org.apache.commons.collections

Class 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:

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.

Since: Commons Collections 1.0

Version: $Revision: 155406 $ $Date: 2005-02-26 12:55:26 +0000 (Sat, 26 Feb 2005) $

Author: Craig R. McClanahan Stephen Colebourne

Field Summary
protected booleanfast
Are we currently operating in "fast" mode?
protected HashMapmap
The underlying map we are managing.
Constructor Summary
FastHashMap()
Construct an empty map.
FastHashMap(int capacity)
Construct an empty map with the specified capacity.
FastHashMap(int capacity, float factor)
Construct an empty map with the specified capacity and load factor.
FastHashMap(Map map)
Construct a new map with the same mappings as the specified map.
Method Summary
voidclear()
Remove all mappings from this map.
Objectclone()
Return a shallow copy of this FastHashMap instance.
booleancontainsKey(Object key)
Return true if this map contains a mapping for the specified key.
booleancontainsValue(Object value)
Return true if this map contains one or more keys mapping to the specified value.
SetentrySet()
Return a collection view of the mappings contained in this map.
booleanequals(Object o)
Compare the specified object with this list for equality.
Objectget(Object key)
Return the value to which this map maps the specified key.
booleangetFast()
Returns true if this map is operating in fast mode.
inthashCode()
Return the hash code value for this map.
booleanisEmpty()
Return true if this map contains no mappings.
SetkeySet()
Return a set view of the keys contained in this map.
Objectput(Object key, Object value)
Associate the specified value with the specified key in this map.
voidputAll(Map in)
Copy all of the mappings from the specified map to this one, replacing any mappings with the same keys.
Objectremove(Object key)
Remove any mapping for this key, and return any previously mapped value.
voidsetFast(boolean fast)
Sets whether this map is operating in fast mode.
intsize()
Return the number of key-value mappings in this map.
Collectionvalues()
Return a collection view of the values contained in this map.

Field Detail

fast

protected boolean fast
Are we currently operating in "fast" mode?

map

protected HashMap map
The underlying map we are managing.

Constructor Detail

FastHashMap

public FastHashMap()
Construct an empty map.

FastHashMap

public FastHashMap(int capacity)
Construct an empty map with the specified capacity.

Parameters: 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.

Parameters: capacity the initial capacity of the empty map factor the load factor of the new map

FastHashMap

public FastHashMap(Map map)
Construct a new map with the same mappings as the specified map.

Parameters: map the map whose mappings are to be copied

Method Detail

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.

Returns: a clone of this map

containsKey

public boolean containsKey(Object key)
Return true if this map contains a mapping for the specified key.

Parameters: key the key to be searched for

Returns: 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.

Parameters: value the value to be searched for

Returns: 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.

Parameters: o the object to be compared to this list

Returns: 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.

Parameters: key the key whose value is to be returned

Returns: the value mapped to that key, or null

getFast

public boolean getFast()
Returns true if this map is operating in fast mode.

Returns: 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.

Returns: suitable integer hash code

isEmpty

public boolean isEmpty()
Return true if this map contains no mappings.

Returns: 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.

Parameters: key the key with which the value is to be associated value the value to be associated with this key

Returns: 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.

Parameters: 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.

Parameters: key the key whose mapping is to be removed

Returns: the value removed, or null

setFast

public void setFast(boolean fast)
Sets whether this map is operating in fast mode.

Parameters: fast true if this map should operate in fast mode

size

public int size()
Return the number of key-value mappings in this map.

Returns: 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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