— Scott McNealy, Sun Microsystems Chairman, President and ceo

interfaces and 
polymorphism
you are here
4
199
Wolf aWolf = new Wolf();
We know we can say:
A Wolf reference to a 
Wolf object.
Wolf
aWolf
W
ol
f
o
b
je
ct 
These two are the same type. 
Animal aHippo = new Hippo();
And we know we can say:
Animal reference to 
a Hippo object.
Animal
aHippo
Hip
po
o
b
je
ct 
These two are NOT the same type. 
Animal anim = new Animal();
But here’s where it gets weird:
Animal reference to 
an Animal object.
Animal
anim
A
n
i
mal
o
bj
e
ct
These two are the same type, but...
what the heck does an Animal object look like?
?

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200
 
chapter 8
scary objects 
What does a new Animal()
object 
look like?
when objects go 
bad 
It makes sense to create a Wolf object or a Hippo 
object or a Tiger object, but what exactly is an 
Animal object? What shape is it? What color, size, 
number of legs...
Trying to create an object of type Animal is like a 
nightmare Star Trek™ transporter accident. The 
one where somewhere in the beam-me--up process 
something bad happened to the buffer.
But how do we deal with this? We need an Animal 
class, for inheritance and polymorphism. But we 
want programmers to instantiate only the less 
abstract subclasses of class Animal, not Animal itself. 
We want Tiger objects and Lion objects, not Animal 
objects
.
Fortunately, there’s a simple way to prevent a class 
from ever being instantiated. In other words, to stop 
anyone from saying “