1. Object-Oriented Programming: Inheritance10
Object-Oriented Programming: Inheritance

2. Good as it is to inherit a library, it is better to collect one.
Say not you know another entirely, till you have divided a inheritance with him.
Johann Kasper Lavater
This method is to define as the number of a class the class of all classes similar to the given class.
Bertrand Russell
Good as it is to inherit a library, it is better to collect one.
Augustine Birrell
Save base authority from others’ books.
William Shakespeare

3. OBJECTIVES In this chapter you will learn:
What inheritance is and how it promotes software reusability.
The notions of base classes and derived classes.
To use keyword Inherits to create a class that inherits attributes and behaviors from another class.
To use the access modifier Protected in a base class to give derived class methods access to base class members.
To access base class members from a derived class with MyBase.
How constructors are used in inheritance hierarchies.
To access the current object with Me and MyClass.
The methods of class Object—the direct or indirect base class of all classes in Visual Basic.

4. 10.1 Introduction
10.2 Base Classes and Derived Classes
10.3 Protected Members
10.4 Relationship between Base Classes and Derived Classes
Creating and Using a CommissionEmployee Class
Creating a BasePlusCommissionEmployee Class without Using Inheritance
Creating a CommissionEmployee– BasePlusCommission Employee Inheritance Hierarchy
CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using Protected Instance Variables
CommissionEmployee–BasePlusCommissionEmployee Inheritance Hierarchy Using Private Instance Variables

5. 10.5 Constructors in Derived Classes
10.6 Software Engineering with Inheritance
10.7 Class Object
10.8 Friend Members
10.9 Wrap-Up

6. 10.1 Introduction Inheritance Software reusability
Create new class from existing class
Absorb existing class’s data and behaviors
Enhance with new capabilities
Derived class extends base class
Derived class
More specialized group of objects
Behaviors inherited from base class
Can customize
Additional behaviors

7. 10.1 Introduction (Cont.) Class hierarchy Direct base class
Inherited explicitly (one level up hierarchy)
Indirect base class
Inherited two or more levels up hierarchy
Single inheritance
Inherits from one base class
Multiple inheritance
Inherits from multiple base classes
Visual Basic does not support multiple inheritance

8. 10.2 Base Classes and Derived Classes
Object of one class “is an” object of another class
Example: Rectangle is a quadrilateral.
Class Rectangle inherits from class Quadrilateral
Quadrilateral: base class
Rectangle: derived class
Base class typically represents larger set of objects than derived classes
Example:
Base class: Vehicle
Cars, trucks, boats, bicycles, …
Derived class: Car
Smaller, more-specific subset of vehicles

9. Fig. 10.1 | Inheritance examples.

10. 10.2 Base Classes and Derived Classes (Cont.)
Inheritance hierarchy
Inheritance relationships: tree-like hierarchy structure
Each class becomes
Base class
Supply members to other classes OR
Derived class
Inherit members from other classes

11. Fig. 10.2 | Inheritance hierarchy for university CommunityMembers.

12. Fig. 10.3 | Inheritance hierarchy for Shapes.

13. 10.3 Protected Members Protected access
Intermediate level of protection between Public and Private
Protected members accessible by
Base class members
Derived class members
Derived class access to base class member
Keyword MyBase and a dot (.)

14. Software Engineering Observation 10.1
Derived class methods cannot directly access Private members of their base class. A derived class can change the state of Private base class instance variables only through non-Private methods provided in the base class and inherited by the derived class.

15. Software Engineering Observation 10.2
Declaring Private instance variables helps programmers test, debug and correctly modify systems. If a derived class could access its base class’s Private instance variables, classes that inherit from that derived class could access the instance variables as well. This would propagate access to what should be Private instance variables, and the benefits of information hiding would be lost.

16. 10.4 Relationship between Base Classes and Derived Classes
Base class and derived class relationship
Example: CommissionEmployee/BasePlusCommissionEmployee inheritance hierarchy
CommissionEmployee
First name, last name, SSN, commission rate, gross sale amount
BasePlusCommissionEmployee
Base salary

17. 10.4.1 Creating and Using a CommissionEmployee Class
Class CommissionEmployee
Extends class Object
Inherits
Every class in Visual Basic extends an existing class
Except object
Every class inherits object’s methods
New class implicitly extends object
To override base class method use keyword Overrides with the same signature (Overridden method must be declared Overridable)
Constructors are not inherited
The first task of any derived class constructor is to call its direct base class’s constructor

18. Outline Class CommissionEmployee Inherits class Object
Declare Private instance variables
Commmission Employee.vb
(1 of 4 )
Implicit call to object constructor
Initialize instance variables
Invoke properties GrossSales and CommissionRate to validate data

19. Outline
Commmission Employee.vb
(2 of 4 )

20. Outline
Commmission Employee.vb
(3 of 4 )
Validation

21. Outline Validation (4 of 4 ) Calculate earnings
Commmission Employee.vb
(4 of 4 )
Calculate earnings
Override method ToString of class Object

22. Common Programming Error 10.1
It is a compilation error to attempt to override a method that is not declared Overridable.

23. Common Programming Error 10.2
It is a compilation error to override a method with a method that has a different access modifier than the method being overridden.

24. Outline Instantiate CommissionEmployee object (1 of 2 )
Commission EmployeeTest.vb
(1 of 2 )
Use CommissionEmployee’s properties to retrieve and change the object’s instance variable values
Explicit call the object’s ToString method

25. Outline Explicit call the object’s CalculateEarnings method (2 of 2 )
Commission EmployeeTest.vb
(2 of 2 )
Explicit call the object’s CalculateEarnings method

26. 10.4.2 Creating a BasePlusCommissionEmployee Class without Using Inheritance
Class BasePlusCommissionEmployee
Much of the code is similar to CommissionEmployee
Private instance variables
Public methods
Constructor
Properties
Additions
Private instance variable baseSalaryValue
BaseSalary property

27. Outline Add instance variable baseSalary (1 of 5 )
BasePlus Commission Employee.vb
(1 of 5 )
Use property BaseSalary to validate data

28. Outline
BasePlus Commission Employee.vb
(2 of 5 )

29. Outline
BasePlus Commission Employee.vb
(3 of 5 )

30. Outline Validates data and sets instance variable baseSalaryValue
BasePlus Commission Employee.vb
(4 of 5 )
Update method CalculateEarnings to calculate the earnings of a base-salaried commission employee

31. Outline (5 of 5 ) Update method ToString to display base salary
BasePlus Commission Employee.vb
(5 of 5 )
Update method ToString to display base salary

32. Outline Instantiate BasePlusCommissionEmployee object (1 of 2 )
BasePlus Commission EmployeeTest.vb
(1 of 2 )
Use BasePluCommissionEmployee’s properties to retrieve and change the object’s instance variable values
Explicitly call the object’s ToString method

33. Outline (2 of 2 ) Call the object’s CalculateEarnings method
BasePlus Commission EmployeeTest.vb
(2 of 2 )

34. Software Engineering Observation 10.3
Copying and pasting code from one class to another can spread errors among multiple source code files. To avoid duplicating code (and possibly errors), use inheritance, rather than the “copy-and-paste” approach, where you want one class to “absorb” the members of another class.

35. Software Engineering Observation 10.4
With inheritance, the common instance variables and methods of all the classes in the hierarchy are declared in a base class. When changes are required for these common features, software developers need to make the changes only in the base class—derived classes then inherit the changes. Without inheritance, the changes would need to be made to all the source code files that contain copies of the code in question.

36. 10.4.3 Creating a CommissionEmployee-BasePlusCommiionEmployee Inheritance Hierarchy
Class BasePlusCommissionEmployee
Inherits class CommissionEmployee
Is a CommissionEmployee
Has instance variable baseSalaryValue
Inherits Public and Protected members
Constructor not inherited

37. Outline
Commmission Employee.vb
(1 of 4 )

38. Outline
Commmission Employee.vb
(2 of 4 )

39. Outline
Commmission Employee.vb
(3 of 4 )

40. Outline
Commmission Employee.vb
(4 of 4 )
Derived classes can now override the CalculateEarnings method

41. Outline
Class BasePluCommissionEmployee is a derived class of CommissionEmployee
BasePlus Commission Employee.vb
(1 of 3 )
Invoke the base class constructor using the base class constructor call syntax

42. Outline
BasePlus Commission Employee.vb
(2 of 3 )
Compiler generates errors because base class’s instance variable are Private

43. Outline
BasePlus Commission Employee.vb
(3 of 3 )

44. CommissionEmployee-BasePlusCommissionEmployee Inheritance Hierarchy Using Protected Instance Variables
Use protected instance variables
Enable class BasePlusCommissionEmployee to directly access base class instance variables
Base class’s Protected members are inherited by all derived classes of that base class

45. Outline Declare Protected instance variables (1 of 4 )
Commmission Employee.vb
(1 of 4 )

46. Outline
Commmission Employee.vb
(2 of 4 )

47. Outline
Commmission Employee.vb
(3 of 4 )

48. Outline
Commmission Employee.vb
(4 of 4 )

49. Outline (1 of 2 ) Must call base class’s constructor
BasePlus Commission Employee.vb
(1 of 2 )
Must call base class’s constructor

50. Outline (2 of 2 ) Overrides base class’s CalculateEarnings method
BasePlus Commission Employee.vb
(2 of 2 )
Overrides base class’s CalculateEarnings method
Directly access base class’s Protected instance variables

51. Outline Instantiate BasePlusCommissionEmployee object (1 of 2 )
BasePlus Commission EmployeeTest.vb
(1 of 2 )
Use BasePluCommissionEmployee’s properties to retrieve and change the object’s instance variable values
Explicitly call the object’s overridden ToString method

52. Outline
BasePlus Commission EmployeeTest.vb
(2 of 2 )

53. CommissionEmployee-BasePlusCommissionEmployee Inheritance Hierarchy Using Protected Instance Variables (Cont.)
Using Protected instance variables
Advantages
Derived classes can modify values directly
Slight increase in performance
Avoid Set/Get accessors call overhead
Disadvantages
No validity checking
Derived class can assign illegal value
Implementation dependent
Derived class methods more likely dependent on base class implementation
Derived class implementation changes may result in derived class modifications
Fragile (brittle) software

54. Software Engineering Observation 10.5
Use the Protected access modifier on a method when a base class is to provide the method to its derived classes but not to other clients.

55. CommissionEmployee-BasePlusCommissionEmployee Inheritance Hierarchy Using Private Instance Variables
Reexamine hierarchy
Use the best software engineering practice
Declare instance variables as Private
Provide Public Get and Set accessors
Use Get accessor to obtain values of instance variables

56. Software Engineering Observation 10.6
Declaring base class instance variables Private (as opposed to Protected) enables the base class implementation of these instance variables to change without affecting derived class implementations.

57. Error-Prevention Tip 10.1
When possible, do not include Protected instance variables in a base class. Instead, include non-Private properties and methods that carefully access Private instance variables. This will ensure that objects of the derived classes of this base class maintain consistent states of the base class instance variables.

58. Outline Declare Private instance variables (1 of 4 )
Commmission Employee.vb
(1 of 4 )

59. Outline
Commmission Employee.vb
(2 of 4 )

60. Outline
Commmission Employee.vb
(3 of 4 )

61. Outline
Commmission Employee.vb
(4 of 4 )
Use properties to obtain the values of instance variables

62. Outline Inherits from CommissionEmployee (1 of 2 )
BasePlus Commission Employee.vb
(1 of 2 )

63. Outline
BasePlus Commission Employee.vb
(2 of 2 )
Invoke an overridden base class method from a derived class
Use properties to obtain the values of instance variables
Invoke an overridden base class method from a derived class

64. Common Programming Error 10.3
When a base class method is overridden in a derived class, the derived class version often calls the base class version to do a portion of the work. Failure to prefix the base class method name with the keyword MyBase and a dot (.) separator when referencing the base class’s method causes the derived class method to call itself, creating an error called infinite recursion.

65. Outline Create BasePlusCommissionEmployee object. (1 of 2 )
BasePlus Commission EmployeeTest.vb
(1 of 2 )
Use inherited properties to access inherited Private instance variables

66. Outline
BasePlus Commission EmployeeTest.vb
(2 of 2 )

67. 10.5 Constructors in Derived Classes
Instantiating derived class object
Chain of constructor calls
Derived class constructor invokes base class constructor
Implicitly or explicitly
Base of inheritance hierarchy
Last constructor called in chain is Object’s constructor
Original derived class constructor’s body finishes executing last
Example: CommissionEmployee-BasePlusCommissionEmployee hierarchy
CommissionEmployee constructor called second last (last is Object constructor)
CommissionEmployee constructor’s body finishes execution second (first is Object constructor’s body)
My Class keyword
Similar to Me
Always invoke the version of the method defined in that particular class
Cannot be used to reference Shared class methods
They can exist when there are no objects of that class

68. Outline
Commission Employee.vb
(1 of 4 )
Constructor outputs message to demonstrate method call order.

69. Outline
Commission Employee.vb
(2 of 4 )

70. Outline
Commission Employee.vb
(3 of 4 )

71. Outline
Commission Employee.vb
(4 of 4 )

72. Outline
BasePlus Commission Employee.vb
(1 of 2 )
Constructor outputs message to demonstrate method call order.

73. Outline
BasePlus Commission Employee.vb
(2 of 2 )

74. Outline Instantiate CommissionEmployee object
Constructor.vb
Instantiate BasePlusCommissionEmployee object to demonstrate order of derived class and base class constructor method calls.
Uses the Me reference

75. Outline
Uses the MyClass reference

76. 10.6 Software Engineering with Inheritance
Customizing existing software
Inherit from existing classes
Include additional members
Redefine base class members
No direct access to base class’s source code
Link to object code
Independent software vendors (ISVs)
Develop proprietary code for sale/license
Available in object-code format
Users derive new classes
Without accessing ISV proprietary source code

77. Software Engineering Observation 10.7
Despite the fact that inheriting from a class does not require access to the class’s source code, developers often insist on seeing the source code to see how the class is implemented. They want to ensure that they are extending a solid class that performs well and is implemented securely.

78. Software Engineering Observation 10.8
At the design stage in an object-oriented system, the designer often finds that certain classes are closely related. The designer should “factor out” common instance variables and methods and place them in a base class. Then the designer should use inheritance to develop derived classes, specializing them with capabilities beyond those inherited from the base class.

79. Performance Tip 10.1
If derived classes are larger than they need to be (i.e., contain too much functionality), memory and processing resources may be wasted. Extend the base class that contains the functionality that is closest to what you need.

80. 10.7 Class Object Class Object methods Equals Finalize GetHashCode
GetType
MemberwiseClone
ReferenceEquals
ToString

81. Fig. 10. 20 | Object methods that are inherited by all classes
Fig | Object methods that are inherited by all classes. (Part 1 of 3.)

82. Fig. 10. 20 | Object methods that are inherited by all classes
Fig | Object methods that are inherited by all classes. (Part 2 of 3.)

83. Fig. 10. 20 | Object methods that are inherited by all classes
Fig | Object methods that are inherited by all classes. (Part 3 of 3.)

84. 10.8 Friend Members Friend Access Another intermediate level
Can be accessed only by code in the same assembly
No “special” effect where there is only one class declaration
To access a non-Shared Friend member:
nameOfObject.nonSharedFriendMember
To access a Shared Friend member:
nameOfClass. SharedFriendMember
Protected Friend Members
Are accessible both from code in the same assembly and by derived classes