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

• Superclass references don't know the full story:

anAccount.deposit(1000); // OK
anAccount.addInterest();
// No--not a method of the class to which anAccount belongs

• When you convert between a subclass object to its superclass type:

  • The value of the reference stays the same – it is the memory location of the object

  • But, less information is known about the object

• Why would anyone want to know less about an object?

  • Reuse code that knows about the superclass but not the subclass:
    
    public void transfer(double amount, BankAccount other)
    {
       withdraw(amount);
       other.deposit(amount);
    }
    
    Can be used to transfer money from any type of BankAccount

• 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;
   . . .
}

Why did the second parameter of the transfer method have to be of type BankAccount and not, for example, SavingsAccount?

   Answer: We want to use the method for all kinds of bank accounts. Had we used a parameter of type SavingsAccount,
   we couldn't have called the method with a CheckingAccount object.

Why can't we change the second parameter of the transfer method to the type Object?

   Answer: We cannot invoke the deposit method on a variable of type Object.