The Java Collections Framework is the recognised standard for collection handling in Java. 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 objectBidiMap
interface for maps that can be looked up from value to key as well and key to valueMapIterator
interface to provide simple and quick iteration over mapsTransforming 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
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.