Package org.apache.groovy.util.concurrent

# Class ConcurrentReferenceHashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.apache.groovy.util.concurrent.ConcurrentReferenceHashMap<K,V>

Type Parameters:
  `K` - the type of keys maintained by this map
  `V` - the type of mapped values

All Implemented Interfaces:
  `Serializable`, `Map<K,V>`

Direct Known Subclasses:
  `ManagedIdentityConcurrentMap`

public class ConcurrentReferenceHashMap<K,V> extends AbstractMap<K,V> implements Serializable
An advanced hash table supporting configurable garbage collection semantics of keys and values, optional referential-equality, full concurrency of retrievals, and adjustable expected concurrency for updates.

This table is designed around specific advanced use-cases. If there is any doubt whether this table is for you, you most likely should be using `ConcurrentHashMap` instead.

This table supports strong, weak, and soft keys and values. By default keys are weak, and values are strong. Such a configuration offers similar behavior to `WeakHashMap`, entries of this table are periodically removed once their corresponding keys are no longer referenced outside of this table. In other words, this table will not prevent a key from being discarded by the garbage collector. Once a key has been discarded by the collector, the corresponding entry is no longer visible to this table; however, the entry may occupy space until a future table operation decides to reclaim it. For this reason, summary functions such as `size` and `isEmpty` might return a value greater than the observed number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during blocking (usually mutating) operations.

Enabling soft keys allows entries in this table to remain until their space is absolutely needed by the garbage collector. This is unlike weak keys which can be reclaimed as soon as they are no longer referenced by a normal strong reference. The primary use case for soft keys is a cache, which ideally occupies memory that is not in use for as long as possible.

By default, values are held using a normal strong reference. This provides the commonly desired guarantee that a value will always have at least the same life-span as its key. For this reason, care should be taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing reclamation. If this is unavoidable, then it is recommended to use the same reference type in use for the key. However, it should be noted that non-strong values may disappear before their corresponding key.

While this table does allow the use of both strong keys and values, it is recommended you use `ConcurrentHashMap` for such a configuration, since it is optimized for that case.

Just like `ConcurrentHashMap`, this class obeys the same functional specification as `Hashtable`, and includes versions of methods corresponding to each method of `Hashtable`. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with `Hashtable` in programs that rely on its thread safety but not on its synchronization details.

Retrieval operations (including `get`) generally do not block, so they may overlap with update operations (including `put` and `remove`). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as `putAll` and `clear`, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw `ConcurrentModificationException`. However, iterators are designed to be used by only one thread at a time.

The allowed concurrency among update operations is guided by the optional `concurrencyLevel` constructor argument (default `16`), which is used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash table is a relatively slow operation, so, when possible, it is a good idea that you provide estimates of expected table sizes in constructors.

This class and its views and iterators implement all of the optional methods of the `Map` and `Iterator` interfaces.

Like `Hashtable` but unlike `HashMap`, this class does not allow `null` to be used as a key or value.

This class is a member of the Java Collections Framework.

See Also:
  - Serialized Form

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Class
  Description
  `protected class`
  `ConcurrentReferenceHashMap.HashIterator`
  Base iterator implementation for traversing the map.
  `protected static class`
  `ConcurrentReferenceHashMap.InitializableEntry<K,V>`
  Reusable entry implementation for cached entry iteration.
  `static enum`
  `ConcurrentReferenceHashMap.Option`
  Behavior-changing configuration options for the map
  `static enum`
  `ConcurrentReferenceHashMap.ReferenceType`
  An option specifying which Java reference type should be used to refer to a key and/or value.
  `protected static class`
  `ConcurrentReferenceHashMap.SimpleEntry<K,V>`
  Simple mutable entry implementation used by the map views.
  `protected class`
  `ConcurrentReferenceHashMap.WriteThroughEntry`
  Custom Entry class used by EntryIterator.next(), that relays setValue changes to the underlying map.

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

  `AbstractMap.SimpleImmutableEntry<K extends Object,V extends Object>`

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

  `Map.Entry<K extends Object,V extends Object>`

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ConcurrentReferenceHashMap()`
  Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong values), default load factor (0.75) and concurrencyLevel (16).
  `ConcurrentReferenceHashMap(int initialCapacity)`
  Creates a new, empty map with the specified initial capacity, and with default reference types (weak keys, strong values), load factor (0.75) and concurrencyLevel (16).
  `ConcurrentReferenceHashMap(int initialCapacity, float loadFactor)`
  Creates a new, empty map with the specified initial capacity and load factor and with the default reference types (weak keys, strong values), and concurrencyLevel (16).
  `ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)`
  Creates a new, empty map with the specified initial capacity, load factor, and concurrency level.
  `ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)`
  Creates a new, empty map with the specified initial capacity, reference types, load factor, and concurrency level.
  `ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)`
  Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16).
  `ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)`
  Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16).
  `ConcurrentReferenceHashMap(Map<? extends K,? extends V> m)`
  Creates a new map with the same mappings as the given map.
  `ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)`
  Creates a new, empty reference map with the specified key and value reference types.
  `ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)`
  Creates a new, empty reference map with the specified reference types and behavioral options.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `V`
  `apply(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)`
  Remaps the value for the supplied key, creating or removing it as needed.
  `V`
  `applyIfAbsent(K key, Function<? super K,? extends V> mappingFunction)`
  `V`
  `applyIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)`
  Remaps the value for the supplied key when it is present.
  `Set<Map.Entry<K,V>>`
  `cachedEntrySet()`
  Returns an entry set view that reuses a mutable entry instance during iteration.
  `void`
  `clear()`
  Removes all of the mappings from this map.
  `boolean`
  `contains(Object value)`
  Legacy method testing if some key maps into the specified value in this table.
  `boolean`
  `containsKey(Object key)`
  Tests if the specified object is a key in this table.
  `boolean`
  `containsValue(Object value)`
  Returns `true` if this map maps one or more keys to the specified value.
  `Enumeration<V>`
  `elements()`
  Returns an enumeration of the values in this table.
  `Set<Map.Entry<K,V>>`
  `entrySet()`
  Returns a `Set` view of the mappings contained in this map.
  `V`
  `get(Object key)`
  Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.
  `protected int`
  `hashOf(Object key)`
  Returns the spread hash for the supplied key.
  `boolean`
  `isEmpty()`
  Returns `true` if this map contains no key-value mappings.
  `Enumeration<K>`
  `keys()`
  Returns an enumeration of the keys in this table.
  `Set<K>`
  `keySet()`
  Returns a `Set` view of the keys contained in this map.
  `void`
  `purgeStaleEntries()`
  Removes any stale entries whose keys have been finalized.
  `V`
  `put(K key, V value)`
  Maps the specified key to the specified value in this table.
  `void`
  `putAll(Map<? extends K,? extends V> m)`
  Copies all of the mappings from the specified map to this one.
  `V`
  `putIfAbsent(K key, V value)`
  `V`
  `remove(Object key)`
  Removes the key (and its corresponding value) from this map.
  `boolean`
  `remove(Object key, Object value)`
  `V`
  `replace(K key, V value)`
  `boolean`
  `replace(K key, V oldValue, V newValue)`
  `int`
  `size()`
  Returns the number of key-value mappings in this map.
  `Collection<V>`
  `values()`
  Returns a `Collection` view of the values contained in this map.

  ### Methods inherited from class java.util.AbstractMap

  `clone, equals, hashCode, toString`

  ### 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, replaceAll`

- ## Constructor Details

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)
    Creates a new, empty map with the specified initial capacity, reference types, load factor, and concurrency level.

    Behavioral changing options such as `ConcurrentReferenceHashMap.Option.IDENTITY_COMPARISONS` can also be specified.

    Parameters:
      `initialCapacity` - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
      `loadFactor` - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
      `concurrencyLevel` - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
      `keyType` - the reference type to use for keys
      `valueType` - the reference type to use for values
      `options` - the behavioral options
    Throws:
      `IllegalArgumentException` - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor, int concurrencyLevel)
    Creates a new, empty map with the specified initial capacity, load factor, and concurrency level.
    Parameters:
      `initialCapacity` - the initial capacity. The implementation performs internal sizing to accommodate this number of elements.
      `loadFactor` - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
      `concurrencyLevel` - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
    Throws:
      `IllegalArgumentException` - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity, float loadFactor)
    Creates a new, empty map with the specified initial capacity and load factor and with the default reference types (weak keys, strong values), and concurrencyLevel (16).
    Parameters:
      `initialCapacity` - The implementation performs internal sizing to accommodate this number of elements.
      `loadFactor` - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative or the load factor is nonpositive
    Since:
      1.6

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)
    Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16).
    Parameters:
      `initialCapacity` - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
      `keyType` - the reference type to use for keys
      `valueType` - the reference type to use for values
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity, ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)
    Creates a new, empty map with the specified initial capacity, reference types, and with a default load factor (0.75) and concurrencyLevel (16). Behavioral changing options such as `ConcurrentReferenceHashMap.Option.IDENTITY_COMPARISONS` can also be specified.
    Parameters:
      `initialCapacity` - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
      `keyType` - the reference type to use for keys
      `valueType` - the reference type to use for values
      `options` - the behavioral options
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType)
    Creates a new, empty reference map with the specified key and value reference types.
    Parameters:
      `keyType` - the reference type to use for keys
      `valueType` - the reference type to use for values
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(ConcurrentReferenceHashMap.ReferenceType keyType, ConcurrentReferenceHashMap.ReferenceType valueType, EnumSet<ConcurrentReferenceHashMap.Option> options)
    Creates a new, empty reference map with the specified reference types and behavioral options.
    Parameters:
      `keyType` - the reference type to use for keys
      `valueType` - the reference type to use for values
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(int initialCapacity)
    Creates a new, empty map with the specified initial capacity, and with default reference types (weak keys, strong values), load factor (0.75) and concurrencyLevel (16).
    Parameters:
      `initialCapacity` - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
    Throws:
      `IllegalArgumentException` - if the initial capacity of elements is negative.

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap()
    Creates a new, empty map with a default initial capacity (16), reference types (weak keys, strong values), default load factor (0.75) and concurrencyLevel (16).

  - ### ConcurrentReferenceHashMap

    public ConcurrentReferenceHashMap(Map<? extends K,? extends V> m)
    Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).
    Parameters:
      `m` - the map

- ## Method Details

  - ### hashOf

    protected int hashOf(Object key)
    Returns the spread hash for the supplied key.
    Parameters:
      `key` - the key to hash
    Returns:
      the spread hash

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this map contains no key-value mappings.
    Specified by:
      `isEmpty` in interface `Map<K,V>`
    Overrides:
      `isEmpty` in class `AbstractMap<K,V>`
    Returns:
      `true` if this map contains no key-value mappings

  - ### size

    public int size()
    Returns the number of key-value mappings in this map. If the map contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.
    Specified by:
      `size` in interface `Map<K,V>`
    Overrides:
      `size` in class `AbstractMap<K,V>`
    Returns:
      the number of key-value mappings in this map

  - ### get

    public V get(Object key)
    Returns the value to which the specified key is mapped, or `null` if this map contains no mapping for the key.

    If this map contains a mapping from a key `k` to a value `v` such that `key.equals(k)`, then this method returns `v`; otherwise it returns `null`. (There can be at most one such mapping.)

    Specified by:
      `get` in interface `Map<K,V>`
    Overrides:
      `get` in class `AbstractMap<K,V>`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### containsKey

    public boolean containsKey(Object key)
    Tests if the specified object is a key in this table.
    Specified by:
      `containsKey` in interface `Map<K,V>`
    Overrides:
      `containsKey` in class `AbstractMap<K,V>`
    Parameters:
      `key` - possible key
    Returns:
      `true` if and only if the specified object is a key in this table, as determined by the `equals` method; `false` otherwise.
    Throws:
      `NullPointerException` - if the specified key is null

  - ### containsValue

    public boolean containsValue(Object value)
    Returns `true` if this map maps one or more keys to the specified value. Note: This method requires a full internal traversal of the hash table, therefore it is much slower than the method `containsKey`.
    Specified by:
      `containsValue` in interface `Map<K,V>`
    Overrides:
      `containsValue` in class `AbstractMap<K,V>`
    Parameters:
      `value` - value whose presence in this map is to be tested
    Returns:
      `true` if this map maps one or more keys to the specified value
    Throws:
      `NullPointerException` - if the specified value is null

  - ### contains

    public boolean contains(Object value)
    Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to `containsValue(java.lang.Object)`, and exists solely to ensure full compatibility with class `Hashtable`, which supported this method prior to introduction of the Java Collections framework.
    Parameters:
      `value` - a value to search for
    Returns:
      `true` if and only if some key maps to the `value` argument in this table as determined by the `equals` method; `false` otherwise
    Throws:
      `NullPointerException` - if the specified value is null

  - ### put

    public V put(K key, V value)
    Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

    The value can be retrieved by calling the `get` method with a key that is equal to the original key.

    Specified by:
      `put` in interface `Map<K,V>`
    Overrides:
      `put` in class `AbstractMap<K,V>`
    Parameters:
      `key` - key with which the specified value is to be associated
      `value` - value to be associated with the specified key
    Returns:
      the previous value associated with `key`, or `null` if there was no mapping for `key`
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### putIfAbsent

    public V putIfAbsent(K key, V value)
    Specified by:
      `putIfAbsent` in interface `Map<K,V>`
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### applyIfAbsent

    public V applyIfAbsent(K key, Function<? super K,? extends V> mappingFunction)

  - ### applyIfPresent

    public V applyIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    Remaps the value for the supplied key when it is present.
    Parameters:
      `key` - the key to update
      `remappingFunction` - the remapping function
    Returns:
      the remapped value, or `null` if the entry was absent or removed

  - ### apply

    public V apply(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
    Remaps the value for the supplied key, creating or removing it as needed.
    Parameters:
      `key` - the key to update
      `remappingFunction` - the remapping function
    Returns:
      the remapped value, or `null` if the entry was removed

  - ### putAll

    public void putAll(Map<? extends K,? extends V> m)
    Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified map.
    Specified by:
      `putAll` in interface `Map<K,V>`
    Overrides:
      `putAll` in class `AbstractMap<K,V>`
    Parameters:
      `m` - mappings to be stored in this map

  - ### remove

    public V remove(Object key)
    Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
    Specified by:
      `remove` in interface `Map<K,V>`
    Overrides:
      `remove` in class `AbstractMap<K,V>`
    Parameters:
      `key` - the key that needs to be removed
    Returns:
      the previous value associated with `key`, or `null` if there was no mapping for `key`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### remove

    public boolean remove(Object key, Object value)
    Specified by:
      `remove` in interface `Map<K,V>`
    Throws:
      `NullPointerException` - if the specified key is null

  - ### replace

    public boolean replace(K key, V oldValue, V newValue)
    Specified by:
      `replace` in interface `Map<K,V>`
    Throws:
      `NullPointerException` - if any of the arguments are null

  - ### replace

    public V replace(K key, V value)
    Specified by:
      `replace` in interface `Map<K,V>`
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `NullPointerException` - if the specified key or value is null

  - ### clear

    public void clear()
    Removes all of the mappings from this map.
    Specified by:
      `clear` in interface `Map<K,V>`
    Overrides:
      `clear` in class `AbstractMap<K,V>`

  - ### purgeStaleEntries

    public void purgeStaleEntries()
    Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary since stale entries are automatically removed lazily, when blocking operations are required. However, there are some cases where this operation should be performed eagerly, such as cleaning up old references to a ClassLoader in a multi-classloader environment.

    Note: this method will acquire locks one at a time across all segments of this table, so this method should be used sparingly.

  - ### keySet

    public Set<K> keySet()
    Returns a `Set` view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `keySet` in interface `Map<K,V>`
    Overrides:
      `keySet` in class `AbstractMap<K,V>`

  - ### values

    public Collection<V> values()
    Returns a `Collection` view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `values` in interface `Map<K,V>`
    Overrides:
      `values` in class `AbstractMap<K,V>`

  - ### entrySet

    public Set<Map.Entry<K,V>> entrySet()
    Returns a `Set` view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from the map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll`, and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's `iterator` is a "weakly consistent" iterator that will never throw `ConcurrentModificationException`, and is guaranteed to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

    Specified by:
      `entrySet` in interface `Map<K,V>`
    Specified by:
      `entrySet` in class `AbstractMap<K,V>`

  - ### cachedEntrySet

    public Set<Map.Entry<K,V>> cachedEntrySet()
    Returns an entry set view that reuses a mutable entry instance during iteration.
    Returns:
      the cached entry set view

  - ### keys

    public Enumeration<K> keys()
    Returns an enumeration of the keys in this table.
    Returns:
      an enumeration of the keys in this table
    See Also:
      - `keySet()`

  - ### elements

    public Enumeration<V> elements()
    Returns an enumeration of the values in this table.
    Returns:
      an enumeration of the values in this table
    See Also:
      - `values()`