public class

Flat3Map

extends Object
implements Serializable Cloneable IterableMap
java.lang.Object
   ↳ org.apache.commons.collections.map.Flat3Map

Class Overview

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 synchronizedMap(Map). This class may throw exceptions when accessed by concurrent threads without synchronization.

Summary

Public Constructors
Flat3Map()
Constructor.
Flat3Map(Map map)
Constructor copying elements from another map.
Public Methods
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.
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.
Protected Methods
AbstractHashedMap createDelegateMap()
Create an instance of the map used for storage when in delegation mode.
[Expand]
Inherited Methods
From class java.lang.Object
From interface java.util.Map
From interface org.apache.commons.collections.IterableMap

Public Constructors

public Flat3Map ()

Constructor.

public Flat3Map (Map map)

Constructor copying elements from another map.

Parameters
map the map to copy
Throws
NullPointerException if the map is null

Public Methods

public void clear ()

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

public Object clone ()

Clones the map without cloning the keys or values.

Returns
  • a shallow clone

public boolean containsKey (Object key)

Checks whether the map contains the specified key.

Parameters
key the key to search for
Returns
  • true if the map contains the key

public boolean containsValue (Object value)

Checks whether the map contains the specified value.

Parameters
value the value to search for
Returns
  • true if the map contains the key

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().

Returns
  • the entrySet view

public boolean equals (Object obj)

Compares this map with another.

Parameters
obj the object to compare to
Returns
  • true if equal

public Object get (Object key)

Gets the value mapped to the key specified.

Parameters
key the key
Returns
  • the mapped value, null if no match

public int hashCode ()

Gets the standard Map hashCode.

Returns
  • the hash code defined in the Map interface

public boolean isEmpty ()

Checks whether the map is currently empty.

Returns
  • true if the map is currently size zero

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().

Returns
  • the keySet view

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.

Returns
  • the map iterator

public Object put (Object key, Object value)

Puts a key-value mapping into this map.

Parameters
key the key to add
value the value to add
Returns
  • the value previously mapped to this key, null if none

public void putAll (Map map)

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

Parameters
map the map to add
Throws
NullPointerException if the map is null

public Object remove (Object key)

Removes the specified mapping from this map.

Parameters
key the mapping to remove
Returns
  • the value mapped to the removed key, null if key not in map

public int size ()

Gets the size of the map.

Returns
  • the size

public String toString ()

Gets the map as a String.

Returns
  • a string version of the map

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().

Returns
  • the values view

Protected Methods

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