org.apache.commons.collections.map

## Class AbstractHashedMap

- java.lang.Object
- - java.util.AbstractMap
  - - org.apache.commons.collections.map.AbstractHashedMap

- All Implemented Interfaces:
    Map, IterableMap
  Direct Known Subclasses:
    AbstractLinkedMap, AbstractReferenceMap, CaseInsensitiveMap, HashedMap, IdentityMap

      public class AbstractHashedMap
      extends AbstractMap
      implements IterableMap

  An abstract implementation of a hash-based map which provides numerous points for subclasses to override.

  This class implements all the features necessary for a subclass hash-based map. Key-value entries are stored in instances of the `HashEntry` class, which can be overridden and replaced. The iterators can similarly be replaced, without the need to replace the KeySet, EntrySet and Values view classes.

  Overridable methods are provided to change the default hashing behaviour, and to change how entries are added to and removed from the map. Hopefully, all you need for unusual subclasses is here.

  NOTE: From Commons Collections 3.1 this class extends AbstractMap. This is to provide backwards compatibility for ReferenceMap between v3.0 and v3.1. This extends clause will be removed in v4.0.

  Since:
    Commons Collections 3.0
  Version:
    $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
  Author:
    java util HashMap, Stephen Colebourne, Christian Siefkes

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `protected static class` | `AbstractHashedMap.EntrySet` EntrySet implementation.
    `protected static class` | `AbstractHashedMap.EntrySetIterator` EntrySet iterator.
    `protected static class` | `AbstractHashedMap.HashEntry` HashEntry used to store the data.
    `protected static class` | `AbstractHashedMap.HashIterator` Base Iterator
    `protected static class` | `AbstractHashedMap.HashMapIterator` MapIterator implementation.
    `protected static class` | `AbstractHashedMap.KeySet` KeySet implementation.
    `protected static class` | `AbstractHashedMap.KeySetIterator` KeySet iterator.
    `protected static class` | `AbstractHashedMap.Values` Values implementation.
    `protected static class` | `AbstractHashedMap.ValuesIterator` Values iterator.

    - ### Nested classes/interfaces inherited from class java.util.AbstractMap

      `AbstractMap.SimpleEntry, AbstractMap.SimpleImmutableEntry`

    - ### Nested classes/interfaces inherited from interface java.util.Map

      `Map.Entry`

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected AbstractHashedMap.HashEntry[]` | `data` Map entries
    `protected static int` | `DEFAULT_CAPACITY` The default capacity to use
    `protected static float` | `DEFAULT_LOAD_FACTOR` The default load factor to use
    `protected static int` | `DEFAULT_THRESHOLD` The default threshold to use
    `protected AbstractHashedMap.EntrySet` | `entrySet` Entry set
    `protected static String` | `GETKEY_INVALID`
    `protected static String` | `GETVALUE_INVALID`
    `protected AbstractHashedMap.KeySet` | `keySet` Key set
    `protected float` | `loadFactor` Load factor, normally 0.75
    `protected static int` | `MAXIMUM_CAPACITY` The maximum capacity allowed
    `protected int` | `modCount` Modification count for iterators
    `protected static String` | `NO_NEXT_ENTRY`
    `protected static String` | `NO_PREVIOUS_ENTRY`
    `protected static Object` | `NULL` An object for masking null
    `protected static String` | `REMOVE_INVALID`
    `protected static String` | `SETVALUE_INVALID`
    `protected int` | `size` The size of the map
    `protected int` | `threshold` Size at which to rehash
    `protected AbstractHashedMap.Values` | `values` Values

  - ### Constructor Summary

    Constructors
    Modifier | Constructor and Description
    --- | ---
    `protected` | `AbstractHashedMap()` Constructor only used in deserialization, do not use otherwise.
    `protected` | `AbstractHashedMap(int initialCapacity)` Constructs a new, empty map with the specified initial capacity and default load factor.
    `protected` | `AbstractHashedMap(int initialCapacity, float loadFactor)` Constructs a new, empty map with the specified initial capacity and load factor.
    `protected` | `AbstractHashedMap(int initialCapacity, float loadFactor, int threshold)` Constructor which performs no validation on the passed in parameters.
    `protected` | `AbstractHashedMap(Map map)` Constructor copying elements from another map.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `protected void` | `addEntry(AbstractHashedMap.HashEntry entry, int hashIndex)` Adds an entry into this map.
    `protected void` | `addMapping(int hashIndex, int hashCode, Object key, Object value)` Adds a new key-value mapping into this map.
    `protected int` | `calculateNewCapacity(int proposedCapacity)` Calculates the new capacity of the map.
    `protected int` | `calculateThreshold(int newCapacity, float factor)` Calculates the new threshold of the map, where it will be resized.
    `protected void` | `checkCapacity()` Checks the capacity of the map and enlarges it if necessary.
    `void` | `clear()` Clears the map, resetting the size to zero and nullifying references to avoid garbage collection issues.
    `protected 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 Object` | `convertKey(Object key)` Converts input keys to another object for storage in the map.
    `protected AbstractHashedMap.HashEntry` | `createEntry(AbstractHashedMap.HashEntry next, int hashCode, Object key, Object value)` Creates an entry to store the key-value data.
    `protected Iterator` | `createEntrySetIterator()` Creates an entry set iterator.
    `protected Iterator` | `createKeySetIterator()` Creates a key set iterator.
    `protected Iterator` | `createValuesIterator()` Creates a values iterator.
    `protected void` | `destroyEntry(AbstractHashedMap.HashEntry entry)` Kills an entry ready for the garbage collector.
    `protected void` | `doReadObject(ObjectInputStream in)` Reads the map data from the stream.
    `protected void` | `doWriteObject(ObjectOutputStream out)` Writes the map data to the stream.
    `protected void` | `ensureCapacity(int newCapacity)` Changes the size of the data structure to the capacity proposed.
    `protected int` | `entryHashCode(AbstractHashedMap.HashEntry entry)` Gets the `hashCode` field from a `HashEntry`.
    `protected Object` | `entryKey(AbstractHashedMap.HashEntry entry)` Gets the `key` field from a `HashEntry`.
    `protected AbstractHashedMap.HashEntry` | `entryNext(AbstractHashedMap.HashEntry entry)` Gets the `next` field from a `HashEntry`.
    `Set` | `entrySet()` Gets the entrySet view of the map.
    `protected Object` | `entryValue(AbstractHashedMap.HashEntry entry)` Gets the `value` field from a `HashEntry`.
    `boolean` | `equals(Object obj)` Compares this map with another.
    `Object` | `get(Object key)` Gets the value mapped to the key specified.
    `protected AbstractHashedMap.HashEntry` | `getEntry(Object key)` Gets the entry mapped to the key specified.
    `protected int` | `hash(Object key)` Gets the hash code for the key specified.
    `int` | `hashCode()` Gets the standard Map hashCode.
    `protected int` | `hashIndex(int hashCode, int dataSize)` Gets the index into the data storage for the hashCode specified.
    `protected void` | `init()` Initialise subclasses during construction, cloning or deserialization.
    `boolean` | `isEmpty()` Checks whether the map is currently empty.
    `protected boolean` | `isEqualKey(Object key1, Object key2)` Compares two keys, in internal converted form, to see if they are equal.
    `protected boolean` | `isEqualValue(Object value1, Object value2)` Compares two values, in external form, to see if they are equal.
    `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.
    `protected void` | `removeEntry(AbstractHashedMap.HashEntry entry, int hashIndex, AbstractHashedMap.HashEntry previous)` Removes an entry from the chain stored in a particular index.
    `protected void` | `removeMapping(AbstractHashedMap.HashEntry entry, int hashIndex, AbstractHashedMap.HashEntry previous)` Removes a mapping from the map.
    `protected void` | `reuseEntry(AbstractHashedMap.HashEntry entry, int hashIndex, int hashCode, Object key, Object value)` Reuses an existing key-value mapping, storing completely new data.
    `int` | `size()` Gets the size of the map.
    `String` | `toString()` Gets the map as a String.
    `protected void` | `updateEntry(AbstractHashedMap.HashEntry entry, Object newValue)` Updates an existing key-value mapping to change the value.
    `Collection` | `values()` Gets the values view of the map.

    - ### Methods inherited from class java.lang.Object

      `finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### NO_NEXT_ENTRY

          protected static final String NO_NEXT_ENTRY

      See Also:
        Constant Field Values

    - #### NO_PREVIOUS_ENTRY

          protected static final String NO_PREVIOUS_ENTRY

      See Also:
        Constant Field Values

    - #### REMOVE_INVALID

          protected static final String REMOVE_INVALID

      See Also:
        Constant Field Values

    - #### GETKEY_INVALID

          protected static final String GETKEY_INVALID

      See Also:
        Constant Field Values

    - #### GETVALUE_INVALID

          protected static final String GETVALUE_INVALID

      See Also:
        Constant Field Values

    - #### SETVALUE_INVALID

          protected static final String SETVALUE_INVALID

      See Also:
        Constant Field Values

    - #### DEFAULT_CAPACITY

          protected static final int DEFAULT_CAPACITY

      The default capacity to use
      See Also:
        Constant Field Values

    - #### DEFAULT_THRESHOLD

          protected static final int DEFAULT_THRESHOLD

      The default threshold to use
      See Also:
        Constant Field Values

    - #### DEFAULT_LOAD_FACTOR

          protected static final float DEFAULT_LOAD_FACTOR

      The default load factor to use
      See Also:
        Constant Field Values

    - #### MAXIMUM_CAPACITY

          protected static final int MAXIMUM_CAPACITY

      The maximum capacity allowed
      See Also:
        Constant Field Values

    - #### NULL

          protected static final Object NULL

      An object for masking null

    - #### loadFactor

          protected transient float loadFactor

      Load factor, normally 0.75

    - #### size

          protected transient int size

      The size of the map

    - #### data

          protected transient AbstractHashedMap.HashEntry[] data

      Map entries

    - #### threshold

          protected transient int threshold

      Size at which to rehash

    - #### modCount

          protected transient int modCount

      Modification count for iterators

    - #### entrySet

          protected transient AbstractHashedMap.EntrySet entrySet

      Entry set

    - #### keySet

          protected transient AbstractHashedMap.KeySet keySet

      Key set

    - #### values

          protected transient AbstractHashedMap.Values values

      Values

  - ### Constructor Detail

    - #### AbstractHashedMap

          protected AbstractHashedMap()

      Constructor only used in deserialization, do not use otherwise.

    - #### AbstractHashedMap

          protected AbstractHashedMap(int initialCapacity,
                           float loadFactor,
                           int threshold)

      Constructor which performs no validation on the passed in parameters.
      Parameters:
        `initialCapacity` - the initial capacity, must be a power of two
        `loadFactor` - the load factor, must be > 0.0f and generally < 1.0f
        `threshold` - the threshold, must be sensible

    - #### AbstractHashedMap

          protected AbstractHashedMap(int initialCapacity)

      Constructs a new, empty map with the specified initial capacity and default load factor.
      Parameters:
        `initialCapacity` - the initial capacity
      Throws:
        `IllegalArgumentException` - if the initial capacity is less than one

    - #### AbstractHashedMap

          protected AbstractHashedMap(int initialCapacity,
                           float loadFactor)

      Constructs a new, empty map with the specified initial capacity and load factor.
      Parameters:
        `initialCapacity` - the initial capacity
        `loadFactor` - the load factor
      Throws:
        `IllegalArgumentException` - if the initial capacity is less than one
        `IllegalArgumentException` - if the load factor is less than or equal to zero

    - #### AbstractHashedMap

          protected AbstractHashedMap(Map map)

      Constructor copying elements from another map.
      Parameters:
        `map` - the map to copy
      Throws:
        `NullPointerException` - if the map is null

  - ### Method Detail

    - #### init

          protected void init()

      Initialise subclasses during construction, cloning or deserialization.

    - #### get

          public Object get(Object key)

      Gets the value mapped to the key specified.
      Specified by:
        `get` in interface `Map`
      Overrides:
        `get` in class `AbstractMap`
      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`
      Overrides:
        `size` in class `AbstractMap`
      Returns:
        the size

    - #### isEmpty

          public boolean isEmpty()

      Checks whether the map is currently empty.
      Specified by:
        `isEmpty` in interface `Map`
      Overrides:
        `isEmpty` in class `AbstractMap`
      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`
      Overrides:
        `containsKey` in class `AbstractMap`
      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`
      Overrides:
        `containsValue` in class `AbstractMap`
      Parameters:
        `value` - the value to search for
      Returns:
        true if the map contains the value

    - #### put

          public Object put(Object key,
                   Object value)

      Puts a key-value mapping into this map.
      Specified by:
        `put` in interface `Map`
      Overrides:
        `put` in class `AbstractMap`
      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.

      This implementation iterates around the specified map and uses `put(Object, Object)`.

      Specified by:
        `putAll` in interface `Map`
      Overrides:
        `putAll` in class `AbstractMap`
      Parameters:
        `map` - the map to add
      Throws:
        `NullPointerException` - if the map is null

    - #### remove

          public Object remove(Object key)

      Removes the specified mapping from this map.
      Specified by:
        `remove` in interface `Map`
      Overrides:
        `remove` in class `AbstractMap`
      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`
      Overrides:
        `clear` in class `AbstractMap`

    - #### convertKey

          protected Object convertKey(Object key)

      Converts input keys to another object for storage in the map. This implementation masks nulls. Subclasses can override this to perform alternate key conversions.

      The reverse conversion can be changed, if required, by overriding the getKey() method in the hash entry.

      Parameters:
        `key` - the key convert
      Returns:
        the converted key

    - #### hash

          protected int hash(Object key)

      Gets the hash code for the key specified. This implementation uses the additional hashing routine from JDK1.4. Subclasses can override this to return alternate hash codes.
      Parameters:
        `key` - the key to get a hash code for
      Returns:
        the hash code

    - #### isEqualKey

          protected boolean isEqualKey(Object key1,
                           Object key2)

      Compares two keys, in internal converted form, to see if they are equal. This implementation uses the equals method and assumes neither key is null. Subclasses can override this to match differently.
      Parameters:
        `key1` - the first key to compare passed in from outside
        `key2` - the second key extracted from the entry via `entry.key`
      Returns:
        true if equal

    - #### isEqualValue

          protected boolean isEqualValue(Object value1,
                             Object value2)

      Compares two values, in external form, to see if they are equal. This implementation uses the equals method and assumes neither value is null. Subclasses can override this to match differently.
      Parameters:
        `value1` - the first value to compare passed in from outside
        `value2` - the second value extracted from the entry via `getValue()`
      Returns:
        true if equal

    - #### hashIndex

          protected int hashIndex(int hashCode,
                      int dataSize)

      Gets the index into the data storage for the hashCode specified. This implementation uses the least significant bits of the hashCode. Subclasses can override this to return alternate bucketing.
      Parameters:
        `hashCode` - the hash code to use
        `dataSize` - the size of the data to pick a bucket from
      Returns:
        the bucket index

    - #### getEntry

          protected AbstractHashedMap.HashEntry getEntry(Object key)

      Gets the entry mapped to the key specified.

      This method exists for subclasses that may need to perform a multi-step process accessing the entry. The public methods in this class don't use this method to gain a small performance boost.

      Parameters:
        `key` - the key
      Returns:
        the entry, null if no match

    - #### updateEntry

          protected void updateEntry(AbstractHashedMap.HashEntry entry,
                         Object newValue)

      Updates an existing key-value mapping to change the value.

      This implementation calls `setValue()` on the entry. Subclasses could override to handle changes to the map.

      Parameters:
        `entry` - the entry to update
        `newValue` - the new value to store

    - #### reuseEntry

          protected void reuseEntry(AbstractHashedMap.HashEntry entry,
                        int hashIndex,
                        int hashCode,
                        Object key,
                        Object value)

      Reuses an existing key-value mapping, storing completely new data.

      This implementation sets all the data fields on the entry. Subclasses could populate additional entry fields.

      Parameters:
        `entry` - the entry to update, not null
        `hashIndex` - the index in the data array
        `hashCode` - the hash code of the key to add
        `key` - the key to add
        `value` - the value to add

    - #### addMapping

          protected void addMapping(int hashIndex,
                        int hashCode,
                        Object key,
                        Object value)

      Adds a new key-value mapping into this map.

      This implementation calls `createEntry()`, `addEntry()` and `checkCapacity()`. It also handles changes to `modCount` and `size`. Subclasses could override to fully control adds to the map.

      Parameters:
        `hashIndex` - the index into the data array to store at
        `hashCode` - the hash code of the key to add
        `key` - the key to add
        `value` - the value to add

    - #### createEntry

          protected AbstractHashedMap.HashEntry createEntry(AbstractHashedMap.HashEntry next,
                                                int hashCode,
                                                Object key,
                                                Object value)

      Creates an entry to store the key-value data.

      This implementation creates a new HashEntry instance. Subclasses can override this to return a different storage class, or implement caching.

      Parameters:
        `next` - the next entry in sequence
        `hashCode` - the hash code to use
        `key` - the key to store
        `value` - the value to store
      Returns:
        the newly created entry

    - #### addEntry

          protected void addEntry(AbstractHashedMap.HashEntry entry,
                      int hashIndex)

      Adds an entry into this map.

      This implementation adds the entry to the data storage table. Subclasses could override to handle changes to the map.

      Parameters:
        `entry` - the entry to add
        `hashIndex` - the index into the data array to store at

    - #### removeMapping

          protected void removeMapping(AbstractHashedMap.HashEntry entry,
                           int hashIndex,
                           AbstractHashedMap.HashEntry previous)

      Removes a mapping from the map.

      This implementation calls `removeEntry()` and `destroyEntry()`. It also handles changes to `modCount` and `size`. Subclasses could override to fully control removals from the map.

      Parameters:
        `entry` - the entry to remove
        `hashIndex` - the index into the data structure
        `previous` - the previous entry in the chain

    - #### removeEntry

          protected void removeEntry(AbstractHashedMap.HashEntry entry,
                         int hashIndex,
                         AbstractHashedMap.HashEntry previous)

      Removes an entry from the chain stored in a particular index.

      This implementation removes the entry from the data storage table. The size is not updated. Subclasses could override to handle changes to the map.

      Parameters:
        `entry` - the entry to remove
        `hashIndex` - the index into the data structure
        `previous` - the previous entry in the chain

    - #### destroyEntry

          protected void destroyEntry(AbstractHashedMap.HashEntry entry)

      Kills an entry ready for the garbage collector.

      This implementation prepares the HashEntry for garbage collection. Subclasses can override this to implement caching (override clear as well).

      Parameters:
        `entry` - the entry to destroy

    - #### checkCapacity

          protected void checkCapacity()

      Checks the capacity of the map and enlarges it if necessary.

      This implementation uses the threshold to check if the map needs enlarging

    - #### ensureCapacity

          protected void ensureCapacity(int newCapacity)

      Changes the size of the data structure to the capacity proposed.
      Parameters:
        `newCapacity` - the new capacity of the array (a power of two, less or equal to max)

    - #### calculateNewCapacity

          protected int calculateNewCapacity(int proposedCapacity)

      Calculates the new capacity of the map. This implementation normalizes the capacity to a power of two.
      Parameters:
        `proposedCapacity` - the proposed capacity
      Returns:
        the normalized new capacity

    - #### calculateThreshold

          protected int calculateThreshold(int newCapacity,
                               float factor)

      Calculates the new threshold of the map, where it will be resized. This implementation uses the load factor.
      Parameters:
        `newCapacity` - the new capacity
        `factor` - the load factor
      Returns:
        the new resize threshold

    - #### entryNext

          protected AbstractHashedMap.HashEntry entryNext(AbstractHashedMap.HashEntry entry)

      Gets the `next` field from a `HashEntry`. Used in subclasses that have no visibility of the field.
      Parameters:
        `entry` - the entry to query, must not be null
      Returns:
        the `next` field of the entry
      Throws:
        `NullPointerException` - if the entry is null
      Since:
        Commons Collections 3.1

    - #### entryHashCode

          protected int entryHashCode(AbstractHashedMap.HashEntry entry)

      Gets the `hashCode` field from a `HashEntry`. Used in subclasses that have no visibility of the field.
      Parameters:
        `entry` - the entry to query, must not be null
      Returns:
        the `hashCode` field of the entry
      Throws:
        `NullPointerException` - if the entry is null
      Since:
        Commons Collections 3.1

    - #### entryKey

          protected Object entryKey(AbstractHashedMap.HashEntry entry)

      Gets the `key` field from a `HashEntry`. Used in subclasses that have no visibility of the field.
      Parameters:
        `entry` - the entry to query, must not be null
      Returns:
        the `key` field of the entry
      Throws:
        `NullPointerException` - if the entry is null
      Since:
        Commons Collections 3.1

    - #### entryValue

          protected Object entryValue(AbstractHashedMap.HashEntry entry)

      Gets the `value` field from a `HashEntry`. Used in subclasses that have no visibility of the field.
      Parameters:
        `entry` - the entry to query, must not be null
      Returns:
        the `value` field of the entry
      Throws:
        `NullPointerException` - if the entry is null
      Since:
        Commons Collections 3.1

    - #### 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. To simply iterate through the entries, use `mapIterator()`.
      Specified by:
        `entrySet` in interface `Map`
      Specified by:
        `entrySet` in class `AbstractMap`
      Returns:
        the entrySet view

    - #### createEntrySetIterator

          protected Iterator createEntrySetIterator()

      Creates an entry set iterator. Subclasses can override this to return iterators with different properties.
      Returns:
        the entrySet iterator

    - #### 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`
      Overrides:
        `keySet` in class `AbstractMap`
      Returns:
        the keySet view

    - #### createKeySetIterator

          protected Iterator createKeySetIterator()

      Creates a key set iterator. Subclasses can override this to return iterators with different properties.
      Returns:
        the keySet iterator

    - #### 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`
      Overrides:
        `values` in class `AbstractMap`
      Returns:
        the values view

    - #### createValuesIterator

          protected Iterator createValuesIterator()

      Creates a values iterator. Subclasses can override this to return iterators with different properties.
      Returns:
        the values iterator

    - #### doWriteObject

          protected void doWriteObject(ObjectOutputStream out)
                                throws IOException

      Writes the map data to the stream. This method must be overridden if a subclass must be setup before `put()` is used.

      Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the `put()` method on read can be affected by subclass state.

      The solution adopted here is to serialize the state data of this class in this protected method. This method must be called by the `writeObject()` of the first serializable subclass.

      Subclasses may override if they have a specific field that must be present on read before this implementation will work. Generally, the read determines what must be serialized here, if anything.

      Parameters:
        `out` - the output stream
      Throws:
        `IOException`

    - #### doReadObject

          protected void doReadObject(ObjectInputStream in)
                               throws IOException,
                                      ClassNotFoundException

      Reads the map data from the stream. This method must be overridden if a subclass must be setup before `put()` is used.

      Serialization is not one of the JDK's nicest topics. Normal serialization will initialise the superclass before the subclass. Sometimes however, this isn't what you want, as in this case the `put()` method on read can be affected by subclass state.

      The solution adopted here is to deserialize the state data of this class in this protected method. This method must be called by the `readObject()` of the first serializable subclass.

      Subclasses may override if the subclass has a specific field that must be present before `put()` or `calculateThreshold()` will work correctly.

      Parameters:
        `in` - the input stream
      Throws:
        `IOException`
        `ClassNotFoundException`

    - #### clone

          protected Object clone()

      Clones the map without cloning the keys or values.

      To implement `clone()`, a subclass must implement the `Cloneable` interface and make this method public.

      Overrides:
        `clone` in class `AbstractMap`
      Returns:
        a shallow clone

    - #### equals

          public boolean equals(Object obj)

      Compares this map with another.
      Specified by:
        `equals` in interface `Map`
      Overrides:
        `equals` in class `AbstractMap`
      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 `AbstractMap`
      Returns:
        the hash code defined in the Map interface

    - #### toString

          public String toString()

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

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