CS 46B - Lecture 4

Cay S. Horstmann

Pre-class reading

• Sections 10.5 - 10.7
• Did you do the quiz?

Converting Between Subclass and Superclass Types

• Ok to convert subclass reference to superclass reference:

  SavingsAccount collegeFund = new SavingsAccount(10);
  BankAccount anAccount = collegeFund;
  Object anObject = collegeFund;

• The three object references stored in collegeFund, anAccount, and anObject all refer to the same object of type SavingsAccount
  

Converting Between Subclass and Superclass Types

• Occasionally you need to convert from a superclass reference to a subclass reference:

  BankAccount anAccount = (BankAccount) anObject;

• This cast is dangerous: if you are wrong, an exception is thrown
• Solution: use the instanceof operator
• instanceof: tests whether an object belongs to a particular type:

  if (anObject instanceof BankAccount)
  {
     BankAccount anAccount = (BankAccount) anObject;
     . . .
  }

• Use sparingly—prefer dynamic binding

Lecture 4 Clicker Question 1

Why was the second parameter of the transfer method of type BankAccount?

public void transfer(double amount, BankAccount other)
{
   withdraw(amount);
   other.deposit(amount);
}

1. That's an error—it should have been of type SavingsAccount
2. That's an error—it should have been of type Object
3. So that the method can be called with SavingsAccount and CheckingAccount objects
4. So that the method can be called with objects of any subclass of BankAccount

Object: The Cosmic Superclass

• All classes defined without an explicit extends clause automatically extend Object:
  

Overriding the toString Method

• Returns a string representation of the object
• Useful for debugging:

  Rectangle box = new Rectangle(5, 10, 20, 30);
  String s = box.toString();
  // Sets s to "java.awt.Rectangle[x=5,y=10,width=20,height=30]"

• toString is called whenever you concatenate a string with an object:

  "box=" + box;
  // Result: "box=java.awt.Rectangle[x=5,y=10,width=20,height=30]"

• Object.toString prints class name and the hash code of the object:

  BankAccount momsSavings = new BankAccount(5000);
  String s = momsSavings.toString();
  // Sets s to something like "BankAccount@d24606bf"

• To provide a nicer representation of an object, override toString:

  public String toString()
  {
     return "BankAccount[balance=" + balance + "]";
  }

toString and Inheritance

• In superclass, print class name

  public class BankAccount
  {
     public String toString()
     {
        return getClass().getName() + "[balance=" + balance + "]";
     }
  }

• In subclass, call super.toString()

• public class SavingsAccount
  {
     public String toString()
     {
        return super.toString() + "[interestRate=" + interestRate + "]";
     }
  }

Lecture 4 Clicker Question 2

BankAccount acct = new SavingsAccount(0.5);
System.out.println(acct);

What does it print?

1. BankAccount[balance=0.0]
2. SavingsAccount[balance=0.0]
3. SavingsAccount[balance=0.0, interestRate=0.5]
4. SavingsAccount[balance=0.0][interestRate=0.5]

Overriding the equals Method

• equals tests for same contents:

  if (coin1.equals(coin2)) . . . 
     // Contents are the same

  

• == tests for references to the same object:

  if (coin1 == coin2) . . . 
     // Objects are the same

  

Overriding the equals Method

• Cannot change parameter type; use a cast instead:

  public class Coin
  {
     . . .
     public boolean equals(Object otherObject)
     {
        Coin other = (Coin) otherObject;
        return name.equals(other.name) && value == other.value;
     }
  }

Lecture 4 Clicker Question 3

public class CollectibleCoin extends Coin
{
   . . .
   public boolean equals(Object otherObject)
   {
      CollectibleCoin other = (CollectibleCoin) otherObject;
      return getName().equals(other.getName()) && getValue == other.getValue()
         && year == other.getYear;
   }
}

Coin c1 = new Coin("quarter", 0.25);
Coin c2 = new CollectibleCoin("quarter", 0.25, 1993);

What do c1.equals(c2) and c2.equals(c1) return?

1. Both return true
2. Both return false
3. The first one returns true, the second one returns false
4. The first one returns true, the second one throws an exception

Equals and Inheritance

• In superclass, check for equality of classes

  public class Coin
  {
     public boolean equals(Object otherObject)
     {
        if (getClass() != otherObject.getClass()) return false;
        ...
     }
  }

• In subclass, call super.equals

  public class CollectibleCoin extends Coin
  {
     public boolean equals(Object otherObject)
     {
        if (!super.equals(otherObject)) return false;
        ...
     }
  }

Lab

• Finally starting Lab 2
• You will work with a buddy
• Buddy list is on schedule page
• No buddy? Come to the front of the room
• One of you (driver) fusses with the code, the other (scribe) types up answers
• Switch roles each class meeting
• Submit lab work to Desire2Learn