— Scott McNealy, Sun Microsystems Chairman, President and ceo

Looking at the Animal class, 
we decide that eat() and 
makeNoise() should be overridden 
by the individual subclasses.
3
Decide if a subclass 
needs behaviors (method 
implementations) that are specifi c 
to that particular subclass type.
designing for 
inheritance 

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inheritance and 
polymorphism
you are here
4
173
We look at our classes and see 
that Wolf and Dog might have some 
behavior in common, and the same goes 
for Lion, Tiger, and Cat. 
4
Look for more opportunities to use 
abstraction, by fi nding two or more 
subclasses that might need common 
behavior.
The class hierarchy is starting to shape up. We 
have each subclass override the makeNoise() and 
eat() methods, so that there’s no mistaking a Dog 
bark from a Cat meow (quite insulting to both 
parties). And a Hippo won’t eat like a Lion. 
But perhaps there’s more we can do. We have to 
look at the subclasses of Animal, and see if two 
or more can be grouped together in some way, 
and given code that’s common to only that new 
group. Wolf and Dog have similarities. So do 
Lion, Tiger, and Cat.
Looking for more inheritance 
opportunities
Animal
size
picture
food
prey
Dog
size
picture
food
prey
Wolf
size
picture
food
prey
Cat
size
picture
food
prey
Tiger
size
picture
food
prey
Hippo
size
picture
food
prey
Lion
makeNoise()
eat()
makeNoise()
eat()
makeNoise()
eat()
makeNoise()
eat()
makeNoise()
eat()
makeNoise()
eat()
Hmmm... I wonder if Lion, 
Tiger, and Cat would have 
something in common.
Wolf and Dog are both canines... 
maybe there’s something that 
BOTH classes could use...
picture
food
hunger
boundaries
location
makeNoise()
eat()
sleep()
roam()

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