Java Reference
In-Depth Information
3.4.3
Polymorphism (from the Greek
poly
meaning “many” and
morph
meaning
“shape”) refers to the language's ability to deal with objects of many different
“shapes,” that is, classes, as if they were all the same. We have already seen that
Java does this via the
extends
and
implements
keywords. You can define an
interface and then define two classes that both implement this interface.
Remember our
Sample
class (Example 3.26). We'll now define another
class,
Employee
, which also implements the
Identifiable
interface
(Example 3.27).
Polymorphism
Example 3.27
The
Employee
class
class
Employee
extends Person
implements Identifiable
{
private int empl_id;
public int getID()
{
return empl_id;
}
}
Notice that the same method,
getID()
, is implemented in the
Employee
class, but that the field from which it gets the ID value is a different field. That's
implementation-specific—the interface defines only the methods that can be
called but not their internal implementation. The
Employee
class not only
implements
Identifiable
, but it also extends the
Person
class, so we better
show you what our example
Person
class looks like (Example 3.28).
To make a really useful
Person
class would take a lot more code than we
need for our example. The important part for our example is only that it is
quite different from the
Sample
class we saw earlier.
Example 3.29 demonstrates the use of polymorphism. We only show some
small relevant snippets of code; there would be a lot more code for this to be-
come an entire, complete example. Don't be distracted by the constructors; we
made up some new ones just for this example, that aren't in the class definitions
above. Can you see where the polymorphism is at work?
