edu.emory.mathcs.backport.java.util.concurrent
public class ConcurrentHashMap extends AbstractMap implements ConcurrentMap, Serializable
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 Java Collections Framework.
Since: 1.5
Constructor Summary | |
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ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
Creates a new, empty map with the specified initial
capacity, load factor and concurrency level.
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ConcurrentHashMap(int initialCapacity, float loadFactor)
Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16).
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ConcurrentHashMap(int initialCapacity)
Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).
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ConcurrentHashMap()
Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and concurrencyLevel (16). | |
ConcurrentHashMap(Map m)
Creates a new map with the same mappings as the given map.
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Method Summary | |
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void | clear()
Removes all of the mappings from this map. |
boolean | contains(Object value)
Legacy method testing if some key maps into the specified value
in this table. |
boolean | containsKey(Object key)
Tests if the specified object is a key in this table.
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boolean | containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value. |
Enumeration | elements()
Returns an enumeration of the values in this table.
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Set | entrySet()
Returns a Set view of the mappings contained in this map.
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Object | get(Object key)
Returns the value to which the specified key is mapped,
or {@code null} if this map contains no mapping for the key.
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boolean | isEmpty()
Returns true if this map contains no key-value mappings.
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Enumeration | keys()
Returns an enumeration of the keys in this table.
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Set | keySet()
Returns a Set view of the keys contained in this map.
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Object | put(Object key, Object value)
Maps the specified key to the specified value in this table.
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void | putAll(Map m)
Copies all of the mappings from the specified map to this one.
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Object | putIfAbsent(Object key, Object value)
{@inheritDoc }
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Object | remove(Object key)
Removes the key (and its corresponding value) from this map.
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boolean | remove(Object key, Object value)
{@inheritDoc }
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boolean | replace(Object key, Object oldValue, Object newValue)
{@inheritDoc }
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Object | replace(Object key, Object value)
{@inheritDoc }
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int | size()
Returns the number of key-value mappings in this map. |
Collection | values()
Returns a Collection view of the values contained in this map.
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Parameters: initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements. loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold. concurrencyLevel the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
Throws: IllegalArgumentException if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.
Parameters: initialCapacity The implementation performs internal sizing to accommodate this many elements. loadFactor the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
Throws: IllegalArgumentException if the initial capacity of elements is negative or the load factor is nonpositive
Since: 1.6
Parameters: initialCapacity the initial capacity. The implementation performs internal sizing to accommodate this many elements.
Throws: IllegalArgumentException if the initial capacity of elements is negative.
Parameters: m the map
Parameters: value a value to search for
Returns: true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
Throws: NullPointerException if the specified value is null
Parameters: key possible key
Returns: true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
Throws: NullPointerException if the specified key is null
Parameters: value value whose presence in this map is to be tested
Returns: true if this map maps one or more keys to the specified value
Throws: NullPointerException if the specified value is null
Returns: an enumeration of the values in this table
See Also: values
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.
More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code key.equals(k)}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)
Throws: NullPointerException if the specified key is null
Returns: true if this map contains no key-value mappings
Returns: an enumeration of the keys in this table
See Also: keySet
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.
The value can be retrieved by calling the get method with a key that is equal to the original key.
Parameters: key key with which the specified value is to be associated value value to be associated with the specified key
Returns: the previous value associated with key, or null if there was no mapping for key
Throws: NullPointerException if the specified key or value is null
Parameters: m mappings to be stored in this map
Returns: the previous value associated with the specified key, or null if there was no mapping for the key
Throws: NullPointerException if the specified key or value is null
Parameters: key the key that needs to be removed
Returns: the previous value associated with key, or null if there was no mapping for key
Throws: NullPointerException if the specified key is null
Throws: NullPointerException if the specified key is null
Throws: NullPointerException if any of the arguments are null
Returns: the previous value associated with the specified key, or null if there was no mapping for the key
Throws: NullPointerException if the specified key or value is null
Returns: the number of key-value mappings in this map
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.