W3cubDocs

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:

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

Methods declared in interface Collection

parallelStream, removeIf, stream, toArray

Methods declared in interface Iterable

forEach

Methods declared in interface Set

clear, containsAll, isEmpty, removeAll, retainAll, size, spliterator, toArray, toArray

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:

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:

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:

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:

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:

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:

© 1993, 2025, Oracle and/or its affiliates. All rights reserved.
Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
Various third party code in OpenJDK is licensed under different licenses (see Debian package).
Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.
https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/ConcurrentHashMap.KeySetView.html