Set

public interface Set implements Collection<E>

Known Indirect Subclasses AbstractSet<E>, ConcurrentHashMap.KeySetView<K, V>, ConcurrentSkipListSet<E>, CopyOnWriteArraySet<E>, EnumSet<E extends Enum<E>>, HashSet<E>, LinkedHashSet<E>, NavigableSet<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.  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 predictable iteration order.  NavigableSet<E> A SortedSet extended with navigation methods reporting closest matches for given search targets.  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.

Immutable Set Static Factory Methods

The ERROR(Set.of()/Set#of(Object...) Set.of()) static factory methods provide a convenient way to create immutable sets. The Set instances created by these methods have the following characteristics:

• They are structurally immutable. Elements cannot be added or removed. Calling any mutator method 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. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones. Therefore, identity-sensitive operations on these instances (reference equality (==), identity hash code, and synchronization) are unreliable and should be avoided.
• 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

Public Method Summary

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.
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).
Spliterator<E>	spliterator() Creates a Spliterator over 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.
abstract Object[]	toArray() Returns an array containing all of the elements in this set.

Inherited Method Summary

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.
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).
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.
Spliterator<E>	spliterator() Creates a Spliterator over the elements in this collection.
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

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.
Spliterator<E>	spliterator() Creates a Spliterator over the elements described by this Iterable.

Public Methods

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	element to be added to this set

Returns

• 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

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 containing elements to be added to this set

Returns

• 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)

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

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	element whose presence in this set is to be tested

Returns

• 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)

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 to be checked for containment in this set

Returns

• 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)

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 to be compared for equality with this set

Returns

• true if the specified object is equal to this set

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

• the hash code value for this set

See Also

• Object.equals(Object)
• equals(Object)

public abstract boolean isEmpty ()

Returns true if this set contains no elements.

Returns

• true if this set contains no elements

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

• an iterator over the elements in this set

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 to be removed from this set, if present

Returns

• 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

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 containing elements to be removed from this set

Returns

• 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

• remove(Object)
• contains(Object)

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 containing elements to be retained in this set

Returns

• 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)

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

• the number of elements in this set (its cardinality)

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.

Returns

• a Spliterator over the elements in this set

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	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

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

• an array containing all the elements in this set