The Google Collections Library is a suite of new collections and collection-related goodness for Java 5.0.
This library is a natural extension of the Java Collections Framework.
The major new types are:
* BiMap. A Map that guarantees unique values, and supports an inverse view.
* Multiset. A Collection that may contain duplicate values like a List, yet has order-independent equality like a Set. Often used to represent a histogram.
* Multimap. Similar to Map, but may contain duplicate keys. Has subtypes SetMultimap and ListMultimap providing more specific behavior.