Objectives:

• To gain an understanding of inheritance and abstract classes.

Prelab: Bring your book to class.

Pair Programming:

1. Find a partner of your choice.
2. Select one person to start as "driver". This person will type at the keyboard for the first 20 mintues.
3. Proceed through the process of completing the lab as described below. Be very careful to ensure that every item for both programs is completed.
4. Swap pairs every 20 minutes.
5. When you are done, be sure to email a copy of the code to the person whose account you are not working in. In other words, make sure that both partners have a copy of the code.

Problem Description

The Library class will use an array of LibraryItem to contain the items in the library (this is very similar to the video list in the Quicklist class in lab 10). The Library class will contain functionality to add an item to the library, to display all the overdue books and charges, and a toString method. (We will not worry about removing items from the library or other functionality at this time.)

Note that all items contained in a library have some common information (title, call number, status (checked out or not) and a number of days overdue which may be zero). In addition, items have common functionality including retrieving the title of the item, setting the number of days overdue, checking whether the item is overdue, and calculating the overdue charge. Since library items have so much in common, we will create a common super class called LibraryItem to contain this data and behavior. However, since libraries do not contain general library items, but specifically books or DVDs, we will never need to make an instance of this class so we will declare the LibraryItem class as abstract. In addition, since we do not know exactly how to calculate the overdue charge since this is dependent upon whether the item is a book or DVD, we will make this functionality abstract. That is, we will declare the calcCharge functionality as abstract and let the Book and Dvd classes define exactly how to calculate the overdue charge.

A Book class will hold information about a book. In addition to the title, call number, and other information inherited from the LibraryItem class, a book will also have an author, publisher, and isbn as well as the overdue charge. Functionality for a book will be limited to a constructor, calculating the overdue charge, and the toString method. Similarly, the Dvd class contains information for the studio that produced the DVD and the playing length of the DVD in addition to the information inherited from the LibraryItem class. The functionality for a DVD will also be limited to a constructor, calculating the overdue charge, and the toString method.

Part 1: Implementation

The UML diagram for the lab is shown below. The diagram shows that one instance of the Library class contains zero or more (up to N) instances of the LibraryItem class or any of its subclasses. This is represented by the line connecting the Library class to the LibraryItem class and the notation with 1 on the Library side and 0..N on the LibraryItem side. The LibraryItem class is shown to be the parent class or superclass of the Book and Dvd classes as modeled by the arrow with the open head that points from the Book and Dvd classes to the LibraryItem class. This open headed arrow is the UML notation for inheritance. Also note that the attributes in the LibraryItem, Book and Dvd classes are preceded by a hash mark. This indicates that the access specifier for these attributes is protected so that the attributes may be accessed by subclasses within the hierarchy, but not by classes outside of the inheritance hierarchy.

LibraryItem

• LibraryItem() : This constructor sets the checkedOut variable to be false and the number of days overdue to be zero.
• getTitle : The getTitle method retrieves and returns the title of the library item.
• isOverdue : The isOverdue method returns true if the number of days overdue is greater than zero, false otherwise.
• setOvedue : The setOverdue method takes an integer that represents the number of days that an item is overdue and sets the daysOverdue attribute to this value.
• calcCharge : The calcCharge method is abstract. A LibraryItem does not know exactly how much to charge for overdue. This functionality is deferred to the specific subtypes.

Book

• Book() : This constructor takes information for the book's title, call number, author, publisher, and initializes the object with these values. It must make a call to the LibraryItem superclass constructor. This looks like: super();
• calcCharge : The calcCharge method calculates and returns the overdue charge for the book. We are providing an implementation of the abstract method inherited from the LibraryItem class. The overdue charge for a book is 10 cents per day.
• toString : The toString method converts the book information to a string so that it can be displayed easily.

Dvd

• Dvd() : This constructor takes information for the Dvd's title, call number, studio, and playing length and initializes the object with these values. It must make a call to the LibraryItem superclass constructor. This looks like: super();
• calcCharge : The calcCharge method calculates and returns the overdue charge for the DVD. We are providing an implementation for the abstract method inherited from the LibraryItem class. The overdue charge for a DVD is $1.00 per day. However, if the DVD has been checked out for more than the overdue maximum limit (in my example this is 10 days), a 20% additional charge is assessed.
• toString : The toString method converts the DVD information to a string so that it can be displayed easily.

Library

• Library() : This default constructor creates a library that can hold up to 100 items.
• Library(int n) : This constructor takes an integer maximum number of items and creates an array of this size.
• addItem : The addItem method is passed an instance of a subtype of LibraryItem to add to the library. The method checks to see if there is room in the collection. If there isn't, an error message is displayed. If there is room, the item is added to the end of the collection.
• displayOverdueCharges : The displayOverdueCharges method displays all overdue items in the library as well as their overdue charge. It does this by looping through the array of LibraryItems, checking to see if each item is overdue, and if the item is overdue it displays the title of the item and calls the calcCharge method on the item to calculate the overdue charge and displays this value.
• toString : The toString method is inherited from the Object class. We override it to step through the collection and call the toString method on each subtype in the collection.

1. Create a new Library instance.
2. Create a Book instance.
3. Make the book 10 days overdue.
4. Add the book to the library.
5. Create another Book instance.
6. Add the book to the library.
7. Create a Dvd instance.
8. Make the DVD 20 days overdue.
9. Add the DVD to the library.
10. Create another Dvd instance.
11. Add the DVD to the library.
12. Display the contents of the library.
13. Display the overdue charges for the library.

Welcome to DrJava.  Working directory is C:\Documents and Settings\hellis2\My Documents\Courses\Introduction to Computing\Spring 2008\Labs\Lab 12
> java LibraryTest
*** CONTENTS OF LIBRARY ***
Title: Gone
Call No: Cob
Author: Harlan Coben
Publisher: Penguin
ISBN: 1234
Checked out? true
Days overdue: 10

Title: Forever
Call No: Bar
Author: Nevada Barr
Publisher: Requim
ISBN: 0987
Checked out? false
Days overdue: 0

Title: Great Minds
Call No: DVD-GM
Studio: Miramax
Playing Length: 120
Checked out? true
Days overdue: 20

Title: Geeksters
Call No: DVD-Gee
Studio: Disney
Playing Length: 90
Checked out? false
Days overdue: 0

*** OVERDUE CHARGES ***
Gone is overdue and has a charge of $1.0
Great Minds is overdue and has a charge of $24.0