## 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

Nested Class Summary

Nested classes inherited from class java.util.Map
`Map.Entry`

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.

  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`
  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`
  Returns:
    the hash code defined in the Map interface

### toString

    public String toString()

  Gets the map as a String.

  Returns:
    a string version of the map

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