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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:

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
Authors:
Craig R. McClanahan
Stephen Colebourne

Field Summary

boolean

fast

Are we currently operating in "fast" mode?

HashMap

map

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

void

clear()

Remove all mappings from this map.

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.

boolean

equals(Object o)

Compare the specified object with this list for equality.

Object

get(Object key)

Return the value to which this map maps the specified key.

boolean

getFast()

Returns true if this map is operating in fast mode.

int

hashCode()

Return the hash code value for this map.

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.

void

putAll(Map in)

Copy all of the mappings from the specified map to this one, replacing any mappings with the same keys.

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.

int

size()

Return the number of key-value mappings in this map.

Collection

values()

Return a collection view of the values contained in this map.

Field Details

fast

protected boolean fast

Are we currently operating in "fast" mode?


map

protected HashMap map

The underlying map we are managing.

Constructor Details

FastHashMap

public FastHashMap()

Construct an 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(int capacity)

Construct an empty map with the specified capacity.

Parameters:
capacity - the initial capacity of the empty 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 Details

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.