inheritanceexamples of inheritance ... summary of (single) inheritance multiple inheritance...

CSE 130 : Fall 2009

Programming Languages

Lecture 15: I h itInheritance

Ranjit Jhala UC San Diego

InheritanceInheritance

• Key concept of OO languages• Key concept of OO languages

S ll h i h i i ?• Someone tell me what inheritance is?

InheritanceInheritance

• Key concept of OO languages• Key concept of OO languages

S ll h i h i i ?• Someone tell me what inheritance is? • “is-a” relationship

• Examples?• Examples?

Examples of inheritanceExamples of inheritance

OverridingOverriding

• Super-class method can be overwritten in • Super class method can be overwritten in sub-class Polymorphism• Polymorphism – external clients write code that handles

different kinds of objects in uniform waydifferent kinds of objects in uniform way

d ’t b t i l t ti d t il – don’t care about implementation details: as long as the object knows to draw itself, that’s good enough (see screensaver on webpage)good enough (see screensaver on webpage)

Polymorphism continuedPolymorphism, continued

• Super-class can have methods that are • Super class can have methods that are not overridden, but that work differently for different sub-classesfor different sub classes

F l l th d • For example: super-class method functionality changes because the super- l ll th d th t t itt class calls a method that gets overwritten

in the sub-class

Simple exampleSimple example class Shape:

def draw(self, screen): # some python code here

def erase(self, screen): screen.setcolor(“white”)screen.setcolor( white ) self.draw(screen) screen.setcolor(“black”)

class Rec(Shape): def draw(self screen):

class Oval(Shape): def draw(self screen):def draw(self, screen):

# some python code here def draw(self, screen):

# some python code here

A QuestionA Question

How can “Oval”’s draw method call the How can Oval s draw method call the superclass’s draw method ?

Stepping away from PythonStepping away from Python

• What are the fundamental issues with • What are the fundamental issues with inheritance?

• How to decide on the inheritance graph? – not always obvious, see next example

Rectangle and SquareRectangle and Square

l R t l l Sclass Rectangle: length = 0 width = 0 def area(this):

class Square: length = 0 def area(this):

return this length *def area(this): return this.length *

this.width

return this.length * this.length

• Which should be a sub-class of which?

Rectangle and SquareRectangle and Square

l R t l l Sclass Rectangle: length = 0 width = 0 def area(this):

class Square: length = 0 def area(this):

return this length *def area(this): return this.length *

this.width

return this.length * this.length

• Which should be a sub-class of which?

• Answer is not clear...

Option 1: Rectangle is-a SquareOption 1: Rectangle is a Square

class Square: length = 0 def area(this):( )

return this.length * this.length

class Rectangle(Square): width = 0width = 0 def area(this):

return this.length * this.width

Option 1: Rectangle is-a SquareOption 1: Rectangle is a Square

+ Store only what is + Store only what is needed (one field for square)

class Square: length = 0 def area(this): square)

―Does not follow “isa” relationship from math

( ) return this.length *

this.length

relationship from math (rectangle is not a square )

class Rectangle(Square): width = 0 square...)

―Have to override area th d

width = 0 def area(this):

return this.length * this.width

method

Option 2: Square is-a RectangleOption 2: Square is a Rectangle

class Rectangle:class Rectangle: length = 0 width = 0 def area(this):( )

return this.length * this.width

class Square(Rectangle): init (self len):__init__(self,len): self.length = len self.width = len

Option 2: Square is-a RectangleOption 2: Square is a Rectangle

class Rectangle: + Follows isa relationship class Rectangle: length = 0 width = 0 def area(this):

+ Follows isa relationship from math

+ Don’t need to write ( )return this.length * this.width

+ Don t need to write two area methods U t fi ld f

class Square(Rectangle): init (self len):

―Use two fields for Square (len and width)

__init__(self,len): self.length = len self.width = len

Option 3: Option 3: class Shape:

class Rectangle(Shape): class Square(Shape):

...

g ( p ) length = 0 width = 0 def area(this):

q ( p ) length = 0 def area(this):

return this.length * return this.length *

this.width this.length

Option 3: Option 3: class Shape:

class Rectangle(Shape): class Square(Shape):

...

g ( p ) length = 0 width = 0 def area(this):

q ( p ) length = 0 def area(this):

return this.length * return this.length *

this.width this.length

St l h t i d d ( fi ld f )+ Store only what is needed (one field for square) ― Does not follow “isa” relationship from math

(rectangle is not a square )(rectangle is not a square...) ― Have to write two area methods

Complex numbersComplex numbers

l R l l C lclass Real: RealPart = 0

class Complex: RealPart = 0 ComplexPart = 0

The same exact options present themselves here, with the same tradeoffs!with the same tradeoffs!

Summary of (single) inheritanceSummary of (single) inheritance

• Inheritance is a powerful mechanismInheritance is a powerful mechanism

• For the programmer • For the programmer, difficulty = defining inheritance diagram

• For the language implementer, difficulty = fast dynamic dispatch

Multiple InheritanceMultiple Inheritance

Next timeNext time

• Multiple inheritance• Multiple inheritance

D• Decorators