CS 46A - Lecture 7

Pre-class reading

• Sections 3.5 - 3.8
• Did you take the quiz?

Providing the Class Implementation

• First determine the data that each bank account object contains

  public BankAccount()
  {
     private double balance;
  }

• Then supply the bodies of the constructors and methods

Implementing Constructors

• Constructors contain instructions to initialize the instance variables of an object

  public BankAccount()
  {
     balance = 0;
  }
  
  public BankAccount(double initialBalance)
  {
     balance = initialBalance;
  }

Constructor Call Example

• Statement:

  BankAccount harrysChecking = new BankAccount(1000);

  • Create a new object of type BankAccount
  • Call the second constructor (because a construction parameter is supplied in the constructor call)
  • Set the parameter variable initialBalance to 1000
  • Set the balance instance variable of the newly created object to initialBalance
  • Return an object reference, that is, the memory location of the object, as the value of the new expression
  • Store that object reference in the harrysChecking variable

Syntax 3.3 Method Declaration

Implementing Methods

• deposit method:

  public void deposit(double amount) 
  {  
     balance = balance + amount; 
  }

Method Call Example

• Statement:

  harrysChecking.deposit(500);

  • Set the parameter variable amount to 500
  • Fetch the balance variable of the object whose location is stored in harrysChecking
  • Add the value of amount to balance
  • Store the sum in the balance instance variable, overwriting the old value

Implementing Methods

• public void withdraw(double amount)
  {
     balance = balance - amount;
  }

• public double getBalance()
  {
     return balance;
  }

Lecture 7 Clicker Question 1

Suppose we modify the BankAccount class so that each bank account has an account number. We add an instance variable

• private int accountNumber;

Where else do we need to make changes?

1. In the constructor
2. In the deposit method
3. In the withdraw method
4. All of the above

Lecture 7 Clicker Question 2

The Rectangle class has four instance variables: x, y, width, and height. Which of the following is an appropriate body for the method

public void translate(int dx, int dy)

1. width = width + dx;
   height = height + dy;

2. int newx = x + dx;
   x = newx;
   int newy = y + dy;
   y = newy;

3. x = x + width;
   y = y + height;

4. dx = x + dx;
   dy = x + dy;

Unit Testing

• Unit test: verifies that a class works correctly in isolation, outside a complete program
• To test a class, use an environment for interactive testing, or write a tester class
• Tester class: a class with a main method that contains statements to test another class
• Typically carries out the following steps:
  1. Construct one or more objects of the class that is being tested
  2. Invoke one or more methods
  3. Print out one or more results
  4. Print the expected results

Unit Testing

• Details for building the program vary. In most environments, you need to carry out these steps:
  1. Make a new subfolder for your program
  2. Make two files, one for each class
  3. Compile both files
  4. Run the test program

Testing With BlueJ

Self Check 3.16

When you run the BankAccountTester program, how many objects of class BankAccount are constructed? How many objects of type BankAccountTester?

1. No objects of BankAccountTester, no objects of BankAccount
2. One object of BankAccountTester, no objects of BankAccount
3. No objects of BankAccountTester, one object of BankAccount
4. One object of BankAccountTester, one object of BankAccount

Local Variables

• A local variable is declared in the body of a method:

  public double giveChange() 
  { 
     double change = payment - purchase; 
     purchase = 0; 
     payment = 0; 
     return change; 
  }

• Parameter variables are declared in the header of a method:

  public void enterPayment(double amount)

Local Variables

• Local and parameter variables belong to methods
  • When a method runs, its local and parameter variables come to life
  • When the method exits, they are removed immediately
• Instance variables belong to objects, not methods
  • When an object is constructed, its instance variables are created
  • The instance variables stay alive until no method uses the object any longer
• In Java, the garbage collector periodically reclaims objects when they are no longer used
• Instance variables are initialized to a default value (0 for numbers, null for objects), but you must initialize local variables

Lecture 7 Clicker Question 4

What does the following program segment print?

public void printArea()
{
   int area;
   int width = 10;
   int height = 20;
   System.out.println(area);
}

1. Nothing—the code contains a compile-time error
2. An error message is printed at run-time

3. 200

4. Expected: 200

Lecture 7 Clicker Question 5

Consider this class:

public class Label
{
   private String text;
   private int start;

   public int end() { return start + text.length();
}

What is the result of executing

System.out.println(new Label().end());

1. Nothing—the code contains a compile-time error
2. An error message is printed at run-time

3. 0

4. null0