Transcript Chapter 3

OO Design with Inheritance
C Sc 335
Rick Mercer
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Justification and Outline
I introduce inheritance different than most books
that just show the mechanics and examples like
the Bicycle hierarchy from Sun
I start with a reason to use inheritance
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Show the objects found for a Library System
Recognize when to use inheritance
Build an inheritance hierarchy
Design Guidelines related to inheritance
See another use of polymorphism
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The 3 Pillars of OOP&D
Object-Oriented Programming
– Encapsulation
• Hide details in a class, provide methods
– Polymorphism
• Same name, different behavior, based on type
– Inheritance
• Capture common attributes and behaviors in a
base class and extend it for different types
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Object-Oriented Technology
OOT began with Simula 67
– developed in Norway
– acronym for simulation language
Why this “new” language (in the 60s)?
– to build accurate models for the description and
simulation of complex man-machine systems.
The modularization occurs at the physical object
level (not at a procedural level)
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The Beginnings
Simula 67 was designed for system simulation (in
Norway by Kristen Nygaard and Ole-Johan Dahl)
– Caller and called subprogram had equal relationship
– First notion of objects including class/instance
distinctions
– Ability to put code inside an instance to be executed
– The class concept was first used here
Kristen Nygaard invented inheritance
– Kristen Won the Turing award for 2002
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One way to Start OOA and D
Identify candidate objects that model
(shape) the system as a natural and
sensible set of abstractions
Determine main responsibility of each
– what an instance of the class must be able to
do and what is should remember
• This is part of Responsibility Driven Design ala
Rebecca Wirfs-Brocks
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System Specification
The college library has requested a system that supports a
small set of library operations. The librarian allows a student
to borrow certain items, return those borrowed items, and
pay fees. Late fees and due dates have been established at the
following rates:
Late fee
Length of Borrow
Books
$0.50 per day
14 days
Video tapes $5.00 plus 1.50 each additional day 2 days
CDs
$2.50 per day
7 days
The due date is set when the borrowed item is checked out.
A student with three (3) borrowed items, one late item, or
late fees greater than $25.00 may not borrow anything new.7
Identify candidate objects
Candidate objects that model a solution with main
responsibility. The model (no GUIs, events, networking)
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Librarian: Coordinates activities
Student, renamed Borrower
Book: Knows due date, late fees, Borrower, checkin...
Video: Knows due date, late fees, Borrower, checkin...
CD: Know due date, late fees, Borrower, checkin...
Three borrowed books: A collection of the things that can
be borrowed, name it LendableList
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– BorrowList: maintains all possible borrowers
A Sketch
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What do Books, Videos, and CDs
have in common?
Common responsibilities (methods and data):
– know call number, borrower, availability
– can be borrowed
– can be returned
Differences:
– compute due date
– compute late fee
– may have additional state
• Books have an author, CDs an artist
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When is inheritance appropriate?
Object-Oriented Design Heuristic:
If two or more classes have common data
and behavior, then those classes should
inherit from a common base class that
captures those data and methods
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An inheritance hierarchy
The abstract class never instantiated
Lendable
Book
CDRom
Video
Lendable is also known as the base class or superclass
Lendable is shown to abstract (in italic)
Book, CD, and Video are concrete subclasses
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Why not have just one class?
Some of the behavior differs
– Determine due date (2, 7, or 14 days)
– Compute late fee not always daysLate * dayLateFee
Data differs
– books have ISBNs and videos may have studio name
Inheritance
– allows you to share implementations
– allows one change in a common method to affect all
– allows other Lendables to be added later more easily
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Examples of inheritance in Java
You've seen HAS-A relationships:
– A Song HAS-A most recent date played
Inheritance models IS-A relationships:
– an oval IS-A shape
– a rectangle IS-A shape
– MyFrame extends JFrame makes MyFrame a
JFrame with additional methods and listeners
for a specific application
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Java Examples of Inheritance
All Exceptions extend Exception:
– RunTimeException IS-AN Exception
– NullPointerException IS-A RunTimeException
Many classes extend the Component class
– a JButton IS-A Component
– a JTextField IS-A Component
A GregorianCalender IS-A Calendar
Vector and ArrayList extends AbstractList
NoSongInQueueException IS-A RuntimeException
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Designing An Inheritance Hierarchy
Start with an abstract class to define commonalities and differences (abstract methods)
public abstract class Lendable {
private instance variables
constructor(s)
methods that Lendable implements when the behavior
is common to all of its subclasses (what is common)
}
abstract methods the subclasses must implement that
to represent what varies
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Some common data fields
Every class in the hierarchy ended with these
private instance variables in class Lendable:
– Subclasses can not directly reference these
private
private
private
private
private
String callNumber;
String title;
boolean availability;
String borrowerID;
DayCounter dueDate;
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Lendable's constructor
Constructor needs a callNumber and title since it
seems that all Lendables will need both
The actual values will later come from the subclasses
constructor
public Lendable(String callNumber, String initTitle) {
callNumber = callNumber;
// from subclass
title = initTitle;
// from subclass
// Initialize others in a special way
borrowerID = null;
dueDate = null;
availability = true;
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}
Common Behavior
public String getCallNumber() {
return callNumber;
}
public String getTitle() {
return title;
}
public boolean isAvailable() {
return availability;
}
public DayCounter getDueDate() {
return dueDate;
}
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Common Behavior continued
public int daysLate() {
// return a positive if dueDate is before today
DayCounter today = new DayCounter();
return dueDate.daysFrom(today);
}
public boolean isOverdue() {
if(this.isAvailable())
return false; // not even checked out
// Or check to see if this Lendable is overdue
DayCounter today = new DayCounter();
// Return true if today is greater than
// the due date for this Lendable
return daysLate() > 0;
}
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Common Behavior continued
public boolean checkSelfIn() {
if(this.isAvailable())
return false;
else { // Adjust state so this is checked out
dueDate = null;
availability = true;
return true;
}
}
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checkSelfOut split between Lendable
and its subclasses
// called from a subclass checkSelfOut
protected
void checkOutAnyLendable(String borrowerID,
int borrowLength) {
// Record who is borrowing this Lendable
this.borrowerID = borrowerID;
// Set the due date
dueDate = new DayCounter(); // today's date
dueDate.adjustDaysBy(borrowLength);
// Mark this as no longer available
availability = false;
}
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Protected
The protected access mode means that
subclasses inherit this method (inherit all public &
protected elements)
It's invoked by the subclass's checkSelfOut
method.
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Design Issues
So far, good design for the following reasons:
– subclasses can't change the private variables of the
superclass, even though the subclass has them
– doesn't require a bunch of setter methods in
Lendable for subclasses to modify their own instance
variables
– the common behavior is in the superclass
• the same thing is done for all subclasses
– We'll get a polymorphic checkSelfOut message
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Abstract methods
– Subclasses differ: setDueDate
getLateFee
• declare the appropriate methods abstract, to force sub-classes to
implement them in their own appropriate ways
public abstract class Lendable {
// Don't really borrow a Lendable,
//
but you borrow a book
// Nor do you or eat a Fruit
//
but you can Eat a Peach
// Subclass must implement these two methods
abstract public void checkSelfOut(String ID);
abstract public double getLateFee();
} // Done with Lendable for now
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What can a subclass do?
General form for inheriting a Java class:
public class subclass extends superclass {
// subclass inherits all public and
// protected methods of superclass
may add class constants
may add instance variables
may add 1 to many constructors
may add new public and protected methods
may override methods in the superclass
may add additional private methods
}
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The Constructor and Super
A subclass typically defines its constructor. If not
– You will still get a default constructor with 0 parameters
that automatically calls the base class constructor
Constructors in a subclass often call the superclass
constructor to initialize the objects
Access superclass with the keyword super
– can pass along arguments
super(callNum, title)
– if used, super must be the first message in the
constructor of the derived classes
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Book extends Lendable
public class Book extends Lendable {
public static final int DAYS_TO_BORROW_BOOK = 14;
public static final double BOOK_LATE_DAY_FEE = 0.50;
private String author;
public Book(String callNumber,
String title,
String author){
super(callNumber, title);
author = author;
}
public String getAuthor() {
return author;
}
}
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Complete both required methods
checkSelfOut delegates some work to
Lendable and passes along the unique
information Book.DAYS_TO_BORROW_BOOK
/** Modify the state of this object so it is borrowed
* @param borrowerID: The identification of borrower
*/
@Override
public void checkSelfOut(String borrowerID) {
checkOutAnyLendable(borrowerID,
Book.DAYS_TO_BORROW_BOOK);
}
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getLateFee differs among the
Lendable subclasses
@Override
public double getLateFee() {
if(this.isAvailable()) // Not even checked out!
return 0.00;
else {
// A positive daysLate means due date has passed
int daysOverdue = this.daysLate();
if(daysOverdue > 0) // This Lendable is overdue
return daysOverdue * Book.BOOK_LATE_DAY_FEE;
else
return 0.00; // The due date has not passed
}
}
}
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A few assertions
@Test
public void testGetters() {
// Show that Book has many methods via inheritance
Book aBook = new Book("QA76.1", "C++", "Jo");
assertTrue(aBook.isAvailable());
assertFalse(aBook.isOverdue());
assertNull(aBook.getBorrowerID());
assertEquals("QA76.1", aBook.getCallNumber());
assertEquals("C++", aBook.getTitle());
assertEquals("Jo", aBook.getAuthor());
// Use the 2 methods that once were abstract and
// now have concrete realizations
assertEquals(0.00, aBook.getLateFee(), 1e-12);
assertTrue(aBook.isAvailable());
aBook.checkSelfOut("Rick");
assertFalse(aBook.isAvailable());
}
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Adding Other Subclasses
Extend Lendable
Optional: add class constants
– days to borrow, late fee amounts
Add a constructor that passes along arguments
to the constructor in Lendable (super)
Add the methods that Lendable requires of all
sublclasses: use Override
– checkSelfOut
– getLateFee
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