Description
Learning Outcomes
Create a sub-class of an existing class
Identify cases of overriding methods and overloading methods
Understand the purpose of the “super” keyword in inherited methods
Observe and experiment with polymorphism and dynamic binding
Use “instanceof” and casting to resolve invalid statements with polymorphism
Pre-Lab
Create a new Java project called Lab2
Download the files: BankAccount.java, CheckingAccount.java, TestBankAccounts.java,
BankMachine.java, and AnswersLab2.txt
Save these downloaded files into the Lab2 src folder
Exercise 1 – Creating a sub-class
1. Create a new SavingsAccount class in your project, which extends the class
BankAccount.
2. Besides the instance variables and methods inherited from class BankAccount, this
class must contain one instance variable of type double called interestRate. We might
need to create several objects of the new class SavingsAccount, each one with its own
balance and interest rate. Should instance variable interestRate be declared as static?
Write your answer in AnswersLab2.txt.
3. This class must also contain the following methods:
a. constructor with two parameters of type double. The method signature is thus:
public SavingsAccount(double initialAmount, double rate).
This constructor must initialize the interestRate instance variable with the value
of rate and it must also initialize the instance variable balance of class
BankAccount with the value of initialAmount (if you do not know how to do this,
please review the lecture notes on inheritance. If you still need additional help,
please see class CheckingAccount to see how to do this).
b. public getter method called getInterestRate() that returns the value of
interestRate.
c. public method calculateInterest(), which calculates one month’s interest
and adds this amount to the balance; the interest is computed by multiplying
interestRate and the balance. How can you get the balance from BankAccount?
CS1027
LAB 2 Computer Science Fundamentals II
How do you add the interest to the balance of BankAccount? (If you are unsure
about how to do this, please read class BankAccount.)
d. public method toString(), which returns a string representation of the
instance variables; this string must be of the form “SavingsAccount: balance
$123, interest rate 0.12”.
4. Add this test harness (copy and paste it) to your SavingsAccount class.
5. Run the program and write the output in AnswersLab2.txt.
Exercise 2 – Experimenting with inheritance
1. Open TestBankAccounts.java. This class has a main method that creates several bank
account objects of the three types: BankAccount, CheckingAccount, and
SavingsAccount.
2. In each step of this exercise, you will be adding statements to the main method and
running it after each addition. Answer the questions at each step and write your answers
in AnswersLab2.txt. If there is a compilation error, make sure you comment out the
offending statement before advancing to the next step of this exercise.
3. Which method(s) of the class CheckingAccount demonstrate overriding? Which
demonstrate overloading?
4. In the main method, add the statement bacc0 = chacc1; which makes the
BankAccount variable bacc0 reference the CheckingAccount object referenced by
chacc1. Is this legal? Why?
5. Add the statement chacc1 = bacc1;. Is this legal? Why not? If not, comment it out.
6. Add the statement BankAccount bacc2 = new CheckingAccount(200.0); then
add the statement chacc1 = bacc2;. Fix this last statement using casting so that it
compiles and runs.
7. Add statements to invoke method deductFees(), without parameters, on all variables
bacc1, chacc1, and sacc1. On which type of objects was this legal? Why?
8. Try to fix the above compilation errors through casting:
((CheckingAccount)variable).deductFees(). Which compilation error(s) could
be fixed this way? Which one(s) could not be fixed this way? Why?
9. If there are compilation errors (cannot be more than 1), comment out the line causing the
error. Then run the program. Did the program run? If the program crashed, why did it
crash?
10. If the program crashed, comment out the statement that caused the program to crash.
Add the statement chacc1.deposit(100.0); Your program should compile and run.
11. Now make the following change to CheckingAccount.java: in the code for the deposit
method, change super.deposit(amount); to deposit(amount); Compile and
run. What was the runtime error?
CS1027
LAB 2 Computer Science Fundamentals II
12. To understand why the program crashed, add a breakpoint in class TestBankAccounts
at the line chacc1.deposit(100.0); (review Lab 2 if you do not remember how to
do this).
13. Run the program using the debugger. The execution of the program will stop at the line
where you added the breakpoint. Repeatedly press F5 to see how the execution of the
program proceeds. How many times is the method deposit() invoked?
14. Terminate the program by clicking on the red square in the tool bar at the top of Eclipse’s
window.
Exercise 3 – Experimenting with polymorphism
1. Open BankMachine.java and read over the code in its main() method.
2. The program produces two compilation errors, one when invoking method
getTransactionCount() and the other when invoking getInterestRate().
Explain why the compiler issues these two errors. Write your answer in AnswersLab2.txt.
3. In the statement String accountInfo = newAcc.toString(); just by looking at
the code can you tell whether the method toString() being invoked is from
SavingsAccount or from CheckingAccount? Explain your answer and write it in
AnswersLab2.txt.
4. Modify the code so that newAcc.getTransactionCount() is invoked only if newAcc
references an object of the class CheckingAccount, and
newAcc.getInterestRate() is invoked only if newAcc references an object of the
class SavingsAccount.
5. Run the program and verify that it works correctly for both account types (run the
program and type either ‘c’ or ‘s’ so the program prints a message indicting that an
account has been created and it also prints information about the account including
either the interest rate or the number of transactions).
Submission
When you have completed the lab, navigate to the weekly module page on OWL and click the
Lab link (where you found this document). Make sure you are in the page for the correct lab.
Upload the files listed below and remember to hit Save and Submit. Check that your submission
went through and look for an automatic OWL email to verify that it was submitted successfully.
CS1027
LAB 2 Computer Science Fundamentals II
Rules
Please only submit the files specified below. Do not attach other files even if they were
part of the lab.
Do not ZIP or use any other form of compressed file for your files. Attach them
individually.
Submit the lab on time. Late submissions will receive a penalty.
Forgetting to hit “Submit” is not a valid excuse for submitting late.
Submitting the files in an incorrect submission page will receive a penalty.
You may re-submit code if your previous submission was not complete or correct,
however, re-submissions after the regular lab deadline will receive a penalty.
Files to submit
SavingsAccount.java
AnswersLab2.txt
