NaiveTicket

The seconf Objects Lab:

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?

Exercise 2.3

• Experiment with inserting different amounts of money before printing tickets.
  • Do you notice anything strange about the machine’s behavior?
  • What happens if you insert too much money into the machine – do you receive any refund?
  • What happens if you do not insert enough and then try to print a ticket?

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?

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.

Exercise 2.7

Does it matter whether we write
public class TicketMachine
or
class public TicketMachine
in the outer wrapper of a class?

• 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?
  • What error message do you get when you now press the compile button?
  • Do you think this message clearly explains what is wrong?

Exercise 2.8

• Check whether or not it is possible to leave out the word public from the outer wrapper of the TicketMachine class.

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.

Exercise 2.10

• Do you notice any features of the constructor that make it significantly different from the other methods of the class?

Exercise 2.11

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

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

Exercise 2.12

• What are the names of the following fields?

private boolean alive;
private Person tutor;
private 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?

• 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?
  • 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?
• 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.

Exercise 2.16

• To what class does the following constructor belong?

public Student(String name)

Exercise 2.17

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

public Book(String title, double price)

Exercise 2.18

• Can you guess what types some of the Book class’s fields might be?
• Can you assume anything about the names of its fields?

READ upto and INCLUDING section 2.15 of this chapter.