# Commons Collections 3.1 API;

Commons-Collections contains implementations, enhancements and utilities that complement the Java Collections Framework.

See:
Description

Packages
org.apache.commons.collections: This package contains the interfaces and utilities shared across all the subpackages of this component.
org.apache.commons.collections.bag: This package contains implementations of the `Bag` and `SortedBag` interfaces.
org.apache.commons.collections.bidimap: This package contains implementations of the `BidiMap`, `OrderedBidiMap` and `SortedBidiMap` interfaces.
org.apache.commons.collections.buffer: This package contains implementations of the `Buffer` interface.
org.apache.commons.collections.collection: This package contains implementations of the `Collection` interface.
org.apache.commons.collections.comparators: This package contains implementations of the `Comparator` interface.
org.apache.commons.collections.functors: This package contains implementations of the `Closure`, `Predicate`, `Transformer` and `Factory` interfaces.
org.apache.commons.collections.iterators: This package contains implementations of the `Iterator` interface.
org.apache.commons.collections.keyvalue: This package contains implementations of collection and map related key/value classes.
org.apache.commons.collections.list: This package contains implementations of the `List` interface.
org.apache.commons.collections.map: This package contains implementations of the `Map`, `IterableMap`, `OrderedMap` and `SortedMap` interfaces.
org.apache.commons.collections.set: This package contains implementations of the `Set` and `SortedSet` interfaces.

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
--- | ---
`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.
`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.
`BidiMap` | A new `Map` subinterface that allows lookup from key to value and from value to key with equal ease.
`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.
`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.
`ResettableIterator` | A new `Iterator` subinterface that allows the iteration to be reset back to the start. Many iterators in this library have this functionality.
`Closure` `Predicate` `Transformer` `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.

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