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,V>`

- ## 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()`