Input/Output - Beginning Java 8 Language Features - page 308

Figure 7-5. Commonly used classes for input streams compared with the decorator pattern

Table 7-1. Comparing the Class Design in the Decorator Pattern, the Drink Application, and Input Streams

Decorator Pattern Drink Application Input Stream

Component Drink InputStream

ConcreteComponentA
ConcreteComponentB

Rum
Vodka
Whisky

FileInputStream
ByteArrayInputStream
PipedInputStream

Decorator DrinkDecorator FilterInputStream

ConcreteDecoratorA
ConcreteDecoratorB

Honey
Spices

BufferedInputStream
PushbackInputStream
DataInputStream
ObjectInputStream

The abstract base component is the InputStream class, which is similar to the Drink class. You have concrete
component classes of FileInputStream , ByteArrayInputStream , and PipedInputStream , which are similar to the
Rum , Vodka, and Whiskey classes. You have a FilterInputStream class, which is similar to the DrinkDecorator
class. Notice the decorator class in the input stream family does not use the word “Decorator” in its class name; it
is named as FilterInputStream instead. It is also not declared abstract as you had declared the DrinkDecorator
class. Not declaring it abstract seems to be an inconsistency in the class design. You have concrete decorator classes
of BufferedInputStream , DataInputStream , and PushbackInputStream, which are similar to the Honey and Spices
classes in the drink application. One noticeable difference is that the ObjectInputStream class is a concrete decorator
and it is inherited from the abstract component InputStream , not from the abstract decorator FilterInputStream .
Note that the requirement for a concrete decorator is that it should have the abstract component class in its
immediate or non-immediate superclass and it should have a constructor that accepts an abstract component as its
argument. The ObjectInputStream class fulfills these requirements.