Originally, Collections was a general place for placing useful collection classes. With version 3.0, this changes. Collections now has a design and package structure of its own. As a result, several well-used classes have been deprecated and moved to new packages.

This release also includes a lot of new functionality, including new interfaces and new implementations in addition to various bug fixes and refactoring changes. All previously deprecated classes have been removed.

The major changes are:

• New package structure - One package now exists for each major interface
• Bidirectional maps - New interface and implementations for maps that need bidirectional lookup
• Map iterator - New interface to enable iteration over map keys and values in one easy step
• Resettable iterator - Interfaces that defines a reset() method to reset the iterator back to the start
• Functors - A default set of functor implementations is now provided for Predicate, Transformer, Closure and Factory
• KeyValue - A new package full of key-value pairs and map entry implementations
• Abstract Hash-based Map - Highly extensible hash-based, and hash-link-based map abstract classes
• PriorityQueue deprecated - The top level interface PriorityQueue has been deprecated in favour of Buffer

These packages are new to Collections 3.0:

• collection - Package of Collection implementations
• list - Package of List implementations
• set - Package of Set implementations
• bag - Package of Bag implementations
• buffer - Package of Buffer implementations
• map - Package of Map implementations
• bidimap - Package of BidiMap implementations
• keyvalue - Package of key-value pair implementations

These implementations are new to Collections 3.0:

• CompositeCollection/Set/Map - Merges multiple collections into a single view
• NodeCachingLinkedList - A list that caches nodes, performing well if long-lived
• HashedMap - An alternative to HashMap that supports a MapIterator
• LinkedMap - A map that retains the order of its entries
• Flat3Map - A map optimised for size 3 or less
• IdentityMap - A map that uses == not equals()
• ListOrderedSet/Map - A set/map that behaves like a List
• SetUniqueList - A list that behaves like a Set
• CircularFifoBuffer - A buffer that eliminates the oldest entry when full
• DualHashBidiMap - BidiMap implementation
• DualTreeBidiMap - SortedBidiMap implementation
• TreeBidiMap - OrderedBidiMap implementation
• FixedOrderComparator - A comparator in a fixed pre-determined order
• BooleanComparator - A comparator over Boolean instances
• ArrayListIterator - A ListIterator over an array of any type (including primitive arrays)
• LoopingIterator - An Iterator that loops repeatedly over the collection
• ObjectArray*Iterator - Iterators over an Object array
• EntrySetMapIterator - A MapIterator implemented using an entry set

These classes have changed in a potentially incompatable way:

• BufferUnderflowException - Now extends NoSuchElementException
• BeanMap.MyMapEntry - Superclass changed
• MultiHashMap - Name facility and getName() method removed
• FilterIterator - Superclass changed
• FilterListIterator - Superclass changed
• TransformIterator - Superclass changed

These collections have changed since Collections 3.0:

• PriorityQueue - Deprecated, use Buffer interface
• BinaryHeap - Deprecated, use PriorityBuffer
• SynchronizedPriorityQueue - Deprecated, use SynchronizedBuffer
• BoundedFifoBuffer - Deprecated, moved to buffer subpackage
• UnboundedFifoBuffer - Deprecated, moved to buffer subpackage
• SequencedHashMap - Deprecated, rewritten as LinkedMap in map subpackage
• DefaultMapBag - Deprecated, rewritten as AbstractMapBag in bag subpackage
• HashBag - Deprecated, moved and rewritten in bag subpackage
• TreeBag - Deprecated, moved and rewritten in bag subpackage
• DefaultMapEntry - Deprecated, moved to keyvalue subpackage
• DoubleOrderedMap - Deprecated, replaced by bidimap interface and implementations
• LRUMap - Deprecated, moved and rewritten in map subpackage
• ReferenceMap - Deprecated, moved to map subpackage
• StaticBucketMap - Deprecated, moved to map subpackage
• ProxyMap - Deprecated, moved as AbstractMapDecorator in map subpackage
• ExtendedProperties - Bug fixes. Please consider using Commons-Configuration PropertiesConfiguration

These comparators have changed since Collections 3.0:

• ComparableComparator - Simpler implementation that respects interface better.
• ComparatorChain - Equals and HashCode added.
• ReverseComparator - Equals and HashCode added.

These iterators have changed since Collections 3.0:

• ArrayIterator - This now implements the ResetableIterator interface. It also supports a start index to iterate from.
• CollatingIterator - The remove() method now throws IllegalStateException not NoSuchElementException.
• FilterIterator - The remove() method is now supported, with some limitations.
• IteratorChain - Now supports a chain of zero iterators. Bug fix for when remove() called before hasNext()/next().
• Singleton*Iterator - These now implement the ResetableIterator interface.
• Proxy*Iterator - Now deprecated in favour of AbstractIteratorDecorator/AbstractListIteratorDecorator.
• Filter*Iterator/TransformIterator - No longer extend ProxyIterator.