Inheritance - Java Concepts - page 624

object of a class that implements the Measurable interface, perhaps a Coin object
or an object of an entirely different class.
What happens when you invoke a method? For example,
BankAccount anAccount = new CheckingAccount();
anAccount.deposit(1000);
Which deposit method is called? The anAccount parameter has type
BankAccount , so it would appear as if BankAccount.deposit is called. On
the other hand, the CheckingAccount class provides its own deposit method
that updates the transaction count. The anAccount field actually refers to an object
of the subclass CheckingAccount , so it would be appropriate if the
CheckingAccount.deposit method were called instead.
In Java, method calls are always determined by the type of the actual object, not the
type of the object reference. That is, if the actual object has the type
CheckingAccount , then the CheckingAccount.deposit method is called.
It does not matter that the object reference is stored in a field of type BankAccount .
As we discussed in Chapter 9 , the ability to refer to objects of multiple types with
varying behavior is called polymorphism.
If polymorphism is so powerful, why not store all account references in variables of
type Object ? This does not work because the compiler needs to check that only
legal methods are invoked. The Object type does not define a deposit methodȌ
the BankAccount type (at least) is required to make a call to the deposit method.
Have another look at the transfer method to see polymorphism at work. Here is
the implementation of the method:
public void transfer(double amount, BankAccount
other)
{
withdraw(amount);
other.deposit(amount);
}
Suppose you call
anAccount.transfer(1000, anotherAccount);
Two method calls are the result: