Prefactoring

5.5. More Than One

You often work with more than one instance of an object. You can aggregate those instances using an array or a collection class. I like to distinguish between two types of aggregations. The first is termed a group, the second a collection. A group is a set of objects, typically coded using an array or a linked list. When I use the [] symbol in a class description, I am implying a group, not necessarily a particular implementation of a group.

A collection is a set of objects, typically implemented by a collection class or template (Vector, List, etc.). Operations on a collection can include performing an operation on the collection as a whole, such as calculating the average value of an attribute for all objects in the collection, or finding a particular object matching a key value.

If you find you are performing more operations on a group, other than just passing it to other methods or enumerating it, you might want to turn it into its own class. Creating a specific collection class, instead of using classwide (static) methods, separates the concerns of operations on all the objects in a collection, from those operations on a single object.^[*]

^[*] A reviewer noted that this is not true in languages such as Ruby and Groovy, which support closures. Closures let you apply arbitrary operations to generic collections without resorting to creating a specific collection class.

For example, suppose that Sam wanted to keep track of the rental history of each ...