Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 575

return false;
}
}
You might think that our instanceof test would allow us to remove the type cast
below it. After all, the instanceof test ensures that the comparison occurs only
when o refers to a Point object. However, the type cast cannot be removed because
the compiler doesn't allow the code to compile without it.
A nice side benefit of the instanceof operator is that it produces a false result
when o is null . Thus, if the client code contains an expression such as
p1.equals(null) , it will correctly return false rather than throwing a
NullPointerException .
Many classes implement an equals method like ours, so much of the preceding
equals code can be reused as boilerplate code. The following is a template for a
well-formed equals method. The instanceof test and type cast are likely the first
two things you'll want to do in any equals method that you write:
public boolean equals(Object o) {
if (o instanceof <type>) {
<type> <name> = (<type>) o;
<compare the data and return the result.>
} else {
return false;
}
}
9.3 Polymorphism
One of the most powerful benefits of inheritance is that it allows client code to treat
different kinds of objects in the same way. For example, with the employee class
hierarchy described earlier, it's possible for client code to create an array or other
data structure that contains both lawyers and legal secretaries, and then perform oper-
ations on each element of that array. The client code will behave differently depend-
ing on the type of object that is used, because each subclass overrides and changes
some of the behavior from the superclass. This ability for the same code to be used
with several different types of objects is called polymorphism.
Polymorphism
The ability for the same code to be used with several different types of
objects and for the code to behave differently depending on the actual type
of object used.
Polymorphism is made possible by the fact that the type of a reference variable
(one that refers to an object) does not have to exactly match the type of the object it