From the book:

In this book, we will focus primarily on four patterns: proxy, adapter, decorator, and composite. They are all structural patterns, which means that they don’t describe behavior or the creation of objects, but rather how classes and objects interrelate.

Our four patterns have a similar class hierarchy, so with a bit of effort we can use dynamic proxies to implement each of them. They are so similar in appearance that we can call them “design-pattern cousins”.