Polymorphism and Abstract Classes - Absolute Java - page 505

changes in the method heading, this definition overrides the method clone inherited
from the class Object . As we noted in Chapter 7 , a change to a more permissive access,
such as from protected to public , is always allowed when overriding a method
definition. Changing the return type from Object to Sale is allowed because Sale
(and every other class, for that matter) is a descendent class of the class Object . This
is an example of a covariant return type, as discussed in the subsection of Chapter 7
entitled “Changing the Return Type of an Overridden Method.”
The clone method for the DiscountSale class can be defined similarly:

public DiscountSale clone()
{

return new DiscountSale( this );

}

The definitions of the classes Sale and DiscountSale on the website that
accompanies this topic each include the method clone defined as we just described.

extra code
on website

PITFALL: Sometimes the clone Method Return Type Is Object

Prior to version 5.0, Java did not allow covariant return types, and so did not
allow any changes whatsoever in the return type of an overridden method. In those
earlier versions of Java, the clone method for all classes had Object as its return
type. This is because the clone method for a class overrides the clone method of
the class Object , and the clone method of the class Object has a return type of
Object . If you encounter a clone method for a class that was designed and coded
before version 5.0 of Java, the clone method will have a return type of Object .
When using such older clone methods, you will need to use a type cast on the value
returned by clone .
For example, suppose the class OldClass was defi ned before Java 5.0. If
original is an object of the class OldClass , then the following will produce a
compiler error message:

OldClass copy = original.clone();
The problem is that original.clone() returns a value of type Object , while the
variable copy is of type OldClass . To correct the situation, you must add a type cast
as follows:

OldClass copy = (OldClass)original.clone();

(continued)