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Summary: Ctors | Methods | Inherited Methods

# CopyOnWriteArraySet

`public class CopyOnWriteArraySet`
`` `` `extends AbstractSet<E>` `implements Serializable`

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

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();
       }
     }

See also:

- `CopyOnWriteArrayList`

## Summary

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

### Public methods
`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)`
`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)`
`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.
`<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.
`Object[]` | `toArray()` Returns an array containing all of the elements in this set.

### Inherited methods
From class `java.util.AbstractSet`
`boolean` | `equals(Object o)` Compares the specified object with this set for equality.
`int` | `hashCode()` Returns the hash code value for this set.
`boolean` | `removeAll(Collection<?> c)` Removes from this set all of its elements that are contained in the specified collection (optional operation).
From class `java.util.AbstractCollection`
`boolean` | `add(E e)` Ensures that this collection contains the specified element (optional operation). This implementation always throws an `UnsupportedOperationException`.
`boolean` | `addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation). This implementation iterates over the specified collection, and adds each object returned by the iterator to this collection, in turn.
`void` | `clear()` Removes all of the elements from this collection (optional operation). This implementation iterates over this collection, removing each element using the `Iterator.remove` operation.
`boolean` | `contains(Object o)` Returns `true` if this collection contains the specified element. This implementation iterates over the elements in the collection, checking each element in turn for equality with the specified element.
`boolean` | `containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection. This implementation iterates over the specified collection, checking each element returned by the iterator in turn to see if it's contained in this collection.
`boolean` | `isEmpty()` Returns `true` if this collection contains no elements. This implementation returns `size() == 0`.
`abstract Iterator<E>` | `iterator()` Returns an iterator over the elements contained in this collection.
`boolean` | `remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation). This implementation iterates over the collection looking for the specified element.
`boolean` | `removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`boolean` | `retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation). This implementation iterates over this collection, checking each element returned by the iterator in turn to see if it's contained in the specified collection.
`abstract int` | `size()` Returns the number of elements in this collection.
`<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. This implementation returns an array containing all the elements returned by this collection's iterator in the same order, stored in consecutive elements of the array, starting with index `0`.
`Object[]` | `toArray()` Returns an array containing all of the elements in this collection. This implementation returns an array containing all the elements returned by this collection's iterator, in the same order, stored in consecutive elements of the array, starting with index `0`.
`String` | `toString()` Returns a string representation of this collection.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.util.Set`
`abstract boolean` | `add(E 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<E>` | `iterator()` Returns an iterator over the elements in this set.
`static <E> Set<E>` | `of(E e1, E e2, E e3)` Returns an immutable set containing three elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9)` Returns an immutable set containing nine elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5)` Returns an immutable set containing five elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an immutable set containing six elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8)` Returns an immutable set containing eight elements.
`static <E> Set<E>` | `of()` Returns an immutable set containing zero elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9, E e10)` Returns an immutable set containing ten elements.
`static <E> Set<E>` | `of(E e1, E e2)` Returns an immutable set containing two elements.
`static <E> Set<E>` | `of(E... elements)` Returns an immutable set containing an arbitrary number of elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an immutable set containing seven elements.
`static <E> Set<E>` | `of(E e1, E e2, E e3, E e4)` Returns an immutable set containing four elements.
`static <E> Set<E>` | `of(E e1)` Returns an immutable set containing one element.
`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).
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this set.
`abstract Object[]` | `toArray()` Returns an array containing all of 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.
From interface `java.util.Collection`
`abstract boolean` | `add(E 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<E>` | `iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>` | `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).
`default 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.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>` | `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.lang.Iterable`
`default 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<E>` | `iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>` | `spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

## Public constructors

### 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` | `Collection`: the collection of elements to initially contain

Throws
`NullPointerException` | if the specified collection is null

## Public methods

### 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`.

Parameters
`e` | `E`: element to be added to this set

Returns
`boolean` | `true` if this set did not already contain the specified element

### 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.

Parameters
`c` | `Collection`: collection containing elements to be added to this set

Returns
`boolean` | `true` if this set changed as a result of the call

Throws
`NullPointerException` | if the specified collection is null

See also:

- `add(Object)`

### clear

    public void clear ()

Removes all of the elements from this set. The set will be empty after this call returns.

### 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)`.

Parameters
`o` | `Object`: element whose presence in this set is to be tested

Returns
`boolean` | `true` if this set contains 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.

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

Returns
`boolean` | `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)`

### 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 Set#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)`.

Parameters
`o` | `Object`: object to be compared for equality with this set

Returns
`boolean` | `true` if the specified object is equal to this set

### forEach

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

Parameters
`action` | `Consumer`

### isEmpty

    public boolean isEmpty ()

Returns `true` if this set contains no elements.

Returns
`boolean` | `true` if this set contains no elements

### 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.

Returns
`Iterator<E>` | an iterator over the elements in this set

### 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.)

Parameters
`o` | `Object`: object to be removed from this set, if present

Returns
`boolean` | `true` if this set contained the specified element

### 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.

Parameters
`c` | `Collection`: collection containing elements to be removed from this set

Returns
`boolean` | `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)`

### removeIf

    public boolean removeIf (Predicate<? super E> filter)

Parameters
`filter` | `Predicate`

Returns
`boolean` |

### 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.

Parameters
`c` | `Collection`: collection containing elements to be retained in this set

Returns
`boolean` | `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)`

### size

    public int size ()

Returns the number of elements in this set.

Returns
`int` | the number of elements in this set

### 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.

Returns
`Spliterator<E>` | a `Spliterator` over the elements in this set

### toArray

    public 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()`.

Parameters
`a` | `T`: 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
`T[]` | 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

### 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.

Returns
`Object[]` | an array containing all the elements in this set

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