Class

1
Class
2
Objectives

• Discuss class basics
• fields
• methods
• access levels
• Present object creation
• new operator

3
Class

• Class represents concept in application domain
• defines a new type
• contains data and operations
• fundamental to object oriented programming

class Stock ...
class CheckingAccount ...
class Rectangle ...
class Aircraft ...
class Rational ...
4
Instance fields

• Class instance fields store object state
• each instance gets own copy
• also called instance variables

class Stock string name double price
int shares ...
instance fields
5
Object creation

• Objects created using new operator
• memory allocated for all instance fields
• object also called instance
• object creation also called instantiation or
  activation

Stock ibm new Stock() Stock sun new Stock()
allocate
ibm
name price shares
objects
sun
name price shares
6
Member access

• Object members accessed using dot operator

Stock ibm new Stock() Stock sun new
Stock() ibm.name "IBM" ibm.price
56.0 ibm.shares 100 sun.name
"SUN" sun.price 10.0 sun.shares 200
access fields
ibm
name IBM price 56.0 shares 100
sun
name SUN price 10.0 shares 200
7
Method

• Class methods implement the behavior of the class
• specify return type, name, parameters, and body
• use void if no value returned

class Stock void Buy(int shares) ...
void Sell(int shares) ... void
SetPrice(double price) ... double
Value() ... ...
methods
8
Method invocation

• Method invoked on object
• use dot operator
• pass parameters

Stock ibm new Stock() ibm.SetPrice(56.0) ibm.
Buy(100) ibm.SetPrice(99.0) ibm.Sell(50)
call methods
9
Method implementation

• Method implementation uses keyword this
• handle to object on which method was called
• used to access members from inside method

class Stock string name double price
int shares void Buy(int s)
this.shares s ...
Stock ibm new Stock() ibm.SetPrice(56.0) ibm.
Buy(100)
ibm/this
name price 56.0 shares 100
implement method
use this
10
Required this

• Must use this when local name conflicts with
  member name

class Stock string name double price
int shares void Buy(int shares)
this.shares shares ...
parameter and field have same name
must use this
11
Omitting this

• Can omit this when there is no ambiguity
• class members accessed implicitly

class Stock string name double price
int shares void Buy(int s) shares
s ...
omit this
12
Member declaration order

• Members may be declared in any order
• no requirement to declare before use
• different rule than local variables

class Stock void Buy(int s) shares
s string name double price int
shares ...
use
declare
13
Return value

• Use return to send data out of method
• value sent back to caller

class Stock double Value() return
price shares ...
return
Stock ibm new Stock() ibm.SetPrice(56.0) ibm.
Buy(100) double v ibm.Value()
14
Method overloading

• Can have several methods with same name
• parameters lists must be different
• compiler selects version to call based on passed
  parameters
• cannot overload on return type alone

class Stock void Buy(int shares)
this.shares shares void Buy(int
shares, double price) this.shares
shares this.price price ...
overload
Stock ibm new Stock() ibm.Buy(100, 56.0)
call (int,double) version of Buy
15
Value parameter

• Pass by value is default parameter passing
  mechanism
• data copied into method
• any changes to parameter inside method affect
  local copy only

void F(int x) x 0
value parameter
change local copy
int y 9 F(y)
y unchanged
16
Ref parameter

• ref parameter passes data in and out
• use keyword ref in definition and call
• must use variable in call
• must initialize passed variable before call

ref parameter, initially 9
void H(ref int x) x x 2
int y 9 H(ref y)
y set to 18
17
Out parameter

• out parameter returns data through parameter
• use keyword out in both definition and call
• actual parameter must be a variable, cannot pass
  literal value
• must assign to parameter inside method or
  compiler error

void G(out int x) x 11
out parameter
assign to parameter
int y G(out y)
y set to 11
18
Member default access

• Class members default to private
• redundant to use keyword private

class Stock string name private
double price private int shares
void Buy (int shares) ... private void
Sell(int shares) ... ...
equivalent
equivalent
19
Meaning of private

• Access to private member limited to code in that
  class only
• compiler error to attempt access from external
  class

class Stock private int shares void
Sell(int shares) this.shares shares
...
private member
access ok in Stock class
class Test static void Main() Stock
ibm new Stock() ... ibm.shares 9

error to access in other class
20
Public member access

• Members can be made public
• can then be accessed outside the class

class Stock private string name private
double price private int shares public
void Buy (int shares) ... public
void Sell (int shares) ... public
void SetPrice(double price ) ... public
double Value () ...
public
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Encapsulation

• Access levels used to separate interface from
  implementation
• interface made public
• implementation made private
• Separation allows implementation to be easily
  changed
• without breaking user code
• supports object-oriented principle of
  encapsulation

22
Naming

• Naming guidelines
• class intercaps with initial capital
• method intercaps with initial capital
• method parameter intercaps with initial lower
  case
• public field intercaps with initial capital
• private field intercaps with initial lower case

follow naming guidelines
class SavingsAccount public void
Deposit(double amountOfDeposit) ... public
int GetAccountNumber() ...
public double Balance private int
accountNumber ...
23
Summary

• Class is basic unit of C programming
• contains data (fields) and operations (methods)
• Several parameter passing options available
• value, out, ref
• Several options for protection level of class
  members
• supports encapsulation and information hiding

24
Initialization
25
Objectives

• Present field initialization options for fields
• default values
• variable initializers
• constructors

26
Instance field default values

• Instance fields set to default values when object
  created
• 0 for numeric types
• false for bool
• '\x0000' for char
• null for references

class Rational int numerator int
denominator ...
instance fields
a
numerator 0 denominator 0
Rational a new Rational()
set to default values
27
Variable initializer

• Instance fields can be initialized at point of
  definition
• called variable initializer
• executed each time an object is created
• convenient way to overwrite default values

class Rational int numerator int
denominator 1 ...
initialize
28
Constructor

• Class can supply constructor to do initialization
• automatically invoked when object created
• implemented using same name as class with no
  return type

class Rational int numerator int
denominator public Rational(int numerator,
int denominator) this.numerator
numerator this.denominator denominator
...
constructor
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Invoking constructor

• Constructor automatically invoked when object
  created

Rational a new Rational(2, 5)
create object, invoke constructor
a
numerator 2 denominator 5
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Multiple constructors

• Class can supply multiple constructors
• parameter lists must be different

class Rational public Rational(int numerator,
int denominator) this.numerator
numerator this.denominator denominator
public Rational(int numerator)
this.numerator numerator
this.denominator 1 ...
constructor
constructor
31
Default constructor

• Can supply constructor that takes no arguments
• often called the default constructor

class Rational public Rational()
this.denominator 1 ...
no argument constructor
32
Selecting constructor to invoke

• Compiler selects constructor version
  automatically
• based on arguments passed

Rational a new Rational(2, 5) Rational b
new Rational(6) Rational c new Rational()
two ints
one int
no arguments
33
Constructor initializer

• One constructor can invoke another constructor
• use this(...) syntax before constructor body
• called constructor initializer
• can put common code in constructor that others
  call

class Rational public Rational(int numerator,
int denominator) this.numerator
numerator this.denominator denominator
public Rational(int numerator)
this(numerator, 1) ... ...
call 2 argument constructor
34
Compiler generated constructor

• Compiler creates default constructor
• only if no constructors supplied by programmer
• Compiler generated constructor
• takes no arguments
• has empty body
• calls no argument constructor for base class

35
Initialization order

• Initialization options executed in well defined
  order
• fields set to default values
• variable initializers run in textual order top to
  bottom
• constructor executed

36
Summary

• Three common options for field initialization
• default values, variable initializers,
  constructors
• executed in well defined order