1. 12 OBJECT-ORIENTED DESIGN CHAPTER
Copyright © 2013 by John Wiley & Sons. All rights reserved.
Slides by Rick Giles

2. OO Class Design
Encapsulation – access to the data and code is tightly controlled by an interface.
Cohesion – the degree in which a class has a single, well-defined role or responsibility. Has to do with how an individual class is designed.
Coupling – the degree to which one class knows about or uses members of another class. Has to do with how classes interact with each other.
Copyright © 2013 by John Wiley & Sons. All rights reserved.

3. Cohesion (1) A class should represent a single concept
The public interface of a class is cohesive if all of its features are related to the concept that the class represents
Copyright © 2013 by John Wiley & Sons. All rights reserved.

4. Cohesion (2) This class lacks cohesion:
public class CashRegister {
public static final double NICKEL_VALUE = 0.05;
public static final double DIME_VALUE = 0.1;
public static final double QUARTER_VALUE = 0.25; …
public void enterPayment(int dollars, int quarters,
int dimes, int nickels, int pennies) }
It involves two concepts: cash register and coin
Copyright © 2013 by John Wiley & Sons. All rights reserved.

5. Cohesion (3) Better: Make two classes:
public class Coin { public Coin(double aValue, String aName) { } public double getValue() { } } public class CashRegister { public void enterPayment(int coinCount, Coin coinType) { } }
Copyright © 2013 by John Wiley & Sons. All rights reserved.

6. Coupling (1)
If many classes depend on each other, the coupling between classes is high
Good practice: minimize coupling between classes
Change in one class may require update of all coupled classes
Using a class in another program requires using all classes on which it depends
Copyright © 2013 by John Wiley & Sons. All rights reserved.

7. Coupling (2)
Copyright © 2013 by John Wiley & Sons. All rights reserved.

8. Good Class Design
Tight encapsulation – hide implementation behind and interface
Instance variables are protected
Getter/Setter methods provide access
High Cohesion – classes whose members support a single, well-focused role or responsibility
Loose Coupling – classes are well encapsulated, minimize references to each other. Use only APIs, not knowledge of implementation.
Copyright © 2013 by John Wiley & Sons. All rights reserved.

9. Benefits of Good Class Design
Tight encapsulation and Loose Coupling – ability to modify implemented code without breaking the code of others who use our code. Gives maintainability, flexibility and extensibility to code.
High Cohesion – classes are easier to maintain and reuse
Copyright © 2013 by John Wiley & Sons. All rights reserved.

10. Class Relationships weak strong Dependency (“knows about”)
Needs another class to do its job
Local variable, parameter variable
Association (“knows about”)
Dependency plus
An attribute is object of another class
Aggregation (“has a”)
Association plus
whole/part relationship
Composition (“has a”)
Aggregation plus
one object manages lifetime of another object
Inheritance (“is a”)
Copyright © 2013 by John Wiley & Sons. All rights reserved.

11. A class depends on another if it uses objects of that class
Dependency
A class depends on another if it uses objects of that class
“knows about” relationship
CashRegister depends on Coin to determine the value of the payment
public class CashRegister { public void enterPayment(int coinCount, Coin coinType) { } }
Copyright © 2013 by John Wiley & Sons. All rights reserved.

12. Dependency Relationship
Copyright © 2013 by John Wiley & Sons. All rights reserved.

13. Association
An association is a structural relationship that indicates that an object of one class has a reference to an object of another class.
An association supports data sharing between classes.
Multiplicity information can be linked to an association to show how many instances of class A are linked with instances of class B. Multiplicity information can be linked to both ends of association relationships.
Copyright © 2013 by John Wiley & Sons. All rights reserved.

14. Aggregation A class contains an instance of another class
public class Address {   }
public class Person {
     private Address personAddress;
     public Person(Address add)
     { personAddress = add;}
    }
Models a “has-a” relationship
Person doesn’t manage lifetime of Address
In main method:
//object instantiated outside of class
Address myAddress = new Address(); Person me(myAddress);

15. Aggregation (1)
A class aggregates another of its objects contain objects of another class
“has-a” relationship
Example: a quiz is made up of questions
Class Quiz aggregates class Question
Copyright © 2013 by John Wiley & Sons. All rights reserved.

16. Aggregation (2)
Finding out about aggregation helps in implementing classes
Example: since a quiz can have any number of questions, use an array or array list for collecting them
public class Quiz {
private ArrayList<Question> questions;
. . . }
Copyright © 2013 by John Wiley & Sons. All rights reserved.

17. Composition A class contains an instance of another class.
public class Engine {   }
public class Car {
   private Engine e;
public Car() { e = new Engine(); } //object instantiated
// inside class }
Models a “part-of” relationship
Car manages the lifetime of Engine

18. Aggregation v. Composition
House:Room
Building:Floor
Bowl:Bowling Ball
Read:Book
Desk:Drawer
DVDWriter:DVD

19. Inheritance (1)
Inheritance is the relationship between a more general class (superclass) and a more specialized class (subclass)
“is-a” relationship
Example: every car is a vehicle; every car has tires
Class Car is a subclass of class Vehicle; class car aggregates class Tire
Copyright © 2013 by John Wiley & Sons. All rights reserved.

20. Inheritance (2)
public class Car extends Vehicle { private Tire[] tires; }
Copyright © 2013 by John Wiley & Sons. All rights reserved.

21. UML Class Diagram
A UML class diagram is a graphical tool that can aid in the design of a class.
The diagram has three main sections.
ClassName
Attributes
Methods
UML diagrams are easily converted to Java class files
The class name should concisely reflect what the class represents.

22. UML Relationship Symbols
Composition
Solid
Solid Diamond
Association
None
Copyright © 2013 by John Wiley & Sons. All rights reserved.

23. Object Oriented Program Design
To discover classes, look for nouns in the problem description
Example: Print an invoice
Candidate classes:
Invoice
LineItem
Customer
Copyright © 2013 by John Wiley & Sons. All rights reserved.

24. Finding Classes & Responsibilities
Finding the classes
Get written description of the problem domain
Identify all nouns, each is a potential class
Refine list to include only classes relevant to the problem
Identify the responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing

25. OOP Oriented Analysis Problem
Problem Domain
Joe’s Automotive Shop services foreign cars, and specializes in servicing cars made by Mercedes, Porsche, and BMW. When a customer brings a car to the shop, the manager gets the customer’s name, address, and telephone number. The manager then determines the make, model and year of the car, and gives the customer a service quote. The service quote shows the estimated parts charges, estimated labor charges, sales tax, and total estimated charges.
Identify class or classes need for this problem
Identify responsibilities of the class or classes

26. Identify All of the Nouns
Joe’s Automotive Shop services foreign cars, and specializes in servicing cars made by Mercedes, Porsche, and BMW. When a customer brings a car to the shop, the manager gets the customer’s name, address, and telephone number. The manager then determines the make, model and year of the car, and gives the customer a service quote. The service quote shows the estimated parts charges, estimated labor charges, sales tax, and total estimated charges.

27. Identify All of the Nouns
Joe’s Automotive Shop foreign cars cars Mercedes Porsche BMW customer car shop manager customer name address
telephone number manager make model year car customer service quote estimated parts charges estimated labor charges sales tax total estimated charges

28. Refine List of Nouns Some of the nouns really mean the same thing.
Some nouns might represent items that we do not need to be concerned with in order to solve the problem.
Some of the nouns might represent objects, not classes.
Some of the nouns might represent simple values that can be stored in a primitive variable and do not require a class.

29. Which nouns really mean the same thing?
Joe’s Automotive Shop foreign cars cars Mercedes Porsche BMW customer car shop name address
telephone number manager make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

30. Which nouns really mean the same thing?
Joe’s Automotive Shop foreign cars cars Mercedes Porsche BMW customer car shop name address
telephone number manager make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

31. Which nouns represent items that we don’t need to solve the problem?
cars Mercedes Porsche BMW customer car shop name address
telephone number manager make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

32. Which nouns represent items that we don’t need to solve the problem?
cars Mercedes Porsche BMW customer car shop name address
telephone number manager make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

33. Which nouns represent objects, not classes?
cars Mercedes Porsche BMW customer car name address
telephone number make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

34. Which nouns represent objects, not classes?
cars Mercedes Porsche BMW customer car name address
telephone number make model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

35. cars customer car name address telephone number make
Which nouns represent simple values that can be stored in primitive variables and don’t require a class?
cars customer car name address telephone number make
model year service quote estimated parts charges estimated labor charges sales tax total estimated charges
Would you use a group of related values to represent the item’s state?
Are there any obvious actions to be performed by the item?

36. Which nouns represent simple values that can be stored in primitive variables and don’t require a class?
cars customer car name address telephone number make
model year service quote estimated parts charges estimated labor charges sales tax total estimated charges

37. Finding Classes Responsibilities
Finding the classes
Get written description of the problem domain
Identify all nouns, each is a potential class
Refine list to include only classes relevant to the problem
Identify the responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing

38. Customer Class Responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing
name address telephone number make model
year estimated parts charges estimated labor charges sales tax total estimated charges

39. Customer Class Responsibilities
Things a class is responsible for knowing
the customer’s name
the customer’s address
the customer’s telephone number
Things a class is responsible for doing
create an object of the Customer class
set and get the customer’s name
set and get the customer’s address
set and get the customer’s telephone number

40. Customer Class UML Diagram

41. Car Class Responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing
name address telephone number make model
year estimated parts charges estimated labor charges sales tax total estimated charges

42. Car Class Responsibilities
Things a class is responsible for knowing
the car’s make
the car’s model
the car’s year
Things a class is responsible for doing
create an object of the Car class
set and get the car’s make
set and get the car’s model
set and get the car’s year

43. Car Class UML Diagram

44. ServiceQuote Class Responsibilities
Things a class is responsible for knowing
Things a class is responsible for doing
name address telephone number make model
year estimated parts charges estimated labor charges sales tax total estimated charges

45. ServiceQuote Class Responsibilities
Things a class is responsible for knowing
the service quote’s estimated parts charges
the service quote’s estimated labor charges
Things a class is responsible for doing
create an object of the ServiceQuote class
set and get the estimated parts charges
set and get the estimated labor charges
calculate sales tax
calculate total estimated charges

46. ServiceQuote UML Diagram

47. Class Collaboration
Collaboration – two classes interact with each other
If an object is to collaborate with another object, it must know something about the second object’s methods and how to call them

48. Examples of Collaboration
ServiceQuote object calls the Customer object’s getName, getAddress and getPhone methods
ServiceQuote object calls the Car object’s getMake, getModel and getYear methods

49. The CRC Card Method
A CRC card describes a class, its responsibilities, and its collaborating classes.
For each responsibility of a class, its collaborators are the other classes needed to fulfill it
Copyright © 2013 by John Wiley & Sons. All rights reserved.

50. CRC Card
Copyright © 2013 by John Wiley & Sons. All rights reserved.