OO Design with Inheritance

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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 a silly example
  (like this from Sun)
• I start with a reason to use inheritance
• Show the objects found for a Library System
• Recognize when to use of inheritance
• Build an inheritance hierarchy
• Design Guidelines related to inheritance
• See another use of polymorphism

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The 3 Pillars of OOPD

• 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?
• to build accurate models of complex working
  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
• 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.

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Identify candidate objects

• Candidate objects that model a solution with main
  responsibility. The model (no GUIs, events,
  networking)
• 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
• BorrowList maintains all possible borrowers

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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 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
• EmptyQueueException IS-A RuntimeException

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Designing An Inheritance Hierarchy

• Start with an abstract class to define
  common-alities 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 String callNumber
• private String title
• private boolean availability
• private String borrowerID
• private 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() gt 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
• // Nor do you or eat a Fruit
• ...
• // 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
• _at_param borrowerID The identification of
  borrower
• /
• _at_Overridepublic void checkSelfOut(String
  borrowerID)
• checkOutAnyLendable(borrowerID,
• Book.DAYS_TO_BORROW_BOOK
  )

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getLateFee differs among the Lendable subclasses

• _at_Overridepublic 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 gt 0) // This Lendable is
  overdue
• return daysOverdue Book.BOOK_LATE_DAY_FE
  E
• else
• return 0.00 // The due date has not
  passed

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A few assertions

• _at_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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