Effective Java (2nd Edition) (Java Series)

A great book that you'll go back to time and again as you encounter the various scenarios addressed in the book. I recommend scanning the TOC every few months to see what's suddenly relevant because of your current project. A great companion to Java Concurrency in Practice.

Greatest Java book ever written. In fact, it's beyond a 'java book', once many tips from here are 'universal tips'.

It's a great book for those who already have at least 4 or 5 programming years (code enough different projects), 'cause you'll notice several (apparently) minor mistakes that everyone make everyday.

Did you realize that enums can implement interfaces? That the simplest safe way to implement a singleton is using an enum? That it's possible to obtain an object reference to a private member using serialization? There are lots of ways to learn the language basics and figure out how to use the libraries. But nowhere do you have such a dense collection of non-obvious, yet important issues that any Java developer ought to be aware of. The book is written from the perspective of someone who creates public APIs, so there is a lot of obsessing over how to prevent users from doing things they shouldn't (intentionally, or not). But even if that's not a big concern for you, it's nevertheless important to be aware of the issues. Having read the first edition a while back, reading the second edition was more that a good refresher -- it's almost a different book. This is in part thanks to Java 5, which has made a lot of the original items (e.g. how create enumerations) obsolete, while introducing the need for a whole new set of items (e.g. regarding the use of generics).

This book is a must-read for all Java programmers. In fact, I think every programmer should read this book before they start programming Java. The book is full of good advice, best practices and guidelines.

Highly recommended

Absolute classic, definitely my favorite Java book. Every professional Java developer should have it on her/his desk.

It's not the easiest book and shouldn't be read if You are novice in Java, there better books to get started with Java, but for experienced developers, this book is invaluable.