Part 2. Built-in classes and modules

In part 2, we come to the heart of the Ruby language: built-in classes and modules.

A great deal of what you’ll do as a Rubyist will involve Ruby’s built-ins. You’ve already seen examples involving many of them: strings, arrays, files, and so forth. Ruby provides you with a rich toolset of out-of-the-box built-in data types that you can use and on which you can build.

That’s the thing: when you design your own classes and modules, you’ll often find that what you need is something similar to an existing Ruby class. If you’re writing a DeckOfCards class, for example, one of your first thoughts will probably be that a deck of cards is a lot like an array. Then you’d want to think about whether your cards class should be a subclass of Array—or perhaps each deck object could store an array in an instance variable and put the cards there—and so forth. The point is that Ruby’s built-in classes provide you with starting points for your own class and object designs as well as with a set of classes extremely useful in their own right.