Composition

So far, we have learned to design systems as a group of interacting objects, where each interaction involves viewing objects at an appropriate level of abstraction. But we don't yet know how to create these levels of abstraction. There are a variety of ways to do this; we'll discuss some advanced design patterns in Chapters 10, 11, and 12. But even most design patterns rely on two basic object-oriented principles known as composition and inheritance. Composition is simpler, so let's start with that.

Composition is the act of collecting several objects together to create a new one. Composition is usually a good choice when one object is part of another object. We've already seen a first hint of composition when talking about cars. A fossil-fueled car is composed of an engine, transmission, starter, headlights, and windshield, among numerous other parts. The engine, in turn, is composed of pistons, a crank shaft, and valves. In this example,...