Module java.base
Package java.util.concurrent

# Class ConcurrentHashMap.KeySetView<K,V>

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

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

Enclosing class:
  ConcurrentHashMap<K,V>

public static 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)`
Indicates whether some other object is "equal to" this one.
`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.
`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 java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

### Methods declared in interface java.util.Collection

`parallelStream, removeIf, stream, toArray`

### Methods declared in interface java.lang.Iterable

`forEach`

### Methods declared in interface java.util.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:
  - `Set.add(Object)`

### hashCode

public int hashCode()
Description copied from class: `Object`
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` 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` 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.

Specified by:
  `hashCode` in interface `Collection<K>`
Specified by:
  `hashCode` in interface `Set<K>`
Overrides:
  `hashCode` in class `Object`
Returns:
  a hash code value for this object.
See Also:
  - `Object.equals(java.lang.Object)`
  - `System.identityHashCode(java.lang.Object)`

### equals

public boolean equals(Object o)
Description copied from class: `Object`
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`.

An equivalence relation partitions the elements it operates on into equivalence classes; all the members of an equivalence class are equal to each other. Members of an equivalence class are substitutable for each other, at least for some purposes.

Specified by:
  `equals` in interface `Collection<K>`
Specified by:
  `equals` in interface `Set<K>`
Overrides:
  `equals` in class `Object`
Parameters:
  `o` - the reference object with which to compare.
Returns:
  `true` if this object is the same as the obj argument; `false` otherwise.
See Also:
  - `Object.hashCode()`
  - `HashMap`

### 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)`