CopyOnWriteArraySet (Java SE 21 & JDK 21)
Description: declaration: module: java.base, package: java.util.concurrent, class: CopyOnWriteArraySet
Keywords: java.util.concurrent.CopyOnWriteArraySet class

Module java.base
Package java.util.concurrent

# Class CopyOnWriteArraySet<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractSet<E>
java.util.concurrent.CopyOnWriteArraySet<E>

Type Parameters:
  `E` - the type of elements held in this set

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

public class CopyOnWriteArraySet<E> extends AbstractSet<E> implements Serializable
A `Set` that uses an internal `CopyOnWriteArrayList` for all of its operations. Thus, it shares the same basic properties:

- It is best suited for applications in which set sizes generally stay small, read-only operations vastly outnumber mutative operations, and you need to prevent interference among threads during traversal.
- It is thread-safe.
- Mutative operations (`add`, `set`, `remove`, etc.) are expensive since they usually entail copying the entire underlying array.
- Iterators do not support the mutative `remove` operation.
- Traversal via iterators is fast and cannot encounter interference from other threads. Iterators rely on unchanging snapshots of the array at the time the iterators were constructed.

Sample Usage. The following code sketch uses a copy-on-write set to maintain a set of Handler objects that perform some action upon state updates.

     class Handler { void handle() { ... } }

     class X {
       private final CopyOnWriteArraySet<Handler> handlers
         = new CopyOnWriteArraySet<>();
       public void addHandler(Handler h) { handlers.add(h); }

       private long internalState;
       private synchronized void changeState() { internalState = ...; }

       public void update() {
         changeState();
         for (Handler handler : handlers)
           handler.handle();
       }
     }

This class is a member of the Java Collections Framework.

Since:
  1.5
See Also:
  - `CopyOnWriteArrayList`
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `CopyOnWriteArraySet()`
  Creates an empty set.
  `CopyOnWriteArraySet(Collection<? extends E> c)`
  Creates a set containing all of the elements of the specified collection.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `boolean`
  `add(E e)`
  Adds the specified element to this set if it is not already present.
  `boolean`
  `addAll(Collection<? extends E> c)`
  Adds all of the elements in the specified collection to this set if they're not already present.
  `void`
  `clear()`
  Removes all of the elements from this set.
  `boolean`
  `contains(Object o)`
  Returns `true` if this set contains the specified element.
  `boolean`
  `containsAll(Collection<?> c)`
  Returns `true` if this set contains all of the elements of the specified collection.
  `boolean`
  `equals(Object o)`
  Compares the specified object with this set for equality.
  `void`
  `forEach(Consumer<? super E> action)`
  Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
  `boolean`
  `isEmpty()`
  Returns `true` if this set contains no elements.
  `Iterator<E>`
  `iterator()`
  Returns an iterator over the elements contained in this set in the order in which these elements were added.
  `boolean`
  `remove(Object o)`
  Removes the specified element from this set if it is present.
  `boolean`
  `removeAll(Collection<?> c)`
  Removes from this set all of its elements that are contained in the specified collection.
  `boolean`
  `removeIf(Predicate<? super E> filter)`
  Removes all of the elements of this collection that satisfy the given predicate.
  `boolean`
  `retainAll(Collection<?> c)`
  Retains only the elements in this set that are contained in the specified collection.
  `int`
  `size()`
  Returns the number of elements in this set.
  `Spliterator<E>`
  `spliterator()`
  Returns a `Spliterator` over the elements in this set in the order in which these elements were added.
  `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.

  ### Methods declared in class java.util.AbstractSet

  `hashCode`

  ### Methods declared in class java.util.AbstractCollection

  `toString`

  ### Methods declared in class java.lang.Object

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

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

  `parallelStream, stream, toArray`

- ## Constructor Details

  - ### CopyOnWriteArraySet

    public CopyOnWriteArraySet()
    Creates an empty set.

  - ### CopyOnWriteArraySet

    public CopyOnWriteArraySet(Collection<? extends E> c)
    Creates a set containing all of the elements of the specified collection.
    Parameters:
      `c` - the collection of elements to initially contain
    Throws:
      `NullPointerException` - if the specified collection is null

- ## Method Details

  - ### size

    public int size()
    Returns the number of elements in this set.
    Specified by:
      `size` in interface `Collection<E>`
    Specified by:
      `size` in interface `Set<E>`
    Returns:
      the number of elements in this set

  - ### isEmpty

    public boolean isEmpty()
    Returns `true` if this set contains no elements.
    Specified by:
      `isEmpty` in interface `Collection<E>`
    Specified by:
      `isEmpty` in interface `Set<E>`
    Overrides:
      `isEmpty` in class `AbstractCollection<E>`
    Returns:
      `true` if this set contains no elements

  - ### contains

    public boolean contains(Object o)
    Returns `true` if this set contains the specified element. More formally, returns `true` if and only if this set contains an element `e` such that `Objects.equals(o, e)`.
    Specified by:
      `contains` in interface `Collection<E>`
    Specified by:
      `contains` in interface `Set<E>`
    Overrides:
      `contains` in class `AbstractCollection<E>`
    Parameters:
      `o` - element whose presence in this set is to be tested
    Returns:
      `true` if this set contains the specified element

  - ### toArray

    public Object[] toArray()
    Returns an array containing all of the elements in this set. If this set 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 set. (In other words, this method must allocate a new array even if this set 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.

    Specified by:
      `toArray` in interface `Collection<E>`
    Specified by:
      `toArray` in interface `Set<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Returns:
      an array containing all the elements in this set

  - ### toArray

    public <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. If the set 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 set.

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

    If this set 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 set known to contain only strings. The following code can be used to dump the set 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()`.
    Specified by:
      `toArray` in interface `Collection<E>`
    Specified by:
      `toArray` in interface `Set<E>`
    Overrides:
      `toArray` in class `AbstractCollection<E>`
    Type Parameters:
      `T` - the component type of the array to contain the collection
    Parameters:
      `a` - the array into which the elements of this set 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 the elements in this set
    Throws:
      `ArrayStoreException` - if the runtime type of the specified array is not a supertype of the runtime type of every element in this set
      `NullPointerException` - if the specified array is null

  - ### clear

    public void clear()
    Removes all of the elements from this set. The set will be empty after this call returns.
    Specified by:
      `clear` in interface `Collection<E>`
    Specified by:
      `clear` in interface `Set<E>`
    Overrides:
      `clear` in class `AbstractCollection<E>`

  - ### remove

    public boolean remove(Object o)
    Removes the specified element from this set if it is present. More formally, removes an element `e` such that `Objects.equals(o, e)`, if this set contains such an element. Returns `true` if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)
    Specified by:
      `remove` in interface `Collection<E>`
    Specified by:
      `remove` in interface `Set<E>`
    Overrides:
      `remove` in class `AbstractCollection<E>`
    Parameters:
      `o` - object to be removed from this set, if present
    Returns:
      `true` if this set contained the specified element

  - ### add

    public boolean add(E e)
    Adds the specified element to this set if it is not already present. More formally, adds the specified element `e` to this set if the set contains no element `e2` such that `Objects.equals(e, e2)`. If this set already contains the element, the call leaves the set unchanged and returns `false`.
    Specified by:
      `add` in interface `Collection<E>`
    Specified by:
      `add` in interface `Set<E>`
    Overrides:
      `add` in class `AbstractCollection<E>`
    Parameters:
      `e` - element to be added to this set
    Returns:
      `true` if this set did not already contain the specified element

  - ### containsAll

    public boolean containsAll(Collection<?> c)
    Returns `true` if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returns `true` if it is a subset of this set.
    Specified by:
      `containsAll` in interface `Collection<E>`
    Specified by:
      `containsAll` in interface `Set<E>`
    Overrides:
      `containsAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection to be checked for containment in this set
    Returns:
      `true` if this set contains all of the elements of the specified collection
    Throws:
      `NullPointerException` - if the specified collection is null
    See Also:
      - `contains(Object)`

  - ### addAll

    public boolean addAll(Collection<? extends E> c)
    Adds all of the elements in the specified collection to this set if they're not already present. If the specified collection is also a set, the `addAll` operation effectively modifies this set so that its value is the union of the two sets. The behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
    Specified by:
      `addAll` in interface `Collection<E>`
    Specified by:
      `addAll` in interface `Set<E>`
    Overrides:
      `addAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be added to this set
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `NullPointerException` - if the specified collection is null
    See Also:
      - `add(Object)`

  - ### removeAll

    public boolean removeAll(Collection<?> c)
    Removes from this set all of its elements that are contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the asymmetric set difference of the two sets.
    Specified by:
      `removeAll` in interface `Collection<E>`
    Specified by:
      `removeAll` in interface `Set<E>`
    Overrides:
      `removeAll` in class `AbstractSet<E>`
    Parameters:
      `c` - collection containing elements to be removed from this set
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `ClassCastException` - if the class of an element of this set is incompatible with the specified collection (optional)
      `NullPointerException` - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
    See Also:
      - `remove(Object)`

  - ### retainAll

    public boolean retainAll(Collection<?> c)
    Retains only the elements in this set that are contained in the specified collection. In other words, removes from this set all of its elements that are not contained in the specified collection. If the specified collection is also a set, this operation effectively modifies this set so that its value is the intersection of the two sets.
    Specified by:
      `retainAll` in interface `Collection<E>`
    Specified by:
      `retainAll` in interface `Set<E>`
    Overrides:
      `retainAll` in class `AbstractCollection<E>`
    Parameters:
      `c` - collection containing elements to be retained in this set
    Returns:
      `true` if this set changed as a result of the call
    Throws:
      `ClassCastException` - if the class of an element of this set is incompatible with the specified collection (optional)
      `NullPointerException` - if this set contains a null element and the specified collection does not permit null elements (optional), or if the specified collection is null
    See Also:
      - `remove(Object)`

  - ### iterator

    public Iterator<E> iterator()
    Returns an iterator over the elements contained in this set in the order in which these elements were added.

    The returned iterator provides a snapshot of the state of the set when the iterator was constructed. No synchronization is needed while traversing the iterator. The iterator does NOT support the `remove` method.

    Specified by:
      `iterator` in interface `Collection<E>`
    Specified by:
      `iterator` in interface `Iterable<E>`
    Specified by:
      `iterator` in interface `Set<E>`
    Specified by:
      `iterator` in class `AbstractCollection<E>`
    Returns:
      an iterator over the elements in this set

  - ### equals

    public boolean equals(Object o)
    Compares the specified object with this set for equality. Returns `true` if the specified object is the same object as this object, or if it is also a `Set` and the elements returned by an iterator over the specified set are the same as the elements returned by an iterator over this set. More formally, the two iterators are considered to return the same elements if they return the same number of elements and for every element `e1` returned by the iterator over the specified set, there is an element `e2` returned by the iterator over this set such that `Objects.equals(e1, e2)`.
    Specified by:
      `equals` in interface `Collection<E>`
    Specified by:
      `equals` in interface `Set<E>`
    Overrides:
      `equals` in class `AbstractSet<E>`
    Parameters:
      `o` - object to be compared for equality with this set
    Returns:
      `true` if the specified object is equal to this set
    See Also:
      - `Object.hashCode()`
      - `HashMap`

  - ### removeIf

    public boolean removeIf(Predicate<? super E> filter)
    Description copied from interface: `Collection`
    Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
    Specified by:
      `removeIf` in interface `Collection<E>`
    Parameters:
      `filter` - a predicate which returns `true` for elements to be removed
    Returns:
      `true` if any elements were removed
    Throws:
      `NullPointerException` - if the specified filter is null

  - ### forEach

    public void forEach(Consumer<? super E> action)
    Description copied from interface: `Iterable`
    Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

    The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

    Specified by:
      `forEach` in interface `Iterable<E>`
    Parameters:
      `action` - The action to be performed for each element
    Throws:
      `NullPointerException` - if the specified action is null

  - ### spliterator

    public Spliterator<E> spliterator()
    Returns a `Spliterator` over the elements in this set in the order in which these elements were added.

    The `Spliterator` reports `Spliterator.IMMUTABLE`, `Spliterator.DISTINCT`, `Spliterator.SIZED`, and `Spliterator.SUBSIZED`.

    The spliterator provides a snapshot of the state of the set when the spliterator was constructed. No synchronization is needed while operating on the spliterator.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    Specified by:
      `spliterator` in interface `Set<E>`
    Returns:
      a `Spliterator` over the elements in this set
    Since:
      1.8