       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="unmodifiable">Unmodifiable Sets</a></h2>
      67  * <p>The {@link Set#of(Object...) Set.of} and
      68  * {@link Set#copyOf Set.copyOf} static factory methods
      69  * provide a convenient way to create unmodifiable sets. The {@code Set}
      70  * instances created by these methods have the following characteristics:
      71  *
      72  * <ul>
      73  * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Elements cannot
      74  * be added or removed. Calling any mutator method on the Set
      75  * will always cause {@code UnsupportedOperationException} to be thrown.
      76  * However, if the contained elements are themselves mutable, this may cause the
      77  * Set to behave inconsistently or its contents to appear to change.
      78  * <li>They disallow {@code null} elements. Attempts to create them with
      79  * {@code null} elements result in {@code NullPointerException}.
      80  * <li>They are serializable if all elements are serializable.
      81  * <li>They reject duplicate elements at creation time. Duplicate elements
      82  * passed to a static factory method result in {@code IllegalArgumentException}.
      83  * <li>The iteration order of set elements is unspecified and is subject to change.
      84  * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
      85  * Callers should make no assumptions about the identity of the returned instances.
      86  * Factories are free to create new instances or reuse existing ones. Therefore,
      87  * identity-sensitive operations on these instances (reference equality ({@code ==}),
      88  * identity hash code, and synchronization) are unreliable and should be avoided.
      89  * <li>They are serialized as specified on the
      90  * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
      91  * page.
      92  * </ul>
      93  *
      94  * <p>This interface is a member of the
      95  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
      96  * Java Collections Framework</a>.
      97  *
      98  * @param <E> the type of elements maintained by this set
      99  *
     100  * @author  Josh Bloch
     101  * @author  Neal Gafter
     102  * @see Collection
     103  * @see List
     104  * @see SortedSet
     105  * @see HashSet
     106  * @see TreeSet
     107  * @see AbstractSet
     108  * @see Collections#singleton(java.lang.Object)
     109  * @see Collections#EMPTY_SET
     110  * @since 1.2
     111  */
     112
     113 public interface Set<E> extends Collection<E> {
     114     // Query Operations
     115
     116     /**
     117      * Returns the number of elements in this set (its cardinality).  If this
     118      * set contains more than {@code Integer.MAX_VALUE} elements, returns
     119      * {@code Integer.MAX_VALUE}.
     120      *
     121      * @return the number of elements in this set (its cardinality)
     122      */
     123     int size();
     124
     125     /**
     126      * Returns {@code true} if this set contains no elements.
     127      *
     128      * @return {@code true} if this set contains no elements
     129      */
     130     boolean isEmpty();
     131
     132     /**
     133      * Returns {@code true} if this set contains the specified element.
     134      * More formally, returns {@code true} if and only if this set
     135      * contains an element {@code e} such that
     136      * {@code Objects.equals(o, e)}.
     137      *
     138      * @param o element whose presence in this set is to be tested
     139      * @return {@code true} if this set contains the specified element
     140      * @throws ClassCastException if the type of the specified element
     141      *         is incompatible with this set
     142      * (<a href="Collection.html#optional-restrictions">optional</a>)
     143      * @throws NullPointerException if the specified element is null and this
     144      *         set does not permit null elements
     145      * (<a href="Collection.html#optional-restrictions">optional</a>)
     146      */
     147     boolean contains(Object o);
     148
     149     /**
     150      * Returns an iterator over the elements in this set.  The elements are
     151      * returned in no particular order (unless this set is an instance of some
     152      * class that provides a guarantee).
     153      *
     154      * @return an iterator over the elements in this set
     155      */
     156     Iterator<E> iterator();
     157
     158     /**
     159      * Returns an array containing all of the elements in this set.
     160      * If this set makes any guarantees as to what order its elements
     161      * are returned by its iterator, this method must return the
     162      * elements in the same order.
     163      *
     164      * <p>The returned array will be "safe" in that no references to it
     165      * are maintained by this set.  (In other words, this method must
     166      * allocate a new array even if this set is backed by an array).
     167      * The caller is thus free to modify the returned array.
     168      *
     169      * <p>This method acts as bridge between array-based and collection-based
     170      * APIs.
     171      *
     172      * @return an array containing all the elements in this set
     173      */
     174     Object[] toArray();
     175
     176     /**
     177      * Returns an array containing all of the elements in this set; the
     178      * runtime type of the returned array is that of the specified array.
     179      * If the set fits in the specified array, it is returned therein.
     180      * Otherwise, a new array is allocated with the runtime type of the
     181      * specified array and the size of this set.
     182      *
     183      * <p>If this set fits in the specified array with room to spare
     184      * (i.e., the array has more elements than this set), the element in
     185      * the array immediately following the end of the set is set to
     186      * {@code null}.  (This is useful in determining the length of this
     187      * set <i>only</i> if the caller knows that this set does not contain
     188      * any null elements.)
     189      *
     190      * <p>If this set makes any guarantees as to what order its elements
     191      * are returned by its iterator, this method must return the elements
     192      * in the same order.
     193      *
     194      * <p>Like the {@link #toArray()} method, this method acts as bridge between
     195      * array-based and collection-based APIs.  Further, this method allows
     196      * precise control over the runtime type of the output array, and may,
     197      * under certain circumstances, be used to save allocation costs.
     198      *
     199      * <p>Suppose {@code x} is a set known to contain only strings.
     200      * The following code can be used to dump the set into a newly allocated
     201      * array of {@code String}:
     202      *
     203      * <pre>
     204      *     String[] y = x.toArray(new String[0]);</pre>
     205      *
     206      * Note that {@code toArray(new Object[0])} is identical in function to
     207      * {@code toArray()}.
     208      *
     209      * @param a the array into which the elements of this set are to be
     210      *        stored, if it is big enough; otherwise, a new array of the same
     211      *        runtime type is allocated for this purpose.
     212      * @return an array containing all the elements in this set
     213      * @throws ArrayStoreException if the runtime type of the specified array
     214      *         is not a supertype of the runtime type of every element in this
     215      *         set
     216      * @throws NullPointerException if the specified array is null
     217      */
     218     <T> T[] toArray(T[] a);
     219
     220
     221     // Modification Operations
     222
     223     /**
     224      * Adds the specified element to this set if it is not already present
     225      * (optional operation).  More formally, adds the specified element
     226      * {@code e} to this set if the set contains no element {@code e2}
     227      * such that
     228      * {@code Objects.equals(e, e2)}.
     229      * If this set already contains the element, the call leaves the set
     230      * unchanged and returns {@code false}.  In combination with the
     231      * restriction on constructors, this ensures that sets never contain
     232      * duplicate elements.
     233      *
     234      * <p>The stipulation above does not imply that sets must accept all
     235      * elements; sets may refuse to add any particular element, including
     236      * {@code null}, and throw an exception, as described in the
     237      * specification for {@link Collection#add Collection.add}.
     238      * Individual set implementations should clearly document any
     239      * restrictions on the elements that they may contain.
     240      *
     241      * @param e element to be added to this set
     242      * @return {@code true} if this set did not already contain the specified
     243      *         element
     244      * @throws UnsupportedOperationException if the {@code add} operation
     245      *         is not supported by this set
     246      * @throws ClassCastException if the class of the specified element
     247      *         prevents it from being added to this set
     248      * @throws NullPointerException if the specified element is null and this
     249      *         set does not permit null elements
     250      * @throws IllegalArgumentException if some property of the specified element
     251      *         prevents it from being added to this set
     252      */
     253     boolean add(E e);
     254
     255
     256     /**
     257      * Removes the specified element from this set if it is present
     258      * (optional operation).  More formally, removes an element {@code e}
     259      * such that
     260      * {@code Objects.equals(o, e)}, if
     261      * this set contains such an element.  Returns {@code true} if this set
     262      * contained the element (or equivalently, if this set changed as a
     263      * result of the call).  (This set will not contain the element once the
     264      * call returns.)
     265      *
     266      * @param o object to be removed from this set, if present
     267      * @return {@code true} if this set contained the specified element
     268      * @throws ClassCastException if the type of the specified element
     269      *         is incompatible with this set
     270      * (<a href="Collection.html#optional-restrictions">optional</a>)
     271      * @throws NullPointerException if the specified element is null and this
     272      *         set does not permit null elements
     273      * (<a href="Collection.html#optional-restrictions">optional</a>)
     274      * @throws UnsupportedOperationException if the {@code remove} operation
     275      *         is not supported by this set
     276      */
     277     boolean remove(Object o);
     278
     279
     280     // Bulk Operations
     281
     282     /**
     283      * Returns {@code true} if this set contains all of the elements of the
     284      * specified collection.  If the specified collection is also a set, this
     285      * method returns {@code true} if it is a <i>subset</i> of this set.
     286      *
     287      * @param  c collection to be checked for containment in this set
     288      * @return {@code true} if this set contains all of the elements of the
     289      *         specified collection
     290      * @throws ClassCastException if the types of one or more elements
     291      *         in the specified collection are incompatible with this
     292      *         set
     293      * (<a href="Collection.html#optional-restrictions">optional</a>)
     294      * @throws NullPointerException if the specified collection contains one
     295      *         or more null elements and this set does not permit null
     296      *         elements
     297      * (<a href="Collection.html#optional-restrictions">optional</a>),
     298      *         or if the specified collection is null
     299      * @see    #contains(Object)
     300      */
     301     boolean containsAll(Collection<?> c);
     302
     303     /**
     304      * Adds all of the elements in the specified collection to this set if
     305      * they're not already present (optional operation).  If the specified
     306      * collection is also a set, the {@code addAll} operation effectively
     307      * modifies this set so that its value is the <i>union</i> of the two
     308      * sets.  The behavior of this operation is undefined if the specified
     309      * collection is modified while the operation is in progress.
     310      *
     311      * @param  c collection containing elements to be added to this set
     312      * @return {@code true} if this set changed as a result of the call
     313      *
     314      * @throws UnsupportedOperationException if the {@code addAll} operation
     315      *         is not supported by this set
     316      * @throws ClassCastException if the class of an element of the
     317      *         specified collection prevents it from being added to this set
     318      * @throws NullPointerException if the specified collection contains one
     319      *         or more null elements and this set does not permit null
     320      *         elements, or if the specified collection is null
     321      * @throws IllegalArgumentException if some property of an element of the
     322      *         specified collection prevents it from being added to this set
     323      * @see #add(Object)
     324      */
     325     boolean addAll(Collection<? extends E> c);
     326
     327     /**
     328      * Retains only the elements in this set that are contained in the
     329      * specified collection (optional operation).  In other words, removes
     330      * from this set all of its elements that are not contained in the
     331      * specified collection.  If the specified collection is also a set, this
     332      * operation effectively modifies this set so that its value is the
     333      * <i>intersection</i> of the two sets.
     334      *
     335      * @param  c collection containing elements to be retained in this set
     336      * @return {@code true} if this set changed as a result of the call
     337      * @throws UnsupportedOperationException if the {@code retainAll} operation
     338      *         is not supported by this set
     339      * @throws ClassCastException if the class of an element of this set
     340      *         is incompatible with the specified collection
     341      * (<a href="Collection.html#optional-restrictions">optional</a>)
     342      * @throws NullPointerException if this set contains a null element and the
     343      *         specified collection does not permit null elements
     344      *         (<a href="Collection.html#optional-restrictions">optional</a>),
     345      *         or if the specified collection is null
     346      * @see #remove(Object)
     347      */
     348     boolean retainAll(Collection<?> c);
     349
     350     /**
     351      * Removes from this set all of its elements that are contained in the
     352      * specified collection (optional operation).  If the specified
     353      * collection is also a set, this operation effectively modifies this
     354      * set so that its value is the <i>asymmetric set difference</i> of
     355      * the two sets.
     356      *
     357      * @param  c collection containing elements to be removed from this set
     358      * @return {@code true} if this set changed as a result of the call
     359      * @throws UnsupportedOperationException if the {@code removeAll} operation
     360      *         is not supported by this set
     361      * @throws ClassCastException if the class of an element of this set
     362      *         is incompatible with the specified collection
     363      * (<a href="Collection.html#optional-restrictions">optional</a>)
     364      * @throws NullPointerException if this set contains a null element and the
     365      *         specified collection does not permit null elements
     366      *         (<a href="Collection.html#optional-restrictions">optional</a>),
     367      *         or if the specified collection is null
     368      * @see #remove(Object)
     369      * @see #contains(Object)
     370      */
     371     boolean removeAll(Collection<?> c);
     372
     373     /**
     374      * Removes all of the elements from this set (optional operation).
     375      * The set will be empty after this call returns.
     376      *
     377      * @throws UnsupportedOperationException if the {@code clear} method
     378      *         is not supported by this set
     379      */
     380     void clear();
     381
     382
     383     // Comparison and hashing
     384
     385     /**
     386      * Compares the specified object with this set for equality.  Returns
     387      * {@code true} if the specified object is also a set, the two sets
     388      * have the same size, and every member of the specified set is
     389      * contained in this set (or equivalently, every member of this set is
     390      * contained in the specified set).  This definition ensures that the
     391      * equals method works properly across different implementations of the
     392      * set interface.
     393      *
     394      * @param o object to be compared for equality with this set
     395      * @return {@code true} if the specified object is equal to this set
     396      */
     397     boolean equals(Object o);
     398
     399     /**
     400      * Returns the hash code value for this set.  The hash code of a set is
     401      * defined to be the sum of the hash codes of the elements in the set,
     402      * where the hash code of a {@code null} element is defined to be zero.
     403      * This ensures that {@code s1.equals(s2)} implies that
     404      * {@code s1.hashCode()==s2.hashCode()} for any two sets {@code s1}
     405      * and {@code s2}, as required by the general contract of
     406      * {@link Object#hashCode}.
     407      *
     408      * @return the hash code value for this set
     409      * @see Object#equals(Object)
     410      * @see Set#equals(Object)
     411      */
     412     int hashCode();
     413
     414     /**
     415      * Creates a {@code Spliterator} over the elements in this set.
     416      *
     417      * <p>The {@code Spliterator} reports {@link Spliterator#DISTINCT}.
     418      * Implementations should document the reporting of additional
     419      * characteristic values.
     420      *
     421      * @implSpec
     422      * The default implementation creates a
     423      * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     424      * from the set's {@code Iterator}.  The spliterator inherits the
     425      * <em>fail-fast</em> properties of the set's iterator.
     426      * <p>
     427      * The created {@code Spliterator} additionally reports
     428      * {@link Spliterator#SIZED}.
     429      *
     430      * @implNote
     431      * The created {@code Spliterator} additionally reports
     432      * {@link Spliterator#SUBSIZED}.
     433      *
     434      * @return a {@code Spliterator} over the elements in this set
     435      * @since 1.8
     436      */
     437     @Override
     438     default Spliterator<E> spliterator() {
     439         return Spliterators.spliterator(this, Spliterator.DISTINCT);
     440     }
     441
     442     /**
     443      * Returns an unmodifiable set containing zero elements.
     444      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     445      *
     446      * @param <E> the {@code Set}'s element type
     447      * @return an empty {@code Set}
     448      *
     449      * @since 9
     450      */
     451     static <E> Set<E> of() {
     452         return ImmutableCollections.Set0.instance();
     453     }
     454
     455     /**
     456      * Returns an unmodifiable set containing one element.
     457      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     458      *
     459      * @param <E> the {@code Set}'s element type
     460      * @param e1 the single element
     461      * @return a {@code Set} containing the specified element
     462      * @throws NullPointerException if the element is {@code null}
     463      *
     464      * @since 9
     465      */
     466     static <E> Set<E> of(E e1) {
     467         return new ImmutableCollections.Set1<>(e1);
     468     }
     469
     470     /**
     471      * Returns an unmodifiable set containing two elements.
     472      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     473      *
     474      * @param <E> the {@code Set}'s element type
     475      * @param e1 the first element
     476      * @param e2 the second element
     477      * @return a {@code Set} containing the specified elements
     478      * @throws IllegalArgumentException if the elements are duplicates
     479      * @throws NullPointerException if an element is {@code null}
     480      *
     481      * @since 9
     482      */
     483     static <E> Set<E> of(E e1, E e2) {
     484         return new ImmutableCollections.Set2<>(e1, e2);
     485     }
     486
     487     /**
     488      * Returns an unmodifiable set containing three elements.
     489      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     490      *
     491      * @param <E> the {@code Set}'s element type
     492      * @param e1 the first element
     493      * @param e2 the second element
     494      * @param e3 the third element
     495      * @return a {@code Set} containing the specified elements
     496      * @throws IllegalArgumentException if there are any duplicate elements
     497      * @throws NullPointerException if an element is {@code null}
     498      *
     499      * @since 9
     500      */
     501     static <E> Set<E> of(E e1, E e2, E e3) {
     502         return new ImmutableCollections.SetN<>(e1, e2, e3);
     503     }
     504
     505     /**
     506      * Returns an unmodifiable set containing four elements.
     507      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     508      *
     509      * @param <E> the {@code Set}'s element type
     510      * @param e1 the first element
     511      * @param e2 the second element
     512      * @param e3 the third element
     513      * @param e4 the fourth element
     514      * @return a {@code Set} containing the specified elements
     515      * @throws IllegalArgumentException if there are any duplicate elements
     516      * @throws NullPointerException if an element is {@code null}
     517      *
     518      * @since 9
     519      */
     520     static <E> Set<E> of(E e1, E e2, E e3, E e4) {
     521         return new ImmutableCollections.SetN<>(e1, e2, e3, e4);
     522     }
     523
     524     /**
     525      * Returns an unmodifiable set containing five elements.
     526      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     527      *
     528      * @param <E> the {@code Set}'s element type
     529      * @param e1 the first element
     530      * @param e2 the second element
     531      * @param e3 the third element
     532      * @param e4 the fourth element
     533      * @param e5 the fifth element
     534      * @return a {@code Set} containing the specified elements
     535      * @throws IllegalArgumentException if there are any duplicate elements
     536      * @throws NullPointerException if an element is {@code null}
     537      *
     538      * @since 9
     539      */
     540     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5) {
     541         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5);
     542     }
     543
     544     /**
     545      * Returns an unmodifiable set containing six elements.
     546      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     547      *
     548      * @param <E> the {@code Set}'s element type
     549      * @param e1 the first element
     550      * @param e2 the second element
     551      * @param e3 the third element
     552      * @param e4 the fourth element
     553      * @param e5 the fifth element
     554      * @param e6 the sixth element
     555      * @return a {@code Set} containing the specified elements
     556      * @throws IllegalArgumentException if there are any duplicate elements
     557      * @throws NullPointerException if an element is {@code null}
     558      *
     559      * @since 9
     560      */
     561     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6) {
     562         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     563                                                e6);
     564     }
     565
     566     /**
     567      * Returns an unmodifiable set containing seven elements.
     568      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     569      *
     570      * @param <E> the {@code Set}'s element type
     571      * @param e1 the first element
     572      * @param e2 the second element
     573      * @param e3 the third element
     574      * @param e4 the fourth element
     575      * @param e5 the fifth element
     576      * @param e6 the sixth element
     577      * @param e7 the seventh element
     578      * @return a {@code Set} containing the specified elements
     579      * @throws IllegalArgumentException if there are any duplicate elements
     580      * @throws NullPointerException if an element is {@code null}
     581      *
     582      * @since 9
     583      */
     584     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7) {
     585         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     586                                                e6, e7);
     587     }
     588
     589     /**
     590      * Returns an unmodifiable set containing eight elements.
     591      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     592      *
     593      * @param <E> the {@code Set}'s element type
     594      * @param e1 the first element
     595      * @param e2 the second element
     596      * @param e3 the third element
     597      * @param e4 the fourth element
     598      * @param e5 the fifth element
     599      * @param e6 the sixth element
     600      * @param e7 the seventh element
     601      * @param e8 the eighth element
     602      * @return a {@code Set} containing the specified elements
     603      * @throws IllegalArgumentException if there are any duplicate elements
     604      * @throws NullPointerException if an element is {@code null}
     605      *
     606      * @since 9
     607      */
     608     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8) {
     609         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     610                                                e6, e7, e8);
     611     }
     612
     613     /**
     614      * Returns an unmodifiable set containing nine elements.
     615      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     616      *
     617      * @param <E> the {@code Set}'s element type
     618      * @param e1 the first element
     619      * @param e2 the second element
     620      * @param e3 the third element
     621      * @param e4 the fourth element
     622      * @param e5 the fifth element
     623      * @param e6 the sixth element
     624      * @param e7 the seventh element
     625      * @param e8 the eighth element
     626      * @param e9 the ninth element
     627      * @return a {@code Set} containing the specified elements
     628      * @throws IllegalArgumentException if there are any duplicate elements
     629      * @throws NullPointerException if an element is {@code null}
     630      *
     631      * @since 9
     632      */
     633     static <E> Set<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E e7, E e8, E e9) {
     634         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     635                                                e6, e7, e8, e9);
     636     }
     637
     638     /**
     639      * Returns an unmodifiable set containing ten elements.
     640      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     641      *
     642      * @param <E> the {@code Set}'s element type
     643      * @param e1 the first element
     644      * @param e2 the second element
     645      * @param e3 the third element
     646      * @param e4 the fourth element
     647      * @param e5 the fifth element
     648      * @param e6 the sixth element
     649      * @param e7 the seventh element
     650      * @param e8 the eighth element
     651      * @param e9 the ninth element
     652      * @param e10 the tenth element
     653      * @return a {@code Set} containing the specified elements
     654      * @throws IllegalArgumentException if there are any duplicate elements
     655      * @throws NullPointerException if an element is {@code null}
     656      *
     657      * @since 9
     658      */
     659     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) {
     660         return new ImmutableCollections.SetN<>(e1, e2, e3, e4, e5,
     661                                                e6, e7, e8, e9, e10);
     662     }
     663
     664     /**
     665      * Returns an unmodifiable set containing an arbitrary number of elements.
     666      * See <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     667      *
     668      * @apiNote
     669      * This method also accepts a single array as an argument. The element type of
     670      * the resulting set will be the component type of the array, and the size of
     671      * the set will be equal to the length of the array. To create a set with
     672      * a single element that is an array, do the following:
     673      *
     674      * <pre>{@code
     675      *     String[] array = ... ;
     676      *     Set<String[]> list = Set.<String[]>of(array);
     677      * }</pre>
     678      *
     679      * This will cause the {@link Set#of(Object) Set.of(E)} method
     680      * to be invoked instead.
     681      *
     682      * @param <E> the {@code Set}'s element type
     683      * @param elements the elements to be contained in the set
     684      * @return a {@code Set} containing the specified elements
     685      * @throws IllegalArgumentException if there are any duplicate elements
     686      * @throws NullPointerException if an element is {@code null} or if the array is {@code null}
     687      *
     688      * @since 9
     689      */
     690     @SafeVarargs
     691     @SuppressWarnings("varargs")
     692     static <E> Set<E> of(E... elements) {
     693         switch (elements.length) { // implicit null check of elements
     694             case 0:
     695                 return ImmutableCollections.Set0.instance();
     696             case 1:
     697                 return new ImmutableCollections.Set1<>(elements[0]);
     698             case 2:
     699                 return new ImmutableCollections.Set2<>(elements[0], elements[1]);
     700             default:
     701                 return new ImmutableCollections.SetN<>(elements);
     702         }
     703     }
     704
     705     /**
     706      * Returns an <a href="#unmodifiable">unmodifiable Set</a> containing the elements
     707      * of the given Collection. The given Collection must not be null, and it must not
     708      * contain any null elements. If the given Collection contains duplicate elements,
     709      * an arbitrary element of the duplicates is preserved. If the given Collection is
     710      * subsequently modified, the returned Set will not reflect such modifications.
     711      *
     712      * @implNote
     713      * If the given Collection is an <a href="#unmodifiable">unmodifiable Set</a>,
     714      * calling copyOf will generally not create a copy.
     715      *
     716      * @param <E> the {@code Set}'s element type
     717      * @param coll a {@code Collection} from which elements are drawn, must be non-null
     718      * @return a {@code Set} containing the elements of the given {@code Collection}
     719      * @throws NullPointerException if coll is null, or if it contains any nulls
     720      * @since 10
     721      */
     722     @SuppressWarnings("unchecked")
     723     static <E> Set<E> copyOf(Collection<? extends E> coll) {
     724         if (coll instanceof ImmutableCollections.AbstractImmutableSet) {
     725             return (Set<E>)coll;
     726         } else {
     727             return (Set<E>)Set.of(new HashSet<>(coll).toArray());
     728         }
     729     }
     730 }