       1 /*
       2  * Copyright (c) 1997, 2017, Oracle and/or its affiliates. All rights reserved.
       3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
       4  *
       5  * This code is free software; you can redistribute it and/or modify it
       6  * under the terms of the GNU General Public License version 2 only, as
       7  * published by the Free Software Foundation.  Oracle designates this
       8  * particular file as subject to the "Classpath" exception as provided
       9  * by Oracle in the LICENSE file that accompanied this code.
      10  *
      11  * This code is distributed in the hope that it will be useful, but WITHOUT
      12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
      13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      14  * version 2 for more details (a copy is included in the LICENSE file that
      15  * accompanied this code).
      16  *
      17  * You should have received a copy of the GNU General Public License version
      18  * 2 along with this work; if not, write to the Free Software Foundation,
      19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
      20  *
      21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
      22  * or visit www.oracle.com if you need additional information or have any
      23  * questions.
      24  */
      25
      26 package java.util;
      27
      28 /**
      29  * A collection that contains no duplicate elements.  More formally, sets
      30  * contain no pair of elements {@code e1} and {@code e2} such that
      31  * {@code e1.equals(e2)}, and at most one null element.  As implied by
      32  * its name, this interface models the mathematical <i>set</i> abstraction.
      33  *
      34  * <p>The {@code Set} interface places additional stipulations, beyond those
      35  * inherited from the {@code Collection} interface, on the contracts of all
      36  * constructors and on the contracts of the {@code add}, {@code equals} and
      37  * {@code hashCode} methods.  Declarations for other inherited methods are
      38  * also included here for convenience.  (The specifications accompanying these
      39  * declarations have been tailored to the {@code Set} interface, but they do
      40  * not contain any additional stipulations.)
      41  *
      42  * <p>The additional stipulation on constructors is, not surprisingly,
      43  * that all constructors must create a set that contains no duplicate elements
      44  * (as defined above).
      45  *
      46  * <p>Note: Great care must be exercised if mutable objects are used as set
      47  * elements.  The behavior of a set is not specified if the value of an object
      48  * is changed in a manner that affects {@code equals} comparisons while the
      49  * object is an element in the set.  A special case of this prohibition is
      50  * that it is not permissible for a set to contain itself as an element.
      51  *
      52  * <p>Some set implementations have restrictions on the elements that
      53  * they may contain.  For example, some implementations prohibit null elements,
      54  * and some have restrictions on the types of their elements.  Attempting to
      55  * add an ineligible element throws an unchecked exception, typically
      56  * {@code NullPointerException} or {@code ClassCastException}.  Attempting
      57  * to query the presence of an ineligible element may throw an exception,
      58  * or it may simply return false; some implementations will exhibit the former
      59  * behavior and some will exhibit the latter.  More generally, attempting an
      60  * operation on an ineligible element whose completion would not result in
      61  * the insertion of an ineligible element into the set may throw an
      62  * exception or it may succeed, at the option of the implementation.
      63  * Such exceptions are marked as "optional" in the specification for this
      64  * interface.
      65  *
      66  * <h2><a id="immutable">Immutable Set Static Factory Methods</a></h2>
      67  * <p>The {@link Set#of(Object...) Set.of()} static factory methods
      68  * provide a convenient way to create immutable sets. The {@code Set}
      69  * instances created by these methods have the following characteristics:
      70  *
      71  * <ul>
      72  * <li>They are <em>structurally immutable</em>. Elements cannot be added or
      73  * removed. Calling any mutator method will always cause
      74  * {@code UnsupportedOperationException} to be thrown.
      75  * However, if the contained elements are themselves mutable, this may cause the
      76  * Set to behave inconsistently or its contents to appear to change.
      77  * <li>They disallow {@code null} elements. Attempts to create them with
      78  * {@code null} elements result in {@code NullPointerException}.
      79  * <li>They are serializable if all elements are serializable.
      80  * <li>They reject duplicate elements at creation time. Duplicate elements
      81  * passed to a static factory method result in {@code IllegalArgumentException}.
      82  * <li>The iteration order of set elements is unspecified and is subject to change.
      83  * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
      84  * Callers should make no assumptions about the identity of the returned instances.
      85  * Factories are free to create new instances or reuse existing ones. Therefore,
      86  * identity-sensitive operations on these instances (reference equality ({@code ==}),
      87  * identity hash code, and synchronization) are unreliable and should be avoided.
      88  * <li>They are serialized as specified on the
      89  * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
      90  * page.
      91  * </ul>
      92  *
      93  * <p>This interface is a member of the
      94  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
      95  * Java Collections Framework</a>.
      96  *
      97  * @param <E> the type of elements maintained by this set
      98  *
      99  * @author  Josh Bloch
     100  * @author  Neal Gafter
     101  * @see Collection
     102  * @see List
     103  * @see SortedSet
     104  * @see HashSet
     105  * @see TreeSet
     106  * @see AbstractSet
     107  * @see Collections#singleton(java.lang.Object)
     108  * @see Collections#EMPTY_SET
     109  * @since 1.2
     110  */
     111
     112 public interface Set<E> extends Collection<E> {
     113     // Query Operations
     114
     115     /**
     116      * Returns the number of elements in this set (its cardinality).  If this
     117      * set contains more than {@code Integer.MAX_VALUE} elements, returns
     118      * {@code Integer.MAX_VALUE}.
     119      *
     120      * @return the number of elements in this set (its cardinality)
     121      */
     122     int size();
     123
     124     /**
     125      * Returns {@code true} if this set contains no elements.
     126      *
     127      * @return {@code true} if this set contains no elements
     128      */
     129     boolean isEmpty();
     130
     131     /**
     132      * Returns {@code true} if this set contains the specified element.
     133      * More formally, returns {@code true} if and only if this set
     134      * contains an element {@code e} such that
     135      * {@code Objects.equals(o, e)}.
     136      *
     137      * @param o element whose presence in this set is to be tested
     138      * @return {@code true} if this set contains the specified element
     139      * @throws ClassCastException if the type of the specified element
     140      *         is incompatible with this set
     141      * (<a href="Collection.html#optional-restrictions">optional</a>)
     142      * @throws NullPointerException if the specified element is null and this
     143      *         set does not permit null elements
     144      * (<a href="Collection.html#optional-restrictions">optional</a>)
     145      */
     146     boolean contains(Object o);
     147
     148     /**
     149      * Returns an iterator over the elements in this set.  The elements are
     150      * returned in no particular order (unless this set is an instance of some
     151      * class that provides a guarantee).
     152      *
     153      * @return an iterator over the elements in this set
     154      */
     155     Iterator<E> iterator();
     156
     157     /**
     158      * Returns an array containing all of the elements in this set.
     159      * If this set makes any guarantees as to what order its elements
     160      * are returned by its iterator, this method must return the
     161      * elements in the same order.
     162      *
     163      * <p>The returned array will be "safe" in that no references to it
     164      * are maintained by this set.  (In other words, this method must
     165      * allocate a new array even if this set is backed by an array).
     166      * The caller is thus free to modify the returned array.
     167      *
     168      * <p>This method acts as bridge between array-based and collection-based
     169      * APIs.
     170      *
     171      * @return an array containing all the elements in this set
     172      */
     173     Object[] toArray();
     174
     175     /**
     176      * Returns an array containing all of the elements in this set; the
     177      * runtime type of the returned array is that of the specified array.
     178      * If the set fits in the specified array, it is returned therein.
     179      * Otherwise, a new array is allocated with the runtime type of the
     180      * specified array and the size of this set.
     181      *
     182      * <p>If this set fits in the specified array with room to spare
     183      * (i.e., the array has more elements than this set), the element in
     184      * the array immediately following the end of the set is set to
     185      * {@code null}.  (This is useful in determining the length of this
     186      * set <i>only</i> if the caller knows that this set does not contain
     187      * any null elements.)
     188      *
     189      * <p>If this set makes any guarantees as to what order its elements
     190      * are returned by its iterator, this method must return the elements
     191      * in the same order.
     192      *
     193      * <p>Like the {@link #toArray()} method, this method acts as bridge between
     194      * array-based and collection-based APIs.  Further, this method allows
     195      * precise control over the runtime type of the output array, and may,
     196      * under certain circumstances, be used to save allocation costs.
     197      *
     198      * <p>Suppose {@code x} is a set known to contain only strings.
     199      * The following code can be used to dump the set into a newly allocated
     200      * array of {@code String}:
     201      *
     202      * <pre>
     203      *     String[] y = x.toArray(new String[0]);</pre>
     204      *
     205      * Note that {@code toArray(new Object[0])} is identical in function to
     206      * {@code toArray()}.
     207      *
     208      * @param a the array into which the elements of this set are to be
     209      *        stored, if it is big enough; otherwise, a new array of the same
     210      *        runtime type is allocated for this purpose.
     211      * @return an array containing all the elements in this set
     212      * @throws ArrayStoreException if the runtime type of the specified array
     213      *         is not a supertype of the runtime type of every element in this
     214      *         set
     215      * @throws NullPointerException if the specified array is null
     216      */
     217     <T> T[] toArray(T[] a);
     218
     219
     220     // Modification Operations
     221
     222     /**
     223      * Adds the specified element to this set if it is not already present
     224      * (optional operation).  More formally, adds the specified element
     225      * {@code e} to this set if the set contains no element {@code e2}
     226      * such that
     227      * {@code Objects.equals(e, e2)}.
     228      * If this set already contains the element, the call leaves the set
     229      * unchanged and returns {@code false}.  In combination with the
     230      * restriction on constructors, this ensures that sets never contain
     231      * duplicate elements.
     232      *
     233      * <p>The stipulation above does not imply that sets must accept all
     234      * elements; sets may refuse to add any particular element, including
     235      * {@code null}, and throw an exception, as described in the
     236      * specification for {@link Collection#add Collection.add}.
     237      * Individual set implementations should clearly document any
     238      * restrictions on the elements that they may contain.
     239      *
     240      * @param e element to be added to this set
     241      * @return {@code true} if this set did not already contain the specified
     242      *         element
     243      * @throws UnsupportedOperationException if the {@code add} operation
     244      *         is not supported by this set
     245      * @throws ClassCastException if the class of the specified element
     246      *         prevents it from being added to this set
     247      * @throws NullPointerException if the specified element is null and this
     248      *         set does not permit null elements
     249      * @throws IllegalArgumentException if some property of the specified element
     250      *         prevents it from being added to this set
     251      */
     252     boolean add(E e);
     253
     254
     255     /**
     256      * Removes the specified element from this set if it is present
     257      * (optional operation).  More formally, removes an element {@code e}
     258      * such that
     259      * {@code Objects.equals(o, e)}, if
     260      * this set contains such an element.  Returns {@code true} if this set
     261      * contained the element (or equivalently, if this set changed as a
     262      * result of the call).  (This set will not contain the element once the
     263      * call returns.)
     264      *
     265      * @param o object to be removed from this set, if present
     266      * @return {@code true} if this set contained the specified element
     267      * @throws ClassCastException if the type of the specified element
     268      *         is incompatible with this set
     269      * (<a href="Collection.html#optional-restrictions">optional</a>)
     270      * @throws NullPointerException if the specified element is null and this
     271      *         set does not permit null elements
     272      * (<a href="Collection.html#optional-restrictions">optional</a>)
     273      * @throws UnsupportedOperationException if the {@code remove} operation
     274      *         is not supported by this set
     275      */
     276     boolean remove(Object o);
     277
     278
     279     // Bulk Operations
     280
     281     /**
     282      * Returns {@code true} if this set contains all of the elements of the
     283      * specified collection.  If the specified collection is also a set, this
     284      * method returns {@code true} if it is a <i>subset</i> of this set.
     285      *
     286      * @param  c collection to be checked for containment in this set
     287      * @return {@code true} if this set contains all of the elements of the
     288      *         specified collection
     289      * @throws ClassCastException if the types of one or more elements
     290      *         in the specified collection are incompatible with this
     291      *         set
     292      * (<a href="Collection.html#optional-restrictions">optional</a>)
     293      * @throws NullPointerException if the specified collection contains one
     294      *         or more null elements and this set does not permit null
     295      *         elements
     296      * (<a href="Collection.html#optional-restrictions">optional</a>),
     297      *         or if the specified collection is null
     298      * @see    #contains(Object)
     299      */
     300     boolean containsAll(Collection<?> c);
     301
     302     /**
     303      * Adds all of the elements in the specified collection to this set if
     304      * they're not already present (optional operation).  If the specified
     305      * collection is also a set, the {@code addAll} operation effectively
     306      * modifies this set so that its value is the <i>union</i> of the two
     307      * sets.  The behavior of this operation is undefined if the specified
     308      * collection is modified while the operation is in progress.
     309      *
     310      * @param  c collection containing elements to be added to this set
     311      * @return {@code true} if this set changed as a result of the call
     312      *
     313      * @throws UnsupportedOperationException if the {@code addAll} operation
     314      *         is not supported by this set
     315      * @throws ClassCastException if the class of an element of the
     316      *         specified collection prevents it from being added to this set
     317      * @throws NullPointerException if the specified collection contains one
     318      *         or more null elements and this set does not permit null
     319      *         elements, or if the specified collection is null
     320      * @throws IllegalArgumentException if some property of an element of the
     321      *         specified collection prevents it from being added to this set
     322      * @see #add(Object)
     323      */
     324     boolean addAll(Collection<? extends E> c);
     325
     326     /**
     327      * Retains only the elements in this set that are contained in the
     328      * specified collection (optional operation).  In other words, removes
     329      * from this set all of its elements that are not contained in the
     330      * specified collection.  If the specified collection is also a set, this
     331      * operation effectively modifies this set so that its value is the
     332      * <i>intersection</i> of the two sets.
     333      *
     334      * @param  c collection containing elements to be retained in this set
     335      * @return {@code true} if this set changed as a result of the call
     336      * @throws UnsupportedOperationException if the {@code retainAll} operation
     337      *         is not supported by this set
     338      * @throws ClassCastException if the class of an element of this set
     339      *         is incompatible with the specified collection
     340      * (<a href="Collection.html#optional-restrictions">optional</a>)
     341      * @throws NullPointerException if this set contains a null element and the
     342      *         specified collection does not permit null elements
     343      *         (<a href="Collection.html#optional-restrictions">optional</a>),
     344      *         or if the specified collection is null
     345      * @see #remove(Object)
     346      */
     347     boolean retainAll(Collection<?> c);
     348
     349     /**
     350      * Removes from this set all of its elements that are contained in the
     351      * specified collection (optional operation).  If the specified
     352      * collection is also a set, this operation effectively modifies this
     353      * set so that its value is the <i>asymmetric set difference</i> of
     354      * the two sets.
     355      *
     356      * @param  c collection containing elements to be removed from this set
     357      * @return {@code true} if this set changed as a result of the call
     358      * @throws UnsupportedOperationException if the {@code removeAll} operation
     359      *         is not supported by this set
     360      * @throws ClassCastException if the class of an element of this set
     361      *         is incompatible with the specified collection
     362      * (<a href="Collection.html#optional-restrictions">optional</a>)
     363      * @throws NullPointerException if this set contains a null element and the
     364      *         specified collection does not permit null elements
     365      *         (<a href="Collection.html#optional-restrictions">optional</a>),
     366      *         or if the specified collection is null
     367      * @see #remove(Object)
     368      * @see #contains(Object)
     369      */
     370     boolean removeAll(Collection<?> c);
     371
     372     /**
     373      * Removes all of the elements from this set (optional operation).
     374      * The set will be empty after this call returns.
     375      *
     376      * @throws UnsupportedOperationException if the {@code clear} method
     377      *         is not supported by this set
     378      */
     379     void clear();
     380
     381
     382     // Comparison and hashing
     383
     384     /**
     385      * Compares the specified object with this set for equality.  Returns
     386      * {@code true} if the specified object is also a set, the two sets
     387      * have the same size, and every member of the specified set is
     388      * contained in this set (or equivalently, every member of this set is
     389      * contained in the specified set).  This definition ensures that the
     390      * equals method works properly across different implementations of the
     391      * set interface.
     392      *
     393      * @param o object to be compared for equality with this set
     394      * @return {@code true} if the specified object is equal to this set
     395      */
     396     boolean equals(Object o);
     397
     398     /**
     399      * Returns the hash code value for this set.  The hash code of a set is
     400      * defined to be the sum of the hash codes of the elements in the set,
     401      * where the hash code of a {@code null} element is defined to be zero.
     402      * This ensures that {@code s1.equals(s2)} implies that
     403      * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
     404      * and {@code s2}, as required by the general contract of
     405      * {@link Object#hashCode}.
     406      *
     407      * @return the hash code value for this set
     408      * @see Object#equals(Object)
     409      * @see Set#equals(Object)
     410      */
     411     int hashCode();
     412
     413     /**
     414      * Creates a {@code Spliterator} over the elements in this set.
     415      *
     416      * <p>The {@code Spliterator} reports {@link Spliterator#DISTINCT}.
     417      * Implementations should document the reporting of additional
     418      * characteristic values.
     419      *
     420      * @implSpec
     421      * The default implementation creates a
     422      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     423      * from the set's {@code Iterator}.  The spliterator inherits the
     424      * <em>fail-fast</em> properties of the set's iterator.
     425      * <p>
     426      * The created {@code Spliterator} additionally reports
     427      * {@link Spliterator#SIZED}.
     428      *
     429      * @implNote
     430      * The created {@code Spliterator} additionally reports
     431      * {@link Spliterator#SUBSIZED}.
     432      *
     433      * @return a {@code Spliterator} over the elements in this set
     434      * @since 1.8
     435      */
     436     @Override
     437     default Spliterator<E> spliterator() {
     438         return Spliterators.spliterator(this, Spliterator.DISTINCT);
     439     }
     440
     441     /**
     442      * Returns an immutable set containing zero elements.
     443      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     444      *
     445      * @param <E> the {@code Set}'s element type
     446      * @return an empty {@code Set}
     447      *
     448      * @since 9
     449      */
     450     static <E> Set<E> of() {
     451         return ImmutableCollections.Set0.instance();
     452     }
     453
     454     /**
     455      * Returns an immutable set containing one element.
     456      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     457      *
     458      * @param <E> the {@code Set}'s element type
     459      * @param e1 the single element
     460      * @return a {@code Set} containing the specified element
     461      * @throws NullPointerException if the element is {@code null}
     462      *
     463      * @since 9
     464      */
     465     static <E> Set<E> of(E e1) {
     466         return new ImmutableCollections.Set1<>(e1);
     467     }
     468
     469     /**
     470      * Returns an immutable set containing two elements.
     471      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     472      *
     473      * @param <E> the {@code Set}'s element type
     474      * @param e1 the first element
     475      * @param e2 the second element
     476      * @return a {@code Set} containing the specified elements
     477      * @throws IllegalArgumentException if the elements are duplicates
     478      * @throws NullPointerException if an element is {@code null}
     479      *
     480      * @since 9
     481      */
     482     static <E> Set<E> of(E e1, E e2) {
     483         return new ImmutableCollections.Set2<>(e1, e2);
     484     }
     485
     486     /**
     487      * Returns an immutable set containing three elements.
     488      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     489      *
     490      * @param <E> the {@code Set}'s element type
     491      * @param e1 the first element
     492      * @param e2 the second element
     493      * @param e3 the third element
     494      * @return a {@code Set} containing the specified elements
     495      * @throws IllegalArgumentException if there are any duplicate elements
     496      * @throws NullPointerException if an element is {@code null}
     497      *
     498      * @since 9
     499      */
     500     static <E> Set<E> of(E e1, E e2, E e3) {
     501         return new ImmutableCollections.SetN<>(e1, e2, e3);
     502     }
     503
     504     /**
     505      * Returns an immutable set containing four elements.
     506      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     507      *
     508      * @param <E> the {@code Set}'s element type
     509      * @param e1 the first element
     510      * @param e2 the second element
     511      * @param e3 the third element
     512      * @param e4 the fourth element
     513      * @return a {@code Set} containing the specified elements
     514      * @throws IllegalArgumentException if there are any duplicate elements
     515      * @throws NullPointerException if an element is {@code null}
     516      *
     517      * @since 9
     518      */
     519     static <E> Set<E> of(E e1, E e2, E e3, E e4) {
     520         return new ImmutableCollections.SetN<>(e1, e2, e3, e4);
     521     }
     522
     523     /**
     524      * Returns an immutable set containing five elements.
     525      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     526      *
     527      * @param <E> the {@code Set}'s element type
     528      * @param e1 the first element
     529      * @param e2 the second element
     530      * @param e3 the third element
     531      * @param e4 the fourth element
     532      * @param e5 the fifth element
     533      * @return a {@code Set} containing the specified elements
     534      * @throws IllegalArgumentException if there are any duplicate elements
     535      * @throws NullPointerException if an element is {@code null}
     536      *
     537      * @since 9
     538      */
     539     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5) {
     540         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5);
     541     }
     542
     543     /**
     544      * Returns an immutable set containing six elements.
     545      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     546      *
     547      * @param <E> the {@code Set}'s element type
     548      * @param e1 the first element
     549      * @param e2 the second element
     550      * @param e3 the third element
     551      * @param e4 the fourth element
     552      * @param e5 the fifth element
     553      * @param e6 the sixth element
     554      * @return a {@code Set} containing the specified elements
     555      * @throws IllegalArgumentException if there are any duplicate elements
     556      * @throws NullPointerException if an element is {@code null}
     557      *
     558      * @since 9
     559      */
     560     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
     561         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     562                                                e6);
     563     }
     564
     565     /**
     566      * Returns an immutable set containing seven elements.
     567      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     568      *
     569      * @param <E> the {@code Set}'s element type
     570      * @param e1 the first element
     571      * @param e2 the second element
     572      * @param e3 the third element
     573      * @param e4 the fourth element
     574      * @param e5 the fifth element
     575      * @param e6 the sixth element
     576      * @param e7 the seventh element
     577      * @return a {@code Set} containing the specified elements
     578      * @throws IllegalArgumentException if there are any duplicate elements
     579      * @throws NullPointerException if an element is {@code null}
     580      *
     581      * @since 9
     582      */
     583     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
     584         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     585                                                e6, e7);
     586     }
     587
     588     /**
     589      * Returns an immutable set containing eight elements.
     590      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     591      *
     592      * @param <E> the {@code Set}'s element type
     593      * @param e1 the first element
     594      * @param e2 the second element
     595      * @param e3 the third element
     596      * @param e4 the fourth element
     597      * @param e5 the fifth element
     598      * @param e6 the sixth element
     599      * @param e7 the seventh element
     600      * @param e8 the eighth element
     601      * @return a {@code Set} containing the specified elements
     602      * @throws IllegalArgumentException if there are any duplicate elements
     603      * @throws NullPointerException if an element is {@code null}
     604      *
     605      * @since 9
     606      */
     607     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
     608         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     609                                                e6, e7, e8);
     610     }
     611
     612     /**
     613      * Returns an immutable set containing nine elements.
     614      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     615      *
     616      * @param <E> the {@code Set}'s element type
     617      * @param e1 the first element
     618      * @param e2 the second element
     619      * @param e3 the third element
     620      * @param e4 the fourth element
     621      * @param e5 the fifth element
     622      * @param e6 the sixth element
     623      * @param e7 the seventh element
     624      * @param e8 the eighth element
     625      * @param e9 the ninth element
     626      * @return a {@code Set} containing the specified elements
     627      * @throws IllegalArgumentException if there are any duplicate elements
     628      * @throws NullPointerException if an element is {@code null}
     629      *
     630      * @since 9
     631      */
     632     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
     633         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     634                                                e6, e7, e8, e9);
     635     }
     636
     637     /**
     638      * Returns an immutable set containing ten elements.
     639      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     640      *
     641      * @param <E> the {@code Set}'s element type
     642      * @param e1 the first element
     643      * @param e2 the second element
     644      * @param e3 the third element
     645      * @param e4 the fourth element
     646      * @param e5 the fifth element
     647      * @param e6 the sixth element
     648      * @param e7 the seventh element
     649      * @param e8 the eighth element
     650      * @param e9 the ninth element
     651      * @param e10 the tenth element
     652      * @return a {@code Set} containing the specified elements
     653      * @throws IllegalArgumentException if there are any duplicate elements
     654      * @throws NullPointerException if an element is {@code null}
     655      *
     656      * @since 9
     657      */
     658     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) {
     659         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     660                                                e6, e7, e8, e9, e10);
     661     }
     662
     663     /**
     664      * Returns an immutable set containing an arbitrary number of elements.
     665      * See <a href="#immutable">Immutable Set Static Factory Methods</a> for details.
     666      *
     667      * @apiNote
     668      * This method also accepts a single array as an argument. The element type of
     669      * the resulting set will be the component type of the array, and the size of
     670      * the set will be equal to the length of the array. To create a set with
     671      * a single element that is an array, do the following:
     672      *
     673      * <pre>{@code
     674      *     String[] array = ... ;
     675      *     Set<String[]> list = Set.<String[]>of(array);
     676      * }</pre>
     677      *
     678      * This will cause the {@link Set#of(Object) Set.of(E)} method
     679      * to be invoked instead.
     680      *
     681      * @param <E> the {@code Set}'s element type
     682      * @param elements the elements to be contained in the set
     683      * @return a {@code Set} containing the specified elements
     684      * @throws IllegalArgumentException if there are any duplicate elements
     685      * @throws NullPointerException if an element is {@code null} or if the array is {@code null}
     686      *
     687      * @since 9
     688      */
     689     @SafeVarargs
     690     @SuppressWarnings("varargs")
     691     static <E> Set<E> of(E... elements) {
     692         switch (elements.length) { // implicit null check of elements
     693             case 0:
     694                 return ImmutableCollections.Set0.instance();
     695             case 1:
     696                 return new ImmutableCollections.Set1<>(elements[0]);
     697             case 2:
     698                 return new ImmutableCollections.Set2<>(elements[0], elements[1]);
     699             default:
     700                 return new ImmutableCollections.SetN<>(elements);
     701         }
     702     }
     703 }