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,V>`

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `Flat3Map()` Constructor.
    `Flat3Map(Map<? extends K,? extends V> map)` Constructor copying elements from another map.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods
    Modifier and Type | Method and 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 Detail

    - #### Flat3Map

          public Flat3Map()

      Constructor.

    - #### 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 Detail

    - #### get

          public V get(Object key)

      Gets the value mapped to the key specified.
      Specified by:
        `get` in interface `Map<K,V>`
      Specified by:
        `get` in interface `Get<K,V>`
      Parameters:
        `key` - the key
      Returns:
        the mapped value, null if no match
      See Also:
        `Map.get(Object)`

    - #### size

          public int size()

      Gets the size of the map.
      Specified by:
        `size` in interface `Map<K,V>`
      Specified by:
        `size` in interface `Get<K,V>`
      Returns:
        the size
      See Also:
        `Map.size()`

    - #### isEmpty

          public boolean isEmpty()

      Checks whether the map is currently empty.
      Specified by:
        `isEmpty` in interface `Map<K,V>`
      Specified by:
        `isEmpty` in interface `Get<K,V>`
      Returns:
        true if the map is currently size zero
      See Also:
        `Map.isEmpty()`

    - #### containsKey

          public boolean containsKey(Object key)

      Checks whether the map contains the specified key.
      Specified by:
        `containsKey` in interface `Map<K,V>`
      Specified by:
        `containsKey` in interface `Get<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 `Map<K,V>`
      Specified by:
        `containsValue` in interface `Get<K,V>`
      Parameters:
        `value` - the value to search for
      Returns:
        true if the map contains the key
      See Also:
        `Map.containsValue(Object)`

    - #### 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)`

    - #### 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

    - #### remove

          public V remove(Object key)

      Removes the specified mapping from this map.
      Specified by:
        `remove` in interface `Map<K,V>`
      Specified by:
        `remove` in interface `Get<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)`

    - #### 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()`

    - #### 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

    - #### 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 `Map<K,V>`
      Specified by:
        `entrySet` in interface `Get<K,V>`
      Returns:
        the entrySet view
      See Also:
        `Map.entrySet()`

    - #### 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 `Map<K,V>`
      Specified by:
        `keySet` in interface `Get<K,V>`
      Returns:
        the keySet view
      See Also:
        `Map.keySet()`

    - #### 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 `Map<K,V>`
      Specified by:
        `values` in interface `Get<K,V>`
      Returns:
        the values view
      See Also:
        `Map.values()`

    - #### 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

    - #### 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

    - #### 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

    - #### toString

          public String toString()

      Gets the map as a String.
      Overrides:
        `toString` in class `Object`
      Returns:
        a string version of the map

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