Collections - The Java Programming Language - page 899

Atomically remove the entry for key if it currently maps to
value . Returns true if the entry was removed. (Optional if the
map does not support remove )
public boolean replace(K key, V oldValue, V newValue)
Atomically replaces the entry for key only if it currently maps
to oldValue . Returns true if the entry was replaced. (Optional
if the map does not support put )
public V replace(K key, V value)
Atomically replaces the mapping for key if and only if a map-
ping currently exists. The old value mapped to key is returned,
or null if the key was not presentbut be aware that null may
be a legitimate value for some maps. (Optional if the map
does not support put )
The ConcurrentLinkedQueue<E> class provides an unbounded thread-safe
queue based on a linked node representation. Operations on the queue
are highly concurrent and do not utilize locks. Because of the concurrent
nature an operation like size requires a complete traversal of the queue
and so is quite expensivehowever, knowing the size that a concur-
rently changing queue used to be is seldom, if ever, useful. The iterator
provided by ConcurrentLinkedQueue is weakly consistent, it guarantees to
traverse the elements that existed when the iterator was created, but
may not reflect subsequent additions.
The CopyOnWriteArrayList<E> is a thread-safe variant of ArrayList . It al-
lows iteration to be done over a snapshot of the contents without ac-
tually making a copy of the original list. Copies are made only when
someone modifies the listhence the name "copy on write." This makes a
very efficient structure for lists that are read much more frequently than
they are changed. Because the iterator sees a snapshot of the elements,
it can never fail; but it does not support the remove operation.