This book is aimed at a reader who might already know java, but wants to learn functional programming. Prior to Groovy, you might have to start with an entirely new language. A major attraction of Groovy is that its creators have gone to great lengths to make most java statements compatible.
Along these lines, these common java packages are implicitly included in every Groovy source file - java.lang, java.util, java.io, java.net. Plus there is a more compact notation that reduces the verbosity of standard java. This translates to hopefully code of yours that is quicker to type and to understand, in a self documenting sense.
The text also shows that Groovy can aid in enterprise applications, where you might already have extensive libraries of java beans, with a concomitant expertise that you have built up. Chapter 3 may provide a pleasant surprise, in showing how Groovy beans, and there are indeed such creatures, can be used in place of your more common java beans. And why? More compact notation. But the Groovy beans also indicate that the Groovy writers are serious about making Groovy have a place in large scale pre-existing enterprise data centers.
At the simplest level, the book has a persuasive example. It shows a typical java bean, with some local variables and associated getter and setter methods. Then it shows the equivalent Groovy bean. No need to explicitly write getters and setters, or default constructors. The sheer brevity of this is so nifty! And I hope you are experienced enough in java to realise that less code means less chance of random bugs creeping in.
But what about the functional aspects of Groovy? There are numerous examples that illustrate annotations. Easy to follow and hopefully you can rapidly write your own cases.
There is much more to the text, of course. But this should give you some of its flavour.