132 | Chapter 4: Decorator Pattern
more like the nesting dolls from Japan and Russia. The innermost doll is the compo-
nent, and it’s placed into a decorator. Then the component and decorator are placed
into another decorator, and then into the next decorator. Figure 4-2 shows this
model.
Probably the most important thing to remember about the Decorator pattern is that
its purpose is to allow additional elements to be added to a class without altering the
base class. So instead of making changes by changing the class, the class is decorated
with subclasses that don’t interfere with the class from which the decorators were
derived.
Key OOP Concepts Used with the Decorator Pattern
Sometimes when you think about key OOP concepts such asinheritance, you have to
consider its larger consequences. The upside of inheritance is that once a superclass
has been established, all the subclasses inherit its features. However, you may not
want or need all the “stuff” for every subclass from a superclass. If you want to add
functionality to an object, subclassing may not be the way to go, because everything
else subclassed from the same superclass may be unnecessarily burdened by unused
functionality.
Unwanted Inheritance
At the end of this chapter, you will see an example of a car dealership where users
select models of cars with a choice of options for those cars. If all the options were
derived from a superclass, that would mean that every car would have all the
options, whether or not you wanted them. One option in the example is a rear view
camera used with minivans and large SUVs with limited rear vision. Such an option
would be superfluous on a sports car such as a Chevrolet Corvette, Morgan, or Fer-
rari. However, with subclassing, that’s exactly what would happen. Every subclass
gets everything from the superclass. So here’s a case where we’d have to look beyond
Figure 4-2. Component wrapped in series of decorators
#3
Concrete decorator
#3#2
Concrete decorator
#2#1
Concrete decorator
#1Concrete component