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 know yet how to create these levels of abstraction. There are a variety of ways to do this; we'll discuss some advanced design patterns in Chapter 8, Strings and Serialization, and Chapter 9, The Iterator Pattern. 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 it.

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 ...

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