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.
| Constructor Summary |
|---|
|
Creates a new, empty map with the specified initial capacity, load factor and concurrency level. |
|
Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16). |
|
Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16). |
|
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16). |
|
Creates a new map with the same mappings as the given map. |
| Method Summary | |
|---|---|
| void |
Removes all of the mappings from this map. |
| boolean |
Legacy method testing if some key maps into the specified value in this table. |
| boolean |
Tests if the specified object is a key in this table. |
| boolean |
Returns true if this map maps one or more keys to the specified value. |
| Enumeration<V> |
Returns an enumeration of the values in this table. |
| Set<Map.Entry<K, V>> |
Returns a Set view of the mappings contained in this map. |
| V |
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. |
| boolean |
Returns true if this map contains no key-value mappings. |
| Enumeration<K> |
Returns an enumeration of the keys in this table. |
| Set<K> |
Returns a Set view of the keys contained in this map. |
| V |
Maps the specified key to the specified value in this table. |
| void |
Copies all of the mappings from the specified map to this one. |
| V |
|
| V |
Removes the key (and its corresponding value) from this map. |
| boolean |
|
| boolean |
|
| V |
|
| int |
Returns the number of key-value mappings in this map. |
| Collection<V> |
Returns a Collection view of the values contained in this map. |
| Methods inherited from java.utilAbstractMap |
|---|
| Methods inherited from java.utilMap |
|---|
| Methods inherited from java.langObject |
|---|
public ConcurrentHashMap
(
int
initialCapacity,
float
loadFactor,
int
concurrencyLevel
)
public ConcurrentHashMap
(
int
initialCapacity,
float
loadFactor
)
public ConcurrentHashMap
(
int
initialCapacity
)
public ConcurrentHashMap
(
)
public
void
clear
(
)
The view's iterator is a "weakly consistent" iterator that will never throw 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.
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.)
public
boolean
isEmpty
(
)
The view's iterator is a "weakly consistent" iterator that will never throw 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.
public
V
put
(
K
key,
V
value
)
The value can be retrieved by calling the get method with a key that is equal to the original key.
public
int
size
(
)
The view's iterator is a "weakly consistent" iterator that will never throw 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.
