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INF230 Basics in C# Programming
AUBG, COS dept
Lecture 25
Title:
Object-Oriented Programming
(part 5 - Interfaces)
Reference: Doyle, chap 11
Chapter 11
Advanced Object-Oriented Programming
Features
C# Programming:
From Problem Analysis to Program Design
4th Edition
Lecture Contents:
•
•
•
•
•
Abstraction
Encapsulation
Inheritance
Polymorphism
Abstract classes
• Interfaces
• Generics
Object-Oriented Language Features
• Abstraction: Generalizing, identifying essential features, hiding
nonessential complexities
• Encapsulation; Packaging data and behaviors into a single unit, hiding
implementation details
• Inheritance: Extending program units to enable reuse of code
• Polymorphism: Providing multiple (different) implementations of same
named behaviors
• Abstract classes: Serve to define a high-level abstract object, that can then
be used as a base class for many different derived related classes.
• Interfaces
– The interface of a class defines how it should behave.
– An interface is for defining common behavior for classes (incl.
unrelated classes).
C# Programming: From Problem Analysis to Program Design
4
Prelude
To
Interfaces, i.e.
Multiple Inheritance
Interfaces
• A superclass defines common behavior for
related subclasses
• An interface can be used to define common
behavior for classes (incl. unrelated classes)
• Inheritance associates “is a” relationship
• Interfaces associate “has a” relationship
Interfaces
• Interfaces enable the developer to
truly separate the “What?” from the
“How?”.
•
J. Sharp
Interfaces & Abstract Classes
The Interface concept
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Interface
No method bodies – just method header.
Must provide implementation when used.
public interface Driver {
void turnWheel(double angle);
void pressAccelerator(double amount);
void pressBrake(double amount);
}
public class BusDriver : Driver {
// must include implementation for
each of the three methods from Driver
}
9
May also have
public class BusDriver : Person, Driver
{
// must include implementation for
each of the three methods from Driver
}
This is a “back door” approach to multiple inheritance
Single inheritance via base class
and extra inheritance via interfaces
10
Interfaces
More details
Interfaces
• C# supports single inheritance
– Only inherit from a single class, abstract or non abstract
– Classes can implement any number of interfaces
• Think of an interface as a class that is totally
abstract; all methods are abstract
– Abstract classes can have abstract methods and regular
methods
– Classes implementing interface agree to define details
for all of the interface’s methods
C# Programming: From Problem Analysis to Program Design
12
Interfaces (continued)
• General form
[modifier] interface InterfaceIdentifier
{
// members - no access modifiers are used
}
• Members can be methods, properties, or events
– No implementations details are provided for any of its
members
C# Programming: From Problem Analysis to Program Design
13
Interfaces
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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Interfaces
 Interface
is class-like construct that contains only
constants and abstract methods.
 Interface is similar to abstract class, but its intent
is to specify common behavior for objects that
belong even to different inheritance hierarchies.
 Reminder1: Abstract class contain regular
methods and abstract methods and polymorphic
approach is within one only specific inheritance
hierarchy.
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
15
Interfaces

To distinguish an interface from a class, C# uses
interface reserved word:
public interface Edible {
string howToEat();
}
You cannot create an instance of interface with new
 You can use interface as data type for ref var, or as a result
of casting

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Interface example

Given: Edible interface and two separate independent
inheritance hierarchies:
===================================================================================================
Animal
↑
↑
Tiger Chicken(Edible)
Fruit(Edible)
↑
↑
Apple Orange
===================================================================================================
Animal – regular base class
 Fruit – base class implements Edible interface
 Chicken – child class implements Edible interface
 Tiger – regular child class
 Apple, Orange – regular child classes

Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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Open file InterfaceExample02Edible.cs
using Edible interface to specify which
object is edible, in other words has implemented
method howToEat().
 Task:
 Browse
the source text
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
18
Open file InterfaceExample02Edible.cs
 Chicken
extends Animal, implements Edible to
specify that chickens are edible, implements
method howToEat()
 Tiger
extends Animal, does not implement Edible
i.e. tigers are not edible, no need to implement
method howToEat()
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
19
Open file InterfaceExample02Edible.cs
public interface Edible
{
string howToEat();
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
20
Open file InterfaceExample02Edible.cs
abstract class Animal { } // empty class
class Chicken : Animal, Edible
{
public string howToEat()
{
return "Chicken: Fry it";
}
}
class Tiger : Animal
{ }
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
21
Open file InterfaceExample02Edible.cs
 Fruit
implements Edible, and does not
implement howToEat() method.
 Therefore Fruit is to be modified as abstract
class and
 concrete subclasses of Fruit must implement
howToEat() method, as it is done with Apple
class and Orange class
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
22
Open file InterfaceExample02Edible.cs
public interface Edible
{
string howToEat();
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
23
Open file InterfaceExample02Edible.cs
abstract class Fruit { }
class Apple : Fruit, Edible {
public string howToEat()
{
return "Apple: Make apple juice";
}
}
class Orange : Fruit, Edible {
public string howToEat()
{
return "Orange: Squeeze it";
}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
24
Open file InterfaceExample02Edible.cs
 Run
the application
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
25
C# supported interfaces
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
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C#(.NET) supported interfaces: IComparable
Problem: we need a method to return the larger of two
objects of the same type:
 Two students, Two dates, Two circles, Two rectangles
 It is must the two (both) objects to be comparable
 Comparability is considered a common behavior
 C# provides IComparable interface for this purpose

public interface Icomparable
{
int CompareTo(object o);
{
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
27
Java supported interfaces: IComparable

Many C# predefined classes (string, Date) implement IComparable
interface to define an order for the objects. String, Date classes
provide CompareTo() method. Their definition is like this:
//------------------------------------public class string : object , IComparable
{
// class body
}
//------------------------------------Public class Date : object, IComparable
{
// class body
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
28
Java supported interfaces: Comparable
 Given
string s;
Date d;
//------------------------------(s instanceof string)
is
true
(s instanceof Object)
is
true
(s instanceof Comparable)
is
true
//------------------------------(d instanceof Date)
is
true
(d instanceof Object)
is
true
(d instanceof Comparable)
is
true
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
29
User Defined class implement interfaces: IComparable
 Reminder
: class Rectangle
 We cannot compare instances of Rectangle,
because Rectangle does not implement
IComparable interface
 How to proceed?
– Re edit Rectangle class to implement IComparable
OR
– Create new class ComparableRectangle to extend
Rectangle and to implement IComparable
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
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User Defined class implement interfaces: IComparable
First option solution
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
31
User Defined class implement interfaces: IComparable
 Solution
scheme for the first option:
GeometricObject
IComparable
|
|
Rectangle - - - - - - - -
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
32
User Defined class implement interfaces: IComparable
Open source file ProgComparableRectangle.cs
 Source text skeleton for Rectangle

public class Rectangle :
GeometricObject , IComparable
{
. . .
public int CompareTo(object o) {…}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
33
User Defined class implement interfaces: IComparable

How to define the CompareTo() method. We need a
comparison criteria. For example, rectangle width size
class Rectangle : GeometricObject, IComparable
{
. . .
public int CompareTo(object o)
{
if (this.width == ((Rectangle)o).width)
return 0;
else if (this.width < ((Rectangle)o).width)
return -1;
else return 1;
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
34
reserved. 0132130807
} // end of class rights
Rectangle
User Defined class implement interfaces: IComparable

How to use the CompareTo() method
Rectangle
Rectangle
Rectangle
Rectangle
r1
r2
r3
r4
=
=
=
=
new
new
new
new
Rectangle(10.0,20.0);
Rectangle(10.0, 20.0);
Rectangle(20.0, 20.0);
Rectangle(5.0, 20.0);
Console.WriteLine(r1.CompareTo(r2));
Console.WriteLine(r1.CompareTo(r3));
Console.WriteLine(r1.CompareTo(r4));
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
35
User Defined class implement interfaces: IComparable
Expected output?
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
36
User Defined class implement interfaces: IComparable
Second option solution
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
37
User Defined class implement interfaces: IComparable
 Solution
scheme for the second option:
GeometricObject
↑
Rectangle
IComparable
↑
↑
ComparableRectangle - - - - -
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
38
User Defined class implement interfaces: IComparable
Open source file ProgComparableRectangle.cs
 Source text skeleton for ComparableRectangle

public class ComparableRectangle :
Rectangle , IComparable {
public ComparableRectangle(…) { …}
public int CompareTo(object o) {…}
}
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
39
User Defined class implement interfaces: IComparable

How to use the CompareTo() method
ComparableRectangle r1 = new ComparableRectangle(4.,5.);
ComparableRectangle r2 = new ComparableRectangle(4.,6.);
ComparableRectangle r3 = new ComparableRectangle(5.,4.);
Console.WriteLine(r1.CompareTo(r2));
Console.WriteLine(r1.CompareTo(r3));
Console.WriteLine(r2.CompareTo(r1));
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
40
User Defined class implement interfaces: IComparable
Expected output?
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
41
Open file InterfaceExample01Traveler.cs
 Run
the application
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
42
Open file InterfaceExample03.cs
 Run
the application
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
43
Conclusion
Abstract
class contain
– regular methods and
– abstract methods and
polymorphic
approach is within one
only specific inheritance hierarchy.
GeometricObject, Circle, Rectangle
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Conclusion
 Interface
contains only abstract methods.
 Interface is similar to abstract class, but its
intent is to specify common behavior for
objects that belong even to different
inheritance hierarchies
 Animal, Tiger, Chicken (Edible interface)
 Fruit (Edible interface), Apple, Orange
 Interface is a back door to implement multiple
inheritance in Java
Liang, Introduction to Java Programming, Eighth Edition, (c) 2011 Pearson Education, Inc. All
rights reserved. 0132130807
Run
Demo programs
on Interfaces
Interfaces
Practice
Defining an Interface
• Can be defined as member of existing namespace
or class OR by compiling it to separate DLL
• Easy approach is to put the interface in a separate
project (like was done with Person and Student)
– Use the Class Library template from Start page
• Unlike abstract classes, it is not necessary to use
the abstract keyword with methods of interfaces
– Because all methods are abstract
Review LibraryFiles Example
C# Programming: From Problem Analysis to Program Design
48
Defining an Interface (continued)
To store in
Class
Library,
Build the
interface
DLL using
Build
option
from
BUILD
menu
Figure 11-15 ITraveler interface
C# Programming: From Problem Analysis to Program Design
49
Implement the Interface
• Follow same steps as with the Person and Student
DLLs …if you want to store in Class Library
– Create Class Library File from Start page
– Compile and Build assembly from BUILD menu
option, creating a .DLL file
– In the class implementing the interface, Add Reference
to the .DLL file
– Type a using statement (for the namespace identifier) in
the class implementing the interface
C# Programming: From Problem Analysis to Program Design
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Implement the Interface (continued)
• Heading for the class implementing the interface
identifies base class and one or more interfaces
following the colon (:) [Base class comes first]
[modifier] class ClassIdentifier : identifier [, identifier]
public class Student : Person, Itraveler
• For testing purposes, PresentationGUI class in the
PresentationGUIAbtractClassAndInterface folder
modified to include calls to interface methods
Review PresentationGUIWithAbstractClassAndInterface Example
C# Programming: From Problem Analysis to Program Design
51
Implement the Interface:
PresentationGUI Application
Populated
by
Interface
methods
Figure 11-16 PresentationGUI output using interface methods
C# Programming: From Problem Analysis to Program Design
52
Implement the Interface:
PresentationGUI Application
• GetStartLocation( ), GetDestination( ), and
DetermineMiles( ) members of ITraveler interface
private void btnTravel_Click(object sender, EventArgs e)
{
// GetStartLocation( ), GetDestination( ) and DetermineMiles( )
// methods all defined as abstract methods in ITraveler interface
txtBxFrom.Text = aStudent.GetStartLocation();
txtBxTo.Text = aStudent.GetDestination();
txtBxMiles.Text = aStudent.DetermineMiles().ToString();
txtBxFrom.Visible = true;
… //More statements
C# Programming: From Problem Analysis to Program Design
53
.NET Framework Interfaces
• Play an important role in the .NET Framework
– Collection classes such as Array class and
HashTable class implement a number of interfaces
• Designing the classes to implement interfaces
provides common functionality among them
C# Programming: From Problem Analysis to Program Design
54
.NET Framework Interfaces
(continued)
• .NET Array class is an abstract class
– Array implements several interfaces (ICloneable; IList;
ICollection; and IEnumerable)
• Includes methods for manipulating arrays, such as:
– Iterating through the elements
– Searching by adding elements to the array
– Copying, cloning, clearing, and removing elements from
the array
– Reversing elements
– Sorting
• Much of functionality is in place because of the contracts
the interfaces are enforcing
C# Programming: From Problem Analysis to Program Design
55
Polymorphism
• Polymorphism is implemented through interfaces,
inheritance, and the use of abstract classes
• Ability for classes to provide different
implementations details for methods with same name
– Determines which method to call or invoke at run time
based on which object calls the method (Dynamic binding)
– Example…ToString( ) method
• Through inheritance, polymorphism is made possible
by allowing classes to override base class members
C# Programming: From Problem Analysis to Program Design
56
Polymorphic Programming in
.NET
• Multiple classes can implement the same interface,
each providing different implementation details
for its abstract methods
– “Black box” concept
• Classes that derive from abstract classes are forced
to include implementation details for any abstract
method
C# Programming: From Problem Analysis to Program Design
57
Polymorphic Programming in
.NET (continued)
• Actual details of the body of interface methods are
left up to the classes that implement the interface
– Method name is the same
– Every class that implements the interface may have a
completely different behavior
C# Programming: From Problem Analysis to Program Design
58
Thank You
For
Your Attention!