Design Patterns Java™ Workbook

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 ...