Excerpted from DomainDrivenDesignBook
Problem: When the relatedness of nested containers is not reflected in the model, common behavior has to be duplicated at each level of the hierarchy, and nesting is rigid (for example, containers can?t usually contain other containers at their own level, and the number of levels is fixed). Clients must deal with different levels of the hierarchy through different interfaces, even though there may be no conceptual difference they care about. Recursion through the hierarchy to produce aggregated information is very complicated.
Solution: Define an abstract type that encompasses all members of the COMPOSITE. Methods that return information are implemented on containers to return aggregated information about their contents. "Leaf" nodes implement those methods based on their own values. Clients deal with the abstract type and have no need to distinguish leaves from containers.
See also: DesignPatternsComposite is mentioned on: MessagesByTopic
