1
CS 112 Introduction to
Programming
Lecture #33:
Abstract Class; Interface
http://flint.cs.yale.edu/cs112/
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Recap: Inheritance for Code Reuse
rClasses are often organized as a class
hierarchy
min C#, all classes are derived from the Object
class
•ToString:converts an object to a string representation
•Equals: checks if two objects are the same
•GetType:returns the type of a type of object
rAn inherited member is passed down the line
meach class can also override a method definition
to favor its own
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Recap: Inheritance for Polymorphism
rAn object reference can refer to an object of its class, or to an
object of any class derived from it by inheritance
rAssigning an object to an ancestor reference is considered to be a
widening conversion, and can be performed by simple assignment
rAssigning an ancestor object to a reference can also be done, but it
is considered to be a narrowing conversion and must be done with
a cast
rThe widening conversion is used to implement polymorphism
mit is the type of the object being referenced, not the reference type, that
determines which method is invoked
Holiday day = new Christmas();
Christmas christ = new Christmas();
Holiday day = christ;
Christmas christ2 = (Christmas)day;
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StaffMember
# name : string
# address : string
# phone : string
+ ToString() : string
+ Pay() : double
Volunteer
+ Pay() : double
Employee
# socialSecurityNumber: String
# payRate: double
+ ToString() : string
+ Pay() : double
Executive
-bonus : double
+ AwardBonus(execBonus : double) : void
+ Pay() : double
Hourly
-hoursWorked: int
+ AddHours(moreHours: int) : void
+ ToString() : string
+ Pay() : double
PayRoll
+ Main (args : string[]) : void
-staffList: staffMemeber[]
Staff
+ Payday() : void
-staffList: StaffMemeber[]
contains multiple objects
Polymorphic references and arrays work well with each other
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Outline
rAdmin.
ØRefining class definition
Øabstract class
mother features
rInterface
6
Abstract Class
rMotivation: if you think no object of a class should be
created, declare the class abstract
rIn general, an abstract class is a placeholder in a class
hierarchy that represents a generic concept
mThe use of abstract classes is a design decision; it helps us
establish common elements in a class that is too general to
instantiate
Vehicle
Car Boat Plane
