Ubiquity

Java comes with many classes. Some of these you might use a few times (like LocalTime), some you will see every day of your coding life (like String).

ArrayList is in the second category. It turns out that a growable list of things is needed for a lot of different tasks. It is ubiquitous in code that you will see in the real world.

I mention this mostly to make sure you are paying attention. Its not particuarly hard, but try not to zone out on these sorts of chapters.