Polymorphism and Abstract Classes - Absolute Java - page 497

PITFALL: (continued)

Now consider the following code:

Sale s = new Sale("floor mat", 10.00);
DiscountSale discount = new DiscountSale("floor mat", 11.00,10);
s.showAdertisement();
discount.showAdertisement();

You might expect the following output:

This is the Sale class.
floor mat Price and total cost = $10.0
This is the DiscountSale class.
floor mat Price = $11.0 Discount = 10.0%
Total cost = $9.9

However, because the defi nition used for the static method announcement , inside of
showAdvertisement , is determined at compile time (based on the type of the variable
holding the calling object), the output actually is the following, where the change is
shown in blue:

This is the Sale class.
floor mat Price and total cost = $10.0
This is the Sale class.
floor mat Price = $11.0 Discount = 10.0%
Total cost = $9.9

Java uses late binding with the nonstatic method toString but static binding with
the static method announcement .

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Downcasting and Upcasting
The following is perfectly legal (given the class definitions in Displays 8.1 and 8.2 ):

Sale saleVariable;
DiscountSale discountVariable =
new DiscountSale("paint", 15, 10);
saleVariable = discountVariable;
System.out.println(saleVariable.toString());

An object of a derived class (in this case, the derived class DiscountSale ) also has the
type of its base class (in this case, Sale ) and so can be assigned to a variable of the base
class type. Now let's consider the invocation of the method toString() on the last
line of the preceding code.