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java.lang.Object | +--java.util.AbstractMap | +--EDU.oswego.cs.dl.util.concurrent.ConcurrentHashMap
A version of Hashtable supporting concurrency for both retrievals and updates:
Iterators and Enumerations (i.e., those returned by keySet().iterator(), entrySet().iterator(), values().iterator(), keys(), and elements()) return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They will return at most one instance of each element (via next()/nextElement()), but might or might not reflect puts and removes that have been processed since they were created. They do not throw ConcurrentModificationException. However, these iterators are designed to be used by only one thread at a time. Passing an iterator across multiple threads may lead to unpredictable results if the table is being concurrently modified.
There is NOT any support for locking the entire table to prevent updates. This makes it imposssible, for example, to add an element only if it is not already present, since another thread may be in the process of doing the same thing. If you need such capabilities, consider instead using the ConcurrentReaderHashMap class.
This class may be used as a direct replacement for java.util.Hashtable in any application that does not rely on the ability to lock the entire table to prevent updates. As of this writing, it performs much faster than Hashtable in typical multi-threaded applications with multiple readers and writers. Like Hashtable but unlike java.util.HashMap, this class does NOT allow null to be used as a key or value.
Implementation note: A slightly faster implementation of this class will be possible once planned Java Memory Model revisions are in place.
[ Introduction to this package. ]
Nested Class Summary |
Nested classes inherited from class java.util.Map |
java.util.Map.Entry |
Field Summary | |
static int |
DEFAULT_INITIAL_CAPACITY
The default initial number of table slots for this table (32). |
static float |
DEFAULT_LOAD_FACTOR
The default load factor for this table (0.75) Used when not otherwise specified in constructor. |
Constructor Summary | |
ConcurrentHashMap()
Constructs a new, empty map with a default initial capacity and default load factor. |
|
ConcurrentHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor. |
|
ConcurrentHashMap(int initialCapacity,
float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor. |
|
ConcurrentHashMap(java.util.Map t)
Constructs a new map with the same mappings as the given map. |
Method Summary | |
void |
clear()
Removes all mappings from this map. |
java.lang.Object |
clone()
Returns a shallow copy of this ConcurrentHashMap instance: the keys and values themselves are not cloned. |
boolean |
contains(java.lang.Object value)
Tests if some key maps into the specified value in this table. |
boolean |
containsKey(java.lang.Object key)
Tests if the specified object is a key in this table. |
boolean |
containsValue(java.lang.Object value)
Returns true if this map maps one or more keys to the specified value. |
java.util.Enumeration |
elements()
Returns an enumeration of the values in this table. |
java.util.Set |
entrySet()
Returns a collection view of the mappings contained in this map. |
java.lang.Object |
get(java.lang.Object key)
Returns the value to which the specified key is mapped in this table. |
boolean |
isEmpty()
Returns true if this map contains no key-value mappings. |
java.util.Enumeration |
keys()
Returns an enumeration of the keys in this table. |
java.util.Set |
keySet()
Returns a set view of the keys contained in this map. |
java.lang.Object |
put(java.lang.Object key,
java.lang.Object value)
Maps the specified key to the specified
value in this table. |
void |
putAll(java.util.Map t)
Copies all of the mappings from the specified map to this one. |
java.lang.Object |
remove(java.lang.Object key)
Removes the key (and its corresponding value) from this table. |
int |
size()
Returns the number of key-value mappings in this map. |
java.util.Collection |
values()
Returns a collection view of the values contained in this map. |
Methods inherited from class java.util.AbstractMap |
equals, hashCode, toString |
Methods inherited from class java.lang.Object |
getClass, notify, notifyAll, wait, wait, wait |
Methods inherited from interface java.util.Map |
equals, hashCode |
Field Detail |
public static int DEFAULT_INITIAL_CAPACITY
public static final float DEFAULT_LOAD_FACTOR
Constructor Detail |
public ConcurrentHashMap(int initialCapacity, float loadFactor)
initialCapacity
- the initial capacity.
The actual initial capacity is rounded to the nearest power of two.loadFactor
- the load factor threshold, used to control resizing.
This value is used in an approximate way: When at least
a quarter of the segments of the table reach per-segment threshold, or
one of the segments itself exceeds overall threshold,
the table is doubled.
This will on average cause resizing when the table-wide
load factor is slightly less than the threshold. If you'd like
to avoid resizing, you can set this to a ridiculously large
value.
java.lang.IllegalArgumentException
- if the load factor is nonpositive.public ConcurrentHashMap(int initialCapacity)
initialCapacity
- the initial capacity of the
ConcurrentHashMap.
java.lang.IllegalArgumentException
- if the initial maximum number
of elements is less
than zero.public ConcurrentHashMap()
public ConcurrentHashMap(java.util.Map t)
Method Detail |
public int size()
size
in interface java.util.Map
size
in class java.util.AbstractMap
public boolean isEmpty()
isEmpty
in interface java.util.Map
isEmpty
in class java.util.AbstractMap
public java.lang.Object get(java.lang.Object key)
get
in interface java.util.Map
get
in class java.util.AbstractMap
key
- a key in the table.
null
if the key is not mapped to any value in
this table.
java.lang.NullPointerException
- if the key is
null
.put(Object, Object)
public boolean containsKey(java.lang.Object key)
containsKey
in interface java.util.Map
containsKey
in class java.util.AbstractMap
key
- possible key.
true
if and only if the specified object
is a key in this table, as determined by the
equals method; false
otherwise.
java.lang.NullPointerException
- if the key is
null
.contains(Object)
public java.lang.Object put(java.lang.Object key, java.lang.Object value)
key
to the specified
value
in this table. Neither the key nor the
value can be null
. (Note that this policy is
the same as for java.util.Hashtable, but unlike java.util.HashMap,
which does accept nulls as valid keys and values.)
The value can be retrieved by calling the get
method
with a key that is equal to the original key.
put
in interface java.util.Map
put
in class java.util.AbstractMap
key
- the table key.value
- the value.
null
if it did not have one.
java.lang.NullPointerException
- if the key or value is
null
.Object.equals(Object)
,
get(Object)
public java.lang.Object remove(java.lang.Object key)
remove
in interface java.util.Map
remove
in class java.util.AbstractMap
key
- the key that needs to be removed.
null
if the key did not have a mapping.
java.lang.NullPointerException
- if the key is
null
.public boolean containsValue(java.lang.Object value)
containsValue
in interface java.util.Map
containsValue
in class java.util.AbstractMap
value
- value whose presence in this map is to be tested.
java.lang.NullPointerException
- if the value is null
.public boolean contains(java.lang.Object value)
containsKey
method.Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the 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.
java.lang.NullPointerException
- if the value is null
.containsKey(Object)
,
containsValue(Object)
,
Map
public void putAll(java.util.Map t)
putAll
in interface java.util.Map
putAll
in class java.util.AbstractMap
t
- Mappings to be stored in this map.public void clear()
clear
in interface java.util.Map
clear
in class java.util.AbstractMap
public java.lang.Object clone()
clone
in class java.util.AbstractMap
public java.util.Set keySet()
keySet
in interface java.util.Map
keySet
in class java.util.AbstractMap
public java.util.Collection values()
values
in interface java.util.Map
values
in class java.util.AbstractMap
public java.util.Set entrySet()
entrySet
in interface java.util.Map
entrySet
in class java.util.AbstractMap
public java.util.Enumeration keys()
Enumeration
,
elements()
,
keySet()
,
Map
public java.util.Enumeration elements()
Enumeration
,
keys()
,
values()
,
Map
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