Also: Collections
package

org.apache.commons.collections

This package contains a small number of collections classes that are needed for beanutils to function. The dependencies will be deprecated in the 1.7.0 release and the need for these classes will be removed in the future. These classes are identical to those in both 2.x and 3.x series of commons-collections releases.

Interfaces

Buffer Defines a collection that allows objects to be removed in some well-defined order. 

Classes

ArrayStack An implementation of the java.util.Stack API that is based on an ArrayList instead of a Vector, so it is not synchronized to protect against multi-threaded access. 
FastHashMap

A customized implementation of java.util.HashMap designed to operate in a multithreaded environment where the large majority of method calls are read-only, instead of structural changes. 

Exceptions

BufferUnderflowException The BufferUnderflowException is used when the buffer is already empty.