Download Interfaces

Interfaces
Reference: Joe Hummel
Objectives
“Good class design starts with good application design — how
many classes, how do they relate to one another, how decoupled
do I want the system, etc. Inheritance and interfaces are the
primary tools for going beyond basic class design into the realm
of application design…”
• Interfaces
• Polymorphic programming
• Interface-based programming
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Part 1
• Interfaces…
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Interfaces
• An interface represents design
• Example:
– Designing an object that can be used for iterating over a data
structure
– Interface: only method signatures, no implementation! (All
methods are abstract)
Also as
generic
public interface IEnumerator
{
void
Reset();
// reset iterator to beginning
bool
MoveNext();
// advance to next element
object Current { get; } // retrieve current element
}
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Why use interfaces?
• Formalise system design before implementation
– especially useful with large systems.
• Contract-based programming
– the interface represents the contract between client and
object.
• Low coupling!
– decouples specification and implementation.
– facilitates reusable client code.
– client code will work with both existing and future objects as
long as the interface is not changed.
• Multiple inheritance
– A class may implement several interfaces, but only inherit
one class. (No competing implementations).
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Example – Iterator Pattern
• This piece of client code iterates over any data structure
that implements IEnumerator:
iterator
IEnumerator
iter;
iter = ...;
// get ref to iterator object
data
structure
iter.Reset();
while ( iter.MoveNext() )
MessageBox.Show( iter.Current.ToString() );
View Demo
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Polymorphism
• Polymorphism: when the same operation is supported across
different types.
• Example:
– classes that inherit some method M from a common base class
B and overrides it.
– classes that implement a common interface I.
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• Polymorphism facilitates reusable code!
• Example:
– This piece of code works on all data structures that
implements IEnumerable!
IEnumerator
iter = ...;
iter;
iter.Reset();
while ( iter.MoveNext() )
MessageBox.Show( iter.Current.ToString() );
•
•
All data structures (Collections) in .NET implement IEnumerator.
All objects in .NET inherit the ToString() method from Object.
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In .NET interfaces is used heavily.
•
•
•
•
•
•
•
•
IComparable
ICloneable
IDisposable
IEnumerable & IEnumerator
IList
ISerializable
IDBConnection, IDBCommand, IDataReader
etc.
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Typical Design in APIs
•
•
•
•
Specification in the interface.
Implementation in the concrete
classes.
Operations with common
implementation are implemented
in the abstract class (no code
duplication).
Operation specific for the
different concrete classes are left
abstract in the abstract class.
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Part 2
• Interface-based programming…
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Example: Sorting
• Goal:
– To write a generic Sort() method as the one found in
System.Array
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Step 1: Define the interface
• Sorting requires a way of comparison of objects:
public interface IComparable
{
int CompareTo(object obj);
}
• returns < 0
• returns == 0
• returns > 0
if this object < obj parameter
if this object = obj parameter
if this object > obj parameter
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Step 2: Classes must implement the interface
• Objects That are to be sorted must implement IComparable
Also as
• Example:
generic
– sort Student objects on id
base class
interface
public class Student : Person, IComparable
{
private int m_ID;
.
.
.
Person
Student
int IComparable.CompareTo(Object obj)
{
Student other;
other = (Student) obj;
return this.m_ID – other.m_ID;
}
}
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Step 3: Clients program towards the interface
• Sort assumes that the elements in the array a implement
IComparable:
public class Array
{
public static void Sort(Array a)
{
IComparable icobj;
for (int i = 0; i < a.Length-1; i++) {
for (int j = i+1; j < a.Length; j++) {
icobj = (IComparable) a.GetValue(i);
if (icobj.CompareTo(a.GetValue(j)) > 0)
swap(a, i, j);
}//for
}//for
}
}
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Step 4: test!
• Example: sort an array of Student objects
Student[]
students;
students = new Student[n];
students[0] = new Student("jane doe", 22, 55630);
students[1] = new Student("kim lee", 19, 81101);
students[2] = new Student("jim bag", 28, 28254);
.
.
.
Array.Sort(students);
foreach(Student s in students)
MessageBox.Show(s.Name);
View Code
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Summing up
• Inheritance is very useful, but…
– Only single inheritance is supported, so use it right
– Remember class A “is-a“ class B (Principle of Substitution).
• Interfaces are useful
– A class may implement any number of interfaces
– Consider interfaces when class A interacts with several classes
B, C, D, …
• Goal:
– Good design – low coupling - “separation of concerns”.
– Exploiting polymorphic programming.
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Exercise
• Make changes to the Student example, so students are
sorted on grade average, on age, on number of courses
passed etc.
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