This release adds various new classes and fixes a number of bugs. All feedback should be directed to commons-user at jakarta.apache.org.

This release is fully source and binary compatible with v3.0. (Checks performed using JDiff and Clirr, thanks).

The only new deprecations are in AbstractDualBidiMap. These only impact non commons-collections subclasses, and existing code will still work.

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]
• ExtendedProperties - Handle comma separated data in multiple keys
• 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]