— Scott McNealy, Sun Microsystems Chairman, President and ceo

380
 
chapter 12
inner 
classes
HeadFirst: 
What makes inner classes important? 
Inner object: 
Where do I start? We give you a chance to 
implement the same interface more than once in a class. 
Remember, you can’t implement a method more than 
once in a normal Java class. But using inner classes, each 
inner class can implement the same interface, so you can 
have all these different implementations of the very same 
interface methods. 
HeadFirst: 
Why would you ever want to implement the 
same method twice?
Inner object: 
Let’s revisit GUI event handlers. Think 
about it... if you want three buttons to each have a 
different event behavior, then use three inner classes, all 
implementing ActionListener—which means each class 
gets to implement its own actionPerformed method.
HeadFirst: 
So are event handlers the only reason to use 
inner classes?
Inner object: 
Oh, gosh no. Event handlers are just an 
obvious example. Anytime you need a separate class, but 
still want that class to behave as if it were part of another 
class, an inner class is the best—and sometimes only—way 
to do it. 
HeadFirst: 
I’m still confused here. If you want the inner 
class to behave like it belongs to the outer class, why have 
a separate class in the first place? Why wouldn’t the inner 
class code just be in the outer class in the first place?
Inner object: 
I just gave you one scenario, where you 
need more than one implementation of an interface. But 
even when you’re not using interfaces, you might need 
two different classes because those classes represent two 
different things. It’s good OO.
HeadFirst: 
Whoa. Hold on here. I thought a big part of
OO design is about reuse and maintenance. You know, the 
idea that if you have two separate classes, they can each 
be modified and used independently, as opposed to stuffing 
it all into one class yada yada yada. But with an inner class, 
you’re still just working with one real class in the end, right? 
The enclosing class is the only one that’s reusable and 
separate from everybody else. Inner classes aren’t exactly 
reusable. In fact, I’ve heard them called “Reuseless—
useless over and over again.”
Inner object: 
Yes it’s true that the inner class is not as 
reusable, in fact sometimes not reusable at all, because it’s 
intimately tied to the instance variables and methods of
the outer class. But it—
HeadFirst: 
—which only proves my point! If they’re not 
reusable, why bother with a separate class? I mean, other 
than the interface issue, which sounds like a workaround 
to me.