Chapter 2:

Chapter 2
Understanding class definitions

Overview

Chapter 1 presented the general concepts behind object oriented programming and the skills needed for creating and using objects from predefined classes. Here we will define and refine these concepts more precisely and more detailed. We will also examine how to take different actions dependent upon a condition being true or false.

2.1 Ticket machines

Train station ticket machines print a ticket when the correct money is inserted. We define a class that models a naive ticket machine where:

a machine is constructed to print tickets of one set amount
customers get the ticket price
customers insert money
customers check balance
customers request ticket to be printed

Exercise 1
In BlueJ

Open project: C201\Projects\Chapter02\naive-ticket-machine
In BlueJ menu:

Click View
Check Show Terminal.

In Terminal Window:

Click Options
Check Record method calls.

Create an instance of the class using TicketMachine(ticketCost) constructor.

Supply an int parameter for the ticketCost in cents for $1.00.

Complete Exercise 2.1-2.4 on page 18 of the text.
Exercise 2.1

Try the getPrice method.

What does it do?
What is the return class?

Insert some money and check the balance using methods:

insertMoney
getBalance

Inspect the object.
Insert more money that a ticket costs.
Call method printTicket.

Exercise 2.2

Check the balance.
What is the balance after printTicket method is called?
Is that reasonable?

Exercise 2.3

Experiment by inserting different amounts of money before calling printTicket method.
What is the balance after printTicket method is called?
What must happen to balance field in printTicket method?

Exercise 2.4

Experiment with the TicketMachine class before we examine in in detail.
Create another TicketMachine object with a different ticket price.
Buy and print a ticket.
Does the ticket look much the same?
Is that reasonable behavior? Why?

2.2 Examining a class definition

The TicketMachine class definition source code of page 20 with comments removed.

public class TicketMachine
{

private int price;
private int balance;
private int total;

public TicketMachine(int ticketCost)
{
       price = ticketCost;
       balance = 0;
       total = 0;
}

public int getPrice()
{
       return price;
}

public int getBalance()
{
       return balance;
}

public void insertMoney(int amount)
{
       balance += amount;
}

public void printTicket()
{
       System.out.println("##################");
       System.out.println("# The BlueJ Line");
       System.out.println("# Ticket");
       System.out.println("# " + price + " cents.");
       System.out.println("##################");
       System.out.println();

       total += balance;
       balance = 0;
}

}

2.3 Fields, constructors and methods

Exercise 2

What are the field names?
What is the constructor name?
What are the method names?
Where does TicketMachine class begin and end?
Give the beginning and ending of classes Picture, Circle, Student and LabClass?
How do constructors and methods appear different?

2.3.1 Fields

Fields - Store data for each object.

Field definitions

private int price;
private int balance;
private int total;
Object named ticketMachine_1,
an instance of TicketMachine class.
It corresponds to the field definitions.

private - More in Chapter 5, for now always use private.
int - The value of the field must be a whole number, an integer.

2.3.2 Constructors

Constructors - Create an instance of the class (object) and initialize the fields to a known state.

TicketMachine Constructor

public TicketMachine(int ticketCost)
{
       price = ticketCost;
       balance = 0;
       total = 0;
}

Object constructed by:

TicketMachine (43)

price = ticketCost;

Field price is assigned value of parameter ticketCost.

balance = 0;

Field balance is assigned value 0.

2.3.3 Passing data via parameters

Parameter - Pass value to constructors and methods.

Formal parameter - Parameter name inside constructors and methods.

public TicketMachine(int ticketCost) - Formal parameter is ticketCost.

Actual parameter - Parameter value outside constructors and methods.

TicketMachine( 43 ) - Actual parameter is 43.

Passing actual parameter 43 to formal parameter ticketCost.

public TicketMachine( int ticketCost)

TicketMachine ticketMachine_1 = new TicketMachine( 43 );

TicketMachine( 43 )

Scope - Limit of where a name is known. Formal parameters are only known in constructor or method.

The scope (where known) of formal parameter ticketCost is TicketMachine constructor.

public TicketMachine(int ticketCost)
{
       price = ticketCost;
       balance = 0;
       total = 0;
}

The scope of field balance is TicketMachine constructor, getBalance, insertMoney and all other TicketMachine methods.

public int getBalance()
{
       return balance;
}

public void insertMoney(int amount)
{
       balance += amount;
}

Exercise 3 - Formal Parameters, Fields and Scope

public int getPrice()
{
return price;
}

private int price;
private int balance;
private int total;

What is the formal parameter of insertMoney method?
What is the formal parameter of getPrice method?
What is the scope of amount?
Can balance be used in method getPrice?
Can amount be used in method getPrice?
Try in BlueJ.

In method getPrice change return price; to return amount;
Compile.
What happened and why?

2.5 Assignment

Assignment - Copies to the variable at the left-side of = from the value of the expression at the right-side.

int balance;
balance = 40*3;                   String name;
name = "Fred";                  boolean onFire;
onFire = true;

For now, both sides must be the same data type: int, String, or boolean.

Expression - Computes values.

Exercise 3.5 - Assignment and Expressions

int balance; String name; boolean onFire;

What is the value of the following?

balance = 40*3+2;
name = "Fred" + "Flintstone";
onFire = true or false;

2.6 Accessor methods

Methods - A method performs action on the object by using the values of the fields.

Four methods of TicketMachine:

public int getPrice()
{
       return price;
}

public int getBalance()
{
       return balance;
}

public void insertMoney(int amount)
{
       balance += amount;
}

public void printTicket()
{
       System.out.println("##################");
       System.out.println("# The BlueJ Line");
       System.out.println("# Ticket");
       System.out.println("# " + price + " cents.");
       System.out.println("##################");
       System.out.println();

       total += balance;
       balance = 0;
}

Comment - For humans to read. Starts with /* and ends with */
Header - Defines method name, return data type, and formal parameters.
Body - Specifies what the method does in form of variable definitions and statements. Starts with { and ends with }.

/**
* Return the price of a ticket.
*/
public int getPrice()
{
return price;
} Comment

Header or signature

Body

Return type - int for getPrice which must return an int value such as 45 or price, void for insertMoney.

/**
* Receive amount of money in cents from a customer.
*/
public void insertMoney(int amount)
{
balance += amount;
} Comment

Header or signature

Body

Accessor Methods - Methods that return information such as getPrice and getBalance.

Exercise 4 -Methods

How does the header of getPrice and insertMoney differ?
How does the body of getPrice and insertMoney differ?
Try in BlueJ.

Remove the return price; statement from getPrice.
Compile
What happened and why?

Write an accessor method getTotal that returns the total field value.

Compile and test.

2.7 Mutator methods

Mutator methods - Change the values of the object's fields.

Assignment changes a variable, such as a field. Calling method printTicket mutates the field balance = 0;

public void insertMoney(int amount)
{
       balance += amount;
}

public void printTicket()
{
       System.out.println("##################");
       System.out.println("# The BlueJ Line");
       System.out.println("# Ticket");
       System.out.println("# " + price + " cents.");
       System.out.println("##################");
       System.out.println();

       total += balance;
       balance = 0;
}

Exercise 5 - Mutators

In BlueJ:

Create a TicketMachine object with a positive ticket price.
Inspect the object doing the following calls:

Call insertMoney method on the object, inserting 53 cents.
Call insertMoney method on the object, inserting 47 cents. Is insertMoney a mutator or accessor method?
Call printTicket method on the object. Is printTicket a mutator or accessor method?

= Assignment only.
+= Add then assignment.

Exercise 6 - Assignment

What is the value of balance after each statement:
balance = 40;
balance = 20;
balance = 10;

What is the value of balance after each statement:
balance = 40;
balance = balance + 20;
balance = balance + 10;

What is the value of balance after each statement:
balance = 40;
balance += 20;
balance += 10;

2.8 Printing from methods

System.out.println( expression ) - Prints the value of the expression followed by a new line to BlueJ Terminal Window.

Printing object ticketMachine_1

public void printTicket()
{
       System.out.println("##################");
       System.out.println("# The BlueJ Line");
       System.out.println("# Ticket");
       System.out.println("# " + price + " cents.");
       System.out.println("##################");
       System.out.println();

       total += balance;
       balance = 0;
}

##################
# The BlueJ Line
# Ticket
# 43 cents.
##################

Concatenation - Combining two Strings to one using the + operator.

"abc"+"def" is "abcdef"
"12"+3 is "123". The integer 3 is converted to a String "3" and then "12" +"3" is concatenated.

Exercise 7 - Printing and String Concatenation

What is printed by the following?

System.out.println("# The BlueJ Line");

What is printed by the following when price is 54?

System.out.println("# " + price + " cents.");

What is the result of the following when price is 54?

"# " + price + " cents."
"ab" + "c"
34 + 5
"34"+"5"
"34"+"5"
"# " +"price" + " cents."
"# + price + cents."

In BlueJ:

Create the TicketMachine object above.

Inspect the object.
Call the printTicket method on the object.

Exercise 2.19, page 33. Add a method prompt to the TicketMachine class. The method has void return type and no parameters. The method body should print:

Please insert the correct amount of money.

Exercise 2.20, page 33. Add a method showPrice to the TicketMachine class. The method has void return type and no parameters. The method body should print:

The price of a ticket is xyz cents.

where xyz is replaced by the value of price field when the method is called.

Exercise 2.22, page 33. Add a method showPrice to the TicketMachine class. The method has void return type and no parameters. The method body should print:

The price of a ticket is xyz cents.

2.9 Summary of the naive ticket machine

Exercise 8 - TicketMachine
In BlueJ

Implement another TicketMachine constructor:

with no ticketPrice parameter
price is fixed at 1000 cents
create a TicketMachine object using your new constructor
create a TicketMachine object using the old constructor and an actual parameter for a ticket price of 54 cents.
Inspect both objects. You should have two inspector windows. Is there any difference in the two objects?
Call insertMoney method using both objects.
Call printTicket method using both objects. Is there any difference in the two objects?

In your head

Exercise 2.26, page 34.
Why does the following version of refundBalance not give the same results as the original?

public int refundBalance()
{
    balance = 0;
    return balance;
}

What tests can you run to demonstrate that it does not?

Exercise 2.27, page 34.
What happens if you try to compile the TicketMachine class with the following version of refundBalance:

public int refundBalance()
{
        return balance;
        balance = 0;
}

What do you know about return statements that explain why this version is incorrect?

2.10 Reflecting on the design of the ticket machine

No check that enough money inserted to pay for ticket.
No check that sensible amount of money inserted, can insert negative amounts of money.
No check that ticket price of constructor is sensible.
No refund when too much money inserted.

2.11 Making choices: the conditional statement

Check that enough money inserted to pay for ticket.
Checks that sensible amount of money inserted, can insert negative amounts of money.
No check that ticket price of constructor is sensible.
No refund when too much money inserted.

Exercise 9 - Conditionals
In BlueJ:

Open the better-ticket-machine project.
Create a TicketMachine object:

with a ticketPrice parameter of 25 cents.

Inspect the object.
Call insertMoney method with amount parameter of -45 cents.

What happened to the object state?

Call insertMoney method with amount parameter of 5 cents.

What happened to the object state?

Call printTicket method.

What happened to the object state?
Explain how total changed.
Explain how balance changed.

Call insertMoney method with amount parameter of 50 cents.

What happened to the object state?

Call printTicket method.

What happened to the object state?
Explain how total changed.
Explain how balance changed.

if else conditional - When condition is true, performs the first statement. When condition is false, performs statement following the else.

Exercise 10 - if else conditionals
What are the following when:

amount = 54
balance = 10
correct = true

4 > 5
if (4 > 5 ) { balance = 6 } else { balance = 7 };
4 <= 5
if ( 4 <= 5) { balance = 6 } else { balance = 7 };
"a" < "b"
"abc" < "ab"
amount > 50
if (amount > 50 ) { balance = 6 } else { balance = 7 };
amount -10 > 50
if (amount - 10 > 50 ) { balance = 6 } else { balance = 7 };
if (amount > 0 ) {
     balance += amount
}
else {
     System.out.println( "Use a positive amount: " + amount );
}
if ( true ) { balance = 6 } else { balance = 7 };
if ( correct ) { balance = 6 } else { balance = 7 };

Changes to naive-ticket-machine to make the better-ticket-machine

public class TicketMachine
{

public void insertMoney(int amount)
{
        if(amount > 0) {
              balance += amount;
       }
       else {
              System.out.println("Use a positive amount: " + amount);
       }
}

public void printTicket()
{
        if(balance >= price) {
              System.out.println("##################");
              System.out.println("# The BlueJ Line");
              System.out.println("# Ticket");
              System.out.println("# " + price + " cents.");
              System.out.println("##################");
              System.out.println();

              total += price;
              balance -= price;
       }
       else {
              System.out.println("You must insert at least: " + (price - balance) + " more cents.");
       }
}

public int refundBalance()
{
       int amountToRefund;
       amountToRefund = balance;
       balance = 0;
       return amountToRefund;
}

}

2.12 A further conditional-statement example

See printTicket above.

Exercise 11 - printTicket

Exercise 2.33, page 39. After a ticket has been printed, could the value of balance field ever be set to a negative value by subtracting price? Justify your answer.

2.13 Local variables

Local variables - Variables defined inside a method. As with a formal parameter, scope is limited to inside the method.

public int refundBalance()
{
       int amountToRefund;

       amountToRefund = balance;
       balance = 0;
       return amountToRefund;
}

Exercise 12 - Local variables
In BlueJ:

Open the better-ticket-machine project.
Create a TicketMachine object:

with a ticketPrice parameter of 25 cents.

Inspect the object.

Is amountToRefund variable a field of the object?

Call insertMoney method with amount parameter of 45 cents.

What happened to the object state?

Call refundBalance method.

What happened to the object state?
What value was returned?

Add the following line in method printTicket:

balance -= price;
amountToRefund = 0;

Compile. What does the error message mean?

Correct printTicket method by deleting the added line.
Change the local variable definition to a comment as follows:

// int amountToRefund;

Compile TicketMachine.

Why is there an error message?

Exercise 2.35, page 40.
Local variables are temporary. Each time a method is called, the local variables are created. When the method returns, the local variables are destroyed.
What is printed by 4 calls to the method:

public void Exercise12() {
     int count = 0;
     count = count + 1;
     System.out.println(count);
}

2.14 Fields, parameters and local variables

Read text, page 40.

Exercise 13 - Fields, parameters and local variables
In BlueJ:

Exercise 2.36, page 41.

Add a new TicketMachine method, emptyMachine, that models emptying the machine of money.
The method should return the value in total and set total to 0.
Create an object with a ticket price of 45 cents.
Inspect.
Call insertMoney method with 73 as the actual parameter.
Call emptyMachine method to test.

Exercise 2.37, page 41. Is emptyMachine an accessor, mutator, or both?
Exercise 2.38, page 41. Rewrite printTicket method:

declare a local variable amountLeftToPay
initialize amountLeftToPay to the difference between price and balance
Rewrite the conditional to check amountLeftToPay <= 0

if true, print a ticket
else print an error message stating the amount still required

Create an object with a ticket price of 45 cents.
Inspect.
Test both conditions:

Call insertMoney method with 44 as the actual parameter.
Call printTicket method. Was the result as expected?
How can the other condition be tested? Test to verify.

2.15 Summary of the better ticket machine

Conditional statement - Performs one of two statements based on a condition test result.
Local variables - Temporary variable inside constructor or method.

2.16 Reviewing a familiar example

public class Student
{

    private String name;
    private String id;
    private int credits;

    public Student(String fullName, String studentID)
   {
        name = fullName;
        id = studentID;
        credits = 0;
    }

    public String getName()
    {
        return name;
    }

    public void changeName(String replacementName)
    {
        name = replacementName;
    }

    public String getStudentID()
    {
        return id;
    }

    public void addCredits(int additionalPoints)
    {
        credits += additionalPoints;
    }

    public int getCredits()
    {
        return credits;
    }

    public String getLoginName()
    {
       return name.substring(0,4) + id.substring(0,3);
    }

    public void print()
    {
        System.out.println(name + " (" + id + ")");
    }

}

Exercise 14 - Review and String methods

"01234567".substring(2,5) is the String "234". Assume the name field is "012345". What is:

"01234567".substring(4,5)
"01234567".substring(4,7)
"01234567".substring(4,10)
name.substring(0,3)
name.substring(-1,3)

"01234567".length() is the integer 8. Assume the name field is "012345". What is:

"01234".length()
"Harvey".length()
name.length()
"01234".substring(1,4).length()
name.substring(0,5).length()

In BlueJ:

Open project C201\Projects\chapter01\lab-classes

Examine in the editor.

Student class contains how many:

fields
constructors
mutators
accessors

What do the accessor names have in common?
Exercise 2.40, page 44.

What would be returned by getLoginName for a student with the name "Henry Moore" and the id "557214"?

Exercise 2.41, page 44.

Create a Student with name "djb" and id "859012". What happens when getLoginName is called on this student? Why do you think this is?

Exercise 2.42, page 44.
Exercise 2.43, page 44.

2.17 Summary

public class Book
{

    private String author;
    private String title;

    public Book( String bookAuthor, String bookTitle )
   {
       author = bookAuthor;
       title = bookTitle;
    }

}

Exercise 15 - Creating a class
In BlueJ:

Open project C201\Projects\chapter01\lab-classes

Examine in the editor.

Exercise 2.45, page 47.
Exercise 2.46, page 47.

Chapter 2Understanding class definitions

Overview

2.1 Ticket machines

2.2 Examining a class definition

2.3 Fields, constructors and methods

2.3.1 Fields

2.3.2 Constructors

2.3.3 Passing data via parameters

2.5 Assignment

2.6 Accessor methods

2.7 Mutator methods

2.8 Printing from methods

2.9 Summary of the naive ticket machine

2.10 Reflecting on the design of the ticket machine

2.11 Making choices: the conditional statement

2.12 A further conditional-statement example

2.13 Local variables

2.14 Fields, parameters and local variables

2.15 Summary of the better ticket machine

2.16 Reviewing a familiar example

2.17 Summary

Chapter 2
Understanding class definitions