This class implements the Set interface, backed by a hash table
(actually a HashMap instance). It makes no guarantees as to the
iteration order of the set; in particular, it does not guarantee that
the order will remain constant over time. This class permits the null
element.
This class offers constant time performance for the basic operations
(add, remove, contains and size), assuming the hash function disperses
the elements properly among the buckets. Iterating over this set
requires time proportional to the sum of the HashSet instance's size
(the number of elements) plus the "capacity" of the backing HashMap
instance (the number of buckets). Thus, it's very important not to set
the initial capacity too high (or the load factor too low) if
iteration performance is important.
Note that this implementation is not synchronized. If multiple threads
access a hash set concurrently, and at least one of the threads
modifies the set, it must be synchronized externally. This is
typically accomplished by synchronizing on some object that naturally
encapsulates the set. If no such object exists, the set should be
"wrapped" using the Collections.synchronizedSet method. This is best
done at creation time, to prevent accidental unsynchronized access to
the set:
Set s = Collections.synchronizedSet(new HashSet(...));
The iterators returned by this class's iterator method are fail-fast:
if the set is modified at any time after the iterator is created, in
any way except through the iterator's own remove method, the Iterator
throws 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.
This class is a member of the Java Collections Framework.
