src/java.base/share/classes/java/util/Set.java - master - githubmirror/riscv-port

src/java.base/share/classes/java/util/Set.java
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8385054: Spec: Clarify ofLazy List/Set/Map equals/hashCode/toString Javadoc
04 июн 2026, 11:50
04 июн 2026, 11:50c6f9c23
/*
 * Copyright (c) 1997, 2026, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.util;

import jdk.internal.javac.PreviewFeature;

import java.io.Serializable;
import java.util.function.Predicate;

/**
 * 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>Unless otherwise specified, 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>.
 * Programmers should treat instances that are {@linkplain #equals(Object) equal}
 * as interchangeable and should not use them for synchronization, or
 * unpredictable behavior may occur. For example, in a future release,
 * synchronization may fail. Callers should make no assumptions
 * about the identity of the returned instances. Factories are free to
 * create new instances or reuse existing ones.
 * <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 {@code Integer.MAX_VALUE} elements, returns
     * {@code Integer.MAX_VALUE}.
     *
     * @return the number of elements in this set (its cardinality)
     */
    int size();

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

    /**
     * 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).
     *
     * @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.
     *
     * <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.
     *
     * <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
     * {@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 {@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 {@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 {@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
     * <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 <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 {@code clear} method
     *         is not supported by this set
     */
    void clear();


    // Comparison and hashing

    /**
     * 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 {@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 {@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 <a href="#unmodifiable">Unmodifiable Sets</a> for details.
     *
     * @param <E> the {@code Set}'s element type
     * @return an empty {@code Set}
     *
     * @since 9
     */
    @SuppressWarnings("unchecked")
    static <E> Set<E> of() {
        return (Set<E>) ImmutableCollections.EMPTY_SET;
    }

    /**
     * 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 <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 <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 <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 <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 <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 <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 <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 <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 <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 <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:
                @SuppressWarnings("unchecked")
                var set = (Set<E>) ImmutableCollections.EMPTY_SET;
                return set;
            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 <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 if (coll.isEmpty()) { // Implicit nullcheck of coll
            return Set.of();
        } else {
            return (Set<E>)Set.of(new HashSet<>(coll).toArray());
        }
    }

    /**
     * {@return a new lazily computed set whose logical membership for each distinct
     *          element candidate in the set of {@code elementCandidates} is
     *          computed via the provided {@code computingFunction} on demand}
     * <p>
     * In the following, the term <em>membership status</em> is used to indicate whether
     * an element belongs to the returned set or not. That is, if the membership status
     * for element {@code E} is {@code true}, then {@code E} <em>is</em> a member of the
     * returned set. Conversely, if the membership status for element {@code E} is
     * {@code false}, then {@code E} <em>is not</em> a member of the returned set.
     * <p>
     * The returned set is an {@linkplain Collection##unmodifiable unmodifiable} set. The
     * elements in the returned set are derived from the element candidates given at
     * construction in combination with evaluating each element's membership status. The
     * set's element membership statuses are lazily computed via the provided
     * {@code computingFunction} when first accessed (e.g., via
     * {@linkplain Set#contains(Object) Set::contains}). Once the membership status has
     * been successfully computed for an element candidate, the associated membership
     * status is initialized (i.e., either as <em>a logical member</em> or as
     * <em>a logical non-member</em>).
     * <p>
     * The provided computing function is guaranteed to be invoked at most once per
     * element candidate, even in a multi-threaded environment. Competing threads
     * accessing an element candidate already under membership status computation will
     * block until the membership status of the element candidate is computed or the
     * computing function completes abnormally.
     * <p>
     * If evaluation of the provided computing function throws an unchecked exception (for
     * an element candidate), the lazy membership status is not initialized but instead
     * transitions to an error state whereafter a {@linkplain NoSuchElementException} is
     * thrown with the unchecked exception as a cause. Subsequent
     * {@linkplain Set#contains(Object) Set::contains} calls for the same membership
     * candidate throw {@linkplain NoSuchElementException} (without ever invoking the
     * computing function again) with no cause and with a message that includes the name
     * of the original unchecked exception's class.
     * <p>
     * All failures are handled in this way. There is a special case that causes
     * unchecked exceptions to be thrown:
     * <p>
     * If the computing function recursively invokes itself (for the same membership
     * candidate) via the returned lazy set, a {@linkplain NoSuchElementException}
     * (with an {@linkplain IllegalStateException} as a cause) will be thrown.
     * <p>
     * The returned set's {@linkplain Object Object methods};
     * {@linkplain Object#equals(Object) equals()},
     * {@linkplain Object#hashCode() hashCode()}, and
     * {@linkplain Object#toString() toString()} methods may trigger initialization of
     * one or more lazy elements. If initialization fails for at least one element,
     * the {@linkplain Object#hashCode() hashCode()} and
     * {@linkplain Object#toString() toString()} methods throw
     * {@linkplain NoSuchElementException}, and the {@linkplain Object#equals(Object)}
     * throw {@linkplain NoSuchElementException} if attempting to compare an element that
     * could not be computed.
     * <p>
     * The returned lazy set strongly references its underlying
     * computing function used to compute membership status at least as long as there are
     * uncomputed element candidates.
     * <p>
     * The returned Set is <em>not</em> {@linkplain Serializable}.
     * <p>
     * If the provided {@code Set} of {@code elementCandidates} is subsequently modified,
     * the returned {@code Set} will not reflect such modifications.
     * <p>
     * The {@code Set} of {@code elementCandidates} must use
     * {@linkplain Set#equals(Object) equals()} as its equivalence relation, or its
     * comparison method must be consistent with {@code equals()}, otherwise the behavior
     * is unspecified.
     * <p>
     * Here is an example involving an application that manages various configurable
     * options -- commonly referred to as "switches" -- that control its behavior. The
     * state of these switches can be determined through the command line, a configuration
     * file, or even a database connection. By using a lazy set, we ensure that the states
     * of these switches are evaluated only once. Once computed, the results are eligible
     * for constant folding by the JVM:
     * {@snippet lang = java:
     * class Application {
     *
     *     enum Option {VERBOSE, DRY_RUN, STRICT}
     *
     *     // Lazily initialized Set of Options
     *     static final Set<Option> OPTIONS =
     *             Set.ofLazy(EnumSet.allOf(Option.class), Application::isEnabled);
     *
     *     // Return true when the given Option is enabled
     *     private static boolean isEnabled(Option option) {
     *         // Parse command line, read configuration file, load database
     *         ...
     *     }
     *
     *     public static void process() {
     *         // The if condition (and subsequent eliminated branch) is
     *         // eligible for constant folding (and code elimination).
     *         if (OPTIONS.contains(Option.DRY_RUN)) {
     *             // Skip processing in DRY_RUN mode
     *             return;
     *         }
     *         // Actual processing logic
     *     }
     *
     * }
     * }
     * <p>
     * The returned {@code Set<E>} can be thought of as a set backed by a
     * {@code Map<E, LazyConstant<Boolean>>} field and where the {@linkplain Set#contains(Object)}
     * operation is equivalent to:
     * {@snippet lang = java:
     * class LazySet<E> extends AbstractCollection<E> implements Set<E> {
     *
     *     private final Map<E, LazyConstant<Boolean>> backingMap;
     *
     *     public LazySet(Set<E> elementCandidates, Predicate<E> computingFunction) {
     *         this.backingMap = elementCandidates.stream()
     *                 .collect(Collectors.toUnmodifiableMap(
     *                         Function.identity(),
     *                         k -> LazyConstant.of(() -> computingFunction.test(k))));
     *     }
     *
     *     @Override
     *     public boolean contains(Object o) {
     *         var lazyConstant = backingMap.get(o);
     *         return lazyConstant == null
     *                 ? false
     *                 : lazyConstant.get();
     *     }
     * }
     *}
     * Except, performance and storage efficiency might be better.
     * <p>
     * Elements in the returned set are eligible for certain performance optimizations
     * such as <em>constant folding</em> as described in
     * {@linkplain LazyConstant##performance LazyConstant}.
     *
     * @implNote  after all element membership statuses have been initialized
     *            successfully or transitioned to an error state, the computing function
     *            is no longer strongly referenced and becomes eligible for garbage
     *            collection.
     *
     * @param elementCandidates the (non-null) element candidates to be evaluated
     * @param computingFunction to invoke whenever the membership status of an element
     *                          candidate is first computed
     * @param <E>               the type of elements maintained by the returned set
     * @throws NullPointerException if the provided set of {@code elementCandidates} is
     *                              {@code null}, if the set of {@code elementCandidates}
     *                              contains a {@code null} element, or if the provided
     *                              {@code computingFunction} is {@code null}
     *
     * @see LazyConstant
     * @since 27
     */
    @PreviewFeature(feature = PreviewFeature.Feature.LAZY_CONSTANTS)
    static <E> Set<E> ofLazy(Set<? extends E> elementCandidates,
                             Predicate<? super E> computingFunction) {
        Objects.requireNonNull(elementCandidates);
        Objects.requireNonNull(computingFunction);
        return LazyCollections.ofLazySet(elementCandidates, computingFunction);
    }

}