APC 03 04 ObjectOrientation

7/27/2019 APC 03 04 ObjectOrientation

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Chair of Business Informatics andSoftware Engineering

Prof. Dr. Stefan Eicker

Prof. Dr. Stefan Eicker, Peter M. Schuler

Winter Semester 2005

Version 2.0

Advanced Programming Concepts

Objectoriented C#


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Object Terminology

Based on: [BiHo2004, p. 31]

a program

types

namespaces

simple

structured

data member function member

fields

variables

constants

operators

constructors

methods

properties

consists of

have

are grouped in

are

are

and

arehave

are int

class or struct

parameters

e.g.

e.g.

indexers

finalizers

contain

events


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Properties

Methods that are dressed to look like a field get accessor takes no parameter and returns the same type as

the property has been declared as

No specification of explicit parameters for the set accesor Implement Read- and Write-Only Property by omitting the

compliment property

Note: C# does not permit to setting different access modifiers Properties can be declared as vi r t ual orabst r act

publ i c st r i ng SomePr oper t y{

get{

r et ur n Thi s i s t he Pr oper t y val ue,}set{

/ / code t o set t he pr oper t y}

}

Source: [Robi2002, p. 117-119]


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Constructors

Compiler will supply a default constructor, if explicitly noother constructor is implemented

If you supply any constructor that takes parameters, then thecompiler will not automatically supply a default one

Constructors can be defined as pr i vat e orpr ot ect ed

It is possibly in C# to write a static no-parameter constructorfor a class (no access modifiers)

publ i c cl ass MyCl ass

{

publ i c MyCl ass ( ) {}

}

Source: [Robi2002, p. 130-140]


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Readonly Fields

Keyword r eadonl y

You can assign values to a r eadonl y field inside a constructor

A r eadonl y field can be an instance rather than a static field.

publ i c cl ass Document{

publ i c r eadonl y Dat eTi me Cr eat i onDat e;

publ i c Document ( )

{

/ / r ead i n creat i on dat e f r om f i l e. Assume r esul t

/ / i s 1 J an 2002 but i n gener al t hi s can be/ / di f f er ent f or di f f er ent i nst ances of t he cl ass

Cr eat i onDat e = new Dat eTi me( 2002, 1, 1) ;

}

} Source: [Robi2002, p. 146-148]


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Structs (2)

newoperator works different for structs

Calls the appropriate constructor, according to the parameters passed to it,

and initializes all fields Legal to write:

Di mensi ons poi nt ;

poi nt . Lengt h = 3;

poi nt . Wi dt h = 6;

Everything must be initialized before use either with- the newoperator

- or when values have been assigned to all of its fields

Structs derive from System.Object

- but a struct can not derive from any other class- and you can not inherit from as struct

You are not permitted to define a constructor that takes no parameters

It is not permitted to define a destructor

Source: [Robi2002, p. 148-151]


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The Object Class

Returns details of the type of the object.publ i cType Get Type( )

Makes a shallow copy of the object.pr ot ect edobj ectMemberwi seCl one( )

This is the .NET version of a destructor.pr ot ect ed vi r t ualvoi d Fi nal i ze( )

Used if implementing dictionaries (hash

tables).

publ i c vi r t uali nt Get HashTabl e( )

Compares instances of the object for

equality.

publ i c vi r t ualbool Equal s ( obj ectobj )

Compares instances of the object for

equality.

publ i c st at i cbool Equal s ( obj ect

obj A, obj ect obj B)

Compares whether two references refer

to the same object.

publ i c st at i cbool Ref er enceEqual s( obj ect obj A, obj ectobj B)

Returns a string representation of the

object.

publ i c vi r t ualst r i ng ToSt r i ng( )

PurposeAccess ModifiersMethod

Source: [Robi2002, p. 120]


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Access Modifiers

The variable or method can only be accessed from

within the type to which it belongs.

pr i vat e

The variable or method can be accessed from the

current assembly, or from types derived from the current

type (that is, from anywhere that could access it if it were

declared as pr ot ect ed ori nt er nal ).

pr ot ect ed i nt er nal

The variable or method can only be accessed from

within the type to which it belongs, or from types derivedfrom that type.

pr ot ect ed

The variable or method can be accessed from anywhereas a field of the type to which it belongs.

publ i c

The variable or method can only be accessed from the

same assembly.

i nt er nal

DescriptionAccessibility



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Interfaces

Interfaces act as contracts

A class, which derives from an interface, is declaring that it

implements certain functions Not permitted to supply implementations of any of the

members of an interface

Interfaces can only obtain declarations of

methods, properties, indexers and events Interface members are always public and cannot be

declared as vi r t ual orst at i c

publ i c i nt er f ace I di sposabl e{

voi d Di spose( ) ;

}

Source: [Robi2002, p. 122-123]


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Constructor Initializer

Special syntax in C# which allows to place code of different constructors inone place

cl ass Car{

pr i vat e st r i ng descr i pt i on;pr i vat e ui nt nWheel s;

publ i c Car ( st r i ng model , ui nt nWheel s){t hi s. descr i pt i on = descr i pt i on;t hi s. nWheel s = nWheel s;

}

publ i c Car ( st r i ng model ) : t hi s( model , 4){}

}Source: [Robi2002, p. 134]


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Constructors (2)

Constructors are called in Order ofSyst em. Obj ect

Then progressed down the hierarchy until it has reached theclass being instantiated

Each constructor handles initializations of the fields in his own

class

Working up the hierarchy a constructor for the derived classcan call up base class methods, properties and any other

members

The base and this keywords are the only keyword allowed in

the line which calls another constructor

Source: [Robi2002, p. 130-140]


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Call Order for Constructors

Source: [Robi2002, p. 439.]

Syst em. Obj ect

MyType

MyAdvType

Publ i c Mat ( )

{

Wer t = 0;

}

Wer t


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Classes and Inheritance


Classes are reference types

cl ass MyCl ass{

pr i vat e i nt someFi el d;

publ i c st r i ng SomeMet hod( bool par amet er s) {}

}

Instantiate an object using the newoperator

MyCl ass my Obj ect ;

myObj ect = new MyCl ass( ) ;


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Single Implementation Inheritance

A class must derive from one another class

Universal base class derives from Syst em. Obj ect

A class may derive from another specific class

cl ass MyDer i vedCl ass : MyBaseCl ass

{

}

Source: [Robi2002, p. 110-111]


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Method Overloading

Methods may be implemented in several versions

Requirement: signatures differ at least in one the following- Name

- Number of parameters

- Parameter types

Signatures that differ only- In their return type

- By virtue of parameter (r ef orout )

are not sufficient!voi d Di spl ayResul t ( st r i ng r esul t ) {}

voi d Di spl ayResul t ( i nt resul t ) {}

Source: [Robi2002, p. 111-112]


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Method Overriding

A base class must be declared is vi r t ual in order to be

overridden

Neither member fields nor static functions can be declared asvi r t ual

publ i c over r i de st r i ng Vi r t ual Met hod( ) {}



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Method Hiding

A method is declared in both base and derived classes and notdeclared as vi r t ual and over r i de

derived class version hides the base class version

cl ass MyDer i vedCl ass : Hi sBaseCl ass

{

publ i c i nt MyMet hod( ) {}

}

Use the keyword new to declare that you intend to hide a method

publ i c new i nt MyMet hod( ) {}

To call a base version of a method from a derived class

base. Cal cul at ePr i ce( ) ;

Source: [Robi2002, p. 112-114]


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Sealed Classes and Methods

You can not inherit from a sealed class

You can not override a sealed method any further

seal ed cl ass Fi nal Cl ass

{publ i c seal ed voi d Fi nal Met hod( ) {}

}

Source: [Robi2002, p. 115-116]


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Chair of Business Informatics and

Software EngineeringProf. Dr. Stefan Eicker

Prof. Dr. Stefan Eicker, Peter M. SchulerWinter Semester 2005

Version 2.0

Advanced Programming Concepts

Objectoriented C#


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Agenda

Operator Overloading

Type Casts

Indexer


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Operator Overloading

Operators like +, - or * are strictly implemented for thepredefined data types

If you want to use the operators for your own classes, youhave to use overloads for the operators

publ i c st at i c Vect or oper at or + ( Vect or l hs, Vect or r hs){

Vect or r esul t = new Vect or ( l hs) ;

r esul t . x += r hs. x;

r esul t . y += r hs. y;

r esul t . z += r hs. z;r et ur n r esul t ;

}

Source: [Robi2002, p. 151-161]


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Which Operators Can You Overload?

none+, - , ++, - -Arithmetic unary

none&, | , , Bitwise binary

none! , ~, t r ue, f al seBitwise unary

must be overloaded in pairs==, ! =, >=,


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Indexers

Are not an essential part of object-oriented programming.

Name of the indexer is the keyword t hi s, other then that it is

quite equal to properties in terms of their definition.

Source: [Robi2002, p. 162-165]


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Indexers (2)

st r uct Vect or{

publ i c doubl e x, y, z ;

publ i c doubl e t hi s [ i nt i ]

{get{

swi t ch ( i ){

case 0:r et ur n x;

case 1:r et ur n y;

case 2:r et ur n z;

def aul t :t hr ow new I ndexOut Of RangeExcept i on( "At t empt t or et r i eve Vect or el ement " + i ) ;

}}

Source: [Robi2002, p. 162-163]


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Indexers (3)

set

{

switch (i)

{

case 0:x = value;

break;

case 1:

y = value;

break;

case 2:z = value;

break;

default:

throw new IndexOutOfRangeException(

"Attempt to set Vector element " + i);

}

}

}

Source: [Robi2002, p. 162-163]


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Comparision of Property and Indexer

Quelle: [MSDN2003]

A set accessor of an indexer has the

same formal parameter list as the

indexer, in addition to the valueparameter.

A set accessor of a property contains the

implicit value parameter.

A get accessor of an indexer has thesame formal parameter list as the

indexer.

A get accessor of a property has noparameters.

Must be an instance member.Can be a static or an instance member.

Accessed through an element access.Accessed through a simple name or a

member access.

Identified by its signature.Identified by its name.

IndexerProperty

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APC 03 04 ObjectOrientation