A hash table supporting full concurrency of retrievals and
adjustable expected concurrency for updates. This class obeys the
same functional specification as Hashtable , and
includes versions of methods corresponding to each method of
Hashtable. However, even though all operations are
thread-safe, retrieval operations do not entail locking,
and there is not any support for locking the entire table
in a way that prevents all access. This class is fully
interoperable with Hashtable in programs that rely on its
thread safety but not on its synchronization details.
Retrieval operations (including get) generally do not
block, so may overlap with update operations (including
put and remove). Retrievals reflect the results
of the most recently completed update operations holding
upon their onset. For aggregate operations such as putAll
and clear, concurrent retrievals may reflect insertion or
removal of only some entries. Similarly, Iterators and
Enumerations return elements reflecting the state of the hash table
at some point at or since the creation of the iterator/enumeration.
They do not throw ConcurrentModificationException .
However, iterators are designed to be used by only one thread at a time.
The allowed concurrency among update operations is guided by
the optional concurrencyLevel constructor argument
(default 16), which is used as a hint for internal sizing. The
table is internally partitioned to try to permit the indicated
number of concurrent updates without contention. Because placement
in hash tables is essentially random, the actual concurrency will
vary. Ideally, you should choose a value to accommodate as many
threads as will ever concurrently modify the table. Using a
significantly higher value than you need can waste space and time,
and a significantly lower value can lead to thread contention. But
overestimates and underestimates within an order of magnitude do
not usually have much noticeable impact. A value of one is
appropriate when it is known that only one thread will modify and
all others will only read. Also, resizing this or any other kind of
hash table is a relatively slow operation, so, when possible, it is
a good idea to provide estimates of expected table sizes in
constructors.
This class and its views and iterators implement all of the
optional methods of the Map and Iterator
interfaces.
Like Hashtable but unlike HashMap , this class
does not allow null to be used as a key or value.
Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException . However, iterators are designed to be used by only one thread at a time.
The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash table is a relatively slow operation, so, when possible, it is a good idea to provide estimates of expected table sizes in constructors.
This class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces.
Like Hashtable but unlike HashMap , this class does not allow null to be used as a key or value.
This class is a member of the Java Collections Framework.