1Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 118PolymorphismPoly => manyMorph => shapeVariables take on many shapes, or manyclasses of objects.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 119PolymorphismPolymorphism in OOP is caused by late-binding of procedure calls (messagelookup).Program can work with any object thathas the right set of methods.2Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 120Object Model for PayrollEmployeeEmployeeTransactionPayrollSystempost:datePaycheckTimecardSalaryChangesalarypay, taxeshoursWorkedvacationpostTo:abstract classabstract methodObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 121Examplespost: aTransactionaTransaction postTo: self.transactations add: aTransactionnumbers inject: 0 into: [:sum :i | sum + i]3Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 122The Case Against CaseStatementsSmalltalk has no case statement.OO programmers tend to avoid casestatements, and to use messagesending, instead.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 123Eliminating Casesexpcase: 1 do: [...];case: 15 do: [...].Make classes for 1 and 15, with a methodcalled msg.exp msg or (dictionary at: exp) msg4Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 124Class UndefinedObjectClass UndefinedObject has one instance --nil.All variables are initialized to nil.Also used to indicate illegal value.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 125Don’t Test ClassesUndefinedObject ObjectisNil isNil^ true ^ falsenotNil notNil^ false ^ trueDon’t test classes: use message sendingand inheritance.5Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 126Choicesbadx class = UndefinedObject ifTrue: [...]OKx = nil ifTrue: [...]bestx isNil ifTrue: [...]Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 127Another choicex ifNil: [ … ]ObjectifNil: aBlock^selfUndefinedObjectifNil: aBlock^aBlock value6Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 128MagnitudeMagnitude ()ArithmeticValue ()Number () ....Point (’x’ ’y’)Character ()Date (’day’ ’year’)Time (’hours’ ’minutes’ ’seconds’)Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 129Number HierarchyNumberFraction (’numerator’ ’denominator’)Integer ()LargeNegativeInteger ()LargePositiveInteger ()SmallInteger ()...7Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 130Magnitude commentThe abstract class Magnitude providescommon protocol for objects that havethe ability to be compared along a lineardimension, such as dates or times.Subclasses of Magnitude include Date,ArithmeticValue, and Time, as well asCharacter and LookupKey.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 131Magnitude commentSubclasses must implement the followingmessages:comparing<=hash8Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 132NumbersNumbers are part of the class hierarchy, notbuilt into compiler.Numbers understand +, -, *, /, <, <=, etc.3 / 6 => 1 / 22 sqrt => 1.414213 + 4 * 2 =>Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 133Points, Numbers, Complex (not in image)squared"Answer the receiver multiplied by itself."^self * selfArithmeticObject9Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 134ArithmeticObject= aNumber"Answer whether the two objects are equal.For numeric objects, the two will be equal ifthe difference between them is zero"^aNumber respondsToArithmeticifTrue: [(self - aNumber) isZero]ifFalse: [false]Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 135NumberNumber is also a magnitude.Adds truncation operationsFixedPoint, Fraction, Integer, Double,Float10Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 136Number// aNumber"Integer quotient defined by division withtruncation toward negativeinfinity. 9//4 = 2, -9//4 = -3. -0.9//0.4 =-3.\\ answers the remainder from thisdivision."^(self / aNumber) floorObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 137Number\\ aNumber"modulo. Remainder defined in termsof //. Answer a Number with the samesign as aNumber. e.g. 9\\4 = 1, -9\\4= 3, 9\\-4 = -3, 0.9\\0.4 = 0.1"^self - (self // aNumber * aNumber)11Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 138For an Abstract Class ...■ Look for "subclassResponsibility"methods to see the core methods.■ Look in subclasses to see examples.■ Flow of control goes up and down theclass hierarchy.• (So don't try to follow it!)■ Don't send "new" to the class.Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 139Uses of InheritanceTo be shared by all subclasses• don't redefineDefault• probably redefine12Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 140Uses of InheritanceParameterized by subclasses (templatemethod)• rarely redefine• change it by redefining the methods itcallsTrue abstract methods• must redefineObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 141Uses of PolymorphismMethod lookup finds the method that isright for the receiver.• method always knows the class ofthe receiver• different classes lead to differentmethods13Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 142Uses of PolymorphismMethods often depend radically on classof receiver.isNilifTrue:ifFalse:double dispatchingObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 143Double-dispatching: the ProblemFloatSmallintegerFractionFixedPointMatrixFrScFiFl M*****FlFlFlFlFlFrFrFlFiFiScScSc ScScScScFiFi14Object-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 144ArithmeticNumber * MatrixMultiply each element by numberMatrix * NumberMultiply each element by numberMatrix * MatrixStandard matrix multiplicationObject-oriented Programming and Design - Copyright 1998 by Ralph E. Johnson 145ArithmeticInteger * Integer Primitive intoperationsInteger * Float Convert int to floatFloat * Integer Convert int to floatFloat * Float Primitive float operation15Object-oriented Programming and Design -