Chapter 8 Lab More Classes and Objects Lab Objectives -Be able to write a copy c

ID: 3695165 • Letter: C

Question

Chapter 8 Lab

More Classes and Objects

Lab Objectives

-Be able to write a copy constructor

-Be able to write equals and toString methods

-Be able to use objects made up of other objects (Aggregation)

-Be able to write methods that pass and return objects

Introduction

We discussed objects in Chapter 6 and we modeled a television in the Chapter 6 lab. We

want build on that lab, and work more with objects. This time, the object that we are

choosing is more complicated. It is made up of other objects. This is called aggregation.

A credit card is an object that is very common, but not as simple as a television.

Attributes of the credit card include information about the owner, as well as a balance and

credit limit. These things would be our instance fields. A credit card allows you to make

payments and charges. These would be methods. As we have seen before, there would

also be other methods associated with this object in order to construct the object and

access its fields.

Examine the UML diagram that follows. Notice that the instance fields in the CreditCard

class are other types of objects, a Person object or a Money object. We can say that the

CreditCard “has a” Person, which means aggregation, and the Person object “has a”

Address object as one of its instance fields. This aggregation structure can create a very

complicated object. We will try to keep this lab reasonably simple.

To start with, we will be editing a partially written class, Money. The constructor that

you will be writing is a copy constructor. This means it should create a new object, but

with the same values in the instance variables as the object that is being copied.

Next, we will write equals and toString methods. These are very common methods that

are needed when you write a class to model an object. You will also see a compareTo

method that is also a common method for objects.

After we have finished the Money class, we will write a CreditCard class. This class

contains Money objects, so you will use the methods that you have written to complete

the Money class. The CreditCard class will explore passing objects and the possible

security problems associated with it. We will use the copy constructor we wrote for the

Money class to create new objects with the same information to return to the user through

the accessor methods.

Task #1 Writing a Copy Constructor

1. Copy the files Address.java (code listing 8.1), Person.java (code listing 8.2),

Money.java (code listing 8.3), MoneyDriver.java (code listing 8.4), and

CreditCardDemo.java (code listing 8.5) from the Student CD or as directed by

your instructor. Address.java, Person.java, MoneyDemo.java, and

CreditCardDemo.java are complete and will not need to be modified. We will

start by modifying Money.java.

2. Overload the constructor. The constructor that you will write will be a copy

constructor. It should use the parameter money object to make a duplicate money

object, by copying the value of each instance variable from the parameter object

to the instance variable of the new object.

Task #2 Writing equals and toString methods

1. Write and document an equals method. The method compares the instance

variables of the calling object with instance variables of the parameter object for

equality and returns true if the dollars and the cents of the calling object are the

same as the dollars and the cents of the parameter object. Otherwise, it returns

false.

2. Write and document a toString method. This method will return a String that

looks like money, including the dollar sign. Remember that if you have less than

10 cents, you will need to put a 0 before printing the cents so that it appears

correctly with 2 decimal places.

3. Compile, debug, and test by running the MoneyDriver.java driver program. You

should get the output:

The current amount is $500.00

Adding $10.02 gives $510.02

Subtracting $10.88 gives $499.14

$10.02 equals $10.02

$10.88 does not equal $10.02

Task #3 Passing and Returning Objects

1. Create a CreditCard class according to the UML Diagram on the back. It should

have data fields that include an owner of type Person, a balance of type Money,

and a creditLimit of type Money.

2. It should have a constructor that has two parameters, a Person to initialize the

owner and a Money value to initialize the creditLimit. The balance can be

initialized to a Money value of zero. Remember you are passing in objects (pass

by reference), so you have passed in the address to an object. If you want your

CreditCard to have its own creditLimit and balance, you should create a new

object of each using the copy constructor in the Money class.

3. It should have accessor methods to get the balance and the available credit. Since

these are objects (pass by reference), we don’t want to create an insecure credit

card by passing out addresses to components in our credit card, so we must return

a new object with the same values. Again, use the copy constructor to create a

new object of type money that can be returned.

4. It should have an accessor method to get the information about the owner, but in

the form of a String that can be printed out. This can be done by calling the

toString method for the owner (who is a Person).

5. It should have a method that will charge to the credit card by adding the amount

of Money in the parameter to the balance if it will not exceed the credit limit. If

the credit limit will be exceeded, the amount should not be added, and an error

message can be printed to the console.

6. It should have a method that will make a payment on the credit card by

subtracting the amount of Money in the parameter from the balance.

7. Compile, debug, and test it out completely by running CreditCardDemo.java.

You should get the output:

Diane Christie, 237J Harvey Hall, Menomonie, WI 54751

Balance: $0.00

Credit Limit: $1000.00

Attempt to charge $200.00

Charge: $200.00

Balance: $200.00

Attempt to charge $10.02

Charge: $10.02

Balance: $210.02

Attempt to pay $25.00

Payment: $25.00

Balance: $185.02

Attempt to charge $990.00

Exceeds credit limit

Balance: $185.02

Code Listing 8.3 (Money.java)

/**Objects represent nonnegative amounts of money*/

public class Money

{

/**A number of dollars*/

private long dollars;

/**A number of cents*/

private long cents;

/**Constructor creates a Money object using the amount of money in

dollars and cents represented with a decimal number

@param amount the amount of money in the conventional decimal

format*/

public Money(double amount)

{

if (amount < 0)

{

System.out.println(

"Error: Negative amounts of money are not allowed.");

System.exit(0);

}

else

{

long allCents = Math.round(amount*100);

dollars = allCents/100;

cents = allCents%100;

}

/**Adds the calling Money object to the parameter Money object.

@param otherAmount the amount of money to add

@return the sum of the calling Money object and the parameter

Money object*/

public Money add(Money otherAmount)

{

Money sum = new Money(0);

sum.cents = this.cents + otherAmount.cents;

long carryDollars = sum.cents/100;

sum.cents = sum.cents%100;

sum.dollars = this.dollars

+ otherAmount.dollars + carryDollars;

return sum;

}

/**Subtracts the parameter Money object from the calling Money

object and returns the difference.

@param amount the amount of money to subtract

@return the difference between the calling Money object and the

parameter Money object*/

public Money subtract (Money amount)

{

Money difference = new Money(0);

if (this.cents < amount.cents)

{

this.dollars = this.dollars - 1;

this.cents = this.cents + 100;

}

difference.dollars = this.dollars - amount.dollars;

difference.cents = this.cents - amount.cents;

return difference;

}

/**Compares instance variable of the calling object with the

parameter object. It returns -1 if the dollars and the cents

of the calling object are less than the dollars and the cents

of the parameter object, 0 if the dollars and the cents of the

calling object are equal to the dollars and cents of the

parameter object, and 1 if the dollars and the cents of the

calling object are more than the dollars and the cents of the

parameter object.

@param amount the amount of money to compare against

@return -1 if the dollars and the cents of the calling object are

less than the dollars and the cents of the parameter object, 0 if

the dollars and the cents of the calling object are equal to the

dollars and cents of the parameter object, and 1 if the dollars

and the cents of the calling object are more than the dollars and

the cents of the parameter object.*/

public int compareTo(Money amount)

{

int value;

if(this.dollars < amount.dollars)

{

value = -1;

}

else if (this.dollars > amount.dollars)

{

value = 1;

}

else if (this.cents < amount.cents)

{

value = -1;

}

else if (this.cents > amount.cents)

{

value = 1;

}

else

{

value = 0;

}

return value;

}

Code Listing 8.4 (MoneyDriver.java)

/**This program tests the money class.*/

public class MoneyDriver

{

//This is a driver for testing the class

public static void main(String[] args)

{

final int BEGINNING = 500;

final Money FIRST_AMOUNT = new Money(10.02);

final Money SECOND_AMOUNT = new Money(10.02);

final Money THIRD_AMOUNT = new Money(10.88);

Money balance = new Money(BEGINNING);

System.out.println("The current amount is " +

balance.toString());

balance = balance.add(SECOND_AMOUNT);

System.out.println("Adding " + SECOND_AMOUNT +

" gives " + balance.toString());

balance = balance.subtract(THIRD_AMOUNT);

System.out.println("Subtracting " + THIRD_AMOUNT +

" gives " + balance.toString());

boolean equal = SECOND_AMOUNT.equals(FIRST_AMOUNT);

if(equal)

System.out.println(SECOND_AMOUNT + " equals "

+ FIRST_AMOUNT);

else

System.out.println(SECOND_AMOUNT +

" does not equal " + FIRST_AMOUNT);

equal = THIRD_AMOUNT.equals(FIRST_AMOUNT);

if(equal)

System.out.println(THIRD_AMOUNT + " equals " +

FIRST_AMOUNT);

else

System.out.println(THIRD_AMOUNT + " does not equal "

+ FIRST_AMOUNT);

}

Code Listing 8.5 (CreditCardDemo.java)

/**Demonstrates the CreditCard class*/

public class CreditCardDemo

{

public static void main(String[] args)

{

final Money LIMIT = new Money(1000);

final Money FIRST_AMOUNT = new Money(200);

final Money SECOND_AMOUNT = new Money(10.02);

final Money THIRD_AMOUNT = new Money(25);

final Money FOURTH_AMOUNT = new Money(990);

Person owner = new Person("Christie", "Diane",

new Address("237J Harvey Hall", "Menomonie",

"WI", "54751"));

CreditCard visa = new CreditCard(owner, LIMIT);

System.out.println(visa.getPersonals());