ConcurrentHashMap.KeySetView

public static class ConcurrentHashMap.KeySetView extends Object
implements Collection<K> Serializable 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(V), newKeySet(), newKeySet(int).

Public Method Summary

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 `Set.add(E)` 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 this instance with the specified object and indicates if they are equal.
void	forEach(Consumer<? super K> action)
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 an integer hash code for this object.
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.
final 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.
Spliterator<K>	spliterator() Creates a `Spliterator` over 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 Object[]	toArray() Returns an array containing all of the elements in this collection.
final String	toString() Returns a string representation of this collection.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this `Object`.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of `Class` that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the `wait()` methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the `notify()` or `notifyAll()` method of this object.

From interface java.util.Collection

abstract boolean	add(K e) Ensures that this collection contains the specified element (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this collection (optional operation).
abstract void	clear() Removes all of the elements from this collection (optional operation).
abstract boolean	contains(Object o) Returns `true` if this collection contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns `true` if this collection contains all of the elements in the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this collection for equality.
abstract int	hashCode() Returns the hash code value for this collection.
abstract boolean	isEmpty() Returns `true` if this collection contains no elements.
abstract Iterator<K>	iterator() Returns an iterator over the elements in this collection.
Stream<K>	parallelStream() Returns a possibly parallel `Stream` with this collection as its source.
abstract boolean	remove(Object o) Removes a single instance of the specified element from this collection, if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes all of this collection's elements that are also contained in the specified collection (optional operation).
boolean	removeIf(Predicate<? super E> filter) Removes all of the elements of this collection that satisfy the given predicate.
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this collection that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this collection.
Spliterator<K>	spliterator() Creates a `Spliterator` over the elements in this collection.
Stream<K>	stream() Returns a sequential `Stream` with this collection as its source.
abstract <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.
abstract Object[]	toArray() Returns an array containing all of the elements in this collection.

From interface java.util.Set

abstract boolean	add(K e) Adds the specified element to this set if it is not already present (optional operation).
abstract boolean	addAll(Collection<? extends E> c) Adds all of the elements in the specified collection to this set if they're not already present (optional operation).
abstract void	clear() Removes all of the elements from this set (optional operation).
abstract boolean	contains(Object o) Returns `true` if this set contains the specified element.
abstract boolean	containsAll(Collection<?> c) Returns `true` if this set contains all of the elements of the specified collection.
abstract boolean	equals(Object o) Compares the specified object with this set for equality.
abstract int	hashCode() Returns the hash code value for this set.
abstract boolean	isEmpty() Returns `true` if this set contains no elements.
abstract Iterator<K>	iterator() Returns an iterator over the elements in this set.
abstract boolean	remove(Object o) Removes the specified element from this set if it is present (optional operation).
abstract boolean	removeAll(Collection<?> c) Removes from this set all of its elements that are contained in the specified collection (optional operation).
abstract boolean	retainAll(Collection<?> c) Retains only the elements in this set that are contained in the specified collection (optional operation).
abstract int	size() Returns the number of elements in this set (its cardinality).
Spliterator<K>	spliterator() Creates a `Spliterator` over the elements in this set.
abstract <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.
abstract Object[]	toArray() Returns an array containing all of the elements in this set.

From interface java.lang.Iterable

void	forEach(Consumer<? super T> action) Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
abstract Iterator<K>	iterator() Returns an iterator over elements of type `T`.
Spliterator<K>	spliterator() Creates a `Spliterator` over the elements described by this `Iterable`.

Public Methods

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.

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

public boolean addAll (Collection<? extends K> c)

Adds all of the elements in the specified collection to this set, as if by calling Set.add(E) on each one.

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

public final void clear ()

Removes all of the elements from this view, by removing all the mappings from the map backing this view.

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 (o==null ? e==null : o.equals(e)).

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

public final boolean containsAll (Collection<?> c)

Returns true if this collection contains all of the elements in the specified collection.

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

public boolean equals (Object o)

Compares this instance with the specified object and indicates if they are equal. In order to be equal, o must represent the same object as this instance using a class-specific comparison. The general contract is that this comparison should be reflexive, symmetric, and transitive. Also, no object reference other than null is equal to null.

The default implementation returns true only if this == o. See Writing a correct equals method if you intend implementing your own equals method.

The general contract for the equals and hashCode() methods is that if equals returns true for any two objects, then hashCode() must return the same value for these objects. This means that subclasses of Object usually override either both methods or neither of them.

Parameters

o	the object to compare this instance with.

Returns

true if the specified object is equal to this Object; false otherwise.

public void forEach (Consumer<? super K> action)

Parameters

action

public ConcurrentHashMap<K, V> getMap ()

Returns the map backing this view.

Returns

the map backing this view

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

public int hashCode ()

Returns an integer hash code for this object. By contract, any two objects for which equals(Object) returns true must return the same hash code value. This means that subclasses of Object usually override both methods or neither method.

Note that hash values must not change over time unless information used in equals comparisons also changes.

See Writing a correct hashCode method if you intend implementing your own hashCode method.

Returns

this object's hash code.

public final boolean isEmpty ()

Returns true if this collection contains no elements.

Returns

true if this collection contains no elements

public Iterator<K> iterator ()

Returns an iterator over the elements in this collection.

The returned iterator is weakly consistent.

Returns

an iterator over the keys of the backing map

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.

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

public final boolean removeAll (Collection<?> c)

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.

Parameters

c	collection containing elements to be removed from this collection

Returns

true if this collection changed as a result of the call

public final boolean retainAll (Collection<?> c)

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.

Parameters

c	collection containing elements to be retained in this collection

Returns

true if this collection changed as a result of the call

public final int size ()

Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns

the number of elements in this collection

public Spliterator<K> spliterator ()

Creates a Spliterator over the elements in this collection. Implementations should document characteristic values reported by the spliterator. Such characteristic values are not required to be reported if the spliterator reports Spliterator.SIZED and this collection contains no elements.

The default implementation should be overridden by subclasses that can return a more efficient spliterator. In order to preserve expected laziness behavior for the stream() and parallelStream()} methods, spliterators should either have the characteristic of IMMUTABLE or CONCURRENT, or be late-binding. If none of these is practical, the overriding class should describe the spliterator's documented policy of binding and structural interference, and should override the stream() and parallelStream() methods to create streams using a Supplier of the spliterator, as in:

Stream<E> s = StreamSupport.stream(() -> spliterator(), spliteratorCharacteristics)

These requirements ensure that streams produced by the stream() and parallelStream() methods will reflect the contents of the collection as of initiation of the terminal stream operation.

Returns

a Spliterator over the elements in this collection

public final 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. 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.

Like the toArray() method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a collection known to contain only strings. The following code can be used to dump the collection into a newly allocated array of String:

     String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to toArray().

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

public final Object[] toArray ()

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 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.

This method acts as bridge between array-based and collection-based APIs.

Returns

an array containing all of the elements in this collection

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).

Returns

a string representation of this collection