidth(); …… } ”AutumnBrown” 23.50 5.0 theCarpet ”Anne’s floor” 4.2 3.5 myFloor Flooring theCarpet = new Flooring("AutumnBrown", 23.50, 5.0); Surface myFloor = new Surface("Anne’s floor", 4.2, 3.5); double noOfMeters = theCarpet.getNoOfMeters(myFloor); What happens when the method getNoOfMeters() is invoked? Behind the scenes: Surface aSurface the method = myFloor client ; aSurface Here is a new reference to the floor-object

16. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 16 A Reference Type as a Parameter for an Access Method, continued What may happen inside getNoOfMeters()? If the Surface class is mutable, the method may change the data inside the Surface object, which is an object "belonging" to the client.

17. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 17 An Instance Variable Refers to an Object that is Created by the Client The Surface class is expanded with information about the flooring which will be used. New constructor: public Surface(String initName, double initLength, double initWidth, Flooring initFlooring) { name = initName; length = initLength; width = initWidth; flooring = initFlooring; } The client program instantiates objects: Flooring theCarpet = new Flooring("AutumnBrown", 23.50, 5.0); Surface myFloor = new Surface("Anne’s floor", 4.2, 3.5, theCarpet); As an alternative, the constructor may create a copy of the flooring object: flooring = new Flooring(initFlooring.getName(), initFlooring.getPrice(), initFlooring.getWidth()); Now, the client and the myFloor object will each have one copy of the flooring object.

18. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 18 Several References to the Same Object, Conclusion Keep the number of classes having access to a mutable object at a minimum – minimal coupling between classes. Consider using immutable classes. Instead of changing the object, a new object is made (The String class is a well known example.) Several classes may work with one copy each, but that seldom agree with the reality modeled. We have to know what we do, and we should document well. Solve problem 1, pp. 201-202.

19. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 19 Passing Arguments We’ve seen that a method can change an object that “belongs” to the client, if a reference to the object is an argument to the method. Can a method, in a similar fashion, change a variable of a primitive data type? As an example, we expand the Flooring class with the following method that change the price of the flooring: public void setPrice(double newPrice) { price = newPrice; newPrice = 0.0; } At the client side: Flooring theFlooring = new Flooring( "Margaret's flooring", 24.50, 5); String newPriceRead =JOptionPane. showInputDialog("New price: "); double newPrice = Double. parseDouble(newPriceRead); theFlooring.setPrice(newPrice); No changes on the client side.

20. Only to be used in connection with the book "Java the UML Way", by Else Lervik and Vegard B. Havdal. ISBN 0-470-84386-1, John Wiley & Sons Ltd 2002 The Research Foundation TISIP, http://tisip.no/engelsk/ Chapter 7, page 20 Summary, Argument Passing In a method call, the values for the arguments are always passed. The parameters are to be used as local variables, with initial values equal to the argument values. We differentiate between the following two cases: –The parameter is of a primitive data type: if the method changes the value for this variable, that doesn’t affect the argument at all. –The parameter is a reference: a copy of the argument means that the method and the client each have their own reference to the same object. If the method makes changes to the object, they will, of course, apply to the object that the argument points to, since that’s the same object we’re talking about. Solve all problems, page 204.