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.

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