org.apache.commons.collections.map
Class Flat3Map

java.lang.Object
  |
  +--org.apache.commons.collections.map.Flat3Map

All Implemented Interfaces:

Cloneable, IterableMap, Map, Serializable

public class Flat3Map

extends Object

implements IterableMap, Serializable, Cloneable

A Map implementation that stores data in simple fields until the size is greater than 3.

This map is designed for performance and can outstrip HashMap. It also has good garbage collection characteristics.

• Optimised for operation at size 3 or less.
• Still works well once size 3 exceeded.
• Gets at size 3 or less are about 0-10% faster than HashMap,
• Puts at size 3 or less are over 4 times faster than HashMap.
• Performance 5% slower than HashMap once size 3 exceeded once.

The design uses two distinct modes of operation - flat and delegate. While the map is size 3 or less, operations map straight onto fields using switch statements. Once size 4 is reached, the map switches to delegate mode and only switches back when cleared. In delegate mode, all operations are forwarded straight to a HashMap resulting in the 5% performance loss.

The performance gains on puts are due to not needing to create a Map Entry object. This is a large saving not only in performance but in garbage collection.

Whilst in flat mode this map is also easy for the garbage collector to dispatch. This is because it contains no complex objects or arrays which slow the progress.

Do not use Flat3Map if the size is likely to grow beyond 3.

Since:

Commons Collections 3.0

Version:

$Revision: 1.18 $ $Date: 2004/05/26 21:56:05 $

Author:

Stephen Colebourne

See Also:

Serialized Form

Constructor Summary

Flat3Map()
Constructor.

Flat3Map(Map map)
Constructor copying elements from another map.

Method Summary

void	clear() Clears the map, resetting the size to zero and nullifying references to avoid garbage collection issues.
Object	clone() Clones the map without cloning the keys or values.
boolean	containsKey(Object key) Checks whether the map contains the specified key.
boolean	containsValue(Object value) Checks whether the map contains the specified value.
protected AbstractHashedMap	createDelegateMap() Create an instance of the map used for storage when in delegation mode.
Set	entrySet() Gets the entrySet view of the map.
boolean	equals(Object obj) Compares this map with another.
Object	get(Object key) Gets the value mapped to the key specified.
int	hashCode() Gets the standard Map hashCode.
boolean	isEmpty() Checks whether the map is currently empty.
Set	keySet() Gets the keySet view of the map.
MapIterator	mapIterator() Gets an iterator over the map.
Object	put(Object key, Object value) Puts a key-value mapping into this map.
void	putAll(Map map) Puts all the values from the specified map into this map.
Object	remove(Object key) Removes the specified mapping from this map.
int	size() Gets the size of the map.
String	toString() Gets the map as a String.
Collection	values() Gets the values view of the map.

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

Flat3Map

public Flat3Map()

Constructor.

Flat3Map

public Flat3Map(Map map)

Constructor copying elements from another map.

Parameters:

map - the map to copy

Throws:

NullPointerException - if the map is null

Method Detail

get

public Object get(Object key)

Gets the value mapped to the key specified.

Specified by:

get in interface Map

Parameters:

key - the key

Returns:

the mapped value, null if no match

size

public int size()

Gets the size of the map.

Specified by:

size in interface Map

Returns:

the size

isEmpty

public boolean isEmpty()

Checks whether the map is currently empty.

Specified by:

isEmpty in interface Map

Returns:

true if the map is currently size zero

containsKey

public boolean containsKey(Object key)

Checks whether the map contains the specified key.

Specified by:

containsKey in interface Map

Parameters:

key - the key to search for

Returns:

true if the map contains the key

containsValue

public boolean containsValue(Object value)

Checks whether the map contains the specified value.

Specified by:

containsValue in interface Map

Parameters:

value - the value to search for

Returns:

true if the map contains the key

put

public Object put(Object key,
                  Object value)

Puts a key-value mapping into this map.

Specified by:

put in interface Map

Parameters:

key - the key to add

value - the value to add

Returns:

the value previously mapped to this key, null if none

putAll

public void putAll(Map map)

Puts all the values from the specified map into this map.

Specified by:

putAll in interface Map

Parameters:

map - the map to add

Throws:

NullPointerException - if the map is null

createDelegateMap

protected AbstractHashedMap createDelegateMap()

Create an instance of the map used for storage when in delegation mode.

This can be overridden by subclasses to provide a different map implementation. Not every AbstractHashedMap is suitable, identity and reference based maps would be poor choices.

Returns:

a new AbstractHashedMap or subclass

Since:

Commons Collections 3.1

remove

public Object remove(Object key)

Removes the specified mapping from this map.

Specified by:

remove in interface Map

Parameters:

key - the mapping to remove

Returns:

the value mapped to the removed key, null if key not in map

clear

public void clear()

Clears the map, resetting the size to zero and nullifying references to avoid garbage collection issues.

Specified by:

clear in interface Map

mapIterator

public MapIterator mapIterator()

Gets an iterator over the map. Changes made to the iterator affect this map.

A MapIterator returns the keys in the map. It also provides convenient methods to get the key and value, and set the value. It avoids the need to create an entrySet/keySet/values object. It also avoids creating the Map Entry object.

Specified by:

mapIterator in interface IterableMap

Returns:

the map iterator

entrySet

public Set entrySet()

Gets the entrySet view of the map. Changes made to the view affect this map. The Map Entry is not an independent object and changes as the iterator progresses. To simply iterate through the entries, use mapIterator().

Specified by:

entrySet in interface Map

Returns:

the entrySet view

keySet

public Set keySet()

Gets the keySet view of the map. Changes made to the view affect this map. To simply iterate through the keys, use mapIterator().

Specified by:

keySet in interface Map

Returns:

the keySet view

values

public Collection values()

Gets the values view of the map. Changes made to the view affect this map. To simply iterate through the values, use mapIterator().

Specified by:

values in interface Map

Returns:

the values view

clone

public Object clone()

Clones the map without cloning the keys or values.

Overrides:

clone in class Object

Returns:

a shallow clone

Since:

Commons Collections 3.1

equals

public boolean equals(Object obj)

Compares this map with another.

Specified by:

equals in interface Map

Overrides:

equals in class Object

Parameters:

obj - the object to compare to

Returns:

true if equal

hashCode

public int hashCode()

Gets the standard Map hashCode.

Specified by:

hashCode in interface Map

Overrides:

hashCode in class Object

Returns:

the hash code defined in the Map interface

toString

public String toString()

Gets the map as a String.

Overrides:

toString in class Object

Returns:

a string version of the map