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Commons-Collections contains implementations, enhancements and utilities
that complement the Java Collections Framework.
The Apache Jakarta Commons Collections Framework component adds a significant
amount of enhancements to the standard JDK collections. These enhancements
come in the form of new interfaces, new implementations and utility classes.
See also the java.util package for the standard Java collections.
Main features
Commons-Collections defines a number of key interfaces:
Interface Description
{@link org.apache.commons.collections.Bag}
A new Collection subinterface that stores each object together
with the number of occurances. Methods are provided to get the number of
occurances, and to add and remove a certain number of that object.
{@link org.apache.commons.collections.Buffer}
A new Collection subinterface that allows objects to be removed
in some well-defined order. Methods enable the next item to be peeked and removed.
{@link org.apache.commons.collections.BidiMap}
A new Map subinterface that allows lookup from key to value and
from value to key with equal ease.
{@link org.apache.commons.collections.OrderedMap}
A new Map subinterface that is used when a map has an order, but is
not sorted. Methods enable bidriectional iteration through the map.
{@link org.apache.commons.collections.MapIterator}
A new Iterator subinterface specially designed for maps. This iterator
avoids the need for entrySet iteration of a map, and is simpler to use.
{@link org.apache.commons.collections.ResettableIterator}
A new Iterator subinterface that allows the iteration to be reset back
to the start. Many iterators in this library have this functionality.
{@link org.apache.commons.collections.Closure}
{@link org.apache.commons.collections.Predicate}
{@link org.apache.commons.collections.Transformer}
{@link org.apache.commons.collections.Factory}
A group of functor interfaces that provide plugin behaviour to various
collections and utilities.
In addition to the interfaces, there are many implementations.
Consult each subpackage for full details of these.
Version 3.0
Commons-Collections was originally created as a place to share collection
implementations created in various places around Jakarta. As a result, there
was no clear design or structure to the component. As from this 3.0 release,
that changes with a clear, consistent package structure being used
As a result of the new structure, several classes have moved packages.
When moving from a previous release to this one it is important to check all your code.
Some implementations that were moved were also optimised, and may contain API changes.
In addition to the new package structure, it was determined that collections was
getting too large.
As a result, two new code donations - primitive collections and event
notification collections - have been separated into their own projects within commons.
Primitives has already released a version 1.0, while Events resides in the sandbox at present.