CS 112 Introduction to Programming, Spring 2001

1
Outline

• Class inheritance why?
• Class inheritance how
• Syntax
• How to control inheritance

2
Software Design and Reuse

• Question What features allow us to better
  organize a program?
• Question What programming language feature we
  have covered allows software reuse?

3
C Classes

• C classes play dual roles
• Program modules containing a list of (static)
  method declarations and (static) data fields
• Blueprints for generating objects
• Instance variables (or fields)
• (Dynamic) methods
• Constructors (for building new objects) and
  destructors
• Properties
• Or a mix of both!
• Classes support inheritance (for code reuse)
  and
• data encapsulation (e.g., private
  components, for abstraction) --- the essence of
  object-oriented programming

4
Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class or
  subclass.
• As the name implies, the child inherits
  characteristics of the parent
• That is, the child class inherits the methods and
  data defined for the parent class

5
Inheritance

• Inheritance relationships are often shown
  graphically in a class diagram, with the arrow
  pointing to the parent class

Inheritance should create an is-a relationship,
meaning the child is a more specific version of
the parent
fly() void
6
Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The existing class is called the parent class, or
  superclass, or base class
• The derived class is called the child class, or
  subclass, or derived class.
• As the name implies, the child inherits
  characteristics of the parent
• That is, the child class inherits the methods and
  data defined for the parent class

7
Inheritance

• Inheritance relationships are often shown
  graphically in a class diagram, with the arrow
  pointing to the parent class

Inheritance should create an is-a relationship,
meaning the child is a more specific version of
the parent
8
Base Classes and Derived Classes
9
Software Engineering with Inheritance

• Can customize derived classes to meet needs by
• Creating new member variables
• Creating new methods
• Override base-class members
• .NET Framework Class Library(FCL) allows full
  reuse of software through inheritance

10
Outline

• Class inheritance why?
• Class inheritance how
• Syntax
• How to control inheritance

11
Deriving Subclasses

• In Java, we use the reserved word extends to
  establish an inheritance relationship
• class Animal
• // class contents
• class Bird extends Animal
• // class contents

12
C Class Declaration

• A class declaration declares a new class Ident
• class-modifiers class Ident base-class
• class-member-declarations
• Sample class modifiers public, protected,
  internal, private, abstract, sealed
• Base-class the parent class if omitted
  the parent is Object

13
C Class Member Declarations

• Constant declarations
• modifiers const type ident1
  constant-expression1,
• 
  
  
  .
• identn
  constant-expressionn
• Modifiers can be private, public, internal,
  protected,
• Examples public const double X
  1.0, Y 2.0, Z 3.0
• Field declarations
• modifiers type ident1
  variable-initializer1 ,
• 
  .
• identn
  variable-initializern
• Modifiers can be private, public, internal,
  protected,
• static, readonly
• Examples public static int X 1, Y, Z
  100

14
C Class Member Declarations (contd)

• Method declarations
• modifiers return-type ident
  (formal-parameter-list)
• 
  
  
  ..
• 
  method-body
• 
  ..
• Modifiers can be private, public, internal,
  protected,
• static,
• new, virtual, sealed, override,
  abstract
• Note abstract methods may not have a
  method body.
• Constructor declarations
• modifiers ClsIdent (formal-parameter-list)
  constructor-initializer
• 
  
  
  ..
• 
  constructor-body
• 
  ..

15
C Class Member Declarations

• Constant declarations
• modifiers const type ident1
  constant-expression1,
• 
  
  
  .
• identn
  constant-expressionn
• Modifiers can be private, public, internal,
  protected,
• Examples public const double X
  1.0, Y 2.0, Z 3.0
• Field declarations
• modifiers type ident1
  variable-initializer1 ,
• 
  .
• identn
  variable-initializern
• Modifiers can be private, public, internal,
  protected,
• static, readonly
• Examples public static int X 1, Y, Z
  100

16
Outline

• Class inheritance why?
• Class inheritance how
• Syntax
• How to control inheritance

17
Controlling Inheritance

• Visibility modifiers determine which class
  members get inherited and which do not
• Members (variables and methods) declared with
  public visibility are inherited, and those with
  private visibility are not
• Problem public variables violate our goal of
  encapsulation
• Solution Java provides a third visibility
  modifier that helps in inheritance situations
  protected

18
Controlling Inheritance

• Visibility modifiers determine which class
  members get inherited and which do not
• Variables and methods declared with public
  visibility are inherited, and those with private
  visibility are not
• But public variables violate our goal of
  encapsulation
• There is a third visibility modifier that helps
  in inheritance situations protected

19
protected and internal Members

• protected members
• can be accessed by base class or any class
  derived from that base class
• internal members
• can only be accessed by classed declared in the
  same assembly
• Overridden base class members can be accessed
• base.member

20
The protected Modifier

• The protected visibility modifier allows a member
  of a base class to be inherited into the child
• protected visibility provides more encapsulation
  than public does
• protected visibility is not as tightly
  encapsulated as private visibility
• The details of each modifier are given in
  Appendix F
• Example
• Book.java
• Dictionary.java
• TestBookDictionary.java

21
Defining Methods in the Child Class the super
Reference

• Constructors are not inherited, even though they
  have public visibility
• Yet we often want to use the parent's constructor
  to set up the "parent's part" of the object
• The super reference can be used to refer to the
  parent class, and is often used to invoke the
  parent's constructor
• Example1
• See Book2.java
• See Dictionary2.java
• See Words2.java
• Example 2
• See FoodItem.java
• See Pizza.java
• See FoodAnalysis.java

22
Defining Methods in the Child Class Overriding
Methods

• A child class can override the definition of an
  inherited method in favor of its own
• That is, a child can redefine a method that it
  inherits from its parent
• The new method must have the same signature as
  the parent's method, but can have different code
  in the body
• The type of the object executing the method
  determines which version of the method is invoked

23
Overriding Methods

• See Thought.java
• See Advice.java
• See Messages.java
• Note
• An overridden parent method can be explicitly
  invoked using the super reference at the child
• If a method is declared with the final modifier,
  it cannot be overridden
• The concept of overriding can be applied to data
  (called shadowing variables), there is generally
  no need for it

24
Overloading vs. Overriding

• Overloading deals with multiple methods in the
  same class with the same name but different
  signatures
• Overloading lets you define a similar operation
  in different ways for different data

• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature
• Overriding lets you define a similar operation in
  different ways for different object types

25
Review Inheritance

• Inheritance allows a software developer to derive
  a new class from an existing one
• The visibility modifier protected
• The super reference can be used to refer to the
  parent class, and is often used to invoke the
  parent's constructor
• A child class can override the definition of an
  inherited method in favor of its own

26
Overloading vs. Overriding

• Overloading deals with multiple methods in the
  same class with the same name but different
  signatures
• Overloading lets you define a similar operation
  in different ways for different data

• Overriding deals with two methods, one in a
  parent class and one in a child class, that have
  the same signature
• Overriding lets you define a similar operation in
  different ways for different object types

27
Outline

• Class hierarchies
• The Object class
• Using class inheritance to implement polymorphism

28
Single vs. Multiple Inheritance

• C and Java support single inheritance, meaning
  that a derived class can have only one parent
  class
• Multiple inheritance allows a class to be derived
  from two or more classes, inheriting the members
  of all parents
• Collisions, such as the same variable name in two
  parents, have to be resolved
• In most cases, the use of interfaces gives us the
  best aspects of multiple inheritance without the
  overhead

29
Class Hierarchies

• Two children of the same parent are called
  siblings
• Good class design puts all common features as
  high in the hierarchy as is reasonable
• An inherited member is continually passed down
  the line
• Class hierarchies often have to be extended and
  modified to keep up with changing needs
• There is no single class hierarchy that is
  appropriate for all situations

30
The Object Class

• A class called Object is defined in the C
  standard class library
• All classes are derived from the Object class
• If a class is not explicitly defined to be the
  child of an existing class, it is assumed to be
  the child of the Object class
• The Object class is therefore the ultimate root
  of all class hierarchies

31
The Object Class

• A class called Object is defined in the java.lang
  package of the Java standard class library
• All classes are derived from the Object class
• If a class is not explicitly defined to be the
  child of an existing class, it is assumed to be
  the child of the Object class
• The Object class is therefore the ultimate root
  of all class hierarchies
• The Object class contains a few useful methods,
  which are inherited by all classes
• toString()
• equals()
• clone()

32
The Object Class the toString Method

• Thats why the println method can call toString
  for any object that is passed to it all objects
  are guaranteed to have a toString method via
  inheritance
• The toString method in the Object class is
  defined to return a string that contains the name
  of the objects class and a hash value
• Every time we have defined toString, we have
  actually been overriding it

33
The Object Class the equals Method

• The equals method of the Object class determines
  if two variables point to the same object (more
  shortly)
• You may choose to override equals to define
  equality in some other way
• Example
• See Student.java
• See GradStudent.java
• See Academia.java

34
Relationship btw Base- and Derived Classes

• Use a point-circle hierarchy to represent
  relationship between base and derived classes
• The first thing a derived class does is call its
  base class constructor, either explicitly or
  implicitly
• override keyword is needed if a derived-class
  method overrides a base-class method
• If a base class method is going to be overridden
  it must be declared virtual

35
Class Hierarchies

• A child class of one parent can be the parent of
  another child, forming class hierarchies

Animal
Reptile
Bird
Mammal
Snake
Lizard
Bat
Horse
Parrot
36
More Examples Base Classes and Derived Classes
CommunityMemeber
Employee
Student
Alumnus
Faculty
Staff
Professor
AssistantProfessor
Fig. 9.2 Inheritance hierarchy for university
CommunityMembers.
37
Yet More Examples Base Classes and Derived
Classes
Shape
TwoDimensionalShape
ThreeDimensionalShape
Sphere
Cube
Cylinder
Triangle
Square
Circle
Fig. 9.3 Portion of a Shape class hierarchy.
38
Class Hierarchies

• Good class design puts all common features as
  high in the hierarchy as is reasonable
• An inherited member is continually passed down
  the lineinheritance is transitive
• Class hierarchies often have to be extended and
  modified to keep up with changing needs
• There is no single class hierarchy that is
  appropriate for all situations

39
Point.cs

• 1 // Fig. 9.4 Point.cs
• 2 // Point class represents an x-y coordinate
  pair.
• 3
• 4 using System
• 5
• 6 // Point class definition implicitly
  inherits from Object
• 7 public class Point
• 8
• 9 // point coordinates
• 10 private int x, y
• 11
• 12 // default (no-argument) constructor
• 13 public Point()
• 14
• 15 // implicit call to Object constructor
  occurs here
• 16
• 17
• 18 // constructor
• 19 public Point( int xValue, int yValue )

40
Point.cs Program Output

• 34 set
• 35
• 36 x value // no need for
  validation
• 37
• 38
• 39 // end property X
• 40
• 41 // property Y
• 42 public int Y
• 43
• 44 get
• 45
• 46 return y
• 47
• 48
• 49 set
• 50
• 51 y value // no need for
  validation
• 52

41
PointTest.cs

• 1 // Fig. 9.5 PointTest.cs
• 2 // Testing class Point.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // PointTest class definition
• 8 class PointTest
• 9
• 10 // main entry point for application
• 11 static void Main( string args )
• 12
• 13 // instantiate Point object
• 14 Point point new Point( 72, 115 )
• 15
• 16 // display point coordinates via X and
  Y properties
• 17 string output "X coordinate is "
  point.X
• 18 "\n" "Y coordinate is "
  point.Y
• 19

42
Circle.cs

• 1 // Fig. 9.6 Circle.cs
• 2 // Circle class contains x-y coordinate pair
  and radius.
• 3
• 4 using System
• 5
• 6 // Circle class definition implicitly
  inherits from Object
• 7 public class Circle
• 8
• 9 private int x, y // coordinates of
  Circle's center
• 10 private double radius // Circle's radius
• 11
• 12 // default constructor
• 13 public Circle()
• 14
• 15 // implicit call to Object constructor
  occurs here
• 16
• 17
• 18 // constructor
• 19 public Circle( int xValue, int yValue,
  double radiusValue )

43
Circle.cs

• 35 set
• 36
• 37 x value // no need for
  validation
• 38
• 39
• 40 // end property X
• 41
• 42 // property Y
• 43 public int Y
• 44
• 45 get
• 46
• 47 return y
• 48
• 49
• 50 set
• 51
• 52 y value // no need for
  validation
• 53

44
Circle.cs

• 70
• 71 // end property Radius
• 72
• 73 // calculate Circle diameter
• 74 public double Diameter()
• 75
• 76 return radius 2
• 77
• 78
• 79 // calculate Circle circumference
• 80 public double Circumference()
• 81
• 82 return Math.PI Diameter()
• 83
• 84
• 85 // calculate Circle area
• 86 public double Area()
• 87
• 88 return Math.PI Math.Pow( radius, 2
  )

45
CircleTest.cs

• 1 // Fig. 9.7 CircleTest.cs
• 2 // Testing class Circle.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // CircleTest class definition
• 8 class CircleTest
• 9
• 10 // main entry point for application.
• 11 static void Main( string args )
• 12
• 13 // instantiate Circle
• 14 Circle circle new Circle( 37, 43,
  2.5 )
• 15
• 16 // get Circle's initial x-y
  coordinates and radius
• 17 string output "X coordinate is "
  circle.X
• 18 "\nY coordinate is " circle.Y
  "\nRadius is "
• 19 circle.Radius

46
CircleTest.cs

• 34 // display Circle's circumference
• 35 output "Circumference is "
• 36 String.Format( "0F",
  circle.Circumference() ) "\n"
• 37
• 38 // display Circle's area
• 39 output "Area is "
• 40 String.Format( "0F",
  circle.Area() )
• 41
• 42 MessageBox.Show( output,
  "Demonstrating Class Circle" )
• 43
• 44 // end method Main
• 45
• 46 // end class CircleTest

47
Circle2.cs

• 1 // Fig. 9.8 Circle2.cs
• 2 // Circle2 class that inherits from class
  Point.
• 3
• 4 using System
• 5
• 6 // Circle2 class definition inherits from
  Point
• 7 class Circle2 Point
• 8
• 9 private double radius // Circle2's
  radius
• 10
• 11 // default constructor
• 12 public Circle2()
• 13
• 14 // implicit call to Point constructor
  occurs here
• 15
• 16
• 17 // constructor
• 18 public Circle2( int xValue, int yValue,
  double radiusValue )
• 19

48
Circle2.cs

• 34 set
• 35
• 36 if ( value gt 0 )
• 37 radius value
• 38
• 39
• 40 // end property Radius
• 41
• 42 // calculate Circle diameter
• 43 public double Diameter()
• 44
• 45 return radius 2
• 46
• 47
• 48 // calculate Circle circumference
• 49 public double Circumference()
• 50
• 51 return Math.PI Diameter()
• 52

49
Circle2.cs program output
50
Point2.cs

• 1 // Fig. 9.9 Point2.cs
• 2 // Point2 class contains an x-y coordinate
  pair as protected data.
• 3
• 4 using System
• 5
• 6 // Point2 class definition implicitly
  inherits from Object
• 7 public class Point2
• 8
• 9 // point coordinate
• 10 protected int x, y
• 11
• 12 // default constructor
• 13 public Point2()
• 14
• 15 // implicit call to Object constructor
  occurs here
• 16
• 17
• 18 // constructor
• 19 public Point2( int xValue, int yValue )

51
Point2.cs

• 34 set
• 35
• 36 x value // no need for
  validation
• 37
• 38
• 39 // end property X
• 40
• 41 // property Y
• 42 public int Y
• 43
• 44 get
• 45
• 46 return y
• 47
• 48
• 49 set
• 50
• 51 y value // no need for
  validation
• 52

52
Circle3.cs

• 1 // Fig. 9.10 Circle3.cs
• 2 // Circle2 class that inherits from class
  Point2.
• 3
• 4 using System
• 5
• 6 // Circle3 class definition inherits from
  Point2
• 7 public class Circle3 Point2
• 8
• 9 private double radius // Circle's radius
• 10
• 11 // default constructor
• 12 public Circle3()
• 13
• 14 // implicit call to Point constructor
  occurs here
• 15
• 16
• 17 // constructor
• 18 public Circle3(
• 19 int xValue, int yValue, double
  radiusValue )

53
Circle3.cs

• 35 set
• 36
• 37 if ( value gt 0 )
• 38 radius value
• 39
• 40
• 41 // end property Radius
• 42
• 43 // calculate Circle diameter
• 44 public double Diameter()
• 45
• 46 return radius 2
• 47
• 48
• 49 // calculate circumference
• 50 public double Circumference()
• 51
• 52 return Math.PI Diameter()
• 53

54
CircleTest3.cs

• 1 / Fig. 9.11 CircleTest3.cs
• 2 // Testing class Circle3.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // CircleTest3 class definition
• 8 class CircleTest3
• 9
• 10 // main entry point for application
• 11 static void Main( string args )
• 12
• 13 // instantiate Circle3
• 14 Circle3 circle new Circle3( 37, 43,
  2.5 )
• 15
• 16 // get Circle3's initial x-y
  coordinates and radius
• 17 string output "X coordinate is "
  circle.X "\n"
• 18 "Y coordinate is " circle.Y
  "\nRadius is "
• 19 circle.Radius

55
CircleTest3.cs

• 35 // display Circle3's Circumference
• 36 output "Circumference is "
• 37 String.Format( "0F",
  circle.Circumference() ) "\n"
• 38
• 39 // display Circle3's Area
• 40 output "Area is "
• 41 String.Format( "0F",
  circle.Area() )
• 42
• 43 MessageBox.Show( output,
  "Demonstrating Class Circle3" )
• 44
• 45 // end method Main
• 46
• 47 // end class CircleTest3

56
Point3.cs

• 1 // Fig. 9.12 Point3.cs
• 2 // Point3 class represents an x-y coordinate
  pair.
• 3
• 4 using System
• 5
• 6 // Point3 class definition implicitly
  inherits from Object
• 7 public class Point3
• 8
• 9 // point coordinate
• 10 private int x, y
• 11
• 12 // default constructor
• 13 public Point3()
• 14
• 15 // implicit call to Object constructor
  occurs here
• 16
• 17
• 18 // constructor
• 19 public Point3( int xValue, int yValue )

57
Point3.cs

• 34 set
• 35
• 36 x value // no need for
  validation
• 37
• 38
• 39 // end property X
• 40
• 41 // property Y
• 42 public int Y
• 43
• 44 get
• 45
• 46 return y
• 47
• 48
• 49 set
• 50
• 51 y value // no need for
  validation
• 52

58
Circle4.cs

• 1 // Fig. 9.13 Circle4.cs
• 2 // Circle4 class that inherits from class
  Point3.
• 3
• 4 using System
• 5
• 6 // Circle4 class definition inherits from
  Point3
• 7 public class Circle4 Point3
• 8
• 9 private double radius
• 10
• 11 // default constructor
• 12 public Circle4()
• 13
• 14 // implicit call to Point constructor
  occurs here
• 15
• 16
• 17 // constructor
• 18 public Circle4( int xValue, int yValue,
  double radiusValue )
• 19 base( xValue, yValue )

59
Circle4.cs

• 36
• 37
• 38 // end property Radius
• 39
• 40 // calculate Circle diameter
• 41 public double Diameter()
• 42
• 43 return Radius 2 // use property
  Radius
• 44
• 45
• 46 // calculate Circle circumference
• 47 public double Circumference()
• 48
• 49 return Math.PI Diameter()
• 50
• 51
• 52 // calculate Circle area
• 53 public virtual double Area()
• 54

60
CircleTest4.cs

• 1 // Fig. 9.14 CircleTest4.cs
• 2 // Testing class Circle4.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // CircleTest4 class definition
• 8 class CircleTest4
• 9
• 10 // main entry point for application
• 11 static void Main( string args )
• 12
• 13 // instantiate Circle4
• 14 Circle4 circle new Circle4( 37, 43,
  2.5 )
• 15
• 16 // get Circle4's initial x-y
  coordinates and radius
• 17 string output "X coordinate is "
  circle.X "\n"
• 18 "Y coordinate is " circle.Y
  "\n"
• 19 "Radius is " circle.Radius

61
CircleTest4.cs

• 35 // display Circle4's Circumference
• 36 output "Circumference is "
• 37 String.Format( "0F",
  circle.Circumference() ) "\n"
• 38
• 39 // display Circle4's Area
• 40 output "Area is "
• 41 String.Format( "0F",
  circle.Area() )
• 42
• 43 MessageBox.Show( output,
  "Demonstrating Class Circle4" )
• 44
• 45 // end method Main
• 46
• 47 // end class CircleTest4

62
Case Study Three-Level Inheritance Hierarchy

• Three-level inheritance example
• Class Cylinder inherits from class Circle4
• Class Circle4 inherits from class Point3

63
Cylinder.cs

• 1 // Fig. 9.15 Cylinder.cs
• 2 // Cylinder class inherits from class
  Circle4.
• 3
• 4 using System
• 5
• 6 // Cylinder class definition inherits from
  Circle4
• 7 public class Cylinder Circle4
• 8
• 9 private double height
• 10
• 11 // default constructor
• 12 public Cylinder()
• 13
• 14 // implicit call to Circle4
  constructor occurs here
• 15
• 16
• 17 // four-argument constructor
• 18 public Cylinder( int xValue, int yValue,
  double radiusValue,
• 19 double heightValue ) base( xValue,
  yValue, radiusValue )

64
Cylinder.cs

• 36
• 37
• 38 // end property Height
• 39
• 40 // override Circle4 method Area to
  calculate Cylinder area
• 41 public override double Area()
• 42
• 43 return 2 base.Area()
  base.Circumference() Height
• 44
• 45
• 46 // calculate Cylinder volume
• 47 public double Volume()
• 48
• 49 return base.Area() Height
• 50
• 51
• 52 // convert Cylinder to string
• 53 public override string ToString()
• 54

65
CylinderTest.cs

• 1 // Fig. 9.16 CylinderTest.cs
• 2 // Tests class Cylinder.
• 3
• 4 using System
• 5 using System.Windows.Forms
• 6
• 7 // CylinderTest class definition
• 8 class CylinderTest
• 9
• 10 // main entry point for application
• 11 static void Main( string args )
• 12
• 13 // instantiate object of class
  Cylinder
• 14 Cylinder cylinder new Cylinder(12,
  23, 2.5, 5.7)
• 15
• 16 // properties get initial x-y
  coordinate, radius and height
• 17 string output "X coordinate is "
  cylinder.X "\n"
• 18 "Y coordinate is " cylinder.Y
  "\nRadius is "
• 19 cylinder.Radius "\n" "Height is
  " cylinder.Height

66
CylinderTest.cs

• 35 // display Cylinder's Circumference
• 36 output "Circumference is "
• 37 String.Format( "0F",
  cylinder.Circumference() ) "\n"
• 38
• 39 // display Cylinder's Area
• 40 output "Area is "
• 41 String.Format( "0F",
  cylinder.Area() ) "\n"
• 42
• 43 // display Cylinder's Volume
• 44 output "Volume is "
• 45 String.Format( "0F",
  cylinder.Volume() )
• 46
• 47 MessageBox.Show( output,
  "Demonstrating Class Cylinder" )
• 48
• 49 // end method Main
• 50
• 51 // end class CylinderTest

67
Abstract Classes

• Java allows abstract classes
• Use the modifier abstract on a class header to
  declare an abstract class abstract class
  Vehicle
• An abstract class is a placeholder in a class
  hierarchy that represents a generic concept

Vehicle
Car
Boat
Plane
68
Constructors and Destructors in Derived Classes

• Instantiating a derived class, causes base class
  constructor to be called, implicitly or
  explicitly
• Can cause chain reaction when a base class is
  also a derived class
• When a destructor is called, it performs its task
  and then invokes the derived class base class
  constructor

69
Point4.cs

• 1 // Fig. 9.17 Point4.cs
• 2 // Point4 class represents an x-y coordinate
  pair.
• 3
• 4 using System
• 5
• 6 // Point4 class definition
• 7 public class Point4
• 8
• 9 // point coordinate
• 10 private int x, y
• 11
• 12 // default constructor
• 13 public Point4()
• 14
• 15 // implicit call to Object constructor
  occurs here
• 16 Console.WriteLine( "Point4
  constructor 0", this )
• 17
• 18
• 19 // constructor

70
Point4.cs

• 36
• 37 get
• 38
• 39 return x
• 40
• 41
• 42 set
• 43
• 44 x value // no need for
  validation
• 45
• 46
• 47 // end property X
• 48
• 49 // property Y
• 50 public int Y
• 51
• 52 get
• 53
• 54 return y

71
Circle5.cs

• 1 // Fig. 9.18 Circle5.cs
• 2 // Circle5 class that inherits from class
  Point4.
• 3
• 4 using System
• 5
• 6 // Circle5 class definition inherits from
  Point4
• 7 public class Circle5 Point4
• 8
• 9 private double radius
• 10
• 11 // default constructor
• 12 public Circle5()
• 13
• 14 // implicit call to Point3 constructor
  occurs here
• 15 Console.WriteLine( "Circle5
  constructor 0", this )
• 16
• 17
• 18 // constructor
• 19 public Circle5( int xValue, int yValue,
  double radiusValue )

72
Circle5.cs

• 35 get
• 36
• 37 return radius
• 38
• 39
• 40 set
• 41
• 42 if ( value gt 0 )
• 43 radius value
• 44
• 45
• 46 // end property Radius
• 47
• 48 // calculate Circle5 diameter
• 49 public double Diameter()
• 50
• 51 return Radius 2
• 52
• 53

73
Circle5.cs

• 69 // use base reference to return Point3
  string
• 70 return "Center " base.ToString()
  
• 71 " Radius " Radius
• 72
• 73
• 74 // end class Circle5

74
ConstructorAndDestructor.cs

• 1 // Fig. 9.19 ConstructorAndDestructor.cs
• 2 // Display order in which base-class and
  derived-class constructors
• 3 // and destructors are called.
• 4
• 5 using System
• 6
• 7 // ConstructorAndFinalizer class definition
• 8 class ConstructorAndFinalizer
• 9
• 10 // main entry point for application.
• 11 static void Main( string args )
• 12
• 13 Circle5 circle1, circle2
• 14
• 15 // instantiate objects
• 16 circle1 new Circle5( 72, 29, 4.5 )
• 17 circle2 new Circle5( 5, 5, 10 )
• 18
• 19 Console.WriteLine()

75
ConstructorAndDestructor.cs program output

• Point4 constructor Center 72, 29 Radius 0
• Circle5 constructor Center 72, 29 Radius
  4.5
• Point4 constructor Center 5, 5 Radius 0
• Circle5 constructor Center 5, 5 Radius 10
•  
• Circle5 destructor Center 5, 5 Radius 10
• Point4 destructor Center 5, 5 Radius 10
• Circle5 destructor Center 72, 29 Radius
  4.5
• Point4 destructor Center 72, 29 Radius
  4.5

76
Outline

• Admin. and review
• Class hierarchies
• The Object class
• Object references
• Using class inheritance to implement polymorphism

77
References and Inheritance

• An object reference can refer to an object of its
  class, or to an object of any class related to it
  by inheritance
• For example, if the Holiday class is used to
  derive a child class called Christmas, then a
  Holiday reference could actually be used to point
  to a Christmas object

Holiday
Holiday day day new Holiday() day new
Christmas()
Christmas
78
References and Inheritance

• Assigning an object to an ancestor reference is
  considered to be a widening conversion, and can
  be performed by simple assignment
• Assigning an ancestor object to a reference can
  also be done, but it is considered to be a
  narrowing conversion and must be done with a cast
• The widening conversion is the most useful
• For implementing polymorphism

Holiday day new Christmas()
Christmas christ new Christmas() Holiday day
christ Christmas christ2 (Christmas)day
79
What is Polymorphism?

• A polymorphic reference is one which can refer to
  different types of objects at different times
• An object reference can refer to one object at
  one time, then it can be changed to refer to
  another object (related by inheritance) at
  another time

80
Polymorphism via Inheritance

• It is the type of the object being referenced,
  not the reference type, that determines which
  method is invoked
• Polymorphic references are therefore resolved at
  run-time, not during compilation this is called
  dynamic binding
• Careful use of polymorphic references can lead to
  elegant, robust software designs

81
Polymorphism via Inheritance

• Inheritance can be used as a basis of
  polymorphism
• Suppose the Holiday class has a method called
  celebrate, and the Christmas class overrode it
• Now consider the following invocation
• day.celebrate()
• If day refers to a Holiday object, it invokes the
  Holiday version of celebrate if it refers to a
  Christmas object, it invokes the Christmas version

82
Example Polymorphism via Inheritance
83
Polymorphism via Inheritance and Arrays of Objects

• Now consider the task of paying all employees
• See StaffMember.java (page 414)
• See Volunteer.java (page 415)
• See Employee.java (page 416)
• See Executive.java (page 417)
• See Hourly.java (page 418)
• See Firm.java (page 410)
• See Staff.java (page 412)

84
Outline

• Admin. and review
• Interface overview
• Interface hierarchies
• Using interface to implement polymorphism

85
Review

• Class inheritance
• What are inherited? FoodItem, Pizza
• Method overriding
• Class hierarchies
• Abstract classes
• The Object classhttp//java.sun.com/j2se/1.4/docs
  /api/java/lang/Object.html
• Object references
• Polymorphism via inheritance and an array of base
  objects

86
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87
Outline

• Admin. and review
• Interface overview
• Interface hierarchies
• Using interface to implement polymorphism

88
Single vs. Multiple Inheritance

• Some object-oriented languages allow Multiple
  inheritance, which allows a class to be derived
  from two or more classes, inheriting the members
  of all parents
• The price collisions, such as the same variable
  name in two parents, have to be resolved
• Java decision single inheritance, meaning that a
  derived class can have only one parent class
• To take the advantages of multiple inheritance,
  Java defines interfaces, which give us the best
  aspects of multiple inheritance without the
  overhead

89
Java Interface

• A Java interface is a collection of constants
  and abstract methods
• An abstract method is a method header without a
  method body
• An abstract method can be declared using the
  modifier abstract, but because all methods in an
  interface are abstract, it is usually left off
• An interface is used to formally define a set of
  methods that a class will implement

90
Interfaces Syntax
public interface Doable public static final
String NAME public void doThis() public
int doThat() public void doThis2 (float
value, char ch) public boolean doTheOther
(int num)
No method in an interface has a definition (body)
91
Interfaces

• An interface cannot be instantiated
• Methods in an interface have public visibility by
  default
• A class formally implements an interface by
• Stating so in the class header in the implements
  clause
• A class can implement multiple interfaces the
  interfaces are listed in the implements clause,
  separated by commas
• If a class asserts that it implements an
  interface, it must define all methods in the
  interface or the compiler will produce errors

92
Implementing Interfaces
public class CanDo implements Doable public
void doThis () // whatever
public void doThat () // whatever
// etc.
public class ManyThings implements Doable,
AnotherDoable
93
Interfaces An Example

• A class that implements an interface can
  implement other methods as well
• See Complexity.java (page 294)
• See Question.java (page 295)
• See MinQuiz.java (page 297)

94
UML Diagram
ltltinterfacegtgt Complexity
getComplexity () int setComplexity (int)
void
1
2
Question
getQuestion () String getAnswer ()
String answerCorrect (String) boolean
toString() String
95
Interfaces Examples from Java Standard Class
Library

• The Java standard class library contains many
  interfaces that are helpful in certain situations
• The Comparable interface contains an abstract
  method called compareTo, which is used to compare
  two objects
• if (obj1.compareTo(obj2) lt 0)
  System.out.println(obj1 is less than obj2)
• The String class implements Comparable which
  gives us the ability to put strings in
  alphabetical order
• The Iterator interface contains methods that
  allow the user to move through a collection of
  objects easily
• hasNext(), next(), remove()

96
Outline

• Admin. and review
• Interface overview
• Interface hierarchies
• Using interface to implement polymorphism

97
Interface Hierarchies

• Inheritance can be applied to interfaces as well
  as classes
• One interface can be used as the parent of
  another
• The child interface inherits all abstract methods
  of the parent
• A class implementing the child interface must
  define all methods from both the parent and child
  interfaces
• Note that class hierarchies and interface
  hierarchies are distinct (they do not overlap)

98
Outline

• Admin. and review
• Interface overview
• Interface hierarchies
• Using interface to implement polymorphism

99
Polymorphism via Interfaces

• An interface name can be used as the type of an
  object reference variable
• Doable obj
• The obj reference can be used to point to any
  object of any class that implements the Doable
  interface
• The version of doThis that the following line
  invokes depends on the type of object that obj is
  referring to
• obj.doThis()

100
Examples

• public interface Speaker
• public void speak()
• class Philosopher extends Human implements
  Speaker
• //
• public void speak()
• public void pontificate()
• class Dog extends Animal implements Speaker
• //
• public void speak()

• Speaker guest
• guest new Philosopher()
• guest.speak()
• guest Dog()
• guest.speak()

Speaker special special new Philosopher() spe
cial.pontificate()
// compiler error
Speaker special special new Philosopher() ((P
hilosopher)special).pontificate()