To push your applications from good enough to great, we look at a variety of more complex topics. We start with optimization in the world of Scala, covering a few handy features in the language that let you reduce the overhead of for certain operations. We also take a thorough look at Scala’'s implicit keyword and the various ways it can be used to employ the Scala's smart compiler’s smarts to make your code easier to extend. Scala’'s interactions with Java can be a sticking point during debugging, profiling, and in more advanced scenarios where Java libraries have useful functions you want to use from Scala; we look at all of these scenarios to get a better understanding of this interaction point. We then look at Scala’'s support for mutability and immutability, and the best ways to decide between the two in practical scenarios. Finally, we touch on a few more advanced tips and tricks---—a grab-bag of useful knowledge that can come in handy any time.