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package java.util;


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. 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.

Author: Josh Bloch,  Neal Gafter
Type parameters:
  • <E> – the type of elements maintained by this set
See Also:
Since:1.2
/**
 * A collection that contains no duplicate elements.  More formally, sets
 * contain no pair of elements {@code e1} and {@code e2} such that
 * {@code e1.equals(e2)}, and at most one null element.  As implied by
 * its name, this interface models the mathematical <i>set</i> abstraction.
 *
 * <p>The {@code Set} interface places additional stipulations, beyond those
 * inherited from the {@code Collection} interface, on the contracts of all
 * constructors and on the contracts of the {@code add}, {@code equals} and
 * {@code hashCode} methods.  Declarations for other inherited methods are
 * also included here for convenience.  (The specifications accompanying these
 * declarations have been tailored to the {@code Set} interface, but they do
 * not contain any additional stipulations.)
 *
 * <p>The additional stipulation on constructors is, not surprisingly,
 * that all constructors must create a set that contains no duplicate elements
 * (as defined above).
 *
 * <p>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 {@code 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.
 *
 * <p>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
 * {@code NullPointerException} or {@code 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.
 *
 * <h2><a id="unmodifiable">Unmodifiable Sets</a></h2>
 * <p>The {@link Set#of(Object...) Set.of} and
 * {@link Set#copyOf Set.copyOf} static factory methods
 * provide a convenient way to create unmodifiable sets. The {@code Set}
 * instances created by these methods have the following characteristics:
 *
 * <ul>
 * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Elements cannot
 * be added or removed. Calling any mutator method on the Set
 * will always cause {@code 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.
 * <li>They disallow {@code null} elements. Attempts to create them with
 * {@code null} elements result in {@code NullPointerException}.
 * <li>They are serializable if all elements are serializable.
 * <li>They reject duplicate elements at creation time. Duplicate elements
 * passed to a static factory method result in {@code IllegalArgumentException}.
 * <li>The iteration order of set elements is unspecified and is subject to change.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * 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 ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 * <li>They are serialized as specified on the
 * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
 * page.
 * </ul>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements maintained by this set
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see Collection
 * @see List
 * @see SortedSet
 * @see HashSet
 * @see TreeSet
 * @see AbstractSet
 * @see Collections#singleton(java.lang.Object)
 * @see Collections#EMPTY_SET
 * @since 1.2
 */

public interface Set<E> extends Collection<E> {
    // Query Operations

    
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)
/**
     * Returns the number of elements in this set (its cardinality).  If this
     * set contains more than {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of elements in this set (its cardinality)
     */
    int size();

    
Returns true if this set contains no elements. 
Returns:true if this set contains no elements
/**
     * Returns {@code true} if this set contains no elements.
     *
     * @return {@code true} if this set contains no elements
     */
    boolean isEmpty();

    
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). 
Params:
  • o – element whose presence in this set is to be tested
Throws:
Returns:true if this set contains the specified element
/**
     * Returns {@code true} if this set contains the specified element.
     * More formally, returns {@code true} if and only if this set
     * contains an element {@code e} such that
     * {@code Objects.equals(o, e)}.
     *
     * @param o element whose presence in this set is to be tested
     * @return {@code true} if this set contains the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this set
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         set does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    boolean contains(Object o);

    
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
/**
     * 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).
     *
     * @return an iterator over the elements in this set
     */
    Iterator<E> iterator();

    
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
/**
     * 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.
     *
     * <p>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.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all the elements in this set
     */
    Object[] toArray();

    
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(). 
Params:
  • 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.
Throws:
Returns:an array containing all the elements in this set
/**
     * 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.
     *
     * <p>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
     * {@code null}.  (This is useful in determining the length of this
     * set <i>only</i> if the caller knows that this set does not contain
     * any null elements.)
     *
     * <p>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.
     *
     * <p>Like the {@link #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.
     *
     * <p>Suppose {@code 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 {@code String}:
     *
     * <pre>
     *     String[] y = x.toArray(new String[0]);</pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param 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.
     * @return 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
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);


    // Modification Operations

    
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. 
Params:
  • e – element to be added to this set
Throws:
Returns:true if this set did not already contain the specified element
/**
     * Adds the specified element to this set if it is not already present
     * (optional operation).  More formally, adds the specified element
     * {@code e} to this set if the set contains no element {@code e2}
     * such that
     * {@code Objects.equals(e, e2)}.
     * If this set already contains the element, the call leaves the set
     * unchanged and returns {@code false}.  In combination with the
     * restriction on constructors, this ensures that sets never contain
     * duplicate elements.
     *
     * <p>The stipulation above does not imply that sets must accept all
     * elements; sets may refuse to add any particular element, including
     * {@code null}, and throw an exception, as described in the
     * specification for {@link Collection#add Collection.add}.
     * Individual set implementations should clearly document any
     * restrictions on the elements that they may contain.
     *
     * @param e element to be added to this set
     * @return {@code true} if this set did not already contain the specified
     *         element
     * @throws UnsupportedOperationException if the {@code add} operation
     *         is not supported by this set
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this set
     * @throws NullPointerException if the specified element is null and this
     *         set does not permit null elements
     * @throws IllegalArgumentException if some property of the specified element
     *         prevents it from being added to this set
     */
    boolean add(E e);


    
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.) 
Params:
  • o – object to be removed from this set, if present
Throws:
Returns:true if this set contained the specified element
/**
     * Removes the specified element from this set if it is present
     * (optional operation).  More formally, removes an element {@code e}
     * such that
     * {@code Objects.equals(o, e)}, if
     * this set contains such an element.  Returns {@code 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.)
     *
     * @param o object to be removed from this set, if present
     * @return {@code true} if this set contained the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this set
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         set does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this set
     */
    boolean remove(Object o);


    // Bulk Operations

    
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.

Params:
  • c – collection to be checked for containment in this set
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:
Returns:true if this set contains all of the elements of the specified collection
/**
     * Returns {@code true} if this set contains all of the elements of the
     * specified collection.  If the specified collection is also a set, this
     * method returns {@code true} if it is a <i>subset</i> of this set.
     *
     * @param  c collection to be checked for containment in this set
     * @return {@code 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
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this set does not permit null
     *         elements
     * (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see    #contains(Object)
     */
    boolean containsAll(Collection<?> 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.

Params:
  • c – collection containing elements to be added to this set
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:
Returns:true if this set changed as a result of the call
/**
     * 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 {@code addAll} operation effectively
     * modifies this set so that its value is the <i>union</i> of the two
     * sets.  The behavior of this operation is undefined if the specified
     * collection is modified while the operation is in progress.
     *
     * @param  c collection containing elements to be added to this set
     * @return {@code true} if this set changed as a result of the call
     *
     * @throws UnsupportedOperationException if the {@code addAll} operation
     *         is not supported by this set
     * @throws ClassCastException if the class of an element of the
     *         specified collection prevents it from being added to this set
     * @throws 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
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this set
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> 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.

Params:
  • c – collection containing elements to be retained in this set
Throws:
See Also:
Returns:true if this set changed as a result of the call
/**
     * 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
     * <i>intersection</i> of the two sets.
     *
     * @param  c collection containing elements to be retained in this set
     * @return {@code true} if this set changed as a result of the call
     * @throws UnsupportedOperationException if the {@code retainAll} operation
     *         is not supported by this set
     * @throws ClassCastException if the class of an element of this set
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this set contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     */
    boolean retainAll(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.

Params:
  • c – collection containing elements to be removed from this set
Throws:
See Also:
Returns:true if this set changed as a result of the call
/**
     * 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 <i>asymmetric set difference</i> of
     * the two sets.
     *
     * @param  c collection containing elements to be removed from this set
     * @return {@code true} if this set changed as a result of the call
     * @throws UnsupportedOperationException if the {@code removeAll} operation
     *         is not supported by this set
     * @throws ClassCastException if the class of an element of this set
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this set contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    
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
/**
     * Removes all of the elements from this set (optional operation).
     * The set will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the {@code clear} method
     *         is not supported by this set
     */
    void clear();


    // Comparison and hashing

    
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. 
Params:
  • o – object to be compared for equality with this set
Returns:true if the specified object is equal to this set
/**
     * Compares the specified object with this set for equality.  Returns
     * {@code 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.
     *
     * @param o object to be compared for equality with this set
     * @return {@code true} if the specified object is equal to this set
     */
    boolean equals(Object o);

    
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. 
See Also:
Returns:the hash code value for this set
/**
     * 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 {@code null} element is defined to be zero.
     * This ensures that {@code s1.equals(s2)} implies that
     * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
     * and {@code s2}, as required by the general contract of
     * {@link Object#hashCode}.
     *
     * @return the hash code value for this set
     * @see Object#equals(Object)
     * @see Set#equals(Object)
     */
    int hashCode();

    
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:a Spliterator over the elements in this set
Since:1.8
/**
     * Creates a {@code Spliterator} over the elements in this set.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#DISTINCT}.
     * Implementations should document the reporting of additional
     * characteristic values.
     *
     * @implSpec
     * The default implementation creates a
     * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     * from the set's {@code Iterator}.  The spliterator inherits the
     * <em>fail-fast</em> properties of the set's iterator.
     * <p>
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SIZED}.
     *
     * @implNote
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SUBSIZED}.
     *
     * @return a {@code Spliterator} over the elements in this set
     * @since 1.8
     */
    @Override
    default Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT);
    }

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

Type parameters:
  • <E> – the Set's element type
Returns:an empty Set
Since:9
/**
     * Returns an unmodifiable set containing zero elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @return an empty {@code Set}
     *
     * @since 9
     */
    static <E> Set<E> of() {
        return ImmutableCollections.emptySet();
    }

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

Params:
  • e1 – the single element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified element
Since:9
/**
     * Returns an unmodifiable set containing one element.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the single element
     * @return a {@code Set} containing the specified element
     * @throws NullPointerException if the element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1) {
        return new ImmutableCollections.Set12<>(e1);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing two elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if the elements are duplicates
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2) {
        return new ImmutableCollections.Set12<>(e1, e2);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing three elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3) {
        return new ImmutableCollections.SetN<>(e1, e2, e3);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing four elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing five elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
  • e6 – the sixth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing six elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                               e6);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
  • e6 – the sixth element
  • e7 – the seventh element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing seven elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                               e6, e7);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
  • e6 – the sixth element
  • e7 – the seventh element
  • e8 – the eighth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing eight elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                               e6, e7, e8);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
  • e6 – the sixth element
  • e7 – the seventh element
  • e8 – the eighth element
  • e9 – the ninth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing nine elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @param e9 the ninth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                               e6, e7, e8, e9);
    }

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

Params:
  • e1 – the first element
  • e2 – the second element
  • e3 – the third element
  • e4 – the fourth element
  • e5 – the fifth element
  • e6 – the sixth element
  • e7 – the seventh element
  • e8 – the eighth element
  • e9 – the ninth element
  • e10 – the tenth element
Type parameters:
  • <E> – the Set's element type
Throws:
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing ten elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @param e1 the first element
     * @param e2 the second element
     * @param e3 the third element
     * @param e4 the fourth element
     * @param e5 the fifth element
     * @param e6 the sixth element
     * @param e7 the seventh element
     * @param e8 the eighth element
     * @param e9 the ninth element
     * @param e10 the tenth element
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null}
     *
     * @since 9
     */
    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) {
        return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
                                               e6, e7, e8, e9, e10);
    }

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

Params:
  • elements – the elements to be contained in the set
Type parameters:
  • <E> – the Set's element type
Throws:
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.
Returns:a Set containing the specified elements
Since:9
/**
     * Returns an unmodifiable set containing an arbitrary number of elements.
     * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @apiNote
     * 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:
     *
     * <pre>{@code
     *     String[] array = ... ;
     *     Set<String[]> list = Set.<String[]>of(array);
     * }</pre>
     *
     * This will cause the {@link Set#of(Object) Set.of(E)} method
     * to be invoked instead.
     *
     * @param <E> the {@code Set}'s element type
     * @param elements the elements to be contained in the set
     * @return a {@code Set} containing the specified elements
     * @throws IllegalArgumentException if there are any duplicate elements
     * @throws NullPointerException if an element is {@code null} or if the array is {@code null}
     *
     * @since 9
     */
    @SafeVarargs
    @SuppressWarnings("varargs")
    static <E> Set<E> of(E... elements) {
        switch (elements.length) { // implicit null check of elements
            case 0:
                return ImmutableCollections.emptySet();
            case 1:
                return new ImmutableCollections.Set12<>(elements[0]);
            case 2:
                return new ImmutableCollections.Set12<>(elements[0], elements[1]);
            default:
                return new ImmutableCollections.SetN<>(elements);
        }
    }

    
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.

Params:
  • coll – a Collection from which elements are drawn, must be non-null
Type parameters:
  • <E> – the Set's element type
Throws:
Implementation Note:
If the given Collection is an unmodifiable Set,
calling copyOf will generally not create a copy.
Returns:a Set containing the elements of the given Collection
Since:10
/**
     * Returns an <a href="#unmodifiable">unmodifiable Set</a> 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.
     *
     * @implNote
     * If the given Collection is an <a href="#unmodifiable">unmodifiable Set</a>,
     * calling copyOf will generally not create a copy.
     *
     * @param <E> the {@code Set}'s element type
     * @param coll a {@code Collection} from which elements are drawn, must be non-null
     * @return a {@code Set} containing the elements of the given {@code Collection}
     * @throws NullPointerException if coll is null, or if it contains any nulls
     * @since 10
     */
    @SuppressWarnings("unchecked")
    static <E> Set<E> copyOf(Collection<? extends E> coll) {
        if (coll instanceof ImmutableCollections.AbstractImmutableSet) {
            return (Set<E>)coll;
        } else {
            return (Set<E>)Set.of(new HashSet<>(coll).toArray());
        }
    }
}