NaiveTicket

The second Objects lab, from the BlueJ book's second chapter.

Look for the Chapter 2 file you need in the doc folder. There is 35 pages of reading and exercises in the chapter.

Work through all these exercises. You edit this file with your answers for these exercises.

Exercise 2.1

• Create a TicketMachine object on the object bench.
• Upon viewing its methods, getBalance, getPrice, insertMoney, printTicket.
• Use getPrice method to view the value of the price of the tickets that was set when this object was created.
• Use insertMoney method to simulate inserting an amount of money into the machine.
• Use getBalance to check that the machine has a record of the amount inserted.
  • You can insert several separate amounts of money into the machine, just like you might insert multiple coins or notes into a real machine. Try inserting the exact amount required for a ticket. As this is a simple machine, a ticket will not be issued automatically, so once you have inserted enough money, call the printTicket method. A facsimile ticket should be printed in the BlueJ terminal window.

Exercise 2.2

• What value is returned if you check the machine’s balance after it has printed a ticket?

ANSWER - The balance resets to 0.

Exercise 2.3

• Experiment with inserting different amounts of money before printing tickets.
  • Do you notice anything strange about the machine’s behavior?
    It tells you how much money it received instead of telling you your change.
  • What happens if you insert too much money into the machine – do you receive any refund? Machine does not issue a refund. The balance just resets to 0.
  • What happens if you do not insert enough and then try to print a ticket? The machine still prints the ticket and the balance goes to 0.

Exercise 2.4

• Try to obtain a good understanding of a ticket machine’s behavior by interacting with it on the object bench before we start looking at how the TicketMachine class is implemented in the next section.

Exercise 2.5

• Create another ticket machine for tickets of a different price.
  • Buy a ticket from that machine.
  • Does the printed ticket look different? No

Exercise 2.6

• Write out what you think the outer wrappers of the Student and LabClass classes might look like – do not worry about the inner part.

public class Student {

}

public class LabClass {

}

Exercise 2.7

Does it matter whether we write
public class TicketMachine
or
class public TicketMachine
in the outer wrapper of a class? It does. If you type class public errors occur and the entire program breaks

• Edit the source of the TicketMachine class to make the change and then close the editor window.
  • Do you notice a change in the class diagram? They appear with lines through them
  • What error message do you get when you now press the compile button? Identifier expected, Invalid method declaration
  • Do you think this message clearly explains what is wrong? I think it does. In this case reversing the words would mean we haven't defined a class around our code, thus breaking everything else. (we haven't identified our class)

Exercise 2.8

• Check whether or not it is possible to leave out the word public from the outer wrapper of the TicketMachine class. It still compiles even if 'public' is removed.

Exercise 2.9

• From your earlier experimentation with the ticket machine objects within BlueJ you can probably remember the names of some of the methods – printTicket, for instance.
  • Look at the class definition in Code 2.1 and use this knowledge, along with the additional information about ordering we have given you, to try to make a list of the names of the fields, constructors, and methods in the TicketMachine class.
  • Hint: There is only one constructor in the class. Fields - price, balance and Total. Methods - getPrice, getTicketNumber, getBalance, insertMoney, calculateTotal, incrementTicketNumber Constructor - TicketMachine

Exercise 2.10

• Do you notice any features of the constructor that make it significantly different from the other methods of the class? It is the only one that is allowed to have the same name as it's class. They can't have a return type.

Exercise 2.11

• What do you think is the type of each of the following fields?

private int count;  int
private Student representative;    String
private Server host;         String

Exercise 2.12

• What are the names of the following fields?

private boolean alive; alive
private Person tutor;  tutor
private Game game;     game

Exercise 2.13

In the following field declaration from the TicketMachine class

private int price;

does it matter which order the three words appear in? The type always needs to be declared before the variable.

• Edit the TicketMachine class to try different orderings. After each change, close the editor.
  • Does the appearance of the class diagram after each change give you a clue as to whether or not other orderings are possible? The diagram appears with lines through it, which lets you know there are errors and it did not compile.
  • Check by pressing the compile button to see if there is an error message.
  • Make sure that you reinstantiate the original version after your experiments!

Exercise 2.14

• Is it always necessary to have a semicolon at the end of a field declaration? Yes, you get an error that a ; is expected.
• Once again, experiment via the editor.
• The rule you will learn here is an important one, so be sure to remember it.

Exercise 2.15

• Write in full the declaration for a field of type int whose name is status. private int status;

Exercise 2.16

• To what class does the following constructor belong?

public Student(String name)

The Student class.(Constructors have the same name as their class)

Exercise 2.17

• How many parameters does the following constructor have and what are their types?

public Book(String title, double price)

The types are string and double. It takes 2 parameters.

Exercise 2.18

• Can you guess what types some of the Book class’s fields might be? Based on the parameters we know each book object will have a string and double type.
• Can you assume anything about the names of its fields? Based on the parameters each will have a title and price field.

READ upto and INCLUDING section 2.15 of this chapter.