Open Source Collections Libraries 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 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