CS1027b Computer Science Fundamentals II

Learning Outcomes

• Implement a subclass of an existing class
• Understand usage of the keyword super in inheritance
• Understand the concepts of overriding methods, polymorphism, and dynamic binding

General Lab Instructions

• Read through the lab instructions before coming to the lab.
• Read the lecture notes on inheritance.

IMPORTANT. Make sure you attend the lab session to which you registered, show the result(s) of each exercise to your TA, and submit the .java files that you wrote and AnswersLab5.txt through OWL by 11:55pm on the same day as your lab session to get your marks for the lab.

• Download the BankAccount and CheckingAccount classes. Download file AnswersLab5 and open it in a text editor.

Exercise 1: Creating another subclass of BankAccount

Create a new project for the classes BankAccount and CheckingAccount.

Create a new SavingsAccount class in your project, which extends the class BankAccount (how should you declare this class?). 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 AnswersLab5.txt; you must explain your answer.

• a constructor with two parameters 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).
• a public getter method called getInterestRate() that returns the value of interestRate.
• a 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? How do you add the interest to the balance of BankAccount? (If you are unsure about how to do this, please read class BankAccont.)
• a 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".
• Add this test harness to your SavingsAccount class and run the program. Write the output in AnswersLab5.txt.

Exercise 2: Experimenting with inheritance

Download the file TestBankAccounts.java and add it to your project. This class has a main method that creates several bank account objects of the three types BankAccount, CheckingAccount, and SavingsAccount.

In each step of this exercise, you will be adding statements to the main method, running it after each addition. Answer the questions at each step and write your answers in AnswersLab5. If there is a compilation error, make sure you comment out the offending statement before advancing to the next step of this exercise.

• Which method(s) of the class CheckingAccount demonstrate overriding? Which demonstrate overloading?

• Add the statement bacc0 = chacc1; which makes the BankAccount variable bacc0 reference the CheckingAccount object referenced by chacc1. Was this legal? Why?
• Now add below the above statement the following: System.out.println(bacc0.toString()); and run. Which toString method did it use, the one from the BankAccount class or the one from the CheckingAccount class? Why?
• Add the statement chacc1 = bacc1. Was this legal? Why not?
• 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.
• Add statements to invoke method deductFees(), without parameters, on all variables bacc1, chacc1, and sacc1. On which type of objects was this legal? Why?
• Try to fix the above compilation errors through casting: ((SavingsAccount)variable).deductFees(). Which compilation error(s) could be fixed this way? Which one(s) could not be fixed this way? Why?
• 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?
• 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.
  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?
• To understand why the program crashed, add a breakpoint in class TestBanckAccounts at the line chacc1.deposit(100.0); (review Lab 3 if you do not remember how to do this).
  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?
  Terminate the program by clicking on the red square in the tool bar at the top of Eclipse's window. Every time that a method is invoked, memory is allocated to that invocation of the method. This memory is taken from a structure called the execution stack. Since the execution stack has a finite size, if a program calls itself continuously, it will exhaust the amount of memory allocated to the execution stack, and this will cause the Java interpreter to crash and issue a "StackOverflowError". We will talk in class more about the execution stack.

Exercise 3: Experimenting with polymorphism

• 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 AnswersLab5.
• In the statement String accountInfo = newAcc.toString(); just by looking at the code can you tell whether the method toString being invoked is the one from the class CheckingAccount or the one form the class SavingsAccount? Explain your answer and write it in AnswersLab5.
• To fix the compilation errors, 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. Run the program and verify that it works correctly (if you run the program and type either 'c' or 's' the program must print 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).
  Please try to answer the above question on your own. If you do not see how to modify the code as required, you can read these hints.