1. Chapter 8 Properties of Objects and Classes

2. Chapter 8: Properties of Objects and Classes
Review from Chapter 5 - Classes.
8.1. Subclasses and Inheritance.
8.2. More About Inheritance.
8.3. Polymorphism and Overriding.
8.4. Abstract Classes.
8.5. Aggregation: Components and Wholes.
8.6. The Difference Between Subclassing and Aggregation.

3. Review from Chapter 5: Objects and Classes
Classes and Classification

4. 8.0. Classes and Classification In the real world
We need to consider:
Attributes,
Behavior,
Relationships, and
Semantics (meaning).

5. 8.0. Classes and Classification In the real world
Definition:
A class is a group of objects with: similar properties (attributes), common behavior (operations), common relationships to other objects, and common semantics (i.e., meaning.)

6. 8.0. Classes and Classification Subclasses In the real world
Some instances of a class may be
grouped together on the basis of:
Features not shared by the rest of the class
Attributes
Behavior
Associations
The key verb is:
“ISAKINDA”

7. Hierarchy Diagram (UML notation)
PERSON
Name, Address
Phone, Sex
Date of Birth
ChangeAddress
EnquireDOB&Sex
CANBEA
ISAKINDA
CUSTOMER
EMPLOYEE
Balance
O/Due 30, 60, 90
Credit Rating
Date Paid
CheckCrRating
AgeBalances
SIN
Marital Status
No. of Dependants
Date Hired
Wage Rate
GiveRaise
CalcMonthPay
This kind of arrowhead indicates that this relationship is one of subclassing

8. 8.0. Classes and Classification Additional Data-world Classes
Abstract Classes
Such classes have no instances other than the ones provided for them by their subclasses
That is, they have no Direct Instances
For this reason we call them Abstract Classes

9. Chapter 8: Properties of Objects and Classes
Review from Chapter 5 - Classes.
8.1. Subclasses and Inheritance.
8.2. More About Inheritance.
8.3. Polymorphism and Overriding.
8.4. Abstract Classes.
8.5. Aggregation: Components and Wholes.
8.6. The Difference Between Subclassing and Aggregation.

10. Chapter 8: Properties of Objects and Classes  8. 1
Chapter 8: Properties of Objects and Classes  8.1. Subclasses and Inheritance.
A Subclass is made up of selected instances from another class, the “Parent class” or “Superclass.”
A Superclass includes ALL the instances of the Subclass, plus possibly more as well.
Inheritance is when a subclass instance, in addition to the attributes and behavior it has by virtue of being in the subclass,
also has all the attributes and behavior
that instances of the superclass have.

11. Definition of Object-Oriented:
Chapter 8: Properties of Objects and Classes  8.1. Subclasses and Inheritance.
Definition of Object-Oriented:
To be considered truly O-O, a language, database, etc. must support:
Objects
Classes
Inheritance
Polymorphism (to be defined shortly)
We can define supporting a feature to mean
that the language makes it “easy and convenient” for a programmer to use that feature.

12. Chapter 8: Properties of Objects and Classes  8. 1
Chapter 8: Properties of Objects and Classes  8.1. Subclasses and Inheritance.
Another thing we can do with subclasses is show more detail about associations on our diagram.
For instance, in most companies, only managers can hire and fire.
In other words, only certain kindsa employees can do certain tasks.
Thus we are able to show that some associations affect only a subclass, and not every instance.

13. Chapter 8: Properties of Objects and Classes  8. 1
Chapter 8: Properties of Objects and Classes  8.1. Subclasses and Inheritance.
Employee
isakinda canbea
Manager
supervises
Regular
Employee 1 * * 1
drives
belongs to * 1
Company
Vehicle
Union
Next: More About Inheritance. . .

14. Chapter 8: Properties of Objects and Classes
Review from Chapter 5 - Classes.
8.1. Subclasses and Inheritance.
8.2. More About Inheritance.
8.3. Polymorphism and Overriding.
8.4. Abstract Classes.
8.5. Aggregation: Components and Wholes.
8.6. The Difference Between Subclassing and Aggregation.

15. Generalization and Specialization
Chapter 8: Properties of Objects and Classes 8.2. More About Inheritance.
Generalization and Specialization
A Payables Account is a special case of an Account.
What makes this Account different (i.e., specialized) is that it is a Payables Account,
And that other one over there is a Receivables Account.
Thus, the Payables and the Receivables subclasses may be viewed as specializing the Account class.
A subclass is said to specialize its superclass(es) by including only certain of the superclass instances.
Now let’s take another look at inheritance. . .

16. Hierarchy Diagram (UML notation)
PERSON
All these attributes
and behaviors
are inherited as is by
both subclasses
Name, Address
Phone, Sex
Date of Birth
ChangeAddress
EnquireDOB&Sex
Each subclass then adds
attributes and behaviors
that it needs but the other
one doesn’t.
CANBEA
ISAKINDA
CUSTOMER
EMPLOYEE
Balance
O/Due 30, 60, 90
Credit Rating
Date Paid
CheckCrRating()
AgeBalances()
SIN
Marital Status
No. of Dependants
Date Hired
Wage Rate
GiveRaise()
CalcMonthPay()
This kind of arrowhead to
indicate that this relationship is one of subclassing

17. Chapter 8: Properties of Objects and Classes
Review from Chapter 5 - Classes.
8.1. Subclasses and Inheritance.
8.2. More About Inheritance.
8.3. Polymorphism and Overriding.
8.4. Abstract Classes.
8.5. Aggregation: Components and Wholes.
8.6. The Difference Between Subclassing and Aggregation.

18. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
Polymorphism
Poly = many
Morph = shape
Polymorphism means “occurring in various forms.”
Along with Inheritance, polymorphism is one of the major keys to the power of objects.
(A “primary key”, perhaps?)

19. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
On the next slide,
You will see a class diagram for handling emergency calls.
Study it well.
Take particular note of the “DeleteCall” method
that appears in the superclass “911 Call” as well as in both subclasses.

20. * * * * * * * * * * 1 1 handles Citizen Response Specialist Fire
911 Call
handles
Response
Specialist
Call ID
Date
Time
Calling Number
Address
Create Call
Display Details
List Calls
DeleteCall *
Police
Officer
Fire
Fighter
Paramedic 1 * 1
investigates *
ISAKINDA
CANBEA *
Vehicle
False Alarm
Severity
CriminalY/N
DeleteCall
Valid Call * *
Problem
Crime/Fire/Acdnt/Poison
Date/Time Dispatched
Date/Time Completed
DispatchCall
CompleteCall
DeleteCall
Weapon * *
Dog * *

21. depends on the kinda call.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
You can see that the amount of work that has to be done deleting a call
depends on the kinda call.
A False Alarm needs some program code that will disconnect it from the investigating officer as well as the offending citizen.
A Valid Call needs program code to disconnect it from:
Many Response Specialists of various kinds,
Many Citizens, and this code it can share with the False Alarm calls,
Many Vehicles, Weapons and Dogs.

22. this is the power of objects!
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
Now you see that we need at least two different pieces of program code to delete these two kindsa 911 calls.
But I have shown them both with the same name!!
This would never work in regular “procedural” programming,
But it can be made to work here, and
this is the power of objects!

23. but they must be subclasses and superclasses.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
Object-Oriented languages allow us to define two functions (i.e., methods) with the same name
Provided they are in different classes
but they must be subclasses and superclasses.
Whichever kinda 911 Call we are handling, the system will choose the right version of the method to suit that particular Call instance.
We say that “The object knows its own type, so it knows what to do.”

24. This one is an instance of the superclass, But not of either subclass.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
And what about a 911 Call instance that needs to be cancelled before it is even evaluated to see if it was a False Alarm or a Valid Call?
This one is an instance of the superclass,
But not of either subclass.
Let’s say our analysis has discovered that the only difference in processing these calls and the False Alarms is the Investigating Officer, so we have decided to use one piece of program code to do both kindsa 911 Calls.
This gives us the diagram on the next slide, with no DeleteCall method in the False Alarm class.

25. * * * * * * * * * * 1 1 handles Citizen Response Specialist Fire
911 Call
handles
Citizen
Response
Specialist
Call ID
Date
Time
Calling Number
Address
Create Call
Display Details
List Calls
DeleteCall *
Police
Officer
Fire
Fighter
Paramedic 1 * 1
investigates * *
Vehicle
False Alarm
Severity
CriminalY/N
Valid Call * *
Problem
Crime/Fire/Acdnt/Poison
Date/Time Dispatched
Date/Time Completed
DispatchCall
CompleteCall
DeleteCall
Weapon * *
Dog * *

26. When an Unevaluated 911 Call is to be deleted,
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
When an Unevaluated 911 Call is to be deleted,
It knows it is an instance only of the superclass, so it looks in there for some code to execute.
Of course it finds a DeleteCall method, so it executes it and deletes the call.
When a Valid 911 Call is to be deleted,
It knows it is an instance in the Valid Call subclass,
(As well as in the superclass)
So it looks in the Valid Call subclass,
Finds a DeleteCall method, and so deletes itself.
But now, what about deleting a False Alarm call?

27. * * * * * * * * * * 1 1 So where does a False Alarm
911 Call
handles
Citizen
Response
Specialist
Call ID
Date
Time
Calling Number
Address
Create Call
Display Details
List Calls
DeleteCall *
Police
Officer
Fire
Fighter
Paramedic 1 * 1
investigates * *
Vehicle
False Alarm
Severity
CriminalY/N
Valid Call * *
Problem
Crime/Fire/Acdnt/Poison
Date/Time Dispatched
Date/Time Completed
DispatchCall
CompleteCall
DeleteCall
Weapon * *
So where does a False Alarm
instance find some program
code for deletions?
Dog * *

28. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
A False Alarm 911 Call
Knows it is an instance of the False Alarm class,
(And also of the superclass),
So it looks in the False Alarm subclass for a DeleteCall method,
But doesn’t find one there.
So now it walks up the hierarchy
And looks in the superclass, 911 Call.
There it finds a DeleteCall method so it executes that one and happily deletes itself.

29. * * * * * * * * * * 1 1 Citizen handles Response Specialist
911 Call
handles
Response
Specialist
Call ID
Date
Time
Calling Number
Address
Create Call
Display Details
List Calls
DeleteCall *
The system walks up
the chain looking for
some program code
to execute.
Police
Officer
Fire
Fighter
Paramedic 1 * 1
investigates * *
Vehicle
False Alarm
Severity
CriminalY/N
Valid Call * *
Problem
Crime/Fire/Acdnt/Poison
Date/Time Dispatched
Date/Time Completed
DispatchCall
CompleteCall
DeleteCall
Weapon * *
This Valid Call finds some
code right here in this class.
Dog * *

30. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
So we say that the DeleteCall method in the superclass (911 Call) is available to all instances of all subclasses.
But it is overridden for instances of the subclass ValidCall by the DeleteCall method (same name!) in the ValidCall subclass.
This also applies to any subclasses of the ValidCall class, and all its generations of descendant classes.

31. for that subclass and all its descendants.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Definition:
We say that a subclass feature (attribute or method)
overrides
the corresponding superclass feature.
That is, the subclass version of the feature actually replaces the superclass feature
for that subclass and all its descendants.

32. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance

33. Take a look at the Paramedic on the next slide.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
Take a look at the Paramedic on the next slide.
Is she “Hospital Staff?”
Or is she a “Response Specialist?” . . .

34. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
Hospital Staff
Response
Specialist
Patient
Care
Physicians
Administrative
Staff
Nurse
Nursing
Assistant
Fire
Fighter
Police
Officer
Paramedic

35. But this can lead to problems. . .
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
Of course, she is both!
Sometimes you find a class that inherits from two (or more) chains of superclasses.
It thus inherits ALL the attributes and behaviors of both chains of superclasses.
But this can lead to problems. . .

36. Chapter 8: Properties of Objects and Classes 8. 3
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
Response
Specialist
GiveRaise
Hospital Staff
GiveRaise
Patient
Care
Physicians
Administrative
Staff
Nurse
Nursing
Assistant
Fire
Fighter
Police
Officer
Paramedic

37. Which one should a Paramedic instance execute?
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
You will notice that both the Hospital Staff class and the Response Specialist class each has its own GiveRaise method.
This could be because the two groups have different union agreements,
which have different procedures for granting a raise in pay,
which means they need different program code to process those raises.
The question is,
Which one should a Paramedic instance execute?

38. Each OOPL and OODBMS has its own way of resolving this difficulty
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. Multiple Inheritance
Each OOPL and OODBMS has its own way of resolving this difficulty
Some simply don’t allow multiple inheritance.
Some have a way of forcing one or the other, or both, to execute.
Some force the programmer to choose.

39. Without having access to the program source code!!
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
Now all this inheritance and polymorphism stuff
brings us to another powerful feature of objects,
that is not available with conventional procedural programming.
This is the ability to add functionality to a subroutine or function library,
Without having access to the program source code!!

40. The vendor doesn’t want us fooling about with their source code,
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
It works like this. . .
If we were to buy an old-fashioned function library,
We can’t modify anything because
The vendor doesn’t want us fooling about with their source code,
And maybe blowing it up!
In which case we’re faced with the old “multi-vendor problem” when deciding whose code is to blame.

41. By defining a subclass that inherits from one of theirs.
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding.
When we buy an object class library, however
We can extend the functionality,
By defining a subclass that inherits from one of theirs.
We then add any new or replacement attributes,
And code any new or replacement methods,
And the new class now has:
All the features (attributes and methods) of the original,
Plus any we changed or added.

42. to the incredible potential for reuse of Object-Oriented program code
Chapter 8: Properties of Objects and Classes 8.3. Polymorphism and Overriding. This factor is what gives rise
to the incredible potential for reuse of Object-Oriented program code
and of Analysis and Design results as well.
Next: Abstract Classes

43. Chapter 8: Properties of Objects and Classes  8.4. Abstract Classes.
In our example of a 911 Call,
check out the
Response Specialist
and subclasses. . .

44. * * * * * * * * * * 1 1 handles Citizen Response Specialist Fire
911 Call
handles
Citizen
Response
Specialist
Call ID
Date
Time
Calling Number
Address
Create Call
Display Details
List Calls
DeleteCall *
Police
Officer
Fire
Fighter
Paramedic 1 * 1
investigates * *
Vehicle
False Alarm
Severity
CriminalY/N
Valid Call * *
Problem
Crime/Fire/Acdnt/Poison
Date/Time Dispatched
Date/Time Completed
DispatchCall
CompleteCall
DeleteCall
Weapon * *
Dog * *

45. The question to ask this time is:
Chapter 8: Properties of Objects and Classes  Abstract Classes. This is the structure we now have:
Response
Specialist
Police
Officer
Fire
Fighter
Paramedic
The question to ask this time is:
Are there any other kindsa Response Specialists besides the three kinds we have shown?

46. Chapter 8: Properties of Objects and Classes  8.4. Abstract Classes.
If the answer is no,
Then we say that the superclass
“Response Specialist”
is completely covered by its subclasses;
We say it has no Direct Instances,
i.e., it is an Abstract Class,
It has no instances other than those provided for it by its subclasses.

47. Chapter 8: Properties of Objects and Classes  8.4. Abstract Classes.
Definition:
An Abstract Class is one with no direct instances.
But why would we bother?
These are often useful to declare the structure, type or interface (protocol) of an attribute or method,
And allow the subclasses to provide the storage for an attribute, or the code for a method.
This firms up the interface and constrains the developers to stick with it as specified.
Next: Components and Wholes.

48. 8.5. Aggregation: Components and Wholes.
Chapter 8: Properties of Objects and Classes
8.5. Aggregation: Components and Wholes.

49. Chapter 8: Properties of Objects and Classes
Review from Chapter 5 - Classes.
8.1. Subclasses and Inheritance.
8.2. More About Inheritance.
8.3. Polymorphism and Overriding.
8.4. Abstract Classes.
8.5. Aggregation: Components and Wholes
8.6. The Difference Between Subclassing and Aggregation.

50. Whole-Part, or Component-Assembly Container-Contents Collection-Member
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Components and Wholes.
There are three relationships that sometimes occur
in an object model:
Whole-Part, or Component-Assembly
Container-Contents
Collection-Member
You may sometimes find these useful for making your model a better tool for understanding and communication.
The model can always be built without these - they do not really affect its use for system design, just for talking to the users.

51. Chapter 8: Properties of Objects and Classes 8. 5
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Components and Wholes. Whole-Part
Taking something apart into its components is a technique we humans often use to understand how something works.
Often we find it improves our understanding to model
A product and its components
A business consists of branches, departments, etc.
A country consists of states, provinces, counties,
cities, boroughs, shires, towns, villages, etc.
A college is made up of Faculties, Departments and Sections

52. Chapter 8: Properties of Objects and Classes 8. 5
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Components and Wholes. Whole-Part
In some cases, we can use this for the classical “Bill of Materials” structure, which models the parts needed for a product, and then how the parts also break down into components.
This structure is an example of a “matrix” structure.
A matrix can be viewed as a two-way tree. 1 *
consists of *
Part *

53. Chapter 8: Properties of Objects and Classes 8. 5
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Container-Contents
Container-Contents is a different and less common relationship.
In some situations we may find it helpful to view a relationship as one of these, e.g.,
Truck or Aircraft and the Products or Shipments that it carries
An actual shipping container and the goods it holds
A ship, bus or airplane and its passengers
A building and the businesses it houses.

54. And the Contents are perfectly OK without the Container
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Container-Contents
The essential difference between Whole-Part and Container-Contents relationships is that:
With Whole-Part, if you take the component away, the assembly (whole) probably won’t work any more
Take a wheel off a car
Take a hand off a clock
Take a leg off a table.
A Container, however, is still a perfectly good Container, even without its Contents
The jar is still OK even without the “water in side.”
And the Contents are perfectly OK without the Container

55. Chapter 8: Properties of Objects and Classes 8. 5
Chapter 8: Properties of Objects and Classes 8.5. Aggregation: Collection-Member
And lastly, Collection-Member is also a different and relatively uncommon relationship.
Sometimes we meet an actual collection:
A library full of books
An art gallery
A stamp or jewelry collection
A fleet of trucks, ships or aircraft.
Other times it may help to use this to describe:
A church, club or regiment and its members
An inventory of furniture or equipment.

56. Chapter 8: Properties of Objects and Classes 8.5. Aggregation.
Definition:
Aggregation is a relationship between two classes
where the instances of one class are in some way
components, members, or contents of the instances of the other class.
These are relationships that appear on the class diagram along with all the subclass hierarchies and the regular 1:M and M:M associations

57. End of Chapter 8