Introduction to Software Development - Defining Classes | CSSE 120, Study notes of Software Engineering

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DEFINING CLASSES

CSSE 120—Rose Hulman Institute of Technology

Review: Using Objects

WIDTH = 400

HEIGHT = 50

REPEAT_COUNT = 20

PAUSE_LENGTH = 0.

win = GraphWin( 'Cubs Win!' , WIDTH, HEIGHT) p = Point(WIDTH/2, HEIGHT/2) t = Text(p, 'Chicago Cubs--National League Central Champs!' ) t.setStyle( 'bold' ) t.draw(win) nextColorIsRed = True t.setFill( 'blue' ) for i in range(REPEAT_COUNT): sleep(PAUSE_LENGTH) if nextColorIsRed: t.setFill( 'red' ) else : t.setFill( 'blue' ) nextColorIsRed = not nextColorIsRed win.close()

Key Concept!

 A class is like an "object factory"

 Calling the constructor tells the classes to make a new

object

 Parameters to constructor are like "factory options",

used to set instance variables

 Or think of class like a "rubber stamp"

 Calling the constructor stamps out a new object shaped

like the class

 Parameters to constructor "fill in the blanks". That is,

they are used to set instance variables.

Example

 Consider:

p = Point(200, 100)

t = Text(p, 'Go Cubs!' )

Point

x _______

y _______

fill _______

outline _______

getX() …

getY() …

'black'

'black'

Text

anchor _______

text _______

getAnchor() …

getText() …

setText(text)

setStyle(style)

p

'Go Cubs'

Point

x _______

y _______

fill _______

outline _______

getX() …

getY() …

'black'

'black'

t

This is a clone of p

class Card: """This class represents a card from a standard deck."""

Code to Define a Class

Declares a class named Card

docstring describes class, used by help() function and by Eclipse help

class Card: """This class represents a card from a standard deck.""" def init( self, card, suit): self.cardName = card self.suitName = suit

Code to Define a Class

Special name, init declares a constructor

Special self parameter is the first formal parameter of each method in a class. self always refers to the current object

Card

def init(self,card,suit):

self.cardName = card

self.suitName = suit

Create instance variables just by assigning to them

A sample constructor call:

c = Card('Ace', 'Hearts')

'Ace'

'Hearts'

cardName ______

suitName ______

c

class Card: """This class represents a card from a standard deck.""" def init( self, card, suit): self.cardName = card self.suitName = suit

def getValue( self): """Returns the value of this card in BlackJack. Aces always count as one, so hands need to adjust to count aces as 11.""" pos = cardNames.index( self.cardName) if pos < 10: return pos + 1 return 10

def str( self): return self.cardName + " of " + self.suitName

Code to Define a Class

Special str method returns a string representation of an object

Sample uses of str method:

print c

msg = "Card is " + str(c)

class Card: """This class represents a card from a standard deck.""" def init( self, card, suit): self.cardName = card self.suitName = suit

def getValue( self): """Returns the value of this card in BlackJack. Aces always count as one, so hands need to adjust to count aces as 11.""" pos = cardNames.index( self.cardName) if pos < 10: return pos + 1 return 10

def str( self): return self.cardName + " of " + self.suitName

Stepping Through Some Code

Sample use:

card = Card( '7','Clubs')

print card.getValue()

print card

Another Example:

Lists of Objects, Lists in Object

class CardCollection: """This class represents a collection of cards, either a single hand or the whole deck.""" def init( self, newDeck = False): """Creates a new collection of cards. By default it's empty, but if newDeck is True then the collection is a full standard deck.""" self.name = None self.cardList = [] self.hideFirst = True if newDeck: # Create an entire deck of cards for s in suits: for c in cardNames: self.insert(Card(c, s)) ...

Optional formal parameter , can construct a CardCollection three ways: deck = CardCollection(True) hand = CardCollection(False) hand = CardCollection()

Instance variable containing a list

Self call , constructor calls another method of the CardCollection class

Another Example (continued):

Lists of Objects, Lists in Object

class CardCollection: """…""" def init( self, newDeck = False): """…""" self.name = None self.cardList = [] self.hideFirst = True if newDeck: # Create an entire deck of cards for s in suits: for c in cardNames: self.insert(Card(c, s))

def insert( self, card): """Adds the given card to this collection.""" self.cardList.append(card)

insert method uses append() method to mutate the list instance variable

We can put objects into lists.

Encapsulation

 Insulating the rest of a program from some details

by "hiding" them inside some structure

 Top-down design encapsulates operations inside

functions

 Object-oriented design encapsulates data and

operations inside classes

 Encapsulation is a form of abstraction—ignoring

irrelevant details

Check It Out

 Use Eclipse SVN Repository Exploring perspective

 Go to your repository:

http://svn.cs.rose-hulman.edu/repos/csse120-fall07- username

 Check-out the BallSim project

 Switch back to the PyDev perspective

 The blackjackWithClasses module is there for you

to review as an example of defining classes