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 Levy, co-creator of Guava-Libraries] 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."