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OO Design with Inheritance

C Sc 335Rick 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– 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 abstractionsDetermine 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 drawn pre-UML

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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

Lendable

Book CDRom Video

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:– MyFrame extends JFrame that makes MyFrame a

JFrame with additional methods and listeners for a specific application

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A Few Examples of Inheritance

All Exceptions extend Exception:Many classes extend the Component class– JButton IS-A Component– JTextField IS-A Component

A GregorianCalender IS-A CalendarVector and ArrayList extend AbstractListDuplicateCardException IS-A RuntimeExceptionJukeboxGUI IS-A JFrame

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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 bothThe actual values will later come from the subclasses constructor

public Lendable(String callNumber, String initTitle) { this.callNumber = callNumber; title = initTitle; 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() { int result = 0; if (!isAvailable()) { DayCounter today = new DayCounter(); result = dueDate.daysFrom(today); } return result; }

public boolean isOverdue() { if (this.isAvailable()) return false; return daysLate() > 0; }

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Common Behavior continued

public boolean checkSelfIn() { if (this.isAvailable()) { return false; } else { dueDate = null; availability = true; return true; } }

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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 { // We Don't really borrow a Lendable,

// but you borrow a book // Nor do you or eat a Fruit

// but you can Eat a Peach // Subclasses 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 subclass do?

General form for inheriting a Java class:

public class subclass extends superclass { may add a constructor may override methods in the superclass may add additional constructors may add class constants may add instance variables may add new 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 constructorConstructors in a subclass often call the superclass constructor to initialize the objectsAccess 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 {

// Class constants public static final int DAYS_TO_BORROW_BOOK = 14; public static final double BOOK_LATE_DAY_FEE = 0.50;

// Instance variable added private String my_author;

public Book(String callNumber, String title, String author) { super(callNumber, title); my_author = author; }

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Complete both abstract methods

public void checkSelfOut(String borrowerID) { // Record who is borrowing this Lendable setBorrowerID(borrowerID); // Set the new due date by the correct days DayCounter due = new DayCounter(); due.adjustDaysBy(Book.DAYS_TO_BORROW_BOOK); setDueDate(due); // Mark this Lendable as no longer available setAvailability(false); }

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Complete both abstract methods

public double getLateFee() { if (this.isAvailable()) return 0.00; else { int daysOverdue = this.daysLate(); if (daysOverdue > 0) // The book is overdue. return daysOverdue * Book.BOOK_LATE_DAY_FEE; else return 0.00; // The due date is in the future } }

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Book IS-A Lendable, cast required @Test public void testGetters() { Lendable 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", ((Book) aBook).getAuthor());

assertEquals(0.00, aBook.getLateFee(), 1e-12); assertTrue(aBook.isAvailable()); aBook.checkSelfOut("Rick"); assertFalse(aBook.isAvailable()); }

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Lendables in a List<E>

ArrayList<Lendable> collection = new ArrayList<>(); collection.add(new Book("QA76.1", "C++", "Jo")); Video aVideo = new Video("VIDEO1", "The Matrix"); collection.add(aVideo); Lendable aLendable = collection.get(0); assertEquals("C++", aLendable.getTitle()); assertEquals("QA76.1", aLendable.getCallNumber());

aLendable = collection.get(1); assertEquals("The Matrix", aLendable.getTitle()); assertEquals("VIDEO1", aLendable.getCallNumber());

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Lendables in a List<E>

collection.get(0).checkSelfOut("Kim");collection.get(1).checkSelfOut("Kim"); for (Lendable thingToBorrow : collection) { System.out.println("Due date: " + thingToBorrow.getDueDate()); System.out.println("Days late: " + thingToBorrow.daysLate());}

Output:Due date: 2014/10/27Days late: -14Due date: 2014/10/15Days late: -2

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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– super used to avoid a bunch of setter methods in

Lendable for subclasses to modify their variables– the common behavior is in the superclass

• the same thing is done for all subclasses– We'll get a polymorphic checkSelfOut and

getLateFee messages

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Adding Other Subclasses

Extend LendableOptional: 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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Java’s 4 Access Modes

Public: known wherever the Object is declared– Modifier public

Private: known only in the class– Modifier private

Package: everywhere within the packageNo modifier: Particular to Java

Protected ….

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Protected

The protected access mode means that only subclasses can reference them – Subclasses can directly reference all public & protected

elements– Subclass inherit private methods and instance variables, but

they can not be directly referenced• Information hiding

This limits method calls to subclasses– Somewhat private

Some people use protected instance variables, we won’t