Package org.apache.commons.collections.map

Class Flat3Map

java.lang.Object
org.apache.commons.collections.map.Flat3Map
All Implemented Interfaces:
Serializable, Cloneable, Map, IterableMap
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.

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:
Commons Collections 3.0
Version:
$Revision: 1713293 $ $Date: 2015-11-08 22:04:34 +0100 (Sun, 08 Nov 2015) $
Author:
Stephen Colebourne
See Also:

  • Constructor Details

    • 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 Details

    • 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