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CS 112 Introduction to
Programming
Lecture #31:
OO Programming and Inheritance
http://flint.cs.yale.edu/cs112/
2
C# Classes
r C# classes play dual roles: m program modules: contain a list of (static) method declarations and (static) data fields m blueprints for generating objects:
r Classes support m “data encapsulation” (e.g., private components, for abstraction) m “inheritance” (for code reuse)
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Inheritance
r Inheritance allows a software developer to derive a new class from an existing one
r The existing class is called the parent class, or superclass , or base class
r The derived class is called the child class, or subclass, or derived class.
r As the name implies, the child inherits characteristics of the parent
r That is, the child class inherits the methods and data defined for the parent class 4
Inheritance
r Inheritance relationships are often shown
graphically in a class diagram , with the arrow
pointing to the parent class
Inheritance should Inheritance should create ancreate an isis--aa relationshiprelationship ,, meaning the childmeaning the child is ais a more specificmore specific version of theversion of the parentparent
Animal
weight : int
Bird
Animal
Bird
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Examples: Base Classes and Derived Classes
Base class Derived classes Student GraduateStudent UndergraduateStudent
Shape Circle Triangle Rectangle Loan CarLoan HomeImprovementLoan MortgageLoan Employee FacultyMember StaffMember Account CheckingAccount SavingsAccount Fig. 9.1 Inheritance examples.
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Declaring a Derived Class
r Define a new class DerivedClass which extends BaseClass
class BaseClass { // class contents } class DerivedClass : BaseClass { // class contents }
r Base class: the “parent” class; if omitted the parent is Object
See Book.cs, Dictionary.cs, BookInheritance.cs
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Controlling Inheritance
r A child class inherits the methods and data defined for the parent class; however, whether a data or method member of a parent class is accessible in the child class depends on the visibility modifier of a member r Variables and methods declared with private visibility are not accessible in the child class m a private data member defined in the parent class is still part of the state of a derived class r Variables and methods declared with public visibility are accessible; but public variables violate our goal of encapsulation r There is a third visibility modifier that helps in inheritance situations: protected
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The protected Modifier
q Variables and methods declared with protected visibility in a parent class are only accessible by a child class or any class derived from that class
q The details of each modifier are linked on the schedule page
r Example m Book.cs m Dictionary.cs m TestBookDictionary.cs
Book # pages : int
- GetNumberOfPages() : void
Dictionary
- public + PrintDefinitionMessage() : void
protected
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Calling Parent’s Constructor in a Child’s
Constructor: the base Reference
r Constructors are not inherited, even though they have public visibility r The first thing a derived class does is to call its base class’ constructor, either explicitly or implicitly m implicitly it is the default constructor m yet we often want to use a specific constructor of the parent to set up the "parent's part" of the object r Syntax: use base public DerivedClass : BaseClass { public DerivedClass(…) : base(…) { // … } } rExample mSee Book.cs mSee Dictionary.cs mSee BookConstructor.cs
rExample mSee FoodItem.cs mSee Pizza.cs mSee FoodAnalysis.cs (^10)
Defining Methods in the Child Class:
Overriding Methods
r A child class can override the definition of an
inherited method in favor of its own
r That is, a child can redefine a method that it
inherits from its parent
r The new method must have the same
signature as the parent's method, but can have
different code in the body
r The type of the object executing the method
determines which version of the method is
invoked
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Overriding Methods: Syntax
r override keyword is needed if a derived-
class method overrides a base-class method
r If a base class method is going to be
overridden it should be declared virtual
rExample mThought.cs mAdvice.cs mThoughtAndAdvice.cs
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Overloading vs. Overriding
r Overloading deals with multiple methods in the same class with the same name but different signatures
r Overloading lets you define a similar operation in different ways for different data
r Overriding deals with two methods, one in a parent class and one in a child class, that have the same signature
r Overriding lets you define a similar operation in different ways for different object types
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References and Inheritance
r An object reference can refer to an object of its
class, or to an object of any class derived from
it by inheritance
r For example, if the Holiday class is used to
derive a child class called Christmas, then a
Holiday reference can be used to point to a
Christmas object
Holiday day; day = new Holiday(); … day = new Christmas();
Holiday
Christmas
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References and Inheritance
r Assigning an object to an ancestor reference is considered to be a widening conversion, and can be performed by simple assignment
r Assigning 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
r The widening conversion is the most useful m for implementing polymorphism
Holiday day = new Christmas();
Christmas christ = new Christmas(); Holiday day = christ; Christmas christ2 = (Christmas)day;
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Polymorphism via Inheritance
r A polymorphic reference is one which can refer to different types of objects at different times
r An object reference can refer to one object at one time, then it can be changed to refer to another object (related by inheritance) at another time m it is the type of the object being referenced, not the reference type, that determines which method is invoked m polymorphic references are therefore resolved at run-time, not during compilation; this is called dynamic binding
r Careful use of polymorphic references can lead to elegant, robust software designs
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Polymorphism via Inheritance
r Suppose the Holiday class has a method
called Celebrate, and the Christmas class
redfines it
r Now consider the following invocation:
day.Celebrate();
r If day refers to a Holiday object, it invokes
the Holiday version of Celebrate; if it
refers to a Christmas object, it invokes the
Christmas version
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Polymorphism via Inheritance: Example 1
r Consider how to model a collection of shapes
r Example
m Shape.cs m Circle.cs m Sphere.cs m Cylinder.cs
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Polymorphism via Inheritance: Example 2
StaffMember
name : string
address : string
phone : string
- ToString() : string + Pay() : double
Volunteer
+ Pay() : double
Employee
socialSecurityNumber : String
payRate : double
- ToString() : string + Pay() : double
Executive
- AwardBonus(execBonus : double) : void + Pay() : double
Hourly
- AddHours(moreHours : int) : void
- ToString() : string + Pay() : double
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Polymorphism via Inheritance and
Arrays of Objects
r Polymorphic references and arrays work well
with each other
r See
m StaffMember.cs m Volunteer.cs
m Employee.cs m Executive.cs m Hourly.cs m Staff.cs m PayRoll.cs
