ConcurrentHashMap.KeySetView (Java SE 26 & JDK 26)
Description: declaration: module: java.base, package: java.util.concurrent, class: ConcurrentHashMap, class: KeySetView
Keywords: java.util.concurrent.ConcurrentHashMap.KeySetView class

# Class ConcurrentHashMap.KeySetView<K,V>

java.lang.Object
java.util.concurrent.ConcurrentHashMap.KeySetView<K,V>

Type Parameters:
  `K` - the type of keys
  `V` - the type of values in the backing map

All Implemented Interfaces:
  `Serializable, Iterable<K>, Collection<K>, Set<K>`

Enclosing class:
  `ConcurrentHashMap<K,V>`

public static final class ConcurrentHashMap.KeySetView<K,V> extends Object implements Set<K>, Serializable
A view of a ConcurrentHashMap as a `Set` of keys, in which additions may optionally be enabled by mapping to a common value. This class cannot be directly instantiated. See `keySet()`, `keySet(V)`, `newKeySet()`, `newKeySet(int)`.

Since:
  1.8
See Also:
  - Serialized Form

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(K e)`
  Adds the specified key to this set view by mapping the key to the default mapped value in the backing map, if defined.
  `boolean`
  `addAll(Collection<? extends K> c)`
  Adds all of the elements in the specified collection to this set, as if by calling `add(K)` on each one.
  `final void`
  `clear()`
  Removes all of the elements from this view, by removing all the mappings from the map backing this view.
  `boolean`
  `contains(Object o)`
  Returns `true` if this collection contains the specified element.
  `final boolean`
  `containsAll(Collection<?> c)`
  Returns `true` if this collection contains all of the elements in the specified collection.
  `boolean`
  `equals(Object o)`
  Compares the specified object with this collection for equality.
  `ConcurrentHashMap<K,V>`
  `getMap()`
  Returns the map backing this view.
  `V`
  `getMappedValue()`
  Returns the default mapped value for additions, or `null` if additions are not supported.
  `int`
  `hashCode()`
  Returns the hash code value for this collection.
  `final boolean`
  `isEmpty()`
  Returns `true` if this collection contains no elements.
  `Iterator<K>`
  `iterator()`
  Returns an iterator over the elements in this collection.
  `boolean`
  `remove(Object o)`
  Removes the key from this map view, by removing the key (and its corresponding value) from the backing map.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes all of this collection's elements that are also contained in the specified collection (optional operation).
  `final boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this collection that are contained in the specified collection (optional operation).
  `final int`
  `size()`
  Returns the number of elements in this collection.
  `final Object[]`
  `toArray()`
  Returns an array containing all of the elements in this collection.
  `final <T> T[]`
  `toArray(T[] a)`
  Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
  `final String`
  `toString()`
  Returns a string representation of this collection.

  ### Methods declared in class Object

  `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`
  Modifier and Type
  Method
  Description
  `protected Object`
  `clone()`
  Creates and returns a copy of this object.
  `boolean`
  `equals(Object obj)`
  Indicates whether some other object is "equal to" this one.
  `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`.
  `int`
  `hashCode()`
  Returns a hash code value for 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 Collection

  `parallelStream, removeIf, stream, toArray`
  Modifier and Type
  Method
  Description
  `default Stream<K>`
  `parallelStream()`
  Returns a possibly parallel `Stream` with this collection as its source.
  `default boolean`
  `removeIf(Predicate<? super K> filter)`
  Removes all of the elements of this collection that satisfy the given predicate (optional operation).
  `default Stream<K>`
  `stream()`
  Returns a sequential `Stream` with this collection as its source.
  `default <T> T[]`
  `toArray(IntFunction<T[]> generator)`
  Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

  ### Methods declared in interface Iterable

  `forEach`
  Modifier and Type
  Method
  Description
  `default void`
  `forEach(Consumer<? super K> action)`
  Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.

  ### Methods declared in interface Set

  `clear, containsAll, isEmpty, removeAll, retainAll, size, spliterator, toArray, toArray`
  Modifier and Type
  Method
  Description
  `void`
  `clear()`
  Removes all of the elements from this set (optional operation).
  `boolean`
  `containsAll(Collection<?> c)`
  Returns `true` if this set contains all of the elements of the specified collection.
  `boolean`
  `isEmpty()`
  Returns `true` if this set contains no elements.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes from this set all of its elements that are contained in the specified collection (optional operation).
  `boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this set that are contained in the specified collection (optional operation).
  `int`
  `size()`
  Returns the number of elements in this set (its cardinality).
  `default Spliterator<K>`
  `spliterator()`
  Creates a `Spliterator` over the elements in this set.
  `Object[]`
  `toArray()`
  Returns an array containing all of the elements in this set.
  `<T> T[]`
  `toArray(T[] a)`
  Returns an array containing all of the elements in this set; the runtime type of the returned array is that of the specified array.

- ## Method Details

  - ### getMappedValue

    public V getMappedValue()
    Returns the default mapped value for additions, or `null` if additions are not supported.
    Returns:
      the default mapped value for additions, or `null` if not supported

  - ### contains

    public boolean contains(Object o)
    Returns `true` if this collection contains the specified element. More formally, returns `true` if and only if this collection contains at least one element `e` such that `Objects.equals(o, e)`.
    Specified by:
      `contains` in interface `Collection<K>`
    Specified by:
      `contains` in interface `Set<K>`
    Parameters:
      `o` - element whose presence in this collection is to be tested
    Returns:
      `true` if this collection contains the specified element
    Throws:
      `NullPointerException` - if the specified key is null

  - ### remove

    public boolean remove(Object o)
    Removes the key from this map view, by removing the key (and its corresponding value) from the backing map. This method does nothing if the key is not in the map.
    Specified by:
      `remove` in interface `Collection<K>`
    Specified by:
      `remove` in interface `Set<K>`
    Parameters:
      `o` - the key to be removed from the backing map
    Returns:
      `true` if the backing map contained the specified key
    Throws:
      `NullPointerException` - if the specified key is null

  - ### iterator

    public Iterator<K> iterator()
    Returns an iterator over the elements in this collection.

    The returned iterator is weakly consistent.

    Specified by:
      `iterator` in interface `Collection<K>`
    Specified by:
      `iterator` in interface `Iterable<K>`
    Specified by:
      `iterator` in interface `Set<K>`
    Returns:
      an iterator over the keys of the backing map

  - ### add

    public boolean add(K e)
    Adds the specified key to this set view by mapping the key to the default mapped value in the backing map, if defined.
    Specified by:
      `add` in interface `Collection<K>`
    Specified by:
      `add` in interface `Set<K>`
    Parameters:
      `e` - key to be added
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `NullPointerException` - if the specified key is null
      `UnsupportedOperationException` - if no default mapped value for additions was provided

  - ### addAll

    public boolean addAll(Collection<? extends K> c)
    Adds all of the elements in the specified collection to this set, as if by calling `add(K)` on each one.
    Specified by:
      `addAll` in interface `Collection<K>`
    Specified by:
      `addAll` in interface `Set<K>`
    Parameters:
      `c` - the elements to be inserted into this set
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `NullPointerException` - if the collection or any of its elements are `null`
      `UnsupportedOperationException` - if no default mapped value for additions was provided
    See Also:
      - `Collection.add(Object)`

  - ### hashCode

    public int hashCode()
    Description copied from interface: `Collection`
    Returns the hash code value for this collection. While the `Collection` interface adds no stipulations to the general contract for the `Object.hashCode` method, programmers should take note that any class that overrides the `Object.equals` method must also override the `Object.hashCode` method in order to satisfy the general contract for the `Object.hashCode` method. In particular, `c1.equals(c2)` implies that `c1.hashCode()==c2.hashCode()`.
    Specified by:
      `hashCode` in interface `Collection<K>`
    Specified by:
      `hashCode` in interface `Set<K>`
    Overrides:
      `hashCode` in class `Object`
    Returns:
      the hash code value for this collection
    See Also:
      - `Object.hashCode()`
      - `Object.equals(Object)`

  - ### equals

    public boolean equals(Object o)
    Description copied from interface: `Collection`
    Compares the specified object with this collection for equality.

    While the `Collection` interface adds no stipulations to the general contract for the `Object.equals`, programmers who implement the `Collection` interface "directly" (in other words, create a class that is a `Collection` but is not a `Set` or a `List`) must exercise care if they choose to override the `Object.equals`. It is not necessary to do so, and the simplest course of action is to rely on `Object`'s implementation, but the implementor may wish to implement a "value comparison" in place of the default "reference comparison." (The `List` and `Set` interfaces mandate such value comparisons.)

    The general contract for the `Object.equals` method states that equals must be symmetric (in other words, `a.equals(b)` if and only if `b.equals(a)`). The contracts for `List.equals` and `Set.equals` state that lists are only equal to other lists, and sets to other sets. Thus, a custom `equals` method for a collection class that implements neither the `List` nor `Set` interface must return `false` when this collection is compared to any list or set. (By the same logic, it is not possible to write a class that correctly implements both the `Set` and `List` interfaces.)

    Specified by:
      `equals` in interface `Collection<K>`
    Specified by:
      `equals` in interface `Set<K>`
    Overrides:
      `equals` in class `Object`
    Parameters:
      `o` - object to be compared for equality with this collection
    Returns:
      `true` if the specified object is equal to this collection
    See Also:
      - `Object.equals(Object)`
      - `Set.equals(Object)`
      - `List.equals(Object)`

  - ### getMap

    public ConcurrentHashMap<K,V> getMap()
    Returns the map backing this view.
    Returns:
      the map backing this view

  - ### clear

    public final void clear()
    Removes all of the elements from this view, by removing all the mappings from the map backing this view.
    Specified by:
      `clear` in interface `Collection<K>`

  - ### size

    public final int size()
    Description copied from interface: `Collection`
    Returns the number of elements in this collection. If this collection contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.
    Specified by:
      `size` in interface `Collection<K>`
    Returns:
      the number of elements in this collection

  - ### isEmpty

    public final boolean isEmpty()
    Description copied from interface: `Collection`
    Returns `true` if this collection contains no elements.
    Specified by:
      `isEmpty` in interface `Collection<K>`
    Returns:
      `true` if this collection contains no elements

  - ### toArray

    public final Object[] toArray()
    Description copied from interface: `Collection`
    Returns an array containing all of the elements in this collection. If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order. The returned array's runtime component type is `Object`.

    The returned array will be "safe" in that no references to it are maintained by this collection. (In other words, this method must allocate a new array even if this collection is backed by an array). The caller is thus free to modify the returned array.

    Specified by:
      `toArray` in interface `Collection<K>`
    Returns:
      an array, whose runtime component type is `Object`, containing all of the elements in this collection

  - ### toArray

    public final <T> T[] toArray(T[] a)
    Description copied from interface: `Collection`
    Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array. If the collection fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this collection.

    If this collection fits in the specified array with room to spare (i.e., the array has more elements than this collection), the element in the array immediately following the end of the collection is set to `null`. (This is useful in determining the length of this collection only if the caller knows that this collection does not contain any `null` elements.)

    If this collection makes any guarantees as to what order its elements are returned by its iterator, this method must return the elements in the same order.

    Specified by:
      `toArray` in interface `Collection<K>`
    Type Parameters:
      `T` - the component type of the array to contain the collection
    Parameters:
      `a` - the array into which the elements of this collection are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose.
    Returns:
      an array containing all of the elements in this collection

  - ### toString

    public final String toString()
    Returns a string representation of this collection. The string representation consists of the string representations of the collection's elements in the order they are returned by its iterator, enclosed in square brackets (`"[]"`). Adjacent elements are separated by the characters `", "` (comma and space). Elements are converted to strings as by `String.valueOf(Object)`.
    Overrides:
      `toString` in class `Object`
    Returns:
      a string representation of this collection

  - ### containsAll

    public final boolean containsAll(Collection<?> c)
    Description copied from interface: `Collection`
    Returns `true` if this collection contains all of the elements in the specified collection.
    Specified by:
      `containsAll` in interface `Collection<K>`
    Parameters:
      `c` - collection to be checked for containment in this collection
    Returns:
      `true` if this collection contains all of the elements in the specified collection
    See Also:
      - `Collection.contains(Object)`

  - ### removeAll

    public boolean removeAll(Collection<?> c)
    Description copied from interface: `Collection`
    Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
    Specified by:
      `removeAll` in interface `Collection<K>`
    Parameters:
      `c` - collection containing elements to be removed from this collection
    Returns:
      `true` if this collection changed as a result of the call
    See Also:
      - `Collection.remove(Object)`
      - `Collection.contains(Object)`

  - ### retainAll

    public final boolean retainAll(Collection<?> c)
    Description copied from interface: `Collection`
    Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
    Specified by:
      `retainAll` in interface `Collection<K>`
    Parameters:
      `c` - collection containing elements to be retained in this collection
    Returns:
      `true` if this collection changed as a result of the call
    See Also:
      - `Collection.remove(Object)`
      - `Collection.contains(Object)`