- Type Parameters:
E
- the type of elements maintained by this set
- All Superinterfaces:
Collection<E>
,Iterable<E>
- All Known Subinterfaces:
NavigableSet<E>
,SortedSet<E>
- All Known Implementing Classes:
AbstractSet
,ConcurrentHashMap.KeySetView
,ConcurrentSkipListSet
,CopyOnWriteArraySet
,EnumSet
,HashSet
,JobStateReasons
,LinkedHashSet
,TreeSet
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 withnull
elements result inNullPointerException
. - 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.
- Since:
- 1.2
- See Also:
-
Method Summary
Modifier and TypeMethodDescriptionboolean
Adds the specified element to this set if it is not already present (optional operation).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).void
clear()
Removes all of the elements from this set (optional operation).boolean
Returnstrue
if this set contains the specified element.boolean
containsAll
(Collection<?> c) Returnstrue
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.boolean
Compares the specified object with this set for equality.int
hashCode()
Returns the hash code value for this set.boolean
isEmpty()
Returnstrue
if this set contains no elements.iterator()
Returns an iterator over the elements in this set.static <E> Set<E>
of()
Returns an unmodifiable set containing zero elements.static <E> Set<E>
of
(E e1) Returns an unmodifiable set containing one element.static <E> Set<E>
of
(E... elements) Returns an unmodifiable set containing an arbitrary number of 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) Returns an unmodifiable set containing three 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, 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) Returns an unmodifiable set containing seven 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 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, E e6, E e7, E e8, E e9, E e10) Returns an unmodifiable set containing ten elements.boolean
Removes the specified element from this set if it is present (optional operation).boolean
removeAll
(Collection<?> c) Removes from this set all of its elements that are contained in the specified collection (optional operation).boolean
retainAll
(Collection<?> c) Retains only the elements in this set that are contained in the specified collection (optional operation).int
size()
Returns the number of elements in this set (its cardinality).default Spliterator<E>
Creates aSpliterator
over the elements in this set.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 inherited from interface java.util.Collection
parallelStream, removeIf, stream, toArray
-
Method Details
-
size
int size()Returns the number of elements in this set (its cardinality). If this set contains more thanInteger.MAX_VALUE
elements, returnsInteger.MAX_VALUE
.- Specified by:
size
in interfaceCollection<E>
- Returns:
- the number of elements in this set (its cardinality)
-
isEmpty
boolean isEmpty()Returnstrue
if this set contains no elements.- Specified by:
isEmpty
in interfaceCollection<E>
- Returns:
true
if this set contains no elements
-
contains
Returnstrue
if this set contains the specified element. More formally, returnstrue
if and only if this set contains an elemente
such thatObjects.equals(o, e)
.- Specified by:
contains
in interfaceCollection<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)
-
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). -
toArray
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 interfaceCollection<E>
- Returns:
- an array containing all the elements in this set
-
toArray
<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 ofString
:String[] y = x.toArray(new String[0]);
Note thattoArray(new Object[0])
is identical in function totoArray()
.- Specified by:
toArray
in interfaceCollection<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 setNullPointerException
- if the specified array is null
-
add
Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified elemente
to this set if the set contains no elemente2
such thatObjects.equals(e, e2)
. If this set already contains the element, the call leaves the set unchanged and returnsfalse
. 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 forCollection.add
. Individual set implementations should clearly document any restrictions on the elements that they may contain.- Specified by:
add
in interfaceCollection<E>
- Parameters:
e
- element to be added to this set- Returns:
true
if this set did not already contain the specified element- Throws:
UnsupportedOperationException
- if theadd
operation is not supported by this setClassCastException
- if the class of the specified element prevents it from being added to this setNullPointerException
- if the specified element is null and this set does not permit null elementsIllegalArgumentException
- if some property of the specified element prevents it from being added to this set
-
remove
Removes the specified element from this set if it is present (optional operation). More formally, removes an elemente
such thatObjects.equals(o, e)
, if this set contains such an element. Returnstrue
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 interfaceCollection<E>
- 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 theremove
operation is not supported by this set
-
containsAll
Returnstrue
if this set contains all of the elements of the specified collection. If the specified collection is also a set, this method returnstrue
if it is a subset of this set.- Specified by:
containsAll
in interfaceCollection<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:
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:
-
addAll
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, theaddAll
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 interfaceCollection<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:
UnsupportedOperationException
- if theaddAll
operation is not supported by this setClassCastException
- if the class of an element of the specified collection prevents it from being added to this setNullPointerException
- if the specified collection contains one or more null elements and this set does not permit null elements, or if the specified collection is nullIllegalArgumentException
- if some property of an element of the specified collection prevents it from being added to this set- See Also:
-
retainAll
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.- Specified by:
retainAll
in interfaceCollection<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:
UnsupportedOperationException
- if theretainAll
operation is not supported by this setClassCastException
- 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:
-
removeAll
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.- Specified by:
removeAll
in interfaceCollection<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:
UnsupportedOperationException
- if theremoveAll
operation is not supported by this setClassCastException
- 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:
-
clear
void clear()Removes all of the elements from this set (optional operation). The set will be empty after this call returns.- Specified by:
clear
in interfaceCollection<E>
- Throws:
UnsupportedOperationException
- if theclear
method is not supported by this set
-
equals
Compares the specified object with this set for equality. Returnstrue
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.- Specified by:
equals
in interfaceCollection<E>
- Overrides:
equals
in classObject
- Parameters:
o
- object to be compared for equality with this set- Returns:
true
if the specified object is equal to this set- See Also:
-
hashCode
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 anull
element is defined to be zero. This ensures thats1.equals(s2)
implies thats1.hashCode()==s2.hashCode()
for any two setss1
ands2
, as required by the general contract ofObject.hashCode()
.- Specified by:
hashCode
in interfaceCollection<E>
- Overrides:
hashCode
in classObject
- Returns:
- the hash code value for this set
- See Also:
-
spliterator
Creates aSpliterator
over the elements in this set.The
Spliterator
reportsSpliterator.DISTINCT
. Implementations should document the reporting of additional characteristic values.- Specified by:
spliterator
in interfaceCollection<E>
- Specified by:
spliterator
in interfaceIterable<E>
- 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 reportsSpliterator.SIZED
. - Implementation Note:
- The created
Spliterator
additionally reportsSpliterator.SUBSIZED
. - Returns:
- a
Spliterator
over the elements in this set - Since:
- 1.8
-
of
Returns an unmodifiable set containing zero elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Returns:
- an empty
Set
- Since:
- 9
-
of
Returns an unmodifiable set containing one element. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the single element- Returns:
- a
Set
containing the specified element - Throws:
NullPointerException
- if the element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing two elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if the elements are duplicatesNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing three elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing four elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing five elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing six elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth elemente6
- the sixth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing seven elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth elemente6
- the sixth elemente7
- the seventh element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing eight elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth elemente6
- the sixth elemente7
- the seventh elemente8
- the eighth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing nine elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth elemente6
- the sixth elemente7
- the seventh elemente8
- the eighth elemente9
- the ninth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
Returns an unmodifiable set containing ten elements. See Unmodifiable Sets for details.- Type Parameters:
E
- theSet
's element type- Parameters:
e1
- the first elemente2
- the second elemente3
- the third elemente4
- the fourth elemente5
- the fifth elemente6
- the sixth elemente7
- the seventh elemente8
- the eighth elemente9
- the ninth elemente10
- the tenth element- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
- Since:
- 9
-
of
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:
This will cause theString[] array = ... ; Set<String[]> list = Set.<String[]>of(array);
Set.of(E)
method to be invoked instead. - Type Parameters:
E
- theSet
's element type- Parameters:
elements
- the elements to be contained in the set- Returns:
- a
Set
containing the specified elements - Throws:
IllegalArgumentException
- if there are any duplicate elementsNullPointerException
- if an element isnull
or if the array isnull
- Since:
- 9
-
copyOf
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.
- Type Parameters:
E
- theSet
's element type- Parameters:
coll
- aCollection
from which elements are drawn, must be non-null- Returns:
- a
Set
containing the elements of the givenCollection
- Throws:
NullPointerException
- if coll is null, or if it contains any nulls- Since:
- 10
-