Inheritance - Absolute Java - page 468

TIP: (continued)

Assuming that the equals method for both Employee and HourlyEmployee are
defined using instanceof (as previously mentioned), consider the following situation:

Employee e =
new Employee("Joe Worker", new Date("January", 1, 2004));
HourlyEmployee hourlyE = new HourlyEmployee("Joe Worker",
new Date("January", 1, 2004), 50.50, 160);
Then, with the definition of equals that uses instanceof , we get that

e.equals(hourlyE)
returns true , because hourlyE is an Employee with the same name and hire date as e .
So far, it sounds reasonable.
However, since we are assuming that we also used instanceof in the defi nition of
equals for the class HourlyEmployee , we also get that

hourlyE.equals(e)
returns false because e instanceof HourlyEmployee returns false . ( e is an
Employee but e is not an HourlyEmployee .)
So, if we defi ne equals in both classes using instanceof , then e equals hourlyE ,
but hourlyE does not equal e . That is no way for equals to behave.
Since instanceof does not yield a suitable defi nition of equals , you should instead
use getClass() as we did in Display 7.10. If we use getClass() in a similar way in
the defi nition of equals for the class HourlyEmployee (see Self-Test Exercise 19 ), then

e.equals(hourlyE)

and

hourlyE.equals(e)

■

both return false .