# Class ConcurrentSkipListMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
java.util.concurrent.ConcurrentSkipListMap<K,V>

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

All Implemented Interfaces:
  `Serializable, Cloneable, ConcurrentMap<K,V>, ConcurrentNavigableMap<K,V>, Map<K,V>, NavigableMap<K,V>, SequencedMap<K,V>, SortedMap<K,V>`

public class ConcurrentSkipListMap<K,V> extends AbstractMap<K,V> implements ConcurrentNavigableMap<K,V>, Cloneable, Serializable
A scalable concurrent `ConcurrentNavigableMap` implementation. The map is sorted according to the natural ordering of its keys, or by a `Comparator` provided at map creation time, depending on which constructor is used.

This class implements a concurrent variant of SkipLists providing expected average log(n) time cost for the `containsKey`, `get`, `put` and `remove` operations and their variants. Insertion, removal, update, and access operations safely execute concurrently by multiple threads.

Iterators and spliterators are weakly consistent.

Ascending key ordered views and their iterators are faster than descending ones.

All `Map.Entry` pairs returned by methods in this class and its views represent snapshots of mappings at the time they were produced. They do not support the `Entry.setValue` method. (Note however that it is possible to change mappings in the associated map using `put`, `putIfAbsent`, or `replace`, depending on exactly which effect you need.)

Beware that bulk operations `putAll`, `equals`, `toArray`, `containsValue`, and `clear` are not guaranteed to be performed atomically. For example, an iterator operating concurrently with a `putAll` operation might view only some of the added elements.

This class and its views and iterators implement all of the optional methods of the `Map` and `Iterator` interfaces. Like most other concurrent collections, this class does not permit the use of `null` keys or values because some null return values cannot be reliably distinguished from the absence of elements.

This class is a member of the Java Collections Framework.

Since:
  1.6
See Also:
  - Serialized Form

- ## Nested Class Summary

  ### Nested classes/interfaces declared in class AbstractMap

  `AbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>`
  Modifier and Type
  Class
  Description
  `static class`
  `AbstractMap.SimpleEntry<K,V>`
  An Entry maintaining a key and a value.
  `static class`
  `AbstractMap.SimpleImmutableEntry<K,V>`
  An unmodifiable Entry maintaining a key and a value.

  ### Nested classes/interfaces declared in interface Map

  `Map.Entry<K,V>`
  Modifier and Type
  Interface
  Description
  `static interface`
  `Map.Entry<K,V>`
  A map entry (key-value pair).

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `ConcurrentSkipListMap()`
  Constructs a new, empty map, sorted according to the natural ordering of the keys.
  `ConcurrentSkipListMap(Comparator<? super K> comparator)`
  Constructs a new, empty map, sorted according to the specified comparator.
  `ConcurrentSkipListMap(Map<? extends K, ? extends V> m)`
  Constructs a new map containing the same mappings as the given map, sorted according to the natural ordering of the keys.
  `ConcurrentSkipListMap(SortedMap<K, ? extends V> m)`
  Constructs a new map containing the same mappings and using the same ordering as the specified sorted map.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `Map.Entry<K,V>`
  `ceilingEntry(K key)`
  Returns a key-value mapping associated with the least key greater than or equal to the given key, or `null` if there is no such entry.
  `K`
  `ceilingKey(K key)`
  Returns the least key greater than or equal to the given key, or `null` if there is no such key.
  `void`
  `clear()`
  Removes all of the mappings from this map.
  `ConcurrentSkipListMap<K,V>`
  `clone()`
  Returns a shallow copy of this `ConcurrentSkipListMap` instance.
  `Comparator<? super K>`
  `comparator()`
  Returns the comparator used to order the keys in this map, or `null` if this map uses the natural ordering of its keys.
  `V`
  `compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)`
  Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping).
  `V`
  `computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)`
  If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unless `null`.
  `V`
  `computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)`
  If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value.
  `boolean`
  `containsKey(Object key)`
  Returns `true` if this map contains a mapping for the specified key.
  `boolean`
  `containsValue(Object value)`
  Returns `true` if this map maps one or more keys to the specified value.
  `NavigableSet<K>`
  `descendingKeySet()`
  Returns a reverse order `NavigableSet` view of the keys contained in this map.
  `ConcurrentNavigableMap<K,V>`
  `descendingMap()`
  Returns a reverse order view of the mappings contained in this map.
  `Set<Map.Entry<K,V>>`
  `entrySet()`
  Returns a `Set` view of the mappings contained in this map.
  `boolean`
  `equals(Object o)`
  Compares the specified object with this map for equality.
  `Map.Entry<K,V>`
  `firstEntry()`
  Returns a key-value mapping associated with the least key in this map, or `null` if the map is empty.
  `K`
  `firstKey()`
  Returns the first (lowest) key currently in this map.
  `Map.Entry<K,V>`
  `floorEntry(K key)`
  Returns a key-value mapping associated with the greatest key less than or equal to the given key, or `null` if there is no such key.
  `K`
  `floorKey(K key)`
  Returns the greatest key less than or equal to the given key, or `null` if there is no such key.
  `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.
  `V`
  `getOrDefault(Object key, V defaultValue)`
  Returns the value to which the specified key is mapped, or the given defaultValue if this map contains no mapping for the key.
  `ConcurrentNavigableMap<K,V>`
  `headMap(K toKey)`
  Returns a view of the portion of this map whose keys are strictly less than `toKey`.
  `ConcurrentNavigableMap<K,V>`
  `headMap(K toKey, boolean inclusive)`
  Returns a view of the portion of this map whose keys are less than (or equal to, if `inclusive` is true) `toKey`.
  `Map.Entry<K,V>`
  `higherEntry(K key)`
  Returns a key-value mapping associated with the least key strictly greater than the given key, or `null` if there is no such key.
  `K`
  `higherKey(K key)`
  Returns the least key strictly greater than the given key, or `null` if there is no such key.
  `NavigableSet<K>`
  `keySet()`
  Returns a `NavigableSet` view of the keys contained in this map.
  `Map.Entry<K,V>`
  `lastEntry()`
  Returns a key-value mapping associated with the greatest key in this map, or `null` if the map is empty.
  `K`
  `lastKey()`
  Returns the last (highest) key currently in this map.
  `Map.Entry<K,V>`
  `lowerEntry(K key)`
  Returns a key-value mapping associated with the greatest key strictly less than the given key, or `null` if there is no such key.
  `K`
  `lowerKey(K key)`
  Returns the greatest key strictly less than the given key, or `null` if there is no such key.
  `V`
  `merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)`
  If the specified key is not already associated with a value, associates it with the given value.
  `NavigableSet<K>`
  `navigableKeySet()`
  Returns a `NavigableSet` view of the keys contained in this map.
  `Map.Entry<K,V>`
  `pollFirstEntry()`
  Removes and returns a key-value mapping associated with the least key in this map, or `null` if the map is empty.
  `Map.Entry<K,V>`
  `pollLastEntry()`
  Removes and returns a key-value mapping associated with the greatest key in this map, or `null` if the map is empty.
  `V`
  `put(K key, V value)`
  Associates the specified value with the specified key in this map.
  `V`
  `putFirst(K k, V v)`
  Throws `UnsupportedOperationException`.
  `V`
  `putIfAbsent(K key, V value)`
  If the specified key is not already associated with a value, associates it with the given value.
  `V`
  `putLast(K k, V v)`
  Throws `UnsupportedOperationException`.
  `V`
  `remove(Object key)`
  Removes the mapping for the specified key from this map if present.
  `boolean`
  `remove(Object key, Object value)`
  Removes the entry for a key only if currently mapped to a given value.
  `V`
  `replace(K key, V value)`
  Replaces the entry for a key only if currently mapped to some value.
  `boolean`
  `replace(K key, V oldValue, V newValue)`
  Replaces the entry for a key only if currently mapped to a given value.
  `ConcurrentNavigableMap<K,V>`
  `subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)`
  Returns a view of the portion of this map whose keys range from `fromKey` to `toKey`.
  `ConcurrentNavigableMap<K,V>`
  `subMap(K fromKey, K toKey)`
  Returns a view of the portion of this map whose keys range from `fromKey`, inclusive, to `toKey`, exclusive.
  `ConcurrentNavigableMap<K,V>`
  `tailMap(K fromKey)`
  Returns a view of the portion of this map whose keys are greater than or equal to `fromKey`.
  `ConcurrentNavigableMap<K,V>`
  `tailMap(K fromKey, boolean inclusive)`
  Returns a view of the portion of this map whose keys are greater than (or equal to, if `inclusive` is true) `fromKey`.
  `Collection<V>`
  `values()`
  Returns a `Collection` view of the values contained in this map.

  ### Methods declared in class AbstractMap

  `hashCode, isEmpty, putAll, size, toString`
  Modifier and Type
  Method
  Description
  `int`
  `hashCode()`
  Returns the hash code value for this map.
  `boolean`
  `isEmpty()`
  Returns `true` if this map contains no key-value mappings.
  `void`
  `putAll(Map<? extends K, ? extends V> m)`
  Copies all of the mappings from the specified map to this map (optional operation).
  `int`
  `size()`
  Returns the number of key-value mappings in this map.
  `String`
  `toString()`
  Returns a string representation of this map.

  ### Methods declared in class Object

  `finalize, getClass, notify, notifyAll, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected void`
  `finalize()`
  Deprecated, for removal: This API element is subject to removal in a future version.
  Finalization is deprecated and subject to removal in a future release.
  `final Class<?>`
  `getClass()`
  Returns the runtime class of this `Object`.
  `final void`
  `notify()`
  Wakes up a single thread that is waiting on this object's monitor.
  `final void`
  `notifyAll()`
  Wakes up all threads that are waiting on this object's monitor.
  `final void`
  `wait()`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted.
  `final void`
  `wait(long timeoutMillis)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
  `final void`
  `wait(long timeoutMillis, int nanos)`
  Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.

  ### Methods declared in interface ConcurrentMap

  `forEach, replaceAll`
  Modifier and Type
  Method
  Description
  `default void`
  `forEach(BiConsumer<? super K, ? super V> action)`
  Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
  `default void`
  `replaceAll(BiFunction<? super K, ? super V, ? extends V> function)`
  Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception (optional operation).

  ### Methods declared in interface NavigableMap

  `reversed`
  Modifier and Type
  Method
  Description
  `default NavigableMap<K,V>`
  `reversed()`
  Returns a reverse-ordered view of this map.

  ### Methods declared in interface SequencedMap

  `sequencedEntrySet, sequencedKeySet, sequencedValues`
  Modifier and Type
  Method
  Description
  `default SequencedSet<Map.Entry<K,V>>`
  `sequencedEntrySet()`
  Returns a `SequencedSet` view of this map's `entrySet`.
  `default SequencedSet<K>`
  `sequencedKeySet()`
  Returns a `SequencedSet` view of this map's `keySet`.
  `default SequencedCollection<V>`
  `sequencedValues()`
  Returns a `SequencedCollection` view of this map's `values` collection.

- ## Constructor Details

  - ### ConcurrentSkipListMap

    public ConcurrentSkipListMap()
    Constructs a new, empty map, sorted according to the natural ordering of the keys.

  - ### ConcurrentSkipListMap

    public ConcurrentSkipListMap(Comparator<? super K> comparator)
    Constructs a new, empty map, sorted according to the specified comparator.
    Parameters:
      `comparator` - the comparator that will be used to order this map. If `null`, the natural ordering of the keys will be used.

  - ### ConcurrentSkipListMap

    public ConcurrentSkipListMap(Map<? extends K, ? extends V> m)
    Constructs a new map containing the same mappings as the given map, sorted according to the natural ordering of the keys.
    Parameters:
      `m` - the map whose mappings are to be placed in this map
    Throws:
      `ClassCastException` - if the keys in `m` are not `Comparable`, or are not mutually comparable
      `NullPointerException` - if the specified map or any of its keys or values are null

  - ### ConcurrentSkipListMap

    public ConcurrentSkipListMap(SortedMap<K, ? extends V> m)
    Constructs a new map containing the same mappings and using the same ordering as the specified sorted map.
    Parameters:
      `m` - the sorted map whose mappings are to be placed in this map, and whose comparator is to be used to sort this map
    Throws:
      `NullPointerException` - if the specified sorted map or any of its keys or values are null

- ## Method Details

  - ### clone

    public ConcurrentSkipListMap<K,V> clone()
    Returns a shallow copy of this `ConcurrentSkipListMap` instance. (The keys and values themselves are not cloned.)
    Overrides:
      `clone` in class `AbstractMap<K,V>`
    Returns:
      a shallow copy of this map
    See Also:
      - `Cloneable`

  - ### containsKey

    public boolean containsKey(Object key)
    Returns `true` if this map contains a mapping for the specified key.
    Specified by:
      `containsKey` in interface `Map<K,V>`
    Overrides:
      `containsKey` in class `AbstractMap<K,V>`
    Parameters:
      `key` - key whose presence in this map is to be tested
    Returns:
      `true` if this map contains a mapping for the specified key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### 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.

    More formally, if this map contains a mapping from a key `k` to a value `v` such that `key` compares equal to `k` according to the map's ordering, 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>`
    Parameters:
      `key` - the key whose associated value is to be returned
    Returns:
      the value to which the specified key is mapped, or `null` if this map contains no mapping for the key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### getOrDefault

    public V getOrDefault(Object key, V defaultValue)
    Returns the value to which the specified key is mapped, or the given defaultValue if this map contains no mapping for the key.
    Specified by:
      `getOrDefault` in interface `ConcurrentMap<K,V>`
    Specified by:
      `getOrDefault` in interface `Map<K,V>`
    Parameters:
      `key` - the key
      `defaultValue` - the value to return if this map contains no mapping for the given key
    Returns:
      the mapping for the key, if present; else the defaultValue
    Throws:
      `NullPointerException` - if the specified key is null
    Since:
      1.8

  - ### put

    public V put(K key, V value)
    Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.
    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 the specified key, or `null` if there was no mapping for the key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key or value is null

  - ### remove

    public V remove(Object key)
    Removes the mapping for the specified key from this map if present.
    Specified by:
      `remove` in interface `Map<K,V>`
    Overrides:
      `remove` in class `AbstractMap<K,V>`
    Parameters:
      `key` - key for which mapping should be removed
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `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. This operation requires time linear in the map size. Additionally, it is possible for the map to change during execution of this method, in which case the returned result may be inaccurate.
    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 a mapping to `value` exists; `false` otherwise
    Throws:
      `NullPointerException` - if the specified 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>`

  - ### computeIfAbsent

    public V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)
    If the specified key is not already associated with a value, attempts to compute its value using the given mapping function and enters it into this map unless `null`. The function is NOT guaranteed to be applied once atomically only if the value is not present.
    Specified by:
      `computeIfAbsent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `computeIfAbsent` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is to be associated
      `mappingFunction` - the function to compute a value
    Returns:
      the current (existing or computed) value associated with the specified key, or null if the computed value is null
    Throws:
      `NullPointerException` - if the specified key is null or the mappingFunction is null
    Since:
      1.8

  - ### computeIfPresent

    public V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
    If the value for the specified key is present, attempts to compute a new mapping given the key and its current mapped value. The function is NOT guaranteed to be applied once atomically.
    Specified by:
      `computeIfPresent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `computeIfPresent` in interface `Map<K,V>`
    Parameters:
      `key` - key with which a value may be associated
      `remappingFunction` - the function to compute a value
    Returns:
      the new value associated with the specified key, or null if none
    Throws:
      `NullPointerException` - if the specified key is null or the remappingFunction is null
    Since:
      1.8

  - ### compute

    public V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)
    Attempts to compute a mapping for the specified key and its current mapped value (or `null` if there is no current mapping). The function is NOT guaranteed to be applied once atomically.
    Specified by:
      `compute` in interface `ConcurrentMap<K,V>`
    Specified by:
      `compute` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is to be associated
      `remappingFunction` - the function to compute a value
    Returns:
      the new value associated with the specified key, or null if none
    Throws:
      `NullPointerException` - if the specified key is null or the remappingFunction is null
    Since:
      1.8

  - ### merge

    public V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)
    If the specified key is not already associated with a value, associates it with the given value. Otherwise, replaces the value with the results of the given remapping function, or removes if `null`. The function is NOT guaranteed to be applied once atomically.
    Specified by:
      `merge` in interface `ConcurrentMap<K,V>`
    Specified by:
      `merge` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is to be associated
      `value` - the value to use if absent
      `remappingFunction` - the function to recompute a value if present
    Returns:
      the new value associated with the specified key, or null if none
    Throws:
      `NullPointerException` - if the specified key or value is null or the remappingFunction is null
    Since:
      1.8

  - ### keySet

    public NavigableSet<K> keySet()
    Returns a `NavigableSet` view of the keys contained in this map.

    The set's iterator returns the keys in ascending order. The set's spliterator additionally reports `Spliterator.CONCURRENT`, `Spliterator.NONNULL`, `Spliterator.SORTED` and `Spliterator.ORDERED`, with an encounter order that is ascending key order.

    The spliterator's comparator is `null` if the map's comparator is `null`. Otherwise, the spliterator's comparator is the same as or imposes the same total ordering as the map's comparator.

    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 iterators and spliterators are weakly consistent.

    This method is equivalent to method `navigableKeySet`.

    Specified by:
      `keySet` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `keySet` in interface `Map<K,V>`
    Specified by:
      `keySet` in interface `SortedMap<K,V>`
    Overrides:
      `keySet` in class `AbstractMap<K,V>`
    Returns:
      a navigable set view of the keys in this map

  - ### navigableKeySet

    public NavigableSet<K> navigableKeySet()
    Description copied from interface: `ConcurrentNavigableMap`
    Returns a `NavigableSet` view of the keys contained in this map. The set's iterator returns the keys in ascending order. 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 iterators and spliterators are weakly consistent.

    Specified by:
      `navigableKeySet` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `navigableKeySet` in interface `NavigableMap<K,V>`
    Returns:
      a navigable set view of the keys in this map

  - ### values

    public Collection<V> values()
    Returns a `Collection` view of the values contained in this map.

    The collection's iterator returns the values in ascending order of the corresponding keys. The collections's spliterator additionally reports `Spliterator.CONCURRENT`, `Spliterator.NONNULL` and `Spliterator.ORDERED`, with an encounter order that is ascending order of the corresponding keys.

    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 the map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll` and `clear` operations. It does not support the `add` or `addAll` operations.

    The view's iterators and spliterators are weakly consistent.

    Specified by:
      `values` in interface `Map<K,V>`
    Specified by:
      `values` in interface `SortedMap<K,V>`
    Overrides:
      `values` in class `AbstractMap<K,V>`
    Returns:
      a collection view of the values contained in this map

  - ### entrySet

    public Set<Map.Entry<K,V>> entrySet()
    Returns a `Set` view of the mappings contained in this map.

    The set's iterator returns the entries in ascending key order. The set's spliterator additionally reports `Spliterator.CONCURRENT`, `Spliterator.NONNULL`, `Spliterator.SORTED` and `Spliterator.ORDERED`, with an encounter order that is ascending key order.

    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 iterators and spliterators are weakly consistent.

    The `Map.Entry` elements traversed by the `iterator` or `spliterator` do not support the `setValue` operation.

    Specified by:
      `entrySet` in interface `Map<K,V>`
    Specified by:
      `entrySet` in interface `SortedMap<K,V>`
    Returns:
      a set view of the mappings contained in this map, sorted in ascending key order

  - ### descendingMap

    public ConcurrentNavigableMap<K,V> descendingMap()
    Description copied from interface: `ConcurrentNavigableMap`
    Returns a reverse order view of the mappings contained in this map. The descending map is backed by this map, so changes to the map are reflected in the descending map, and vice-versa.

    The returned map has an ordering equivalent to `Collections.reverseOrder``(comparator())`. The expression `m.descendingMap().descendingMap()` returns a view of `m` essentially equivalent to `m`.

    Specified by:
      `descendingMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `descendingMap` in interface `NavigableMap<K,V>`
    Returns:
      a reverse order view of this map

  - ### descendingKeySet

    public NavigableSet<K> descendingKeySet()
    Description copied from interface: `ConcurrentNavigableMap`
    Returns a reverse order `NavigableSet` view of the keys contained in this map. The set's iterator returns the keys in descending order. 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 iterators and spliterators are weakly consistent.

    Specified by:
      `descendingKeySet` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `descendingKeySet` in interface `NavigableMap<K,V>`
    Returns:
      a reverse order navigable set view of the keys in this map

  - ### equals

    public boolean equals(Object o)
    Compares the specified object with this map for equality. Returns `true` if the given object is also a map and the two maps represent the same mappings. More formally, two maps `m1` and `m2` represent the same mappings if `m1.entrySet().equals(m2.entrySet())`. This operation may return misleading results if either map is concurrently modified during execution of this method.
    Specified by:
      `equals` in interface `Map<K,V>`
    Overrides:
      `equals` in class `AbstractMap<K,V>`
    Parameters:
      `o` - object to be compared for equality with this map
    Returns:
      `true` if the specified object is equal to this map
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### putIfAbsent

    public V putIfAbsent(K key, V value)
    If the specified key is not already associated with a value, associates it with the given value. This is equivalent to, for this `map`:

        if (!map.containsKey(key))
          return map.put(key, value);
        else
          return map.get(key);

    except that the action is performed atomically.
    Specified by:
      `putIfAbsent` in interface `ConcurrentMap<K,V>`
    Specified by:
      `putIfAbsent` in interface `Map<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 the specified key, or `null` if there was no mapping for the key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key or value is null

  - ### remove

    public boolean remove(Object key, Object value)
    Removes the entry for a key only if currently mapped to a given value. This is equivalent to, for this `map`:

        if (map.containsKey(key)
            && Objects.equals(map.get(key), value)) {
          map.remove(key);
          return true;
        } else {
          return false;
        }

    except that the action is performed atomically.
    Specified by:
      `remove` in interface `ConcurrentMap<K,V>`
    Specified by:
      `remove` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is associated
      `value` - value expected to be associated with the specified key
    Returns:
      `true` if the value was removed
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### replace

    public boolean replace(K key, V oldValue, V newValue)
    Replaces the entry for a key only if currently mapped to a given value. This is equivalent to, for this `map`:

        if (map.containsKey(key)
            && Objects.equals(map.get(key), oldValue)) {
          map.put(key, newValue);
          return true;
        } else {
          return false;
        }

    except that the action is performed atomically.
    Specified by:
      `replace` in interface `ConcurrentMap<K,V>`
    Specified by:
      `replace` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is associated
      `oldValue` - value expected to be associated with the specified key
      `newValue` - value to be associated with the specified key
    Returns:
      `true` if the value was replaced
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if any of the arguments are null

  - ### replace

    public V replace(K key, V value)
    Replaces the entry for a key only if currently mapped to some value. This is equivalent to, for this `map`:

        if (map.containsKey(key))
          return map.put(key, value);
        else
          return null;

    except that the action is performed atomically.
    Specified by:
      `replace` in interface `ConcurrentMap<K,V>`
    Specified by:
      `replace` in interface `Map<K,V>`
    Parameters:
      `key` - key with which the specified value is associated
      `value` - value to be associated with the specified key
    Returns:
      the previous value associated with the specified key, or `null` if there was no mapping for the key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key or value is null

  - ### comparator

    public Comparator<? super K> comparator()
    Description copied from interface: `SortedMap`
    Returns the comparator used to order the keys in this map, or `null` if this map uses the natural ordering of its keys.
    Specified by:
      `comparator` in interface `SortedMap<K,V>`
    Returns:
      the comparator used to order the keys in this map, or `null` if this map uses the natural ordering of its keys

  - ### firstKey

    public K firstKey()
    Description copied from interface: `SortedMap`
    Returns the first (lowest) key currently in this map.
    Specified by:
      `firstKey` in interface `SortedMap<K,V>`
    Returns:
      the first (lowest) key currently in this map
    Throws:
      `NoSuchElementException` - if this map is empty

  - ### lastKey

    public K lastKey()
    Description copied from interface: `SortedMap`
    Returns the last (highest) key currently in this map.
    Specified by:
      `lastKey` in interface `SortedMap<K,V>`
    Returns:
      the last (highest) key currently in this map
    Throws:
      `NoSuchElementException` - if this map is empty

  - ### putFirst

    public V putFirst(K k, V v)
    Throws `UnsupportedOperationException`. The encounter order induced by this map's comparison method determines the position of mappings, so explicit positioning is not supported.
    Specified by:
      `putFirst` in interface `SequencedMap<K,V>`
    Specified by:
      `putFirst` in interface `SortedMap<K,V>`
    Parameters:
      `k` - the key
      `v` - the value
    Returns:
      the value previously associated with k, or null if none
    Throws:
      `UnsupportedOperationException` - always
    Since:
      21

  - ### putLast

    public V putLast(K k, V v)
    Throws `UnsupportedOperationException`. The encounter order induced by this map's comparison method determines the position of mappings, so explicit positioning is not supported.
    Specified by:
      `putLast` in interface `SequencedMap<K,V>`
    Specified by:
      `putLast` in interface `SortedMap<K,V>`
    Parameters:
      `k` - the key
      `v` - the value
    Returns:
      the value previously associated with k, or null if none
    Throws:
      `UnsupportedOperationException` - always
    Since:
      21

  - ### subMap

    public ConcurrentNavigableMap<K,V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys range from `fromKey` to `toKey`. If `fromKey` and `toKey` are equal, the returned map is empty unless `fromInclusive` and `toInclusive` are both true. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside of its range, or to construct a submap either of whose endpoints lie outside its range.

    Specified by:
      `subMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `subMap` in interface `NavigableMap<K,V>`
    Parameters:
      `fromKey` - low endpoint of the keys in the returned map
      `fromInclusive` - `true` if the low endpoint is to be included in the returned view
      `toKey` - high endpoint of the keys in the returned map
      `toInclusive` - `true` if the high endpoint is to be included in the returned view
    Returns:
      a view of the portion of this map whose keys range from `fromKey` to `toKey`
    Throws:
      `ClassCastException` - if `fromKey` and `toKey` cannot be compared to one another using this map's comparator (or, if the map has no comparator, using natural ordering). Implementations may, but are not required to, throw this exception if `fromKey` or `toKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `fromKey` or `toKey` is null
      `IllegalArgumentException` - if `fromKey` is greater than `toKey`; or if this map itself has a restricted range, and `fromKey` or `toKey` lies outside the bounds of the range

  - ### headMap

    public ConcurrentNavigableMap<K,V> headMap(K toKey, boolean inclusive)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys are less than (or equal to, if `inclusive` is true) `toKey`. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside its range.

    Specified by:
      `headMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `headMap` in interface `NavigableMap<K,V>`
    Parameters:
      `toKey` - high endpoint of the keys in the returned map
      `inclusive` - `true` if the high endpoint is to be included in the returned view
    Returns:
      a view of the portion of this map whose keys are less than (or equal to, if `inclusive` is true) `toKey`
    Throws:
      `ClassCastException` - if `toKey` is not compatible with this map's comparator (or, if the map has no comparator, if `toKey` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `toKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `toKey` is null
      `IllegalArgumentException` - if this map itself has a restricted range, and `toKey` lies outside the bounds of the range

  - ### tailMap

    public ConcurrentNavigableMap<K,V> tailMap(K fromKey, boolean inclusive)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys are greater than (or equal to, if `inclusive` is true) `fromKey`. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside its range.

    Specified by:
      `tailMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `tailMap` in interface `NavigableMap<K,V>`
    Parameters:
      `fromKey` - low endpoint of the keys in the returned map
      `inclusive` - `true` if the low endpoint is to be included in the returned view
    Returns:
      a view of the portion of this map whose keys are greater than (or equal to, if `inclusive` is true) `fromKey`
    Throws:
      `ClassCastException` - if `fromKey` is not compatible with this map's comparator (or, if the map has no comparator, if `fromKey` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `fromKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `fromKey` is null
      `IllegalArgumentException` - if this map itself has a restricted range, and `fromKey` lies outside the bounds of the range

  - ### subMap

    public ConcurrentNavigableMap<K,V> subMap(K fromKey, K toKey)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys range from `fromKey`, inclusive, to `toKey`, exclusive. (If `fromKey` and `toKey` are equal, the returned map is empty.) The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside its range.

    Equivalent to `subMap(fromKey, true, toKey, false)`.

    Specified by:
      `subMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `subMap` in interface `NavigableMap<K,V>`
    Specified by:
      `subMap` in interface `SortedMap<K,V>`
    Parameters:
      `fromKey` - low endpoint (inclusive) of the keys in the returned map
      `toKey` - high endpoint (exclusive) of the keys in the returned map
    Returns:
      a view of the portion of this map whose keys range from `fromKey`, inclusive, to `toKey`, exclusive
    Throws:
      `ClassCastException` - if `fromKey` and `toKey` cannot be compared to one another using this map's comparator (or, if the map has no comparator, using natural ordering). Implementations may, but are not required to, throw this exception if `fromKey` or `toKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `fromKey` or `toKey` is null
      `IllegalArgumentException` - if `fromKey` is greater than `toKey`; or if this map itself has a restricted range, and `fromKey` or `toKey` lies outside the bounds of the range

  - ### headMap

    public ConcurrentNavigableMap<K,V> headMap(K toKey)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys are strictly less than `toKey`. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside its range.

    Equivalent to `headMap(toKey, false)`.

    Specified by:
      `headMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `headMap` in interface `NavigableMap<K,V>`
    Specified by:
      `headMap` in interface `SortedMap<K,V>`
    Parameters:
      `toKey` - high endpoint (exclusive) of the keys in the returned map
    Returns:
      a view of the portion of this map whose keys are strictly less than `toKey`
    Throws:
      `ClassCastException` - if `toKey` is not compatible with this map's comparator (or, if the map has no comparator, if `toKey` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `toKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `toKey` is null
      `IllegalArgumentException` - if this map itself has a restricted range, and `toKey` lies outside the bounds of the range

  - ### tailMap

    public ConcurrentNavigableMap<K,V> tailMap(K fromKey)
    Description copied from interface: `NavigableMap`
    Returns a view of the portion of this map whose keys are greater than or equal to `fromKey`. The returned map is backed by this map, so changes in the returned map are reflected in this map, and vice-versa. The returned map supports all optional map operations that this map supports.

    The returned map will throw an `IllegalArgumentException` on an attempt to insert a key outside its range.

    Equivalent to `tailMap(fromKey, true)`.

    Specified by:
      `tailMap` in interface `ConcurrentNavigableMap<K,V>`
    Specified by:
      `tailMap` in interface `NavigableMap<K,V>`
    Specified by:
      `tailMap` in interface `SortedMap<K,V>`
    Parameters:
      `fromKey` - low endpoint (inclusive) of the keys in the returned map
    Returns:
      a view of the portion of this map whose keys are greater than or equal to `fromKey`
    Throws:
      `ClassCastException` - if `fromKey` is not compatible with this map's comparator (or, if the map has no comparator, if `fromKey` does not implement `Comparable`). Implementations may, but are not required to, throw this exception if `fromKey` cannot be compared to keys currently in the map.
      `NullPointerException` - if `fromKey` is null
      `IllegalArgumentException` - if this map itself has a restricted range, and `fromKey` lies outside the bounds of the range

  - ### lowerEntry

    public Map.Entry<K,V> lowerEntry(K key)
    Returns a key-value mapping associated with the greatest key strictly less than the given key, or `null` if there is no such key. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `lowerEntry` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      an entry with the greatest key less than `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### lowerKey

    public K lowerKey(K key)
    Description copied from interface: `NavigableMap`
    Returns the greatest key strictly less than the given key, or `null` if there is no such key.
    Specified by:
      `lowerKey` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      the greatest key less than `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### floorEntry

    public Map.Entry<K,V> floorEntry(K key)
    Returns a key-value mapping associated with the greatest key less than or equal to the given key, or `null` if there is no such key. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `floorEntry` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      an entry with the greatest key less than or equal to `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### floorKey

    public K floorKey(K key)
    Description copied from interface: `NavigableMap`
    Returns the greatest key less than or equal to the given key, or `null` if there is no such key.
    Specified by:
      `floorKey` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      the greatest key less than or equal to `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### ceilingEntry

    public Map.Entry<K,V> ceilingEntry(K key)
    Returns a key-value mapping associated with the least key greater than or equal to the given key, or `null` if there is no such entry. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `ceilingEntry` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      an entry with the least key greater than or equal to `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### ceilingKey

    public K ceilingKey(K key)
    Description copied from interface: `NavigableMap`
    Returns the least key greater than or equal to the given key, or `null` if there is no such key.
    Specified by:
      `ceilingKey` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      the least key greater than or equal to `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### higherEntry

    public Map.Entry<K,V> higherEntry(K key)
    Returns a key-value mapping associated with the least key strictly greater than the given key, or `null` if there is no such key. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `higherEntry` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      an entry with the least key greater than `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### higherKey

    public K higherKey(K key)
    Description copied from interface: `NavigableMap`
    Returns the least key strictly greater than the given key, or `null` if there is no such key.
    Specified by:
      `higherKey` in interface `NavigableMap<K,V>`
    Parameters:
      `key` - the key
    Returns:
      the least key greater than `key`, or `null` if there is no such key
    Throws:
      `ClassCastException` - if the specified key cannot be compared with the keys currently in the map
      `NullPointerException` - if the specified key is null

  - ### firstEntry

    public Map.Entry<K,V> firstEntry()
    Returns a key-value mapping associated with the least key in this map, or `null` if the map is empty. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `firstEntry` in interface `NavigableMap<K,V>`
    Specified by:
      `firstEntry` in interface `SequencedMap<K,V>`
    Returns:
      an entry with the least key, or `null` if this map is empty

  - ### lastEntry

    public Map.Entry<K,V> lastEntry()
    Returns a key-value mapping associated with the greatest key in this map, or `null` if the map is empty. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `lastEntry` in interface `NavigableMap<K,V>`
    Specified by:
      `lastEntry` in interface `SequencedMap<K,V>`
    Returns:
      an entry with the greatest key, or `null` if this map is empty

  - ### pollFirstEntry

    public Map.Entry<K,V> pollFirstEntry()
    Removes and returns a key-value mapping associated with the least key in this map, or `null` if the map is empty. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `pollFirstEntry` in interface `NavigableMap<K,V>`
    Specified by:
      `pollFirstEntry` in interface `SequencedMap<K,V>`
    Returns:
      the removed first entry of this map, or `null` if this map is empty

  - ### pollLastEntry

    public Map.Entry<K,V> pollLastEntry()
    Removes and returns a key-value mapping associated with the greatest key in this map, or `null` if the map is empty. The returned entry does not support the `Entry.setValue` method.
    Specified by:
      `pollLastEntry` in interface `NavigableMap<K,V>`
    Specified by:
      `pollLastEntry` in interface `SequencedMap<K,V>`
    Returns:
      the removed last entry of this map, or `null` if this map is empty