SWEN20003
Object Oriented Software Development
Workshop 2 (Solutions)
Semester 1, 2021
1. Create a Circle class with a radius, x coordinate, and y coordinate.
(a) Add a default constructor public Circle() that sets the radius to 1 and the coordinates to (0,
0).
(b) Add a constructor public Circle(double radius) that sets the radius to the argument value, and
the coordinates to (0, 0).
(c) Add a constructor public Circle(double radius, double x, double y) with the appropriate
actions.
(d) Add toString and equals methods.
Solution:
public class Circle {
private double radius;
private double x;
private double y;
public Circle() {
this.radius = 1;
this.x = 0;
this.y = 0;
}
public Circle(double radius) {
this.radius = radius;
this.x = 0;
this.y = 0;
}
public Circle(double radius, double x, double y) {
this.radius = radius;
this.x = x;
this.y = y;
}
public String toString() {
return String.format(“Circle at (%f, %f) with radius %f”, x, y, radius);
}
public boolean equals(Circle rhs) {
double EPSILON = 1e-5;
return Math.abs(radius – rhs.radius) < EPSILON && Math.abs(x - rhs.x) < EPSILON && Math.abs(y - rhs.y) < EPSILON; } } 1 2. Create a similar Rectangle class with a left coordinate, a top coordinate, a width, and a height. Solution: public class Rectangle { private double left; private double top; private double width; private double height; public Rectangle() { this.left = 0; this.top = 0; this.width = 1; this.height = 1; } // etc. public String toString() { return String.format("Rectangle from (%f, %f) to (%f, %f)", left, top, left + width, top + height); } public boolean equals(Rectangle rhs) { double EPSILON = 1e-5; return Math.abs(left - rhs.left) < EPSILON && Math.abs(top - rhs.top) < EPSILON && Math.abs(width - rhs.width) < EPSILON && Math.abs(height - rhs.height) < EPSILON; } } 3. (a) Create a Book class to represent a book in a library. Books have an author, a title, and can either be borrowed or not borrowed. (b) Write a constructor for your class. (c) Define getters for your class. (d) Add appropriate toString and equals methods to your class. Solution: public class Book { private String author; private String title; private boolean borrowed = false; public Book(String author, String title) { this.author = author; this.title = title; } public String getAuthor() { return author; } public String getTitle() { return title; } public boolean isBorrowed() { return borrowed; } public String toString() { return author + ": " + title; } 2 public boolean equals(Book rhs) { return author.equals(rhs.author) && title.equals(rhs.title); } } (e) Define a method void borrow(String borrowedBy) that marks the book as borrowed. You’ll need to add an attribute to the class to store who has borrowed the book. (f) Define a method void returnBook() that returns the book to the library. (g) Add a static attribute to count the number of books that are currently borrowed. (h) Define a static method that returns the number of books currently borrowed. Solution: private String borrowedBy = null; private boolean borrowed = false; public void borrow(String borrowedBy) { if (!borrowed) { borrowed = true; this.borrowedBy = borrowedBy; ++Book.numBorrowed; } } public void returnBook() { if (borrowed) { borrowed = false; borrowedBy = null; --Book.numBorrowed; } } public static int numBorrowed() { return numBorrowed; } 4. (a) Create a Library class with an appropriate constructor to represent a library that can hold up to 10 books. (Hint: use an array!) (b) Define a method to add a book to the library. If the library is already full, it should do nothing. (c) Define a method Book lookup(String title) that looks up a book by title and returns the first book with that title in the library. If there is no such book, it should return null. (d) Add an overloaded method Book lookup(String title, String author) that looks up a book by title and author. (e) Add a method String getCatalogue() that returns a string containing each book in the library on separate lines, in the following format: Charles Dickens: Great Expectations Sun Tzu: The Art of War Brian Kernighan & Denis Ritchie: The C Programming Language (Hint: if you define Book’s toString method carefully, this problem is easy.) Solution: public class Library { private static final int MAX_BOOKS = 10; private int numBooks = 0; private Book[] books; public Library() { this.books = new Book[10]; } 3 public void addBook(Book book) { if (numBooks < MAX_BOOKS) { books[numBooks] = book; ++numBooks; } } public Book lookup(String title) { for (int i = 0; i < numBooks; ++i) { if (books[i].getTitle().equals(title)) { return books[i]; } } return null; } public Book lookup(String title, String author) { for (int i = 0; i < numBooks; ++i) { Book book = books[i]; if (book.getTitle().equals(title) && book.getAuthor().equals(author)) { return book; } } return null; } public String getCatalogue() { String result = ""; for (int i = 0; i < numBooks; ++i) { result += books[i].toString() + "\n"; } return result; } } (f) Replace the static attribute and method in the Book class with an instance variable and method in the Library class. Solution: public int numBorrowed() { int count = 0; for (int i = 0; i < numBooks; ++i) { if (books[i].isBorrowed()) { ++count; } } return count; } (g) Write a main method to create some books, add them to a library, look up books, and borrow them. Solution: public static void main(String[] args) { Library library = new Library(); library.addBook(new Book("Charles Dickens", "Great Expectations")); library.addBook(new Book("Sun Tzu", "The Art of War")); library.addBook(new Book("Brian Kernighan & Denis Ritchie", "The C Programming Language")); System.out.print(library.getCatalogue()); Book dickens = library.lookup("Great Expectations"); dickens.borrow("Eleanor McMurtry"); 4 System.out.println("Number of books borrowed: " + library.numBorrowed()); } 5