Google Collections Library

The Google Collections Library is a suite of new collections and collection-related goodness for Java 5.0, brought to you by Google.

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.

Static utility classes include:

• Comparators. Natural order, compound, null-friendly, ad-hoc . . .
• Iterators and Iterables. Element-based equality, cycle, concat, partition, filter with predicate, transform with function . . .
• Lists, Sets and Maps. A plethora of convenient factory methods and much more.
• PrimitiveArrays: "boxing"/"unboxing" of primitive arrays

http://code.google.com/p/google-collections/