org.apache.commons.collections
Class FastHashMap
HashMap
org.apache.commons.collections.FastHashMap
public class FastHashMap
extends HashMap
A customized implementation of
java.util.HashMap
designed
to operate in a multithreaded environment where the large majority of
method calls are read-only, instead of structural changes. When operating
in "fast" mode, read calls are non-synchronized and write calls perform the
following steps:
- Clone the existing collection
- Perform the modification on the clone
- Replace the existing collection with the (modified) clone
When first created, objects of this class default to "slow" mode, where
all accesses of any type are synchronized but no cloning takes place. This
is appropriate for initially populating the collection, followed by a switch
to "fast" mode (by calling
setFast(true)
) after initialization
is complete.
NOTE: If you are creating and accessing a
HashMap
only within a single thread, you should use
java.util.HashMap
directly (with no synchronization), for
maximum performance.
NOTE:
This class is not cross-platform.
Using it may cause unexpected failures on some architectures.
It suffers from the same problems as the double-checked locking idiom.
In particular, the instruction that clones the internal collection and the
instruction that sets the internal reference to the clone can be executed
or perceived out-of-order. This means that any read operation might fail
unexpectedly, as it may be reading the state of the internal collection
before the internal collection is fully formed.
For more information on the double-checked locking idiom, see the
Double-Checked Locking Idiom Is Broken Declaration.
$Revision: 1.17 $ $Date: 2004/02/18 01:15:42 $- Craig R. McClanahan
- Stephen Colebourne
protected boolean | fast - Are we currently operating in "fast" mode?
|
protected HashMap | map - The underlying map we are managing.
|
FastHashMap() - Construct an empty map.
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FastHashMap(Map map) - Construct a new map with the same mappings as the specified map.
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FastHashMap(int capacity) - Construct an empty map with the specified capacity.
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FastHashMap(int capacity, float factor) - Construct an empty map with the specified capacity and load factor.
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void | clear() - Remove all mappings from this map.
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Object | clone() - Return a shallow copy of this
FastHashMap instance.
|
boolean | containsKey(Object key) - Return
true if this map contains a mapping for the
specified key.
|
boolean | containsValue(Object value) - Return
true if this map contains one or more keys mapping
to the specified value.
|
Set | entrySet() - Return a collection view of the mappings contained in this map.
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boolean | equals(Object o) - Compare the specified object with this list for equality.
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Object | get(Object key) - Return the value to which this map maps the specified key.
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boolean | getFast() - Returns true if this map is operating in fast mode.
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int | hashCode() - Return the hash code value for this map.
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boolean | isEmpty() - Return
true if this map contains no mappings.
|
Set | keySet() - Return a set view of the keys contained in this map.
|
Object | put(Object key, Object value) - Associate the specified value with the specified key in this map.
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void | putAll(Map in) - Copy all of the mappings from the specified map to this one, replacing
any mappings with the same keys.
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Object | remove(Object key) - Remove any mapping for this key, and return any previously
mapped value.
|
void | setFast(boolean fast) - Sets whether this map is operating in fast mode.
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int | size() - Return the number of key-value mappings in this map.
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Collection | values() - Return a collection view of the values contained in this map.
|
fast
protected boolean fast
Are we currently operating in "fast" mode?
map
protected HashMap map
The underlying map we are managing.
FastHashMap
public FastHashMap()
Construct an empty map.
FastHashMap
public FastHashMap(Map map)
Construct a new map with the same mappings as the specified map.
map
- the map whose mappings are to be copied
FastHashMap
public FastHashMap(int capacity)
Construct an empty map with the specified capacity.
capacity
- the initial capacity of the empty map
FastHashMap
public FastHashMap(int capacity,
float factor)
Construct an empty map with the specified capacity and load factor.
capacity
- the initial capacity of the empty mapfactor
- the load factor of the new map
clear
public void clear()
Remove all mappings from this map.
clone
public Object clone()
Return a shallow copy of this FastHashMap
instance.
The keys and values themselves are not copied.
containsKey
public boolean containsKey(Object key)
Return true
if this map contains a mapping for the
specified key.
key
- the key to be searched for
- true if the map contains the key
containsValue
public boolean containsValue(Object value)
Return true
if this map contains one or more keys mapping
to the specified value.
value
- the value to be searched for
- true if the map contains the value
entrySet
public Set entrySet()
Return a collection view of the mappings contained in this map. Each
element in the returned collection is a Map.Entry
.
equals
public boolean equals(Object o)
Compare the specified object with this list for equality. This
implementation uses exactly the code that is used to define the
list equals function in the documentation for the
Map.equals
method.
o
- the object to be compared to this list
- true if the two maps are equal
get
public Object get(Object key)
Return the value to which this map maps the specified key. Returns
null
if the map contains no mapping for this key, or if
there is a mapping with a value of null
. Use the
containsKey()
method to disambiguate these cases.
key
- the key whose value is to be returned
- the value mapped to that key, or null
getFast
public boolean getFast()
Returns true if this map is operating in fast mode.
- true if this map is operating in fast mode
hashCode
public int hashCode()
Return the hash code value for this map. This implementation uses
exactly the code that is used to define the list hash function in the
documentation for the Map.hashCode
method.
- suitable integer hash code
isEmpty
public boolean isEmpty()
Return true
if this map contains no mappings.
- is the map currently empty
keySet
public Set keySet()
Return a set view of the keys contained in this map.
put
public Object put(Object key,
Object value)
Associate the specified value with the specified key in this map.
If the map previously contained a mapping for this key, the old
value is replaced and returned.
key
- the key with which the value is to be associatedvalue
- the value to be associated with this key
- the value previously mapped to the key, or null
putAll
public void putAll(Map in)
Copy all of the mappings from the specified map to this one, replacing
any mappings with the same keys.
in
- the map whose mappings are to be copied
remove
public Object remove(Object key)
Remove any mapping for this key, and return any previously
mapped value.
key
- the key whose mapping is to be removed
- the value removed, or null
setFast
public void setFast(boolean fast)
Sets whether this map is operating in fast mode.
fast
- true if this map should operate in fast mode
size
public int size()
Return the number of key-value mappings in this map.
- the current size of the map
values
public Collection values()
Return a collection view of the values contained in this map.
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