This release adds various new classes and fixes a number of bugs. No deprecations have occurred. All feedback should be directed to commons-user at jakarta.apache.org.

This release is source and binary compatible with v3.0 with one known exception:

• ReferenceMap (map package) - No longer extends AbstractMap, only an issue if you previously cast to AbstractMap

This release also enables a workaround of the binary compatability issue with IteratorUtils in v3.0. Collections v2.1.1 (with deprecations solved) and v3.1 are compatible with regards to iterators.

• TreeList - A list implementation that is faster than ArrayList for insertions and removals
• MultiKeyMap - A map that allows multiple keys to be used to map the value
• SingletonMap - fully featured singleton Map implementation
• ObjectGraphIterator - An iterator that can iterate over a graph of objects
• MapBackedSet - Set created by decorating a map
• ReferenceIdentityMap - Similar to ReferenceMap, but matching keys and values by identity [26503]
• AbstractReferenceMap - New base class for reference maps [26503]
• Empty*Iterator - Iterators over empty collections
• TransformedPredicate - A predicate where the input object is transformed [26946]
• PredicateDecorator - An interface to provide a uniform interface to the decorated predicates
• AbstractSerializable*Decorator - Abstract classes for decorators that are Serializable

• ReferenceMap - Changed to extend AbstractReferenceMap, thus gaining a mapIterator() and subclassability [26503]
• Fast3Map - Add clone() method
• MultiKey - Add getKey(index) and size() methods and make constructor public
• MultiHashMap - Add five methods to improve the API
• AbstractHashedMap,AbstractLinkedMap - Add methods to access entry methods when protected scope blocks
• Functors - Add get methods to retrieve internal state [27515]
• Functors - Add additional getInstance() methods for consistency [27856,27857]
• CollectionUtils - get(Object,int) method now supports primitive arrays
• CollectionUtils - cardinality() now works faster for Set and Bag instances [28629]
• CollectionUtils - Add size(Object) method to find the size of various collection-like objects [27909]
• SingletonIterator - make remove() functionality optional
• AbstractLinkedList/NodeCachingLinkedList - added getValue() and setValue() to Node, and made everything use them
• LRUMap - The addMapping() method now uses isFull() to determine whether it is full
• LRUMap - Add boolean flag, scanUntilRemovable, giving the removeLRU() method more power [28887]
• InvokerTransformer - Add additional getInstance() method
• Reduced inter-class and inter-package dependencies, especially via *Utils classes
• ListOrderedSet - Add new factory method decorate(Set,List)
• ListOrderedSet - Add public constructor that uses a HashSet and ArrayList
• ListOrderedMap - Add public constructor that uses a HashMap and ArrayList

Made Serializable

• Fast3Map [27946]
• ReferenceMap
• UnboundedFifoBuffer
• BoundedFifoBuffer
• CircularFifoBuffer
• TiedMapEntry
• FixedSize* [18815]
• Predicated* [18815]
• Synchronized* [18815]
• Transformed* [18815]
• Typed* [18815]
• Unmodifiable* [18815]
• Lazy* [18815]
• ListOrdered* [18815]
• SetUniqueList [18815]

• Map/BidiMap implementations only checked key and not value in entry set contains(Object) and remove(Object)
• AbstractHashedMap subclasses failed to clone() correctly [27159]
• ExtendedProperties - Close input stream in constructor [27737]
• Flat3Map - Handle infinite loops in toString
• LRUMap - The removeLRU() method was passed the wrong LinkEntry [28433]
• EnumIterator/MapUtils - Changed enum references to enable JDK 1.5 compliance
• UnmodifiableSortedBag - Fix to ensure unmodifiable
• MultiHashMap - Fix copy constructor and clone to work properly [28972]
• ListOrderedSet - Fix to throw IllegalArgumentException instead of NPE on null factory decorate(List)
• *Dual*BidiMap - Fix poorly designed subclass method call from superclass constructor [29519]

• Bag - Indicate that no changes are likely to the interface despite its problems
• TreeBidiMap - Add javadoc about requiring Comparable entries [26470]
• MultiKey - Add extra explanatations, examples and warnings
• MultiMap,MultiHashMap - Add extra documentation to clarify the interface and implementation
• XxxUtils - Clarify the internal workings of the predicated decorator classes [28115]
• BidiMap - Clarify 1 to 1 nature of the map
• AbstractHashedMap - Fix isEqualValue() javadoc to refer to value not key
• LRUMap - Added extra javadoc about method implementations
• BoundedCollection/UnmodifiableBoundedCollection - reword to avoid misunderstandings
• Closure/Predicate/Transformer/Factory - Additional javadoc with links
• ClosureUtils/PredicateUtils/TransformerUtils/FactoryUtils - Additional links to implementations
• Predicated* - Additional usage explanation [29018]