The Google Collections library provides more out-of-the-box features that what can be achieved with the standard Java collections framework. Both Kevin Bourrillion and Jared Levy -- creators of the Google Collections library -- talk about their motivations behind the project as well as the benefits for Java developers.
"The library's functionality simplifies your code so it's easier to write, read, and maintain. Some library methods make commonly used one-line commands, such as creating a HashSet of a particular generic type, more concise. Other new collection classes, such as a multimap, multiset, or bimap, are helpful in certain situations. While you can survive without such features, the Google Collections Library will improve your productivity as a developer, while reducing the amount of boilerplate low-level code you need to write."
"The same basic problems that the Java Collections Framework solves, just taken to the next level. Additional abstractions to more closely fit your needs, new data structures that speed up your processing, conveniences that wage war on boilerplate code everywhere. Jared and I both love to play board games and, when there's a board game that's successful, the publisher inevitably comes back and releases an 'expansion' for that game. With the expansion, you can still play the original game you already love, but you can do a little more as well -- sail over to the neighboring islands, or go build your train lines in Switzerland instead, etc. So we'd love for you to see our library as 'the unofficial expansion to the Java Collections Framework'! Whether you feel we do or don't measure up to that standard, it's the mindset we have had since the beginning. We find the gaps in java.util and we fill those in; we pick up where the JDK leaves off. Most importantly, we conform to the principles and design choices in the Java Collections as much as possible, only deviating when there's a very clear win."
Read the complete post: http://www.javalobby.org/articles/google-collections/
The Google Collections library: http://code.google.com/p/google-collections/