Commons-Collections seek to build upon the JDK classes by providing
new interfaces, implementations and utilities. There are many
features, including:

* Bag interface for collections that have a number of copies of
each object
* Buffer interface for collections that have a well defined removal
order, like FIFOs
* BidiMap interface for maps that can be looked up from value to
key as well and key to value
* MapIterator interface to provide simple and quick iteration over
maps
* Type checking decorators to ensure that only instances of a
certain type can be added
* Transforming decorators that alter each object as it is added to
the collection
* Composite collections that make multiple collections look like
one
* Ordered maps and sets that retain the order elements are added
in, including an LRU based map
* Identity map that compares objects based on their identity (==)
instead of the equals method
* Reference map that allows keys and/or values to be garbage
collected under close control
* Many comparator implementations
* Many iterator implementations
* Adapter classes from array and enumerations to collections
* Utilities to test or create typical set-theory properties of
collections such as union, intersection, and closure