Programming Fundamentals

PROGRAMMING FUNDAMENTALS

Neal [email protected]

OBJECT-ORIENTED PROGRAMMING CONCEPTS

Key Concepts Classes Objects Inheritance Polymorphism Encapsulation

Classes and Objects A class describes what an object looks like

AttributesBehavior

An object is an instance of a particular class

A class is like the definition for “mouse” in the dictionary.

Object instances are like a dozen of them sitting on desks in the room.

Attributes An object’s attributes are represented by a class’

instance variables (or member variables, or fields) The content of all of an object’s instance variables

define its state

class Door bool opened bool lockedendClass

Each instance of Door can have one of four states

Behaviors An object’s behaviors are represented by its methods

class Door void closeDoor return

void lockDoor return

void openDoor return

void unlockDoor returnendClass

Similar Yet Different Consider two objects

MovieSong

What attributes do they have in common?

What behaviors do they have in common?

What attributes and behaviors are unique to each one?

Inheritance If we consider a Movie and a Song to

both be types of a MediaItem, we can define a MediaItem and say Movie and Song both inherit certain attributes and behaviors from MediaItem

Heirarchy:MediaItem

○ Song○ Movie

Polymorphism We know a MediaItem has behaviors to play

and pause, but the behavior is different depending on whether we are referring to a Movie or Song

Defining play and pause methods on a Movie or Song class is an example of polymorphism

The method call looks the same for both objects, but the specific behavior is differentA MediaItem can take on many (poly) shapes

(morph)

Encapsulation Every class encapsulates internal

information by hiding it from other objects (information hiding)

Hiding an object’s internal information makes it possible to modify the internal structure of an object without affecting other objects that may use it

Defining Classes Every class has a unique name Most classes contain data (fields) Most classes contain methods

A class that only contains simple data values may not define any methods, but this is rare

Our Program classes have always contained methods, but no data – again, this is rare

An Employee Classclass Employee string name

string getName() return name

void setName(string newName) name = newName returnendClass

The class has a unique name

An Employee Classclass Employee string name

string getName() return name

void setName(string newName) name = newName returnendClass

The class contains a field

An Employee Classclass Employee string name

string getName() return name

void setName(string newName) name = newName returnendClass

The class contains methods

A CardStack Classclass CardStack num cards[52]

num draw() num card = the next card in the stack return card

void shuffle() rearrange the cards in the stack returnendClass

A Card Classclass Card num value // 0 – 51

num getRank() return (value % 13) + 1 // 1 – 13

num getSuit() return value / 13 // 0 – 3

bool isFaceCard return getRank() > 10 // true or falseendClass

Declaring Instances Since a class is like a definition in a

dictionary, we need to create an instance of the class to actually do anything

Creating an instance allocates memory for the class variables

Declaring Instances Declaring variables uses the computer’s

memory Declaring class instances works the

same way

int

int

int

stringint

CardStack

. . .int

int

int

int

int

int

int

int

int

int

int

int

int

int

int

int

Declaring Instances Each instance declaration creates a new object

num counter = 0string name = “John Smith”

Employee assistantEmployee managerEmployee president

Employee Employee Employeeint

string

An Employee with More Fields

class Employee int id string name string hireDate

…Methods for the class…endClass

Memory for Each Instance

Employee Employee Employee

int

string string

int

string string

int

string string

int

string

Each instance of Employee has its own allocation of class variables

Using Methods Methods are accessed using a “dot” notation on an

instance of the class

// Create an instance of the Employee classEmployee manager// We now have an employee object.

// Set and get the name for an instance of// the Employee classmanager.setName(“John Smith”)output manager.getName() // John Smith

Using Multiple InstancesEmployee managermanager.setName(“John Smith”)

Employee assistantassistant.setName(“Fred Jones”)

output manager.getName() // John Smithoutput assistant.getName() // Fred Jones

ReviewWhich of the following is the same as

object?

a) classb) fieldc) instanced) category

ReviewWhich of the following is the same as

object?

a) classb) fieldc) instanced) category

Common Types of Methods “Set” methods are used to set fields in the class

class Employee string name

void setName(string newName) name = newName returnendClass

Common Types of Methods “Get” methods are used to retrieve data

from the class

class Employee string name

string getName() return nameendClass

Common Types of Methods “Get” methods may combine fields

class Employee string firstName string lastName

string getFullName() string name = lastName + ", " + firstName return nameendClass

Common Types of Methods “Work” methods may calculate a return value

class Circle num radius

num calcArea() num area = PI * radius * radius return area

num calcCircumference() num circumference = 2 * PI * radius return circumferenceendClass

Public and Private Access We may want to control access to some

fields and methods Fields and methods can be declared as

public or private Public fields and methods can be

accessed from outside the class Private fields and methods can only be

accessed from within the class

Back to the CardStack Which fields and methods do we want to protect?

class CardStack num cards[52]

num draw() num card = the next card in the stack return card

void shuffle() swap random cards in the stack return

void swap(num cardIndex1, num cardIndex2) switch the card at index 1 with the card at index 2 returnendClass

Back to the CardStack We don’t want free access to any card in the stack

class CardStack private num cards[52]

num draw() num card = the next card in the stack return card

void shuffle() swap random cards in the stack return

void swap(num cardIndex1, num cardIndex2) switch the card at index 1 with the card at index 2 returnendClass

Back to the CardStack We do want access to the card on top of the stack

class CardStack private num cards[52]

public num draw() num card = the next card in the stack return card

void shuffle() swap random cards in the stack return

void swap(num cardIndex1, num cardIndex2) switch the card at index 1 with the card at index 2 returnendClass

Back to the CardStack We do want ability to shuffle the stack

class CardStack private num cards[52]

public num draw() num card = the next card in the stack return card

public void shuffle() swap random cards in the stack return

void swap(num cardIndex1, num cardIndex2) switch the card at index 1 with the card at index 2 returnendClass

Back to the CardStack We don’t want the ability to swap any two cards in the stack

class CardStack private num cards[52]

public num draw() num card = the next card in the stack return card

public void shuffle() swap random cards in the stack return

private void swap(num cardIndex1, num cardIndex2) switch the card at index 1 with the card at index 2 returnendClass

Back to the Card Class Which fields and methods do we want to protect?

class Card num value // 0 – 51

num getRank() return (value % 13) + 1 // 1 – 13

num getSuit() return value / 13 // 0 – 3

bool isFaceCard return getRank() > 10 // true or falseendClass

Back to the Card Class We don’t want the card to be changed

class Card private num value // 0 – 51

num getRank() return (value % 13) + 1 // 1 – 13

num getSuit() return value / 13 // 0 – 3

bool isFaceCard return getRank() > 10 // true or falseendClass

Back to the Card Class We do want to access the card’s rank

class Card private num value // 0 – 51

public num getRank() return (value % 13) + 1 // 1 – 13

num getSuit() return value / 13 // 0 – 3

bool isFaceCard return getRank() > 10 // true or falseendClass

Back to the Card Class We do want to access the card’s suit

class Card private num value // 0 – 51

public num getRank() return (value % 13) + 1 // 1 – 13

public num getSuit() return value / 13 // 0 – 3

bool isFaceCard return getRank() > 10 // true or falseendClass

Back to the Card Class We do want to know if it’s a face card

class Card private num value // 0 – 51

public num getRank() return (value % 13) + 1 // 1 – 13

public num getSuit() return value / 13 // 0 – 3

public bool isFaceCard return getRank() > 10 // true or falseendClass

ReviewAssume that a working program contains the

following statements:

Dog myDogmyDog.setName(“Bowser”)

Which of the following do you know?a) setName() is a public method of the Dog classb) setName() accepts a string parameterc) both of the aboved) none of the above

ReviewAssume that a working program contains the

following statements:

Dog myDogmyDog.setName(“Bowser”)

Which of the following do you know?a) setName() is a public method of the Dog classb) setName() accepts a string parameterc) both of the aboved) none of the above

Instance Methods The methods we’ve seen so far are called

instance methods They operate on a particular instance of a

class

// Get the name from a specific instance// of the Employee classEmployee managermanager.setName(newName)output manager.getName()

But which instance? From outside a method we reference an

instance of a class using an identifier The identifier determines the instance

Employee manageroutput manager.getName()

But which instance?class Employee private string name

public string getName() return nameendClass

output manager.getName()output assistant.getName()

The compiler generates object code that knows which instance of name should be used when calling getName()

manager- name

assistant- name

But which instance? What if we want an instance to

reference itself? There’s no identifier defined within the

class and there may be many instances of the class

Use “this” one… From within a method, you can reference the instance that is

“in scope” using the “this” keyword

class Employee private string name

public string getName() return this.name

void setName(string newName) this.name = newName returnendClass

Why do I care? One example…

class Employee private string name

// What if a parameter identifier overlaps // a field identifier? void setName(string name) name = name returnendClass

“this” Resolves Ambiguity One example…

class Employee private string name

// What if a parameter identifier overlaps // a field identifier? void setName(string name) this.name = name returnendClass

You’re right… Why not just avoid using the same identifier?

class Employee private string mName

// What if a parameter identifier overlaps // a field identifier? void setName(string name) mName = name returnendClass

Or… More likely, change the parameter name…

class Employee private string mName

// What if a parameter identifier overlaps // a field identifier? void setName(string inName) mName = inName returnendClass

But… We will see “this” again where it is more

useful. For now, just know it’s there.

Static Methods Instance methods operate on a specific

instance of a class Static methods do not operate on a

specific instance of the classWhich means they cannot modify fields defined

within the class Static methods are typically useful

methods that are closely related to the class, but not associated with a particular instance

ExamplesFrom our C# examples…

Console.ReadLine();Console.WriteLine();

Console is the class name, but we never create an instance of the class

The Console class is used to group together all the functionality for interacting with a console window

Using Objects (Instances) We can pass class instances as method parameters and

return values

class CardStack void add(Card newCard) add a new card to the stack return

Card draw() Card nextCard = find the next card in the stack return nextCardendClass

Using Objects (Instances) But object are passed by reference

Counter hiccupshiccups.reset() // count = 0

useCounter(hiccups)output hiccups.getCount() // count = 1

void useCounter(Counter c) // The variables “c” and “hiccups” refer // to the same object. Changing one changes // the other. c.increment()return

Using Objects (Instances) Objects can be stored in an array

Employee[] newHires = Employee[10]

ReviewAssume that a working program contains the

following statements:

Cat myCatmyCat.setName(“Socks”)

Which of the following do you know?a) myCat is an object of a class named Catb) setName() is a static methodc) both of the aboved) none of the above

ReviewAssume that a working program contains the

following statements:

Cat myCatmyCat.setName(“Socks”)

Which of the following do you know?a) myCat is an object of a class named Catb) setName() is a static methodc) both of the aboved) none of the above

Bringing It All Together Let’s create a Card class and a

CardStack class A Card represent any one of the standard

52 playing cards A CardStack can be shuffled A Card can be drawn from a CardStack A Card can be added to a CardStack We should be able to determine how

many Cards are in a CardStack

Graphical Objects Let’s take a look at GUI programming in

Visual Studio

Class Diagrams Represent the definition of a class Language independent way of

describing the class p. 272 Programming tools can often consume

or generate class diagrams

Summary Principles of object-oriented

programming Using objects Public and private access Instance methods Static methods GUI object Class diagrams