Here are a couple of excerpts from that book (found in the intro section):
Activities marked with the Exercise (running shoe) logo are mandatory! Don't skip them is you're serious about learning Java.
...
We included more than 50 exercuses, because your brain is tuned to learn and remember more when you do things than when you read about things. And we made the exercises challenging-yet-do-able, because that's what most people prefer.
...
If you see the puzzle logo, the activity is optional, and if you don't like twisty logic or cross-word puzzles, you won't like these either.
So there you have it. If you want to get the most out of your book, be sure to at least do the exercises. The puzzles are optional but, having tested a number of them myself, I'm recommend doing a few and see if you like them or not. You might even find that they work well for you.