Session 06:C# OOP-3

Inheritance and Polymorphism. Static and dynamic type of an object.

FEN 2013-03-12 1AK - IT: Softwarekonstruktion

FEN 2013-03-12 AK - IT: Softwarekonstruktion 2

Object-Oriented Programming

“ The Three Pillars of OOP”:Encapsulation

InheritancePolymorphism

The Substitution Principle

Software Quality Factors

FEN 2013-03-12 AK - IT: Softwarekonstruktion 3

• The most important ones:– Reliability:

– Correctness – Robustness

– Modularity:– Extendibility– Reusability

• This is addressed through:

– Inheritance and polymorphism

The DoME example

"Database of Multimedia Entertainment"

• stores details about CDs and DVDs– CD: title, artist, # tracks, playing time, got-it,

comment– DVD: title, director, playing time, got-it, comment

• allows (later) to search for information or print lists

FEN 2013-03-12 4AK - IT: Softwarekonstruktion

DoME objects

FEN 2013-03-12 5AK - IT: Softwarekonstruktion

DoME object model

FEN 2013-03-12 6AK - IT: Softwarekonstruktion

Class diagram

View Source (dome-v1)

FEN 2013-03-12 7AK - IT: Softwarekonstruktion

Critique of DoME

• code duplication– CD and DVD classes very similar (large

part are identical)– makes maintenance difficult/more work– introduces danger of bugs through

incorrect maintenance• code duplication also in Database class

FEN 2013-03-12 8AK - IT: Softwarekonstruktion

Using inheritance

FEN 2013-03-12 9AK - IT: Softwarekonstruktion

Using inheritance

• define one base or super class: Item• define subclasses for DVD and CD• the super class defines common

attributes• the subclasses inherit the super class

attributes• the subclasses add own attributes

FEN 2013-03-12 10AK - IT: Softwarekonstruktion

Inheritance in C#public class Item{ ...}

public class CD : Item{ ...}

public class DVD : Item { ...}

no change here

change here

View Source (dome-v2)

FEN 2013-03-12 11AK - IT: Softwarekonstruktion

Subtyping

First, we had:public void AddCD(CD theCD)public void AddDVD(DVD theDVD)

Now, we have:public void AddItem(Item theItem)

We call this method with:DVD dvd = new DVD(...);

myDB.AddItem(myDVD);

Static type

Dynamic type

FEN 2013-03-12 12AK - IT: Softwarekonstruktion

Static and dynamic type

• The declared type of a variable is its static type.• The type of the object a variable refers to is its

dynamic type.• The compiler’s job is to check for static-type

violations.

foreach(Item item in items) { item.Print(); // Item must have

// declared a Print method.}

FEN 2013-03-12 13AK - IT: Softwarekonstruktion

Subclasses and subtyping

• Classes define types.• Subclasses define subtypes.• Objects of subclasses can be used

where objects of supertypes are required.(This is called substitution .)

FEN 2013-03-12 14AK - IT: Softwarekonstruktion

Polymorphic variables

• Object variables in C# are polymorphic.(They can reference objects of more than one type.)

• They can reference objects of the declared type, or of subtypes of the declared type.

FEN 2013-03-12 15AK - IT: Softwarekonstruktion

Object diagram

Static type

Dynamic type

FEN 2013-03-12 16AK - IT: Softwarekonstruktion

Conflicting outputCD: A Swingin' Affair (64 mins)* Frank Sinatra tracks: 16 my favourite Sinatra album DVD: O Brother, Where Art Thou? (106 mins) Joel & Ethan Coen The Coen brothers’ best movie!

title: A Swingin' Affair (64 mins)* my favourite Sinatra album

title: O Brother, Where Art Thou? (106 mins) The Coen brothers’ best movie!

What we want

What we have

FEN 2013-03-12 17AK - IT: Softwarekonstruktion

The inheritance hierarchy

Here we only know information in Item

FEN 2013-03-12 18AK - IT: Softwarekonstruktion

Overriding: the solution

print method in both super-

and subclasses.

Satisfies both static and

dynamic type checking.

View Source (dome-v3)

FEN 2013-03-12 19AK - IT: Softwarekonstruktion

Overriding

• Superclass and subclass define methods with the same signature.

• Each has access to the fields of its class.• Superclass satisfies static type check.• Subclass method is called at runtime – it

overrides the superclass version.• What becomes of the superclass version?

FEN 2013-03-12 20AK - IT: Softwarekonstruktion

Method lookup

No inheritance or polymorphism.

The obvious method is selected.FEN 2013-03-12 21AK - IT: Softwarekonstruktion

Method lookup

Inheritance but no overriding. The

inheritance hierarchy is ascended, searching for

a match.FEN 2013-03-12 22AK - IT: Softwarekonstruktion

Method lookup

Polymorphism and overriding. The ‘first’

version found (starting at the

bottom of the hierarchy) is used.FEN 2013-03-12 23AK - IT: Softwarekonstruktion

Method lookup summary

• The variable is accessed.• The object stored in the variable is found.• The class of the object is found.• The class is searched for a method match.• If no match is found, the superclass is

searched.• This is repeated until a match is found, or the

class hierarchy is exhausted.• Overriding methods take precedence.FEN 2013-03-12 24AK - IT: Softwarekonstruktion

Call to base in methods

• Overridden methods are hidden ...• ... but we often still want to be able to

call them.• An overridden method can be called

from the method that overrides it.– base.Method(...)– Compare with the use of base in

constructors.FEN 2013-03-12 25AK - IT: Softwarekonstruktion

Defining and Calling an overridden method

public class CD : Item{ ... public override void Print() { base.Print(); --- } ...}

public class Item{ ... public virtual void Print() {

--- } ...}

FEN 2013-03-12 26AK - IT: Softwarekonstruktion

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Example:

• On Employee there is a method GiveBonus() which may have different implementations in the superclass and in the subclasses.

ManagernoOfOpts

SalesPersonsale

Employeenamesaleryposition

WorksOnhours

0..*1 0..*1

Projectnamedepartment10..* 10..*

View Source (EmpProjectV2.rar)

FEN 2013-03-12 AK - IT: Softwarekonstruktion 28

C# - overriding pre-defined methods- When are objects equal?

• Classes ought to override the Equals-method inherited from Object

public class Customer{ . . .

public override bool Equals(object obj) { Customer other; if ((obj == null) || (!(obj is Customer))) return false; // surely not equal

other = (Customer) obj; // typecast to gain access return this.id == other.id; // equal, if ids are... }

FEN 2013-03-12 AK - IT: Softwarekonstruktion 29

Exercises

• Session06.docx