Java Reference
In-Depth Information
Your choice:
This is a Duck
It's Daffy the Aylesbury
Quack quackquack
Your choice:
This is a Cat
It's Max the Abyssinian
Miiaooww
The chances are that you will get a different set from this, and a different set again when you re-run the
example. The output from the example clearly shows that the methods are being selected at runtime,
depending on which object happens to get stored in the variable
petChoice
.
How It Works
The definition of the
sound()
method in the
Animal
class has no statements in the body, so it will do
nothing if it is executed. We will see a little later in this chapter how we can avoid including the empty
definition for the method, but still get polymorphic behavior in the derived classes.
We need the
import
statement because we use a
Random
class object in our example to produce
pseudo-random index values in the way we have seen before. The array
theAnimals
of type
Animal
contains a
Dog
object, a
Cat
object and a
Duck
object. We select objects randomly from this array in
the
for
loop using the
Random
object,
select
, and store the selection in
petChoice
. We can then
call the
toString()
and
sound()
methods using the object reference stored. The effect is that the
appropriate method is selected automatically to suit the object stored, so our program operates
differently depending on what type of object is referenced by
petChoice
.
Of course, we call the
toString()
method implicitly in the argument to
println()
. The compiler
will insert a call to this method to produce a
String
representation of the
petChoice
object. The
particular
toString()
method will automatically be selected to correspond with the type of object
referenced by
petChoice
. This would still work even if we had not included the
toString()
method in the base class. We will see a little later in this chapter there is a
toString()
method in
every class that you define, regardless of whether you define one or not.
Polymorphism is a fundamental part of object-oriented programming. We will be making extensive use
of polymorphism in many of the examples we will develop later in the topic, you will find that you will
use it often in your own applications and applets. But this is not all there is to polymorphism in Java,
and we will come back to it again later in this chapter.
Multiple Levels of Inheritance
As we indicated at the beginning of the chapter, there is nothing to prevent a derived class being used as
a base class. For example, we could derive a class
Spaniel
from the class
Dog
without any problem:
