Decorator - Design Patterns Java - page 223

Chapter 27. Decorator
Ordinarily, an object inherits behaviors from its superclasses. As earlier chapters have shown,
you can apply the S TATE and the S TRATEGY patterns to alter the behavior of an object
dynamically. Like S TATE and S TRATEGY , the D ECORATOR pattern relies on polymorphism,
but D ECORATOR combines polymorphism with delegation to let you extend an object's
behavior. The intent of D ECORATOR is to let you compose an object's behavior dynamically.
A Classic Example of Decorator: Streams
The Java class libraries provide a classic example of the D ECORATOR pattern in the overall
design of Java input and output streams. A stream is a serial collection of bytes or characters,
such as those that appear in a document. In Java, each stream object usually contains another
stream object to which it forwards its operations. This structure is typical of D ECORATOR .
The D ECORATOR pattern arranges for each decorator object to contain another decorator
object. In this regard, a decorator is like a slim composite whose elements each have a single
child. Unlike the C OMPOSITE pattern, whose purpose is to compose aggregate objects,
the purpose of D ECORATOR is to compose behavior.
Structurally, D ECORATOR arranges classes in a hierarchy and distributes operations across
this hierarchy. Each class in the hierarchy typically has a constructor that requires another
instance of a class in the hierarchy. For example, Figure 27.1 shows a portion of
the OutputStream hierarchy from the Java class libraries.