The self-test quiz for week 8 is here.

ArrayLists and Classes

Download the BlueJ project club.zip for this week’s exercises. There are three classes in the project that help you to represent a club and its members.

The Member class is fully defined and contains all the methods you need. Note a couple of things we haven’t covered in lectures. The constructor uses a throw statement to create a Java runtime error. Try entering a month with a number outside the range 1 .. 12 and see what happens.

The Member class also defines a method called toString. This returns a string representation of the object. If you try to print the object using System.out.println, the system will used toString to convert your object into a string for printing.

The ClubDemo class is the “main” class that we use to create a club and members and test the other classes.

The Club class is incomplete. The methods in the class are empty and your job is to write the code inside to make them work.

Question 1

The club class needs a way of storing a collection of members. You should use an ArrayList to do that. The class includes two constructors. Let’s start with the first one. It should initialise the ArrayList and leave it empty.

Question 2

The Join method is passed a new member as an argument. You should add this member to the ArrayList.

Question 3

The numberOfMembers should return the number of elements in the ArrayList.

Question 4

The listMembers method should go through the ArrayList of members and print each one, one per line.

Question 5

The removeMember method should remove a member from the ArrayList when a member decides to leave the club. The argument to the method is a string, which is the member’s name. You have to search the ArrayList to find the member with that name. Use a simple algorithm, just go through the list, one-by-one until you find the member with the given name and remove it from the ArrayList.

Question 6

The second constructor in the class allows you to pass an array of members. Initialise the club membership with the members in the array.

The self-test quiz for this week is here

Task 1: Catch Up

If you haven’t completed exercises from previous labs, you may use this week’s lab to catch up on unfinished work.

If you are up-to-date, you can carry on with the following exercises.

Task 2: Return to the Ticket Machine

Go back to the ticket machine project you worked on in last week’s tut.

1. Create a new class Ticket which represents a ticket.
2. Create appropriate fields. What information should a Ticket carry?
3. Create a constructor to initialise these fields.
4. Now modify purchaseTicket on the TicketMachine class so that it creates a new Ticket object and returns it, rather than printing it.

Let’s suppose now that a ticket has 10 charges. Each time we use it, one charge is lost.

1. Change your Ticket class to track charges. What changes do you need to make to do this?

Now let’s add a recharge method to the TicketMachine. It should take a Ticket as a parameter and reset its charges to 10 but at the cost of 1/10th of the ticketPrice for every charge added (if the user has enough balance). Implement this in stages:

1. Make the method just recharge the Ticket for free.
2. Compute how much the recharge will cost. You will need to use an accessor method on the Ticket to find out how much charge it has.
3. Add a test to see if the user has enough balance.
4. If there is not enough balance print an error message and do not recharge.
5. Subtract the charge from the balance.

Task 3: Clock (if time permits)

Take a look at the clock project from Chapter 3 of the textbook.

1. Investigate the NumberDisplay class first. How does it work?
2. Look at the setValue method. What does the && operator do? This is a boolean operator. Try experimenting with it in the Code Pad.
3. Look at the increment method. What does the % operator do? Try experimenting with it in the BlueJ Code Pad.
4. Now take a look at the ClockDisplay class. Try to understand how it works.
5. ClockDisplay has two constructors. How do they differ?
6. The second constructor does not call updateDisplay. Is this a bug? Why/why not?
7. Change this class to have a seconds field as well as hours and minutes.
8. Change the class to show a 12 hour clock with AM/PM.

The self-test quiz for this week is here.

Assignment 1

1. If you haven’t started working on assignment 1, follow the assignment instructions to download the skeleton project.
2. Make sure you understand what is required. If not ask your tutor.
3. Assignment submission will be enabled later in the week.

Investigating the TicketMachine:

1. Download the TicketMachine project we’ve started looking at in lectures. Unzip it on your desktop and open the project with BlueJ.
2. Create an instance of the TicketMachine class.
3. Experiment with the addMoney() method. What happens if you:
   • Add 0 cents?
   • Add an negative amount?

   Is this behaviour correct?

4. Experiment with the purchaseTicket() method. Does it work correctly? Try a number of different tests to make sure.
5. Open up the source and place a breakpoint at the beginning of the purchaseTicket() method by clicking in the grey bar on the left. A little stop sign should appear.
6. Call the method again and use the Debugger to step through the code.
7. Try this with different balances to see how the execution varies.
8. There is a bug in this method. Can you find it?

Extending the TicketMachine

Let us extend the ticket machine to keep track of the total amount of cash collected and to allow us to change ticket prices.

1. Add a new field to the class to store the total cash collected.
2. When does this value change? What methods will you need to modify to track these changes?
3. Add a new accessor method getTotal() to return the value.
4. Test your code with a variety of method calls to make sure it does the right thing.
5. Add a new mutator method setPrice() to allow you to change the ticket price.
6. What happens if you set the ticket price to zero? a negative number?
7. Add a conditional statement to set-price to stop the price from being set to a negative value.

Design a Class to represent an E-Book.
It should record the following information:

• An author
• An title
• The number of pages
• The current page being read.

Implement your class in the following steps:

1. Create a class definition with an appropriate name.
2. Add appropriate fields. What types will they have? What names will you give them?
3. Create a constructor to initialise these fields. What starting values will you use?
4. Create accessor methods to access these fields.
5. Create mutator methods to allow the use to:
   • turn pages forwards and backwards
   • jump to a specific page
6. What should happen if they try to access a page outside of the sensible range? How can you implement this?
7. Create several different books to test your code. What happens if:
   • Your book has only one page?
   • Your book has no pages?
   • Your book has 1000000000000000000000 pages?
8. Comment your class to describe what it is for and what each of your methods does.

In this lab, we’ll practice creating a new BlueJ project from scratch. Use the “New” menu item to make a new class. Call it Week4. All the exercises below can be done in the same class file. You may not be able to finish all the questions during the lab time. Make sure that you do at least one question from each section in the lab and finish the rest as homework.

The self-test quiz for this week is here.

Basic Output

1. Write a method that prints the string “Hello World” 8 times. The output should be:

Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World
Hello World

2. Write a method that prints your name, age and favourite number, using integers for the numbers. For example:

My name is Fred, I am 18 years old, my favourite number is 2.

Simple Calculations

1. Write a method, which takes one parameter n and returns the number squared.

2. Write a method, which takes three parameters a, b and c to calculate the area of a triangle using Heron’s formula: area = sqrt(s(s-a)(s-b)(s-c)). Where s is the “semi-perimeter” given by s = (a+b+c)/2. The sqrt can be calculated by Math.sqrt()

3. Write a method that performs a calculation that you learnt in high school mathematics. The method should take as parameters any variables of the calculations and return the result of the calculation.

4. Write a method that takes a 4 digit number and then prints the number of units, tens, hundreds and thousands. For example:

Number: 1234
4 units
3 tens
2 hundreds
1 thousands

Using conditions

1. Write a method that takes in one parameter year and prints “This is a leap year”, if the year is a leap year or “This is not a leap year” if it is not. Recall that years are leap years if they are divisible by 4. Except years divisible by 100 are not leap years unless they are also divisible by 400.
HINT: the code x % y returns the remainder when x is divided by y

2. Write a method that takes three integers and prints them out in increasing order.

3. Write a method which takes in three real numbers a, b and c representing the coefficients of a quadratic equation ax² + bx + cand uses the quadratic formula to determine one solution of the equation ax² + bx + c = 0. Use an if statement to ensure that the program works correctly for all numeric input.

4. Write a method that takes a single integer, n between 0 and 9 and prints out the word for that number. For example

Number: 0
zero
Number: 1
one
Number: 9
nine

Using Loops

1. Write a method, which takes two parameters n and pow and returns the value of n^pow

2. Write a method with a while loop that prints out the numbers from 1 to 10 and their squares.

3. Change your method in question 1 to use a for loop.

4. Modify your method in question 2 so that is prints “Number is even”, if the number (or its square) is even. Otherwise print “Number is odd”.

5. Write a method that takes one parameter n and outputs an n x n square of asterisks. For example

Square: 5
*****
*****
*****
*****
*****

6. Write a method that takes one parameter n and outputs an n x n triangle. For example

Triangle: 5
    *
   **
  ***
 ****
*****

7. Write a method, which takes one parameter, a positive integer n and prints all the factors of n. For example:

n: 1001
The factors of 1001 are:
1
7
11
13
77
91
143
1001

Using Arrays

1. Write the method, void stars(int[] values) which, for each element of values prints out the number of asterisks corresponding to the integer. For example:

 values: {3, 2, 1, 5, 5, 3, 1, 0, 2}
***
**
*
*****
*****
***
*

**

2. Write the method, boolean isIncreasing (int[] values) which returns true if the values in the array are in increasing order. It should return false otherwise