Polymorphism

• In Java, type of a variable doesn't completely determine type of object to which it refers

BankAccount aBankAccount = new SavingsAccount(1000);
//  aBankAccount holds a reference to a SavingsAccount

• Method calls are determined by type of actual object, not type of object reference

BankAccount anAccount = new CheckingAccount();
anAccount.deposit(1000); // Calls "deposit" from CheckingAccount

• Compiler needs to check that only legal methods are invoked

Object anObject = new BankAccount();
anObject.deposit(1000); // Wrong!

• Polymorphism: ability to refer to objects of multiple types with varying behavior

• Polymorphism at work:

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

• Depending on types of amount and other, different versions of withdraw and deposit are called

Click here for Programs of Bank Account Tester

If a is a variable of type BankAccount that holds a non-null reference, what do you know about the object to which a refers?

   Answer: The object is an instance of BankAccount or one of its subclasses.

If a refers to a checking account, what is the effect of calling a.transfer(1000, a)?

   Answer: The balance of a is unchanged, and the transaction count is incremented twice.