We've been focused on objects and their attributes and methods. Now, we'll take a look at designing higher-level objects; the kind of objects that manage other objects – the objects that tie everything together.
The difference between these objects and most of the previous examples is that the latter usually represent concrete ideas. Management objects are more like office managers; they don't do the actual visible work out on the floor, but without them, there would be no communication between departments and nobody would know what they are supposed to do (although, this can be true anyway if the organization is badly managed!). Analogously, the attributes on a management class tend to refer to other objects that do the visible work; the behaviors on such a class delegate to those other classes at the right time, and pass messages between them.
As an example, we'll write a program that does a find-and-replace action for text files stored in a compressed ZIP file. We'll need...