A hash table supporting full concurrency of retrievals and
adjustable expected concurrency for updates. This class obeys the
same functional specification as
java.util.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
java.util.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
java.util.Hashtable
but unlike
java.util.HashMap
, this class
does
not allow
null to be used as a key or value.
This class is a member of the
../../../../../../../../guide/collections/index.html">
Java Collections Framework.
clear
public void clear()
Removes all of the mappings from this map.
contains
public boolean contains(Object value)
Legacy method testing if some key maps into the specified value
in this table. This method is identical in functionality to
containsValue(Object)
, and exists solely to ensure
full compatibility with class
java.util.Hashtable
,
which supported this method prior to introduction of the
Java Collections framework.
value
- a value to search for
- true if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwise
containsKey
public boolean containsKey(Object key)
Tests if the specified object is a key in this table.
- true if and only if the specified object
is a key in this table, as determined by the
equals method; false otherwise.
containsValue
public boolean containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. Note: This method requires a full internal
traversal of the hash table, and so is much slower than
method containsKey.
value
- value whose presence in this map is to be tested
- true if this map maps one or more keys to the
specified value
elements
public Enumeration elements()
Returns an enumeration of the values in this table.
- an enumeration of the values in this table
entrySet
public Set entrySet()
Returns a
Set
view of the mappings contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. The set supports element
removal, which removes the corresponding mapping from the map,
via the
Iterator.remove,
Set.remove,
removeAll,
retainAll, and
clear
operations. It does not support the
add or
addAll operations.
The view's
iterator is a "weakly consistent" iterator
that will never throw
java.util.ConcurrentModificationException
,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.
get
public Object get(Object key)
Returns the value to which the specified key is mapped,
or
null
if this map contains no mapping for the key.
More formally, if this map contains a mapping from a key
k
to a value
v
such that
key.equals(k)
,
then this method returns
v
; otherwise it returns
null
. (There can be at most one such mapping.)
isEmpty
public boolean isEmpty()
Returns true if this map contains no key-value mappings.
- true if this map contains no key-value mappings
keySet
public Set keySet()
Returns a
Set
view of the keys contained in this map.
The set is backed by the map, so changes to the map are
reflected in the set, and vice-versa. The set supports element
removal, which removes the corresponding mapping from this map,
via the
Iterator.remove,
Set.remove,
removeAll,
retainAll, and
clear
operations. It does not support the
add or
addAll operations.
The view's
iterator is a "weakly consistent" iterator
that will never throw
java.util.ConcurrentModificationException
,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.
- keySet in interface AbstractMap
keys
public Enumeration keys()
Returns an enumeration of the keys in this table.
- an enumeration of the keys in this table
put
public Object put(Object key,
Object value)
Maps the specified key to the specified value in this table.
Neither the key nor the value can be null.
The value can be retrieved by calling the
get method
with a key that is equal to the original key.
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified key
- the previous value associated with key, or
null if there was no mapping for key
putAll
public void putAll(Map m)
Copies all of the mappings from the specified map to this one.
These mappings replace any mappings that this map had for any of the
keys currently in the specified map.
m
- mappings to be stored in this map
putIfAbsent
public Object putIfAbsent(Object key,
Object value)
- putIfAbsent in interface ConcurrentMap
- the previous value associated with the specified key,
or null if there was no mapping for the key
remove
public Object remove(Object key)
Removes the key (and its corresponding value) from this map.
This method does nothing if the key is not in the map.
key
- the key that needs to be removed
- the previous value associated with key, or
null if there was no mapping for key
replace
public Object replace(Object key,
Object value)
- replace in interface ConcurrentMap
- the previous value associated with the specified key,
or null if there was no mapping for the key
replace
public boolean replace(Object key,
Object oldValue,
Object newValue)
- replace in interface ConcurrentMap
size
public int size()
Returns the number of key-value mappings in this map. If the
map contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.
- the number of key-value mappings in this map
values
public Collection values()
Returns a
Collection
view of the values contained in this map.
The collection is backed by the map, so changes to the map are
reflected in the collection, and vice-versa. The collection
supports element removal, which removes the corresponding
mapping from this map, via the
Iterator.remove,
Collection.remove,
removeAll,
retainAll, and
clear operations. It does not
support the
add or
addAll operations.
The view's
iterator is a "weakly consistent" iterator
that will never throw
java.util.ConcurrentModificationException
,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.