Composite

Intent

Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

-Design Patterns, GoF

The other day I was listening to NPR’s Car Talk, a popular weekly broadcast during which callers ask questions about their vehicles. Before every program break, the show’s hosts ask callers to dial 1-800-CAR-TALK, which corresponds to 1-800-227-8255. Of course, the former proves much easier to remember than the latter, in part because the words “CAR TALK” are composite: two words that represent seven digits. Humans generally find it easier to deal with composites, rather than their individual components. Likewise, when you develop object-oriented software, it’s often convenient to manipulate composites just like you manipulate individual components.

-David Geary, Javaworld.com

Questions

1) Is it possible to restrict the components of a composite?

2) Can a component have multiple parents?

3) Does a component have a reference to its parent?

4) Where should the child management methods (i.e. add and remove) be declared? In the composite or in the component? What are the advantages of each?

5) Under what circumstances does the Composite pattern violate standard OO design principles?

Examples – Which are Composites?

1) Java Swing Component

2) Apache Struts Tiles

3) Caching

Implement It!

Imagine you are creating a feature for HR to calculate employee salary. There are two main types of employees: managers and workers. The salary for each employee is the sum of the individual’s salary plus everyone underneath her. How could this be implemented using the composite pattern.

References

· A Look at the Composite Design Pattern, David Geary (Javaworld.com, September 2003)