Added in API level 1

Summary: Methods | Inherited Methods

Set

public interface Set
implements Collection<E>

java.util.Set<E>

Known indirect subclasses

AbstractSet<E>, ArraySet<E>, ConcurrentHashMap.KeySetView<K, V>, ConcurrentSkipListSet<E>, CopyOnWriteArraySet<E>, EnumSet<E extends Enum<E>>, HashSet<E>, LinkedHashSet<E>, NavigableSet<E>, SequencedSet<E>, SortedSet<E>, TreeSet<E>

AbstractSet<E>	This class provides a skeletal implementation of the `Set` interface to minimize the effort required to implement this interface.
ArraySet<E>	ArraySet is a generic set data structure that is designed to be more memory efficient than a traditional `HashSet`.
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.
ConcurrentSkipListSet<E>	A scalable concurrent `NavigableSet` implementation based on a `ConcurrentSkipListMap`.
CopyOnWriteArraySet<E>	A `Set` that uses an internal `CopyOnWriteArrayList` for all of its operations.
EnumSet<E extends Enum<E>>	A specialized `Set` implementation for use with enum types.
HashSet<E>	This class implements the `Set` interface, backed by a hash table (actually a `HashMap` instance).
LinkedHashSet<E>	Hash table and linked list implementation of the `Set` interface, with well-defined encounter order.
NavigableSet<E>	A `SortedSet` extended with navigation methods reporting closest matches for given search targets.
SequencedSet<E>	A collection that is both a `SequencedCollection` and a `Set`.
SortedSet<E>	A `Set` that further provides a total ordering on its elements.
TreeSet<E>	A `NavigableSet` implementation based on a `TreeMap`.

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element. As implied by its name, this interface models the mathematical set abstraction.

The Set interface places additional stipulations, beyond those inherited from the Collection interface, on the contracts of all constructors and on the contracts of the add, equals and hashCode methods. Declarations for other inherited methods are also included here for convenience. (The specifications accompanying these declarations have been tailored to the Set interface, but they do not contain any additional stipulations.)

The additional stipulation on constructors is, not surprisingly, that all constructors must create a set that contains no duplicate elements (as defined above).

Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.

Some set implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException. Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the set may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

Unmodifiable Sets

The Set.of and Set.copyOf static factory methods provide a convenient way to create unmodifiable sets. The Set instances created by these methods have the following characteristics:

They are unmodifiable. Elements cannot be added or removed. Calling any mutator method on the Set will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the Set to behave inconsistently or its contents to appear to change.
They disallow null elements. Attempts to create them with null elements result in NullPointerException.
They are serializable if all elements are serializable.
They reject duplicate elements at creation time. Duplicate elements passed to a static factory method result in IllegalArgumentException.
The iteration order of set elements is unspecified and is subject to change.
They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
They are serialized as specified on the Serialized Form page.

This interface is a member of the Java Collections Framework.

See also:

Collection
List
SortedSet
HashSet
TreeSet
AbstractSet
Collections.singleton(java.lang.Object)
Collections.EMPTY_SET

Summary

Public methods
`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.
`static <E> Set<E>`	`copyOf(Collection<? extends E> coll)` Returns an unmodifiable Set containing the elements of the given 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 unmodifiable 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 unmodifiable set containing nine elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5)` Returns an unmodifiable set containing five elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6)` Returns an unmodifiable 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 unmodifiable set containing eight elements.
`static <E> Set<E>`	`of(E... elements)` Returns an unmodifiable set containing an arbitrary number of elements.
`static <E> Set<E>`	`of()` Returns an unmodifiable 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 unmodifiable set containing ten elements.
`static <E> Set<E>`	`of(E e1, E e2)` Returns an unmodifiable set containing two elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4, E e5, E e6, E e7)` Returns an unmodifiable set containing seven elements.
`static <E> Set<E>`	`of(E e1, E e2, E e3, E e4)` Returns an unmodifiable set containing four elements.
`static <E> Set<E>`	`of(E e1)` Returns an unmodifiable 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.

Inherited methods

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.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

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 methods

add

Added in API level 1

public abstract boolean add (E e)

Adds the specified element to this set if it is not already present (optional operation). 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. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.

The stipulation above does not imply that sets must accept all elements; sets may refuse to add any particular element, including null, and throw an exception, as described in the specification for Collection.add. Individual set implementations should clearly document any restrictions on the elements that they may contain.

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

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

Throws
`UnsupportedOperationException`	if the `add` operation is not supported by this set
`ClassCastException`	if the class of the specified element prevents it from being added to this set
`NullPointerException`	if the specified element is null and this set does not permit null elements
`IllegalArgumentException`	if some property of the specified element prevents it from being added to this set

addAll

Added in API level 1

public 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). 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
`UnsupportedOperationException`	if the `addAll` operation is not supported by this set
`ClassCastException`	if the class of an element of the specified collection prevents it from being added to this set
`NullPointerException`	if the specified collection contains one or more null elements and this set does not permit null elements, or if the specified collection is null
`IllegalArgumentException`	if some property of an element of the specified collection prevents it from being added to this set

See also:

add(Object)

clear

Added in API level 1

public abstract void clear ()

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

Throws
`UnsupportedOperationException`	if the `clear` method is not supported by this set

contains

Added in API level 1

public abstract 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

Throws
`ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`NullPointerException`	if the specified element is null and this set does not permit null elements (optional)

containsAll

Added in API level 1

public abstract 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
`ClassCastException`	if the types of one or more elements in the specified collection are incompatible with this set (optional)
`NullPointerException`	if the specified collection contains one or more null elements and this set does not permit null elements (optional), or if the specified collection is null

See also:

contains(Object)

copyOf

Added in API level 31

public static Set<E> copyOf (Collection<? extends E> coll)

Returns an unmodifiable Set containing the elements of the given Collection. The given Collection must not be null, and it must not contain any null elements. If the given Collection contains duplicate elements, an arbitrary element of the duplicates is preserved. If the given Collection is subsequently modified, the returned Set will not reflect such modifications.

Implementation Note:

If the given Collection is an unmodifiable Set, calling copyOf will generally not create a copy.

Parameters
`coll`	`Collection`: a `Collection` from which elements are drawn, must be non-null

Returns
`Set<E>`	a `Set` containing the elements of the given `Collection`

Throws
`NullPointerException`	if coll is null, or if it contains any nulls

equals

Added in API level 1

public abstract boolean equals (Object o)

Compares the specified object with this set for equality. Returns true if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.

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

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

hashCode

Added in API level 1

public abstract int hashCode ()

Returns the hash code value for this set. The hash code of a set is defined to be the sum of the hash codes of the elements in the set, where the hash code of a null element is defined to be zero. This ensures that s1.equals(s2) implies that s1.hashCode()==s2.hashCode() for any two sets s1 and s2, as required by the general contract of Object#hashCode.

Returns
`int`	the hash code value for this set

See also:

isEmpty

Added in API level 1

public abstract boolean isEmpty ()

Returns true if this set contains no elements.

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

iterator

Added in API level 1

public abstract Iterator<E> iterator ()

Returns an iterator over the elements in this set. The elements are returned in no particular order (unless this set is an instance of some class that provides a guarantee).

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

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3)

Returns an unmodifiable set containing three elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4, 
                E e5, 
                E e6, 
                E e7, 
                E e8, 
                E e9)

Returns an unmodifiable set containing nine elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element
`e6`	`E`: the sixth element
`e7`	`E`: the seventh element
`e8`	`E`: the eighth element
`e9`	`E`: the ninth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4, 
                E e5)

Returns an unmodifiable set containing five elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4, 
                E e5, 
                E e6)

Returns an unmodifiable set containing six elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element
`e6`	`E`: the sixth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4, 
                E e5, 
                E e6, 
                E e7, 
                E e8)

Returns an unmodifiable set containing eight elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element
`e6`	`E`: the sixth element
`e7`	`E`: the seventh element
`e8`	`E`: the eighth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E... elements)

Returns an unmodifiable set containing an arbitrary number of elements. See Unmodifiable Sets for details.

API Note:

This method also accepts a single array as an argument. The element type of the resulting set will be the component type of the array, and the size of the set will be equal to the length of the array. To create a set with a single element that is an array, do the following:
```
String[] array = ... ;
     Set<String[]> list = Set.<String[]>of(array);
 
```
This will cause the Set.of(E) method to be invoked instead.

Parameters
`elements`	`E`: the elements to be contained in the set

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null` or if the array is `null`

of

Added in API level 30

public static Set<E> of ()

Returns an unmodifiable set containing zero elements. See Unmodifiable Sets for details.

Returns
`Set<E>`	an empty `Set`

of

Added in API level 30

public static 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 unmodifiable set containing ten elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element
`e6`	`E`: the sixth element
`e7`	`E`: the seventh element
`e8`	`E`: the eighth element
`e9`	`E`: the ninth element
`e10`	`E`: the tenth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2)

Returns an unmodifiable set containing two elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if the elements are duplicates
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4, 
                E e5, 
                E e6, 
                E e7)

Returns an unmodifiable set containing seven elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element
`e5`	`E`: the fifth element
`e6`	`E`: the sixth element
`e7`	`E`: the seventh element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1, 
                E e2, 
                E e3, 
                E e4)

Returns an unmodifiable set containing four elements. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the first element
`e2`	`E`: the second element
`e3`	`E`: the third element
`e4`	`E`: the fourth element

Returns
`Set<E>`	a `Set` containing the specified elements

Throws
`IllegalArgumentException`	if there are any duplicate elements
`NullPointerException`	if an element is `null`

of

Added in API level 30

public static Set<E> of (E e1)

Returns an unmodifiable set containing one element. See Unmodifiable Sets for details.

Parameters
`e1`	`E`: the single element

Returns
`Set<E>`	a `Set` containing the specified element

Throws
`NullPointerException`	if the element is `null`

remove

Added in API level 1

public abstract boolean remove (Object o)

Removes the specified element from this set if it is present (optional operation). 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

Throws
`ClassCastException`	if the type of the specified element is incompatible with this set (optional)
`NullPointerException`	if the specified element is null and this set does not permit null elements (optional)
`UnsupportedOperationException`	if the `remove` operation is not supported by this set

removeAll

Added in API level 1

public abstract boolean removeAll (Collection<?> c)

Removes from this set all of its elements that are contained in the specified collection (optional operation). 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
`UnsupportedOperationException`	if the `removeAll` operation is not supported by this set
`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:

retainAll

Added in API level 1

public abstract boolean retainAll (Collection<?> c)

Retains only the elements in this set that are contained in the specified collection (optional operation). 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
`UnsupportedOperationException`	if the `retainAll` operation is not supported by this set
`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

Added in API level 1

public abstract int size ()

Returns the number of elements in this set (its cardinality). If this set contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns
`int`	the number of elements in this set (its cardinality)

spliterator

Added in API level 24

public Spliterator<E> spliterator ()

Creates a Spliterator over the elements in this set.

The Spliterator reports Spliterator#DISTINCT. Implementations should document the reporting of additional characteristic values.

Implementation Requirements:

The default implementation creates a late-binding spliterator from the set's Iterator. The spliterator inherits the fail-fast properties of the set's iterator.
The created Spliterator additionally reports Spliterator#SIZED.

Implementation Note:

The created Spliterator additionally reports Spliterator#SUBSIZED.

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

toArray

Added in API level 1

public abstract 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

toArray

Added in API level 1

public abstract 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