A Red-Black tree based
implementation. The map is sorted according to the natural ordering
of its keys, or by a
provided at map creation time, depending on which constructor is used.
This implementation provides guaranteed log(n) time cost for the
remove operations. Algorithms are adaptations of those in Cormen, Leiserson, and Rivest's Introduction to Algorithms .
Note that the ordering maintained by a tree map, like any sorted map, and whether or not an explicit comparator is provided, must be consistent with
equals if this sorted map is to correctly implement the
Map interface. (See
Comparator for a precise definition of consistent with equals .) This is so because the
Map interface is defined in terms of the
equals operation, but a sorted map performs all key comparisons using its
compare) method, so two keys that are deemed equal by this method are, from the standpoint of the sorted map, equal. The behavior of a sorted map is well-defined even if its ordering is inconsistent with
equals; it just fails to obey the general contract of the
Note that this implementation is not synchronized. If multiple threads access a map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with an existing key is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the
Collections.synchronizedSortedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:
SortedMap m = Collections.synchronizedSortedMap(new TreeMap(...));
The iterators returned by the
iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own
remove method, the iterator will throw a
ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw
ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
Map.Entry pairs returned by methods in this class and its views represent snapshots of mappings at the time they were produced. They do not support the
Entry.setValue method. (Note however that it is possible to change mappings in the associated map using
This class is a member of the Java Collections Framework .