The Java Collections Framework was a major addition in JDK 1.2. It added many powerful data structures that accelerate development of most significant Java applications. Since that time it has become 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 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

A getting started User's Guide is available as are various project reports.

The JavaDoc API documents are available online:

• The current release 3.3
• The latest 2.x release - version 2.1.1
• The latest SVN

The subversion repository can be browsed.

The latest version is v3.3 - Download now!
The release notes are also available.

For previous releases, see the Apache Archive

The commons mailing lists act as the main support forum. The user list is suitable for most library usage queries. The dev list is intended for the development discussion. Please remember that the lists are shared between all commons components, so prefix your email by [collections].

Issues may be reported via ASF JIRA. Please read the instructions carefully to submit a useful bug report or enhancement request.