Package org.apache.commons.collections4.map

Class Flat3Map<K,V>

java.lang.Object

org.apache.commons.collections4.map.Flat3Map<K,V>

Type Parameters:

K - the type of the keys in this map

V - the type of the values in this map

All Implemented Interfaces:

Serializable, Cloneable, Map<K,V>, Get<K,V>, IterableGet<K,V>, IterableMap<K,V>, Put<K,V>

public class Flat3Map<K,V>
extends Object
implements IterableMap<K,V>, 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.

Note that Flat3Map is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using Collections.synchronizedMap(Map). This class may throw exceptions when accessed by concurrent threads without synchronization.

Since:

3.0

See Also:

• Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K extends Object,V extends Object>

Constructor Summary

Constructors

Constructor	Description
Flat3Map()	Constructs a new instance.
Flat3Map(Map<? extends K,? extends V> map)	Constructor copying elements from another map.

Method Summary

Modifier and Type	Method	Description
void	clear()	Clears the map, resetting the size to zero and nullifying references to avoid garbage collection issues.
Flat3Map<K,V>	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<K,V>	createDelegateMap()	Create an instance of the map used for storage when in delegation mode.
Set<Map.Entry<K,V>>	entrySet()	Gets the entrySet view of the map.
boolean	equals(Object obj)	Compares this map with another.
V	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<K>	keySet()	Gets the keySet view of the map.
MapIterator<K,V>	mapIterator()	Gets an iterator over the map.
V	put(K key, V value)	Puts a key-value mapping into this map.
void	putAll(Map<? extends K,? extends V> map)	Puts all the values from the specified map into this map.
V	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<V>	values()	Gets the values view of the map.

Methods inherited from class java.lang.Object

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

Methods inherited from interface java.util.Map

compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAll

Constructor Details

Flat3Map

public Flat3Map()

Constructs a new instance.

Flat3Map

public Flat3Map(Map<? extends K,? extends V> map)

Constructor copying elements from another map.

Parameters:

map - the map to copy

Throws:

NullPointerException - if the map is null

Method Details

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<K,V>

Specified by:

clear in interface Put<K,V>

See Also:

• Map.clear()

clone

public Flat3Map<K,V> clone()

Clones the map without cloning the keys or values.

Overrides:

clone in class Object

Returns:

a shallow clone

Since:

3.1

containsKey

public boolean containsKey(Object key)

Checks whether the map contains the specified key.

Specified by:

containsKey in interface Get<K,V>

Specified by:

containsKey in interface Map<K,V>

Parameters:

key - the key to search for

Returns:

true if the map contains the key

See Also:

• Map.containsKey(Object)

containsValue

public boolean containsValue(Object value)

Checks whether the map contains the specified value.

Specified by:

containsValue in interface Get<K,V>

Specified by:

containsValue in interface Map<K,V>

Parameters:

value - the value to search for

Returns:

true if the map contains the key

See Also:

• Map.containsValue(Object)

createDelegateMap

protected AbstractHashedMap<K,V> 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:

3.1

entrySet

public Set<Map.Entry<K,V>> entrySet()

Gets the entrySet view of the map. Changes made to the view affect this map.

NOTE: from 4.0, the returned Map Entry will be an independent object and will not change anymore as the iterator progresses. To avoid this additional object creation and simply iterate through the entries, use mapIterator().

Specified by:

entrySet in interface Get<K,V>

Specified by:

entrySet in interface Map<K,V>

Returns:

the entrySet view

See Also:

• Map.entrySet()

equals

public boolean equals(Object obj)

Compares this map with another.

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

obj - the object to compare to

Returns:

true if equal

get

public V get(Object key)

Gets the value mapped to the key specified.

Specified by:

get in interface Get<K,V>

Specified by:

get in interface Map<K,V>

Parameters:

key - the key

Returns:

the mapped value, null if no match

See Also:

• Map.get(Object)

hashCode

public int hashCode()

Gets the standard Map hashCode.

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the hash code defined in the Map interface

isEmpty

public boolean isEmpty()

Checks whether the map is currently empty.

Specified by:

isEmpty in interface Get<K,V>

Specified by:

isEmpty in interface Map<K,V>

Returns:

true if the map is currently size zero

See Also:

• Map.isEmpty()

keySet

public Set<K> 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 Get<K,V>

Specified by:

keySet in interface Map<K,V>

Returns:

the keySet view

See Also:

• Map.keySet()

mapIterator

public MapIterator<K,V> 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 IterableGet<K,V>

Returns:

the map iterator

put

public V put(K key,
 V value)

Puts a key-value mapping into this map.

Specified by:

put in interface Map<K,V>

Specified by:

put in interface Put<K,V>

Parameters:

key - the key to add

value - the value to add

Returns:

the value previously mapped to this key, null if none

See Also:

• Map.put(Object, Object)

putAll

public void putAll(Map<? extends K,? extends V> map)

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

Specified by:

putAll in interface Map<K,V>

Specified by:

putAll in interface Put<K,V>

Parameters:

map - the map to add

Throws:

NullPointerException - if the map is null

See Also:

• Map.putAll(Map)

remove

public V remove(Object key)

Removes the specified mapping from this map.

Specified by:

remove in interface Get<K,V>

Specified by:

remove in interface Map<K,V>

Parameters:

key - the mapping to remove

Returns:

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

See Also:

• Map.remove(Object)

size

public int size()

Gets the size of the map.

Specified by:

size in interface Get<K,V>

Specified by:

size in interface Map<K,V>

Returns:

the size

See Also:

• Map.size()

toString

public String toString()

Gets the map as a String.

Overrides:

toString in class Object

Returns:

a string version of the map

values

public Collection<V> 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 Get<K,V>

Specified by:

values in interface Map<K,V>

Returns:

the values view

See Also:

• Map.values()