CS100A Lecture 22

1
CS100A Lecture 22

• Previous Lecture
• MatLab demonstration
• This Lecture
• Inheritance
• Method overriding
• Polymorphism
• Reading Lewis and Loftus, Chapter 8

2
Object-Oriented Programming

• Object-oriented programming aspires to model the
  real world as a collection of objects.
• Objects are classified into categories called
  classes.
• Objects in the real world and in mathematics can
  be further classified by hierarchical taxonomies.
  
• For the taxonomy
• we say every Y is-a X. For example,
• every kitten is a cat.

X
Y
Cat
Kitten
3
Taxonomy of Polygons

4
Two Taxonomies of Person
Person
Female
Male
boy
man
girl
woman
Person
Old
Young
boy
girl
man
woman
5
Not a Taxonomy
Person
Female
Male
Old
Young
boy
man
girl
woman

• Taxonomies are strictly hierarchical

6
The Class Hierarchy

• In Java, all classes are organized into a
  taxonomy known as the class hierarchy, Class
  Object is at the top of the hierarchy.
• If class s is below class c in the class
  hierarchy, class s is called a subclass of c, a c
  is a superclass of s.
• The Java class definition
• class class-name
• . . .
• implicitly defines class-name to be a subclass
  of class Object.
• Example. Classes Account, Room, Person, and
  Matrix are each subclasses of class Object

Object
Person
Room
Matrix
Account
7
Defining a Subclass

• The Java class definition
• class class-name1 extends class-name2
• . . .
• explicitly defines class-name1 to be a subclass
  of class-name2.
• Example.
• class Male extends Person
• . . .
• class Female extends Person
• . . .

Object
Room
Matrix
Person
Account
Female
Male
8
Inheritance

• Objects of a given class have all characteristics
  (fields and methods) of objects above them in the
  hierarchy.
• A subclass is said to inherit the fields and
  methods of its superclass.
• The class hierarchy is sometime called the
  inheritance hierarchy.

9
Method Resolution

• Let o be an object of type t, i.e., o was
  contructed by a constructor t.
• Question. Suppose you invoke method m on object
  o. Which definition of m is invoked?
• Answer. The first definition of m found (at run
  time) in the class hierarchy, starting at t,
  working up through its superclasses, to Object.

10
Field Selection

• Let e be a reference expression of type t, where
  t is some class. i.e., at run time expression e
  will evaluate to a reference to some object of
  type t, where t is t or a subtype of t.
• Question. Suppose you select field f on whatever
  object e refers to . Which field f is invoked?
• Answer. The first definition of f found (at
  compile time) in the class hierarchy, starting at
  t, working up through its superclasses, to
  Object. Note that t and not t is used to resolve
  the field reference.

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Inheritance of Fields

• class Account
• int balance 0 // current balance
• . . .
• class SavingsAccount extends Account
• double rate 0 // interest rate
• . . .
• // Client code
• Account act1 new Account()
• SavingsAccount act2 new SavingsAccount()
• // A SavingsAccount has both balance and rate.
• System.out.println(act2.balance) // LEGAL!
• System.out.println(act2.rate)

12
Inheritance of Methods

• class Room
• int id // Id number of room
• . . .
• public String toString()
• return Room id
• class Bathroom extends Room
• boolean shower // true if room has shower.
• . . .
• // Client code
• Room r1 new Room()
• Bathroom r2 new Bathroom()
• / Rooms toString method is available for both
  Rooms and Bathrooms. /

13
Method Overriding

• Recall that if you dont define a toString
  methods in a class, a default toString method
  is used.
• Where does that default method come from? It is
  the toString method of class Object.
• Redefining a method that is already defined in a
  superclass is called method overriding (not to be
  confused with method overloading).
• Method overriding lets you have a method that is
  specialized for a subclass.

Object
Room
Bathroom
14
Types of Variables

• Recall that the type of a variable determines the
  types of values that can be stored in the
  variable.
• A type t variable can contain type t objects.
• Room r new Room() // LEGAL!
• Account act new Acount() // LEGAL!
• A type t variable may not contain objects of type
  t if t and t are unrelated types.
• // Room r new Account() NOT LEGAL!
• // Account act new Room() NOT LEGAL!

15
Polymorphism

• A type t variable can contain any object whose
  type is t or a subtype of t
• // Client Code.
• / Because every Bathroom is a Room, both of the
  following statements are legal. /
• Room r1 new Room() // LEGAL!
• Room r2 new Bathroom() // LEGAL!
• / You can only access fields that are guaranteed
  to exist based on the type of the object
  reference. /
• // System.out.println(r2.shower) NOT LEGAL!
• / Because not every Room is a Bathroom, only the
  second of the following statements is legal. /
• // Bathroom r3 new Room() NOT LEGAL!
• Bathroom r4 new Bathroom() // LEGAL!

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Polymorphism, continued

• class Room
• int id // Id number of room
• . . .
• public String toString()
• return Room id
• class Bathroom extends Room
• . . .
• public String toString()
• return Bathroom id
• / Client code. The class of the object, not the
  type of the variable, determines which method is
  invoked. /
• Room r1 new Room()
• System.out.println(r1) // output Room