added in API level 24

ConcurrentHashMap.KeySetView

public static class ConcurrentHashMap.KeySetView
extends Object implements Collection<K>, Serializable, Set<K>, Serializable

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


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).

Summary

Public methods

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(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)

Indicates whether some other object is "equal to" this one.

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 a hash code value for the 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 methods

Public methods

add

added in API level 24
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 K: key to be added

Returns
boolean 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

added in API level 24
boolean addAll (Collection<? extends K> c)

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

Parameters
c Collection: the elements to be inserted into this set

Returns
boolean 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

clear

added in API level 24
void clear ()

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

contains

added in API level 24
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 Object: element whose presence in this collection is to be tested

Returns
boolean true if this collection contains the specified element

Throws
NullPointerException if the specified key is null

containsAll

added in API level 24
boolean containsAll (Collection<?> c)

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

Parameters
c Collection: collection to be checked for containment in this collection

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

equals

added in API level 24
boolean equals (Object o)

Indicates whether some other object is "equal to" this one.

The equals method implements an equivalence relation on non-null object references:

  • It is reflexive: for any non-null reference value x, x.equals(x) should return true.
  • It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
  • It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
  • It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
  • For any non-null reference value x, x.equals(null) should return false.

The equals method for class Object implements the most discriminating possible equivalence relation on objects; that is, for any non-null reference values x and y, this method returns true if and only if x and y refer to the same object (x == y has the value true).

Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.

Parameters
o Object: the reference object with which to compare.

Returns
boolean true if this object is the same as the obj argument; false otherwise.

forEach

added in API level 24
void forEach (Consumer<? super K> action)

Parameters
action Consumer

getMap

added in API level 24
ConcurrentHashMap<K, V> getMap ()

Returns the map backing this view.

Returns
ConcurrentHashMap<K, V> the map backing this view

getMappedValue

added in API level 24
V getMappedValue ()

Returns the default mapped value for additions, or null if additions are not supported.

Returns
V the default mapped value for additions, or null if not supported

hashCode

added in API level 24
int hashCode ()

Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

The general contract of hashCode is:

  • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
  • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
  • It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java™ programming language.)

Returns
int a hash code value for this object.

isEmpty

added in API level 24
boolean isEmpty ()

Returns true if this collection contains no elements.

Returns
boolean true if this collection contains no elements

iterator

added in API level 24
Iterator<K> iterator ()

Returns an iterator over the elements in this collection.

The returned iterator is weakly consistent.

Returns
Iterator<K> an iterator over the keys of the backing map

remove

added in API level 24
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 Object: the key to be removed from the backing map

Returns
boolean true if the backing map contained the specified key

Throws
NullPointerException if the specified key is null

removeAll

added in API level 24
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: collection containing elements to be removed from this collection

Returns
boolean true if this collection changed as a result of the call

retainAll

added in API level 24
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: collection containing elements to be retained in this collection

Returns
boolean true if this collection changed as a result of the call

size

added in API level 24
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
int the number of elements in this collection

spliterator

added in API level 24
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 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
Spliterator<K> a Spliterator over the elements in this collection

toArray

added in API level 24
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 T: 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
T[] an array containing all of the elements in this collection

toArray

added in API level 24
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
Object[] an array containing all of the elements in this collection

toString

added in API level 24
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 valueOf(Object).

Returns
String a string representation of this collection