object-oriented programming. agenda classes & objects attributes methods objects in memory...

Object-Oriented Programming

Agenda• Classes & Objects• Attributes• Methods• Objects in memory• Visibility• Properties (get & set)• Constructors

Classes

• Encapsulate data and functions

• Enable more complex programming– Large-scale programming

• Requires “Object think”– Security/reliability– Design by contract

Defining a Class

class NAME {

ATTRIBUTES (data)

METHODS (functions)

}

Class Exampleclass BMW_Z4 {

int ModelYear;

string LicensePlate;

bool TopUp;

void Drive()

{

Console.WriteLine(“Roadin’ and Rockin’“);

}

void OpenTop()

{

TopUp = false;

}

}

Class Example

class BMW_Z4 {

int ModelYear;

string LicensePlate;

bool TopUp;

void Drive()

{

Console.WriteLine(“Roadin’ and Rockin’“);

}

void OpenTop()

{

TopUp = true;

}

}

Methods(functions)

Attributes (data)

Instantiating Classes

• Defining the class is akin to defining a type

• An object is an instance of a class

• Class:Object::Type:Variable

• Use new() to instantiate the class

Instantiating Class & Using ObjectBMW_Z4 myCar;myCar = new BMW_Z4();

myCar.LicensePlate = "BMR4ME";myCar.ModelYear = 2004;myCar.Drive();myCar.OpenTop();myCar.Drive();

Memory TraceDog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

null

Memory Trace(declare the variables; they are dead)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

null

d1 d2

Memory Trace(bring d1 to life)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

// Remember, new opens

// up space and calls the

// class constructor

null

d1 d2

Memory Trace(bring d1 to life)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

// Remember, new opens

// up space and calls the

// class constructor

null

d1 d2

Memory Trace(bring d1 to life)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

// Remember, new opens

// up space and calls the

// class constructor

null

d1 d2

rabid=false

weight=5

name=“bob”

Memory Trace(bring d2 to life)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

// Remember, new opens

// up space and calls the

// class constructor

null

d1 d2

rabid=false

weight=5

name=“bob”

Memory Trace(bring d2 to life)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

// Remember, new opens

// up space and calls the

// class constructor

null

d1 d2

rabid=false

weight=5

name=“bob”

rabid=false

weight=5

name=“bob”

Memory Trace(comparison)

d1 and d2 do NOT occupy

the same space in memory,

so they are NOT ==

Example:

d1 == d2

evaluates to false

null

d1 d2

rabid=false

weight=5

name=“bob”

rabid=false

weight=5

name=“bob”

Memory Trace(have d1 point to d2)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

null

d1 d2

rabid=false

weight=5

name=“bob”

rabid=false

weight=5

name=“bob”

Memory Trace(have d1 point to d2)

Dog d1, d2;

d1 = new Dog (5, “bob”);

d2 = new Dog (5, “bob”);

d1 = d2;

null

d1 d2

rabid=false

weight=5

name=“bob”

rabid=false

weight=5

name=“bob”

Memory Trace(have d1 point to d2)

Now, d1 and d2 are ==

But no one is pointing to this

Because no one is pointing to

it, we can no longer reference

it. We call this piece of memory

garbage. The garbage collector will deallocate it.

null

d1 d2

rabid=false

weight=5

name=“bob”

rabid=false

weight=5

name=“bob”

Visibility

• OO motivation: protection/security

• We need a way of selectively “publishing” parts of a class and “hiding” other parts of the class

• Public & private

class BMW_Z4 {

private int ModelYear;

public string LicensePlate;

private bool TopUp;

public void Drive()

{

Console.WriteLine("Roadin’ and Rockin’");

}

public void OpenTop()

{

TopUp = false;

}

}

VisibilityExample

Method aretypically public

Note the visibility change(most attributes will be private)

Object Method & Attribute Visibility

BMW_Z4 myCar;myCar = new BMW_Z4();

myCar.LicensePlate = "BMR4ME";myCar.ModelYear = 2004;myCar.Drive();myCar.OpenTop();myCar.Drive();

Illegal b/c private

Properties• Combines field/attribute with method

• Standard:– Make attributes private

– Lower-case first letter of attribute

– Make properties public

– Upper-case first letter of properties

– Define “get” and “set” for each property (selectively)

class BMW_Z4 {

private int modelYear;

private string licensePlate;

private bool topUp;

public int ModelYear

{

get { return modelYear; }

set { if (value >= 2003) modelYear = value; }

}

public string LicensePlate

{

get { return licensePlate; }

set { if (value.Length() == 6) licensePlate = value; }

}

...

}

PropertiesExample

Constructors• So far, we’ve seen attributes and methods• Constructor is a unique method

– Named same as the class name– Automatically called when class is instantiated– Useful for setting attributes & initializing the instance– No return type (not even void)– Must be public

• Default constructor is used unless you specify a constructor

• For each attribute in the class, assign something to it in the constructor (initialize)

class BMW_Z4 {

private int modelYear;

private string licensePlate;

private bool topUp;

public BMW_Z4 ()

{

ModelYear = 2004;

TopUp = false;

LicensePlate = "DEALER";

}

...

}

ConstructorExample

FIN