| |||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | ||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
ConcurrentHashMap c = new ConcurrentHashMap(5);
Based on 68 examples
ConcurrentHashMap empty = new ConcurrentHashMap();
Based on 50 examples
public class ConcurrentHashMap extends AbstractMap implements ConcurrentMap, Serializable
A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. This class obeys the same functional specification as {@link 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 {@link 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 {@link Map} and {@link Iterator} interfaces.
Like {@link Hashtable} but unlike {@link 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.
Nested Class Summary |
---|
Nested classes/interfaces inherited from class java.util.AbstractMap |
---|
AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry |
Constructor Summary | |
---|---|
Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16). |
|
ConcurrentHashMap(int initialCapacity) Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16). |
|
ConcurrentHashMap(int initialCapacity, float loadFactor) Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16). |
|
ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) Creates a new, empty map with the specified initial capacity, load factor and concurrency level. |
|
Creates a new map with the same mappings as the given map. |
Method Summary | |
---|---|
void |
clear() Removes all of the mappings from this map. |
boolean |
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. |
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. |
Set |
entrySet() Returns a java.util.Set view of the mappings contained in this map. |
Object |
Returns the value to which the specified key is mapped, or if this map contains no mapping for the key. |
boolean |
isEmpty() Returns true if this map contains no key-value mappings. |
Enumeration |
keys() Returns an enumeration of the keys in this table. |
Set |
keySet() Returns a java.util.Set view of the keys contained in this map. |
Object |
Maps the specified key to the specified value in this table. |
void |
Copies all of the mappings from the specified map to this one. |
Object |
putIfAbsent(Object key, Object value) |
Object |
Removes the key (and its corresponding value) from this map. |
boolean |
|
Object |
|
boolean |
|
int |
size() Returns the number of key-value mappings in this map. |
Collection |
values() Returns a java.util.Collection view of the values contained in this map. |
Methods inherited from class java.util.AbstractMap |
---|
clear, clone, containsKey, containsValue, entrySet, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
---|
public ConcurrentHashMap()
public ConcurrentHashMap(int initialCapacity)
initialCapacity
- the initial capacity. The implementation
performs internal sizing to accommodate this many elements.public ConcurrentHashMap(int initialCapacity, float loadFactor)
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.public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
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.public ConcurrentHashMap(Map m)
m
- the mapMethod Detail |
---|
public void clear()
clear
in class AbstractMap
public boolean contains(Object value)
value
- a value to search forpublic boolean containsKey(Object key)
containsKey
in class AbstractMap
key
- possible keypublic boolean containsValue(Object value)
containsValue
in class AbstractMap
value
- value whose presence in this map is to be testedpublic Enumeration elements()
public Set entrySet()
The view's iterator is a "weakly consistent" iterator that will never throw {@link 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.
entrySet
in class AbstractMap
public Object get(Object key)
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.)
get
in class AbstractMap
key
public boolean isEmpty()
isEmpty
in class AbstractMap
public Enumeration keys()
public Set keySet()
The view's iterator is a "weakly consistent" iterator that will never throw {@link 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 class AbstractMap
public Object put(Object key, Object value)
The value can be retrieved by calling the get method with a key that is equal to the original key.
put
in class AbstractMap
key
- key with which the specified value is to be associatedvalue
- value to be associated with the specified keypublic void putAll(Map m)
putAll
in class AbstractMap
m
- mappings to be stored in this mappublic Object putIfAbsent(Object key, Object value)
key
value
public Object remove(Object key)
remove
in class AbstractMap
key
- the key that needs to be removedpublic boolean remove(Object key, Object value)
key
value
public Object replace(Object key, Object value)
key
value
public boolean replace(Object key, Object oldValue, Object newValue)
key
oldValue
newValue
public int size()
size
in class AbstractMap
public Collection values()
The view's iterator is a "weakly consistent" iterator that will never throw {@link 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.
values
in class AbstractMap
| |||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | ||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |