1. Chapter 8 More Object Concepts
An Object-Oriented Approach to Programming Logic and Design Fourth Edition
Chapter 8
More Object Concepts

2. Objectives In this chapter, you will learn about: Constructors
Destructors
The concept of inheritance
Inheritance terminology
Accessing private members of a parent class
An Object-Oriented Approach to Programming Logic and Design

3. Objectives (cont’d) Overriding base class methods
How constructors are called during inheritance
The concept that a derived class object “is an” instance of the base class
Using inheritance to achieve good software design
An Object-Oriented Approach to Programming Logic and Design

4. An Introduction to Constructors
Can create classes to encapsulate data and methods
Can instantiate objects from these classes
Figure 8-1
Figure 8-1 shows an Employee class
Contains the fields lastName and hourlyWage
Contains methods that set and return values for those fields
An Object-Oriented Approach to Programming Logic and Design

5. An Introduction to Constructors (cont’d)
Method that establishes an object
Provides its name and reserves memory space
Created automatically by compiler for every class
Declaring Employee object
Constructor establishes one Employee instance
Depending on language, may provide initial values for object’s data fields
An Object-Oriented Approach to Programming Logic and Design

6. An Introduction to Constructors (cont’d)
Can write your own constructor
If you want different default values
If you want to perform additional tasks
Must have same name as class it constructs
Cannot have a return type
Place anywhere inside the class, outside another method
An Object-Oriented Approach to Programming Logic and Design

7. A default constructor has no parameters
Figure 8-2
Employee class with a default constructor that sets a value for hourlyWage
A default constructor has no parameters
Constructor calls setHourlyWage() method and passes it 10.00
An Object-Oriented Approach to Programming Logic and Design

8. Program that declares Employee objects using class in Figure 8-2
setHourlyWage()not used in the program
Figure 8-3
Figure 8-4
Sample program output for Figure 8-3
Wage: set by constructor
An Object-Oriented Approach to Programming Logic and Design

9. Constructors with Parameters
Allow each object instantiation to have different initial values
Figure 8-5: Employee class with a constructor that accepts a parameter
Must pass a numeric value to the constructor
Figure 8-5
An Object-Oriented Approach to Programming Logic and Design

10. Overloading Instance Methods and Constructors
Same concept as overloading methods
Figure 8-6: Overloaded Employee class constructors
Figure 8-6
One version requires no argument (default constructor)
Other requires numeric argument
An Object-Oriented Approach to Programming Logic and Design

11. Overloading Instance Methods and Constructors (cont’d)
When using the class in Figure 8-6 (previous slide)
Can make the following statements
Figure 8-7 shows overloaded constructors
Parameterless constructor calls one that requires a parameter
Figure 8-7
An Object-Oriented Approach to Programming Logic and Design

12. Understanding Destructors
Contains actions you require when instance of a class is destroyed
Most often destroyed when it goes out of scope
Identifier (class name) is preceded by a tilde (~)
No arguments can be provided
Cannot be overloaded
No return type
An Object-Oriented Approach to Programming Logic and Design

13. Program never explicitly calls Student class destructor
Student class with destructor
Unusual for a constructor or destructor to display anything
Figure 8-8
Figure 8-9
Program never explicitly calls Student class destructor
Invoked automatically
Figure 8-10
DemoStudentDestructor program output: first object created is last object destroyed
An Object-Oriented Approach to Programming Logic and Design

14. Understanding Inheritance
Introduced in Chapter 7
Can apply knowledge of a general category to more specific objects
Classes can inherit data and methods from existing classes
An Object-Oriented Approach to Programming Logic and Design

15. Understanding Inheritance (cont’d)
Figure 8-11
Figure 8-11 shows Customer class
contains two data fields
contains methods that get and set each field
Sample Customer class objects
Customer firstCustomer
Customer SecondCustomer
An Object-Oriented Approach to Programming Logic and Design

16. Figure 8-13 shows pseudocode for PreferredCustomer class
InheritsFrom Customer indicates inheritance
Each programming language uses its own syntax
Java: extends
Figure 8-12 shows diagram of how PreferredCustomer class inherits from Customer class
An Object-Oriented Approach to Programming Logic and Design

17. Understanding Inheritance (cont’d)
Benefits of using inheritance
Saves time by not recreating Customer fields and methods
Reduces chance of errors (Customer methods already used and tested)
Makes it easier for anyone who used Customer class to understand PreferredCustomer class
Reduces chance of inconsistencies in shared fields
Makes programs easier to write, easier to understand, and less proven to errors
Makes code reusable
An Object-Oriented Approach to Programming Logic and Design

18. Understanding Inheritance Terminology
Base class
Class that is used as a basis for inheritance
Derived class (or extended class)
Created class that inherits from a base class
“Is a” case or instance of the base class
PreferredCustomer “is a” Customer
Superclass and subclass; parent class and child class
Synonyms for base class and derived class
Evergreen class can be considered a subclass of the Tree superclass
An Object-Oriented Approach to Programming Logic and Design

19. Understanding Inheritance Terminology (cont’d)
To discover which is base and which is derived class
Say names together; more specific name is first
Look at size; derived class generally larger than base
Derived class can be further extended
A subclass can have a child of its own
Example: Evergreen (derived class from Tree class)
Can create Spruce class from Evergreen
Ancestors are an entire list of parent classes
An Object-Oriented Approach to Programming Logic and Design

20. Understanding Inheritance Terminology (cont’d)
Possible ancestors and descendents of Dog class
Figure 8-14
An Object-Oriented Approach to Programming Logic and Design

21. Understanding Inheritance Terminology (cont’d)
customer2 object can use all methods of its parent
Figure 8-15
Figure 8-16
An Object-Oriented Approach to Programming Logic and Design

22. Understanding Inheritance Terminology (cont’d)
A child class contains the data fields and methods of its parent
Parent class object cannot access child’s data and methods
An Object-Oriented Approach to Programming Logic and Design

23. Accessing Private Members of a Parent Class
Methods are public, but data is often private
No outside class can access data directly
Child class cannot directly access parent class’s data
Figure 8-17
An Object-Oriented Approach to Programming Logic and Design

24. Accessing Private Members of a Parent Class (cont’d)
Protected access
Used when you want no outside classes except descendent classes to use a data field
Figure 8-18 shows how data and methods are accessible by child and outside classes
Figure 8-18
An Object-Oriented Approach to Programming Logic and Design

25. Customer class uses the protected access specifier on its purchaseTotal field
Figure 8-19
An Object-Oriented Approach to Programming Logic and Design

26. PreferredCustomer class when purchaseTotal remains private
Figure 8-20
PreferredCustomer class when purchaseTotal remains private
Class uses the public method setPurchaseTotal()
An Object-Oriented Approach to Programming Logic and Design

27. Accessing Private Members of a Parent Class (cont’d)
Benefits of using public method to access private data
No protected access specifiers are needed in parent class
Creators of parent class did not have to foresee need for protected field
If parent class method contains additional code to enforce limits on values, they would be enforced for parent and child objects
Likelihood of errors decreases
Best approach is often to use public method
An Object-Oriented Approach to Programming Logic and Design

28. Overriding Base Class Methods
Override a method in a parent class
Create a method with same signature as parent version
Parent version becomes hidden from objects of the child class
When using method name with child class object, child class’s version is used
When using method name with parent class object, parent class’s version is used
An Object-Oriented Approach to Programming Logic and Design

29. Overriding Base Class Methods (cont’d)
Figure 8-21
Student class contains three fields and a get method for each field
There is no set method for tuition because clients cannot set tuition directly
An Object-Oriented Approach to Programming Logic and Design

30. Figure 8-22 shows how Student class is implemented
An Object-Oriented Approach to Programming Logic and Design

31. Overriding Base Class Methods (cont’d)
Three ways of creating ScholarshipStudent class
Create a special method in the ScholarshipStudent class with a name such as setScholarshipStudentCredits()
Create a method in the ScholarshipStudent class with same name as parent class method but different parameter list
Override the setCredits()method in Student class
Best solution
Child class method is only used with child class objects
An Object-Oriented Approach to Programming Logic and Design

32. Overriding Base Class Methods (cont’d)
Calls superclass version of the method setCredits()
Figure 8-23
Class contains setCredits() method
overrides setCredits()method in Student class
actualCredits stores # of credits for which student is enrolled
Child class method setCredits()calls parent class method with same name, passing 0 as the number of billableCredits
An Object-Oriented Approach to Programming Logic and Design

33. Overriding Base Class Methods (cont’d)
Figure 8-24
Application that declares a full-time Student and a full-time ScholarshipStudent
Figure 8-25
An Object-Oriented Approach to Programming Logic and Design

34. Understanding How Constructors are Called during Inheritance
Creating an object
Calling a constructor with the same name as the class
Instantiating an object that is a member of a subclass
Calling two constructors: one for base class and one for derived class
Superclass constructor executes first and then subclass
If superclass has a default constructor
Can create a subclass with or without its own constructor
An Object-Oriented Approach to Programming Logic and Design

35. Understanding How Constructors are Called during Inheritance (cont’d)
If superclass has only constructors that require arguments
Subclass constructors can contain any number of statements
Each subclass constructor must call superclass constructor and pass required arguments
Parent class constructor must be fully executed before child class constructor can operate
Must include at least one constructor for each subclass
Must call the superclass constructor and pass the required arguments to it
An Object-Oriented Approach to Programming Logic and Design

36. Employee class contains single constructor that requires two parameters
Figure 8-26
Child class constructor calling base class constructor with constant arguments
An Object-Oriented Approach to Programming Logic and Design

37. Employee class contains single constructor that requires two parameters
Child class constructor calling base class constructor with variable arguments
Figure 8-27
An Object-Oriented Approach to Programming Logic and Design

38. Understanding How a Derived Class Object “is an” Instance of the Base Class
Every derived class object “is a” specific instance of both the derived class and the base class
MyCar “is a” Car as well as a Vehicle
Can assign a derived class object to an object of any type that is an ancestor
Implicit conversion
Made when a derived class object is assigned to an object of any of its superclass types
An Object-Oriented Approach to Programming Logic and Design

39. Program passes a Student object and a ScholarshipStudent object to the display()method
Figure 8-28
An Object-Oriented Approach to Programming Logic and Design

40. Using Inheritance to Achieve Good Software Design
Benefits of using inheritance
Saves development time because much of class code is already written
Saves testing time because superclass code has been written and tested; it is reliable
Reduces time to learn the new class features
Superclass code maintains its integrity
An Object-Oriented Approach to Programming Logic and Design

41. Summary Constructor Destructor Method that establishes an object
Default constructor created automatically by the compiler
Can be created by the programmer
Must have the same name as the class it constructs and cannot have return type
May receive arguments
Destructor
Contains actions required when class instance is destroyed
Automatically provided unless explicitly created
An Object-Oriented Approach to Programming Logic and Design

42. Summary (cont’d) Inheritance
Principle of applying knowledge of a general category to specific objects
New class contains all fields and methods of an existing class plus added members
Makes programs easier to write and understand
Less prone to errors
Base class: class that is the basis for inheritance
Derived or extended class: class that inherits from a base class
An Object-Oriented Approach to Programming Logic and Design

43. Summary (cont’d) Relationship terminology: Private data fields
parent and child class; super and subclass
Private data fields
Child classes cannot access parent data fields
protected access modifier can be used
Can always use public methods of the base class
Overriding methods in a child class
Create a method with same signature as parent version
Instantiating subclass objects
Calls two constructors: one for base and one for derived class
An Object-Oriented Approach to Programming Logic and Design

44. Summary (cont’d) Derived class objects Use inheritance to:
“Is a” specific instance of the derived class and base class
Use inheritance to:
Reduce development and testing time
Provide reliable code
Make it easier for clients to learn to use the new class
An Object-Oriented Approach to Programming Logic and Design