Composite - Design Patterns Java - page 41

Chapter 5. Composite
A composite is a group of objects in which some objects may contain others; thus, one object
may represent groups, and another may represent an individual item, or leaf . When you model
a composite, two powerful concepts emerge. One important modeling idea is to design groups
so that they can contain either individual items or other groups. (A common error is to define
groups so that they can contain only leaves.) A second powerful concept is to define common
behaviors for individual objects and for compositions. You can bring these ideas together by
defining a common type for groups and items and by modeling groups as containing
a collection of objects of this type. This fulfills the intent of the C OMPOSITE pattern:
C OMPOSITE lets clients treat individual objects and compositions of objects uniformly.
An Ordinary Composite
Figure 5.1 shows an ordinary composite structure. The Leaf and Composite classes share
a common interface that Component abstracts, and a Composite object retains a collection
of other Composite and Leaf objects. Note that the Component class in Figure 5.1 is
an abstract class with no concrete operations, so you could define it as an interface that Leaf
and Composite implement.
Figure 5.1. The key ideas of Composite are that composites can contain other
composites—not just leaves—and that composite and leaf nodes share a common
interface.
CHALLENGE 5.1
Give two reasons why the interface that Leaf and Composite share usually
appears in an abstract class rather than in an interface.