The foreword of the book characterizes its contents very well. There are three parts to master a programming language: understand the core language (object-oriented, functional, etc.), know the vocabulary (operations, standard libraries, etc.) and use the language effectively. This book covers the third part. You can, of course, write Java programs without this book, but with its provided knowledge your Java programs will become better. Definitively! It should be on the desk of every serious Java developer. I haven't read it from start to finish. I use it as a reference to continuously improve my Java programs.