    rev 48045 : 8177290: add copy factory methods for unmodifiable List, Set, Map
    8184690: add Collectors for collecting into unmodifiable List, Set, and Map
    Reviewed-by: alanb, briangoetz, dholmes, jrose, rriggs, scolebourne

       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 import java.util.function.BiConsumer;
      29 import java.util.function.BiFunction;
      30 import java.util.function.Function;
      31 import java.io.Serializable;
      32
      33 /**
      34  * An object that maps keys to values.  A map cannot contain duplicate keys;
      35  * each key can map to at most one value.
      36  *
      37  * <p>This interface takes the place of the {@code Dictionary} class, which
      38  * was a totally abstract class rather than an interface.
      39  *
      40  * <p>The {@code Map} interface provides three <i>collection views</i>, which
      41  * allow a map's contents to be viewed as a set of keys, collection of values,
      42  * or set of key-value mappings.  The <i>order</i> of a map is defined as
      43  * the order in which the iterators on the map's collection views return their
      44  * elements.  Some map implementations, like the {@code TreeMap} class, make
      45  * specific guarantees as to their order; others, like the {@code HashMap}
      46  * class, do not.
      47  *
      48  * <p>Note: great care must be exercised if mutable objects are used as map
      49  * keys.  The behavior of a map is not specified if the value of an object is
      50  * changed in a manner that affects {@code equals} comparisons while the
      51  * object is a key in the map.  A special case of this prohibition is that it
      52  * is not permissible for a map to contain itself as a key.  While it is
      53  * permissible for a map to contain itself as a value, extreme caution is
      54  * advised: the {@code equals} and {@code hashCode} methods are no longer
      55  * well defined on such a map.
      56  *
      57  * <p>All general-purpose map implementation classes should provide two
      58  * "standard" constructors: a void (no arguments) constructor which creates an
      59  * empty map, and a constructor with a single argument of type {@code Map},
      60  * which creates a new map with the same key-value mappings as its argument.
      61  * In effect, the latter constructor allows the user to copy any map,
      62  * producing an equivalent map of the desired class.  There is no way to
      63  * enforce this recommendation (as interfaces cannot contain constructors) but
      64  * all of the general-purpose map implementations in the JDK comply.
      65  *
      66  * <p>The "destructive" methods contained in this interface, that is, the
      67  * methods that modify the map on which they operate, are specified to throw
      68  * {@code UnsupportedOperationException} if this map does not support the
      69  * operation.  If this is the case, these methods may, but are not required
      70  * to, throw an {@code UnsupportedOperationException} if the invocation would
      71  * have no effect on the map.  For example, invoking the {@link #putAll(Map)}
      72  * method on an unmodifiable map may, but is not required to, throw the
      73  * exception if the map whose mappings are to be "superimposed" is empty.
      74  *
      75  * <p>Some map implementations have restrictions on the keys and values they
      76  * may contain.  For example, some implementations prohibit null keys and
      77  * values, and some have restrictions on the types of their keys.  Attempting
      78  * to insert an ineligible key or value throws an unchecked exception,
      79  * typically {@code NullPointerException} or {@code ClassCastException}.
      80  * Attempting to query the presence of an ineligible key or value may throw an
      81  * exception, or it may simply return false; some implementations will exhibit
      82  * the former behavior and some will exhibit the latter.  More generally,
      83  * attempting an operation on an ineligible key or value whose completion
      84  * would not result in the insertion of an ineligible element into the map may
      85  * throw an exception or it may succeed, at the option of the implementation.
      86  * Such exceptions are marked as "optional" in the specification for this
      87  * interface.
      88  *
      89  * <p>Many methods in Collections Framework interfaces are defined
      90  * in terms of the {@link Object#equals(Object) equals} method.  For
      91  * example, the specification for the {@link #containsKey(Object)
      92  * containsKey(Object key)} method says: "returns {@code true} if and
      93  * only if this map contains a mapping for a key {@code k} such that
      94  * {@code (key==null ? k==null : key.equals(k))}." This specification should
      95  * <i>not</i> be construed to imply that invoking {@code Map.containsKey}
      96  * with a non-null argument {@code key} will cause {@code key.equals(k)} to
      97  * be invoked for any key {@code k}.  Implementations are free to
      98  * implement optimizations whereby the {@code equals} invocation is avoided,
      99  * for example, by first comparing the hash codes of the two keys.  (The
     100  * {@link Object#hashCode()} specification guarantees that two objects with
     101  * unequal hash codes cannot be equal.)  More generally, implementations of
     102  * the various Collections Framework interfaces are free to take advantage of
     103  * the specified behavior of underlying {@link Object} methods wherever the
     104  * implementor deems it appropriate.
     105  *
     106  * <p>Some map operations which perform recursive traversal of the map may fail
     107  * with an exception for self-referential instances where the map directly or
     108  * indirectly contains itself. This includes the {@code clone()},
     109  * {@code equals()}, {@code hashCode()} and {@code toString()} methods.
     110  * Implementations may optionally handle the self-referential scenario, however
     111  * most current implementations do not do so.
     112  *
     113  * <h2><a id="unmodifiable">Unmodifiable Maps</a></h2>
     114  * <p>The {@link Map#of() Map.of},
     115  * {@link Map#ofEntries(Map.Entry...) Map.ofEntries}, and
     116  * {@link Map#copyOf Map.copyOf}
     117  * static factory methods provide a convenient way to create unmodifiable maps.
     118  * The {@code Map}
     119  * instances created by these methods have the following characteristics:
     120  *
     121  * <ul>
     122  * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Keys and values
     123  * cannot be added, removed, or updated. Calling any mutator method on the Map
     124  * will always cause {@code UnsupportedOperationException} to be thrown.
     125  * However, if the contained keys or values are themselves mutable, this may cause the
     126  * Map to behave inconsistently or its contents to appear to change.
     127  * <li>They disallow {@code null} keys and values. Attempts to create them with
     128  * {@code null} keys or values result in {@code NullPointerException}.
     129  * <li>They are serializable if all keys and values are serializable.
     130  * <li>They reject duplicate keys at creation time. Duplicate keys
     131  * passed to a static factory method result in {@code IllegalArgumentException}.
     132  * <li>The iteration order of mappings is unspecified and is subject to change.
     133  * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
     134  * Callers should make no assumptions about the identity of the returned instances.
     135  * Factories are free to create new instances or reuse existing ones. Therefore,
     136  * identity-sensitive operations on these instances (reference equality ({@code ==}),
     137  * identity hash code, and synchronization) are unreliable and should be avoided.
     138  * <li>They are serialized as specified on the
     139  * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
     140  * page.
     141  * </ul>
     142  *
     143  * <p>This interface is a member of the
     144  * <a href="{@docRoot}/java/util/package-summary.html#CollectionsFramework">
     145  * Java Collections Framework</a>.
     146  *
     147  * @param <K> the type of keys maintained by this map
     148  * @param <V> the type of mapped values
     149  *
     150  * @author  Josh Bloch
     151  * @see HashMap
     152  * @see TreeMap
     153  * @see Hashtable
     154  * @see SortedMap
     155  * @see Collection
     156  * @see Set
     157  * @since 1.2
     158  */
     159 public interface Map<K, V> {
     160     // Query Operations
     161
     162     /**
     163      * Returns the number of key-value mappings in this map.  If the
     164      * map contains more than {@code Integer.MAX_VALUE} elements, returns
     165      * {@code Integer.MAX_VALUE}.
     166      *
     167      * @return the number of key-value mappings in this map
     168      */
     169     int size();
     170
     171     /**
     172      * Returns {@code true} if this map contains no key-value mappings.
     173      *
     174      * @return {@code true} if this map contains no key-value mappings
     175      */
     176     boolean isEmpty();
     177
     178     /**
     179      * Returns {@code true} if this map contains a mapping for the specified
     180      * key.  More formally, returns {@code true} if and only if
     181      * this map contains a mapping for a key {@code k} such that
     182      * {@code Objects.equals(key, k)}.  (There can be
     183      * at most one such mapping.)
     184      *
     185      * @param key key whose presence in this map is to be tested
     186      * @return {@code true} if this map contains a mapping for the specified
     187      *         key
     188      * @throws ClassCastException if the key is of an inappropriate type for
     189      *         this map
     190      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     191      * @throws NullPointerException if the specified key is null and this map
     192      *         does not permit null keys
     193      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     194      */
     195     boolean containsKey(Object key);
     196
     197     /**
     198      * Returns {@code true} if this map maps one or more keys to the
     199      * specified value.  More formally, returns {@code true} if and only if
     200      * this map contains at least one mapping to a value {@code v} such that
     201      * {@code Objects.equals(value, v)}.  This operation
     202      * will probably require time linear in the map size for most
     203      * implementations of the {@code Map} interface.
     204      *
     205      * @param value value whose presence in this map is to be tested
     206      * @return {@code true} if this map maps one or more keys to the
     207      *         specified value
     208      * @throws ClassCastException if the value is of an inappropriate type for
     209      *         this map
     210      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     211      * @throws NullPointerException if the specified value is null and this
     212      *         map does not permit null values
     213      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     214      */
     215     boolean containsValue(Object value);
     216
     217     /**
     218      * Returns the value to which the specified key is mapped,
     219      * or {@code null} if this map contains no mapping for the key.
     220      *
     221      * <p>More formally, if this map contains a mapping from a key
     222      * {@code k} to a value {@code v} such that
     223      * {@code Objects.equals(key, k)},
     224      * then this method returns {@code v}; otherwise
     225      * it returns {@code null}.  (There can be at most one such mapping.)
     226      *
     227      * <p>If this map permits null values, then a return value of
     228      * {@code null} does not <i>necessarily</i> indicate that the map
     229      * contains no mapping for the key; it's also possible that the map
     230      * explicitly maps the key to {@code null}.  The {@link #containsKey
     231      * containsKey} operation may be used to distinguish these two cases.
     232      *
     233      * @param key the key whose associated value is to be returned
     234      * @return the value to which the specified key is mapped, or
     235      *         {@code null} if this map contains no mapping for the key
     236      * @throws ClassCastException if the key is of an inappropriate type for
     237      *         this map
     238      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     239      * @throws NullPointerException if the specified key is null and this map
     240      *         does not permit null keys
     241      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     242      */
     243     V get(Object key);
     244
     245     // Modification Operations
     246
     247     /**
     248      * Associates the specified value with the specified key in this map
     249      * (optional operation).  If the map previously contained a mapping for
     250      * the key, the old value is replaced by the specified value.  (A map
     251      * {@code m} is said to contain a mapping for a key {@code k} if and only
     252      * if {@link #containsKey(Object) m.containsKey(k)} would return
     253      * {@code true}.)
     254      *
     255      * @param key key with which the specified value is to be associated
     256      * @param value value to be associated with the specified key
     257      * @return the previous value associated with {@code key}, or
     258      *         {@code null} if there was no mapping for {@code key}.
     259      *         (A {@code null} return can also indicate that the map
     260      *         previously associated {@code null} with {@code key},
     261      *         if the implementation supports {@code null} values.)
     262      * @throws UnsupportedOperationException if the {@code put} operation
     263      *         is not supported by this map
     264      * @throws ClassCastException if the class of the specified key or value
     265      *         prevents it from being stored in this map
     266      * @throws NullPointerException if the specified key or value is null
     267      *         and this map does not permit null keys or values
     268      * @throws IllegalArgumentException if some property of the specified key
     269      *         or value prevents it from being stored in this map
     270      */
     271     V put(K key, V value);
     272
     273     /**
     274      * Removes the mapping for a key from this map if it is present
     275      * (optional operation).   More formally, if this map contains a mapping
     276      * from key {@code k} to value {@code v} such that
     277      * {@code Objects.equals(key, k)}, that mapping
     278      * is removed.  (The map can contain at most one such mapping.)
     279      *
     280      * <p>Returns the value to which this map previously associated the key,
     281      * or {@code null} if the map contained no mapping for the key.
     282      *
     283      * <p>If this map permits null values, then a return value of
     284      * {@code null} does not <i>necessarily</i> indicate that the map
     285      * contained no mapping for the key; it's also possible that the map
     286      * explicitly mapped the key to {@code null}.
     287      *
     288      * <p>The map will not contain a mapping for the specified key once the
     289      * call returns.
     290      *
     291      * @param key key whose mapping is to be removed from the map
     292      * @return the previous value associated with {@code key}, or
     293      *         {@code null} if there was no mapping for {@code key}.
     294      * @throws UnsupportedOperationException if the {@code remove} operation
     295      *         is not supported by this map
     296      * @throws ClassCastException if the key is of an inappropriate type for
     297      *         this map
     298      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     299      * @throws NullPointerException if the specified key is null and this
     300      *         map does not permit null keys
     301      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     302      */
     303     V remove(Object key);
     304
     305
     306     // Bulk Operations
     307
     308     /**
     309      * Copies all of the mappings from the specified map to this map
     310      * (optional operation).  The effect of this call is equivalent to that
     311      * of calling {@link #put(Object,Object) put(k, v)} on this map once
     312      * for each mapping from key {@code k} to value {@code v} in the
     313      * specified map.  The behavior of this operation is undefined if the
     314      * specified map is modified while the operation is in progress.
     315      *
     316      * @param m mappings to be stored in this map
     317      * @throws UnsupportedOperationException if the {@code putAll} operation
     318      *         is not supported by this map
     319      * @throws ClassCastException if the class of a key or value in the
     320      *         specified map prevents it from being stored in this map
     321      * @throws NullPointerException if the specified map is null, or if
     322      *         this map does not permit null keys or values, and the
     323      *         specified map contains null keys or values
     324      * @throws IllegalArgumentException if some property of a key or value in
     325      *         the specified map prevents it from being stored in this map
     326      */
     327     void putAll(Map<? extends K, ? extends V> m);
     328
     329     /**
     330      * Removes all of the mappings from this map (optional operation).
     331      * The map will be empty after this call returns.
     332      *
     333      * @throws UnsupportedOperationException if the {@code clear} operation
     334      *         is not supported by this map
     335      */
     336     void clear();
     337
     338
     339     // Views
     340
     341     /**
     342      * Returns a {@link Set} view of the keys contained in this map.
     343      * The set is backed by the map, so changes to the map are
     344      * reflected in the set, and vice-versa.  If the map is modified
     345      * while an iteration over the set is in progress (except through
     346      * the iterator's own {@code remove} operation), the results of
     347      * the iteration are undefined.  The set supports element removal,
     348      * which removes the corresponding mapping from the map, via the
     349      * {@code Iterator.remove}, {@code Set.remove},
     350      * {@code removeAll}, {@code retainAll}, and {@code clear}
     351      * operations.  It does not support the {@code add} or {@code addAll}
     352      * operations.
     353      *
     354      * @return a set view of the keys contained in this map
     355      */
     356     Set<K> keySet();
     357
     358     /**
     359      * Returns a {@link Collection} view of the values contained in this map.
     360      * The collection is backed by the map, so changes to the map are
     361      * reflected in the collection, and vice-versa.  If the map is
     362      * modified while an iteration over the collection is in progress
     363      * (except through the iterator's own {@code remove} operation),
     364      * the results of the iteration are undefined.  The collection
     365      * supports element removal, which removes the corresponding
     366      * mapping from the map, via the {@code Iterator.remove},
     367      * {@code Collection.remove}, {@code removeAll},
     368      * {@code retainAll} and {@code clear} operations.  It does not
     369      * support the {@code add} or {@code addAll} operations.
     370      *
     371      * @return a collection view of the values contained in this map
     372      */
     373     Collection<V> values();
     374
     375     /**
     376      * Returns a {@link Set} view of the mappings contained in this map.
     377      * The set is backed by the map, so changes to the map are
     378      * reflected in the set, and vice-versa.  If the map is modified
     379      * while an iteration over the set is in progress (except through
     380      * the iterator's own {@code remove} operation, or through the
     381      * {@code setValue} operation on a map entry returned by the
     382      * iterator) the results of the iteration are undefined.  The set
     383      * supports element removal, which removes the corresponding
     384      * mapping from the map, via the {@code Iterator.remove},
     385      * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
     386      * {@code clear} operations.  It does not support the
     387      * {@code add} or {@code addAll} operations.
     388      *
     389      * @return a set view of the mappings contained in this map
     390      */
     391     Set<Map.Entry<K, V>> entrySet();
     392
     393     /**
     394      * A map entry (key-value pair).  The {@code Map.entrySet} method returns
     395      * a collection-view of the map, whose elements are of this class.  The
     396      * <i>only</i> way to obtain a reference to a map entry is from the
     397      * iterator of this collection-view.  These {@code Map.Entry} objects are
     398      * valid <i>only</i> for the duration of the iteration; more formally,
     399      * the behavior of a map entry is undefined if the backing map has been
     400      * modified after the entry was returned by the iterator, except through
     401      * the {@code setValue} operation on the map entry.
     402      *
     403      * @see Map#entrySet()
     404      * @since 1.2
     405      */
     406     interface Entry<K, V> {
     407         /**
     408          * Returns the key corresponding to this entry.
     409          *
     410          * @return the key corresponding to this entry
     411          * @throws IllegalStateException implementations may, but are not
     412          *         required to, throw this exception if the entry has been
     413          *         removed from the backing map.
     414          */
     415         K getKey();
     416
     417         /**
     418          * Returns the value corresponding to this entry.  If the mapping
     419          * has been removed from the backing map (by the iterator's
     420          * {@code remove} operation), the results of this call are undefined.
     421          *
     422          * @return the value corresponding to this entry
     423          * @throws IllegalStateException implementations may, but are not
     424          *         required to, throw this exception if the entry has been
     425          *         removed from the backing map.
     426          */
     427         V getValue();
     428
     429         /**
     430          * Replaces the value corresponding to this entry with the specified
     431          * value (optional operation).  (Writes through to the map.)  The
     432          * behavior of this call is undefined if the mapping has already been
     433          * removed from the map (by the iterator's {@code remove} operation).
     434          *
     435          * @param value new value to be stored in this entry
     436          * @return old value corresponding to the entry
     437          * @throws UnsupportedOperationException if the {@code put} operation
     438          *         is not supported by the backing map
     439          * @throws ClassCastException if the class of the specified value
     440          *         prevents it from being stored in the backing map
     441          * @throws NullPointerException if the backing map does not permit
     442          *         null values, and the specified value is null
     443          * @throws IllegalArgumentException if some property of this value
     444          *         prevents it from being stored in the backing map
     445          * @throws IllegalStateException implementations may, but are not
     446          *         required to, throw this exception if the entry has been
     447          *         removed from the backing map.
     448          */
     449         V setValue(V value);
     450
     451         /**
     452          * Compares the specified object with this entry for equality.
     453          * Returns {@code true} if the given object is also a map entry and
     454          * the two entries represent the same mapping.  More formally, two
     455          * entries {@code e1} and {@code e2} represent the same mapping
     456          * if<pre>
     457          *     (e1.getKey()==null ?
     458          *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
     459          *     (e1.getValue()==null ?
     460          *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
     461          * </pre>
     462          * This ensures that the {@code equals} method works properly across
     463          * different implementations of the {@code Map.Entry} interface.
     464          *
     465          * @param o object to be compared for equality with this map entry
     466          * @return {@code true} if the specified object is equal to this map
     467          *         entry
     468          */
     469         boolean equals(Object o);
     470
     471         /**
     472          * Returns the hash code value for this map entry.  The hash code
     473          * of a map entry {@code e} is defined to be: <pre>
     474          *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
     475          *     (e.getValue()==null ? 0 : e.getValue().hashCode())
     476          * </pre>
     477          * This ensures that {@code e1.equals(e2)} implies that
     478          * {@code e1.hashCode()==e2.hashCode()} for any two Entries
     479          * {@code e1} and {@code e2}, as required by the general
     480          * contract of {@code Object.hashCode}.
     481          *
     482          * @return the hash code value for this map entry
     483          * @see Object#hashCode()
     484          * @see Object#equals(Object)
     485          * @see #equals(Object)
     486          */
     487         int hashCode();
     488
     489         /**
     490          * Returns a comparator that compares {@link Map.Entry} in natural order on key.
     491          *
     492          * <p>The returned comparator is serializable and throws {@link
     493          * NullPointerException} when comparing an entry with a null key.
     494          *
     495          * @param  <K> the {@link Comparable} type of then map keys
     496          * @param  <V> the type of the map values
     497          * @return a comparator that compares {@link Map.Entry} in natural order on key.
     498          * @see Comparable
     499          * @since 1.8
     500          */
     501         public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K, V>> comparingByKey() {
     502             return (Comparator<Map.Entry<K, V>> & Serializable)
     503                 (c1, c2) -> c1.getKey().compareTo(c2.getKey());
     504         }
     505
     506         /**
     507          * Returns a comparator that compares {@link Map.Entry} in natural order on value.
     508          *
     509          * <p>The returned comparator is serializable and throws {@link
     510          * NullPointerException} when comparing an entry with null values.
     511          *
     512          * @param <K> the type of the map keys
     513          * @param <V> the {@link Comparable} type of the map values
     514          * @return a comparator that compares {@link Map.Entry} in natural order on value.
     515          * @see Comparable
     516          * @since 1.8
     517          */
     518         public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K, V>> comparingByValue() {
     519             return (Comparator<Map.Entry<K, V>> & Serializable)
     520                 (c1, c2) -> c1.getValue().compareTo(c2.getValue());
     521         }
     522
     523         /**
     524          * Returns a comparator that compares {@link Map.Entry} by key using the given
     525          * {@link Comparator}.
     526          *
     527          * <p>The returned comparator is serializable if the specified comparator
     528          * is also serializable.
     529          *
     530          * @param  <K> the type of the map keys
     531          * @param  <V> the type of the map values
     532          * @param  cmp the key {@link Comparator}
     533          * @return a comparator that compares {@link Map.Entry} by the key.
     534          * @since 1.8
     535          */
     536         public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
     537             Objects.requireNonNull(cmp);
     538             return (Comparator<Map.Entry<K, V>> & Serializable)
     539                 (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
     540         }
     541
     542         /**
     543          * Returns a comparator that compares {@link Map.Entry} by value using the given
     544          * {@link Comparator}.
     545          *
     546          * <p>The returned comparator is serializable if the specified comparator
     547          * is also serializable.
     548          *
     549          * @param  <K> the type of the map keys
     550          * @param  <V> the type of the map values
     551          * @param  cmp the value {@link Comparator}
     552          * @return a comparator that compares {@link Map.Entry} by the value.
     553          * @since 1.8
     554          */
     555         public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
     556             Objects.requireNonNull(cmp);
     557             return (Comparator<Map.Entry<K, V>> & Serializable)
     558                 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
     559         }
     560     }
     561
     562     // Comparison and hashing
     563
     564     /**
     565      * Compares the specified object with this map for equality.  Returns
     566      * {@code true} if the given object is also a map and the two maps
     567      * represent the same mappings.  More formally, two maps {@code m1} and
     568      * {@code m2} represent the same mappings if
     569      * {@code m1.entrySet().equals(m2.entrySet())}.  This ensures that the
     570      * {@code equals} method works properly across different implementations
     571      * of the {@code Map} interface.
     572      *
     573      * @param o object to be compared for equality with this map
     574      * @return {@code true} if the specified object is equal to this map
     575      */
     576     boolean equals(Object o);
     577
     578     /**
     579      * Returns the hash code value for this map.  The hash code of a map is
     580      * defined to be the sum of the hash codes of each entry in the map's
     581      * {@code entrySet()} view.  This ensures that {@code m1.equals(m2)}
     582      * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
     583      * {@code m1} and {@code m2}, as required by the general contract of
     584      * {@link Object#hashCode}.
     585      *
     586      * @return the hash code value for this map
     587      * @see Map.Entry#hashCode()
     588      * @see Object#equals(Object)
     589      * @see #equals(Object)
     590      */
     591     int hashCode();
     592
     593     // Defaultable methods
     594
     595     /**
     596      * Returns the value to which the specified key is mapped, or
     597      * {@code defaultValue} if this map contains no mapping for the key.
     598      *
     599      * @implSpec
     600      * The default implementation makes no guarantees about synchronization
     601      * or atomicity properties of this method. Any implementation providing
     602      * atomicity guarantees must override this method and document its
     603      * concurrency properties.
     604      *
     605      * @param key the key whose associated value is to be returned
     606      * @param defaultValue the default mapping of the key
     607      * @return the value to which the specified key is mapped, or
     608      * {@code defaultValue} if this map contains no mapping for the key
     609      * @throws ClassCastException if the key is of an inappropriate type for
     610      * this map
     611      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     612      * @throws NullPointerException if the specified key is null and this map
     613      * does not permit null keys
     614      * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     615      * @since 1.8
     616      */
     617     default V getOrDefault(Object key, V defaultValue) {
     618         V v;
     619         return (((v = get(key)) != null) || containsKey(key))
     620             ? v
     621             : defaultValue;
     622     }
     623
     624     /**
     625      * Performs the given action for each entry in this map until all entries
     626      * have been processed or the action throws an exception.   Unless
     627      * otherwise specified by the implementing class, actions are performed in
     628      * the order of entry set iteration (if an iteration order is specified.)
     629      * Exceptions thrown by the action are relayed to the caller.
     630      *
     631      * @implSpec
     632      * The default implementation is equivalent to, for this {@code map}:
     633      * <pre> {@code
     634      * for (Map.Entry<K, V> entry : map.entrySet())
     635      *     action.accept(entry.getKey(), entry.getValue());
     636      * }</pre>
     637      *
     638      * The default implementation makes no guarantees about synchronization
     639      * or atomicity properties of this method. Any implementation providing
     640      * atomicity guarantees must override this method and document its
     641      * concurrency properties.
     642      *
     643      * @param action The action to be performed for each entry
     644      * @throws NullPointerException if the specified action is null
     645      * @throws ConcurrentModificationException if an entry is found to be
     646      * removed during iteration
     647      * @since 1.8
     648      */
     649     default void forEach(BiConsumer<? super K, ? super V> action) {
     650         Objects.requireNonNull(action);
     651         for (Map.Entry<K, V> entry : entrySet()) {
     652             K k;
     653             V v;
     654             try {
     655                 k = entry.getKey();
     656                 v = entry.getValue();
     657             } catch (IllegalStateException ise) {
     658                 // this usually means the entry is no longer in the map.
     659                 throw new ConcurrentModificationException(ise);
     660             }
     661             action.accept(k, v);
     662         }
     663     }
     664
     665     /**
     666      * Replaces each entry's value with the result of invoking the given
     667      * function on that entry until all entries have been processed or the
     668      * function throws an exception.  Exceptions thrown by the function are
     669      * relayed to the caller.
     670      *
     671      * @implSpec
     672      * <p>The default implementation is equivalent to, for this {@code map}:
     673      * <pre> {@code
     674      * for (Map.Entry<K, V> entry : map.entrySet())
     675      *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
     676      * }</pre>
     677      *
     678      * <p>The default implementation makes no guarantees about synchronization
     679      * or atomicity properties of this method. Any implementation providing
     680      * atomicity guarantees must override this method and document its
     681      * concurrency properties.
     682      *
     683      * @param function the function to apply to each entry
     684      * @throws UnsupportedOperationException if the {@code set} operation
     685      * is not supported by this map's entry set iterator.
     686      * @throws ClassCastException if the class of a replacement value
     687      * prevents it from being stored in this map
     688      * @throws NullPointerException if the specified function is null, or the
     689      * specified replacement value is null, and this map does not permit null
     690      * values
     691      * @throws ClassCastException if a replacement value is of an inappropriate
     692      *         type for this map
     693      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     694      * @throws NullPointerException if function or a replacement value is null,
     695      *         and this map does not permit null keys or values
     696      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     697      * @throws IllegalArgumentException if some property of a replacement value
     698      *         prevents it from being stored in this map
     699      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     700      * @throws ConcurrentModificationException if an entry is found to be
     701      * removed during iteration
     702      * @since 1.8
     703      */
     704     default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
     705         Objects.requireNonNull(function);
     706         for (Map.Entry<K, V> entry : entrySet()) {
     707             K k;
     708             V v;
     709             try {
     710                 k = entry.getKey();
     711                 v = entry.getValue();
     712             } catch (IllegalStateException ise) {
     713                 // this usually means the entry is no longer in the map.
     714                 throw new ConcurrentModificationException(ise);
     715             }
     716
     717             // ise thrown from function is not a cme.
     718             v = function.apply(k, v);
     719
     720             try {
     721                 entry.setValue(v);
     722             } catch (IllegalStateException ise) {
     723                 // this usually means the entry is no longer in the map.
     724                 throw new ConcurrentModificationException(ise);
     725             }
     726         }
     727     }
     728
     729     /**
     730      * If the specified key is not already associated with a value (or is mapped
     731      * to {@code null}) associates it with the given value and returns
     732      * {@code null}, else returns the current value.
     733      *
     734      * @implSpec
     735      * The default implementation is equivalent to, for this {@code
     736      * map}:
     737      *
     738      * <pre> {@code
     739      * V v = map.get(key);
     740      * if (v == null)
     741      *     v = map.put(key, value);
     742      *
     743      * return v;
     744      * }</pre>
     745      *
     746      * <p>The default implementation makes no guarantees about synchronization
     747      * or atomicity properties of this method. Any implementation providing
     748      * atomicity guarantees must override this method and document its
     749      * concurrency properties.
     750      *
     751      * @param key key with which the specified value is to be associated
     752      * @param value value to be associated with the specified key
     753      * @return the previous value associated with the specified key, or
     754      *         {@code null} if there was no mapping for the key.
     755      *         (A {@code null} return can also indicate that the map
     756      *         previously associated {@code null} with the key,
     757      *         if the implementation supports null values.)
     758      * @throws UnsupportedOperationException if the {@code put} operation
     759      *         is not supported by this map
     760      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     761      * @throws ClassCastException if the key or value is of an inappropriate
     762      *         type for this map
     763      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     764      * @throws NullPointerException if the specified key or value is null,
     765      *         and this map does not permit null keys or values
     766      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     767      * @throws IllegalArgumentException if some property of the specified key
     768      *         or value prevents it from being stored in this map
     769      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     770      * @since 1.8
     771      */
     772     default V putIfAbsent(K key, V value) {
     773         V v = get(key);
     774         if (v == null) {
     775             v = put(key, value);
     776         }
     777
     778         return v;
     779     }
     780
     781     /**
     782      * Removes the entry for the specified key only if it is currently
     783      * mapped to the specified value.
     784      *
     785      * @implSpec
     786      * The default implementation is equivalent to, for this {@code map}:
     787      *
     788      * <pre> {@code
     789      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     790      *     map.remove(key);
     791      *     return true;
     792      * } else
     793      *     return false;
     794      * }</pre>
     795      *
     796      * <p>The default implementation makes no guarantees about synchronization
     797      * or atomicity properties of this method. Any implementation providing
     798      * atomicity guarantees must override this method and document its
     799      * concurrency properties.
     800      *
     801      * @param key key with which the specified value is associated
     802      * @param value value expected to be associated with the specified key
     803      * @return {@code true} if the value was removed
     804      * @throws UnsupportedOperationException if the {@code remove} operation
     805      *         is not supported by this map
     806      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     807      * @throws ClassCastException if the key or value is of an inappropriate
     808      *         type for this map
     809      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     810      * @throws NullPointerException if the specified key or value is null,
     811      *         and this map does not permit null keys or values
     812      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     813      * @since 1.8
     814      */
     815     default boolean remove(Object key, Object value) {
     816         Object curValue = get(key);
     817         if (!Objects.equals(curValue, value) ||
     818             (curValue == null && !containsKey(key))) {
     819             return false;
     820         }
     821         remove(key);
     822         return true;
     823     }
     824
     825     /**
     826      * Replaces the entry for the specified key only if currently
     827      * mapped to the specified value.
     828      *
     829      * @implSpec
     830      * The default implementation is equivalent to, for this {@code map}:
     831      *
     832      * <pre> {@code
     833      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     834      *     map.put(key, newValue);
     835      *     return true;
     836      * } else
     837      *     return false;
     838      * }</pre>
     839      *
     840      * The default implementation does not throw NullPointerException
     841      * for maps that do not support null values if oldValue is null unless
     842      * newValue is also null.
     843      *
     844      * <p>The default implementation makes no guarantees about synchronization
     845      * or atomicity properties of this method. Any implementation providing
     846      * atomicity guarantees must override this method and document its
     847      * concurrency properties.
     848      *
     849      * @param key key with which the specified value is associated
     850      * @param oldValue value expected to be associated with the specified key
     851      * @param newValue value to be associated with the specified key
     852      * @return {@code true} if the value was replaced
     853      * @throws UnsupportedOperationException if the {@code put} operation
     854      *         is not supported by this map
     855      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     856      * @throws ClassCastException if the class of a specified key or value
     857      *         prevents it from being stored in this map
     858      * @throws NullPointerException if a specified key or newValue is null,
     859      *         and this map does not permit null keys or values
     860      * @throws NullPointerException if oldValue is null and this map does not
     861      *         permit null values
     862      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     863      * @throws IllegalArgumentException if some property of a specified key
     864      *         or value prevents it from being stored in this map
     865      * @since 1.8
     866      */
     867     default boolean replace(K key, V oldValue, V newValue) {
     868         Object curValue = get(key);
     869         if (!Objects.equals(curValue, oldValue) ||
     870             (curValue == null && !containsKey(key))) {
     871             return false;
     872         }
     873         put(key, newValue);
     874         return true;
     875     }
     876
     877     /**
     878      * Replaces the entry for the specified key only if it is
     879      * currently mapped to some value.
     880      *
     881      * @implSpec
     882      * The default implementation is equivalent to, for this {@code map}:
     883      *
     884      * <pre> {@code
     885      * if (map.containsKey(key)) {
     886      *     return map.put(key, value);
     887      * } else
     888      *     return null;
     889      * }</pre>
     890      *
     891      * <p>The default implementation makes no guarantees about synchronization
     892      * or atomicity properties of this method. Any implementation providing
     893      * atomicity guarantees must override this method and document its
     894      * concurrency properties.
     895      *
     896      * @param key key with which the specified value is associated
     897      * @param value value to be associated with the specified key
     898      * @return the previous value associated with the specified key, or
     899      *         {@code null} if there was no mapping for the key.
     900      *         (A {@code null} return can also indicate that the map
     901      *         previously associated {@code null} with the key,
     902      *         if the implementation supports null values.)
     903      * @throws UnsupportedOperationException if the {@code put} operation
     904      *         is not supported by this map
     905      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     906      * @throws ClassCastException if the class of the specified key or value
     907      *         prevents it from being stored in this map
     908      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     909      * @throws NullPointerException if the specified key or value is null,
     910      *         and this map does not permit null keys or values
     911      * @throws IllegalArgumentException if some property of the specified key
     912      *         or value prevents it from being stored in this map
     913      * @since 1.8
     914      */
     915     default V replace(K key, V value) {
     916         V curValue;
     917         if (((curValue = get(key)) != null) || containsKey(key)) {
     918             curValue = put(key, value);
     919         }
     920         return curValue;
     921     }
     922
     923     /**
     924      * If the specified key is not already associated with a value (or is mapped
     925      * to {@code null}), attempts to compute its value using the given mapping
     926      * function and enters it into this map unless {@code null}.
     927      *
     928      * <p>If the mapping function returns {@code null}, no mapping is recorded.
     929      * If the mapping function itself throws an (unchecked) exception, the
     930      * exception is rethrown, and no mapping is recorded.  The most
     931      * common usage is to construct a new object serving as an initial
     932      * mapped value or memoized result, as in:
     933      *
     934      * <pre> {@code
     935      * map.computeIfAbsent(key, k -> new Value(f(k)));
     936      * }</pre>
     937      *
     938      * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
     939      * supporting multiple values per key:
     940      *
     941      * <pre> {@code
     942      * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
     943      * }</pre>
     944      *
     945      * <p>The mapping function should not modify this map during computation.
     946      *
     947      * @implSpec
     948      * The default implementation is equivalent to the following steps for this
     949      * {@code map}, then returning the current value or {@code null} if now
     950      * absent:
     951      *
     952      * <pre> {@code
     953      * if (map.get(key) == null) {
     954      *     V newValue = mappingFunction.apply(key);
     955      *     if (newValue != null)
     956      *         map.put(key, newValue);
     957      * }
     958      * }</pre>
     959      *
     960      * <p>The default implementation makes no guarantees about detecting if the
     961      * mapping function modifies this map during computation and, if
     962      * appropriate, reporting an error. Non-concurrent implementations should
     963      * override this method and, on a best-effort basis, throw a
     964      * {@code ConcurrentModificationException} if it is detected that the
     965      * mapping function modifies this map during computation. Concurrent
     966      * implementations should override this method and, on a best-effort basis,
     967      * throw an {@code IllegalStateException} if it is detected that the
     968      * mapping function modifies this map during computation and as a result
     969      * computation would never complete.
     970      *
     971      * <p>The default implementation makes no guarantees about synchronization
     972      * or atomicity properties of this method. Any implementation providing
     973      * atomicity guarantees must override this method and document its
     974      * concurrency properties. In particular, all implementations of
     975      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     976      * whether the mapping function is applied once atomically only if the value
     977      * is not present.
     978      *
     979      * @param key key with which the specified value is to be associated
     980      * @param mappingFunction the mapping function to compute a value
     981      * @return the current (existing or computed) value associated with
     982      *         the specified key, or null if the computed value is null
     983      * @throws NullPointerException if the specified key is null and
     984      *         this map does not support null keys, or the mappingFunction
     985      *         is null
     986      * @throws UnsupportedOperationException if the {@code put} operation
     987      *         is not supported by this map
     988      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     989      * @throws ClassCastException if the class of the specified key or value
     990      *         prevents it from being stored in this map
     991      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     992      * @throws IllegalArgumentException if some property of the specified key
     993      *         or value prevents it from being stored in this map
     994      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     995      * @since 1.8
     996      */
     997     default V computeIfAbsent(K key,
     998             Function<? super K, ? extends V> mappingFunction) {
     999         Objects.requireNonNull(mappingFunction);
    1000         V v;
    1001         if ((v = get(key)) == null) {
    1002             V newValue;
    1003             if ((newValue = mappingFunction.apply(key)) != null) {
    1004                 put(key, newValue);
    1005                 return newValue;
    1006             }
    1007         }
    1008
    1009         return v;
    1010     }
    1011
    1012     /**
    1013      * If the value for the specified key is present and non-null, attempts to
    1014      * compute a new mapping given the key and its current mapped value.
    1015      *
    1016      * <p>If the remapping function returns {@code null}, the mapping is removed.
    1017      * If the remapping function itself throws an (unchecked) exception, the
    1018      * exception is rethrown, and the current mapping is left unchanged.
    1019      *
    1020      * <p>The remapping function should not modify this map during computation.
    1021      *
    1022      * @implSpec
    1023      * The default implementation is equivalent to performing the following
    1024      * steps for this {@code map}, then returning the current value or
    1025      * {@code null} if now absent:
    1026      *
    1027      * <pre> {@code
    1028      * if (map.get(key) != null) {
    1029      *     V oldValue = map.get(key);
    1030      *     V newValue = remappingFunction.apply(key, oldValue);
    1031      *     if (newValue != null)
    1032      *         map.put(key, newValue);
    1033      *     else
    1034      *         map.remove(key);
    1035      * }
    1036      * }</pre>
    1037      *
    1038      * <p>The default implementation makes no guarantees about detecting if the
    1039      * remapping function modifies this map during computation and, if
    1040      * appropriate, reporting an error. Non-concurrent implementations should
    1041      * override this method and, on a best-effort basis, throw a
    1042      * {@code ConcurrentModificationException} if it is detected that the
    1043      * remapping function modifies this map during computation. Concurrent
    1044      * implementations should override this method and, on a best-effort basis,
    1045      * throw an {@code IllegalStateException} if it is detected that the
    1046      * remapping function modifies this map during computation and as a result
    1047      * computation would never complete.
    1048      *
    1049      * <p>The default implementation makes no guarantees about synchronization
    1050      * or atomicity properties of this method. Any implementation providing
    1051      * atomicity guarantees must override this method and document its
    1052      * concurrency properties. In particular, all implementations of
    1053      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
    1054      * whether the remapping function is applied once atomically only if the
    1055      * value is not present.
    1056      *
    1057      * @param key key with which the specified value is to be associated
    1058      * @param remappingFunction the remapping function to compute a value
    1059      * @return the new value associated with the specified key, or null if none
    1060      * @throws NullPointerException if the specified key is null and
    1061      *         this map does not support null keys, or the
    1062      *         remappingFunction is null
    1063      * @throws UnsupportedOperationException if the {@code put} operation
    1064      *         is not supported by this map
    1065      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1066      * @throws ClassCastException if the class of the specified key or value
    1067      *         prevents it from being stored in this map
    1068      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1069      * @throws IllegalArgumentException if some property of the specified key
    1070      *         or value prevents it from being stored in this map
    1071      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1072      * @since 1.8
    1073      */
    1074     default V computeIfPresent(K key,
    1075             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
    1076         Objects.requireNonNull(remappingFunction);
    1077         V oldValue;
    1078         if ((oldValue = get(key)) != null) {
    1079             V newValue = remappingFunction.apply(key, oldValue);
    1080             if (newValue != null) {
    1081                 put(key, newValue);
    1082                 return newValue;
    1083             } else {
    1084                 remove(key);
    1085                 return null;
    1086             }
    1087         } else {
    1088             return null;
    1089         }
    1090     }
    1091
    1092     /**
    1093      * Attempts to compute a mapping for the specified key and its current
    1094      * mapped value (or {@code null} if there is no current mapping). For
    1095      * example, to either create or append a {@code String} msg to a value
    1096      * mapping:
    1097      *
    1098      * <pre> {@code
    1099      * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
    1100      * (Method {@link #merge merge()} is often simpler to use for such purposes.)
    1101      *
    1102      * <p>If the remapping function returns {@code null}, the mapping is removed
    1103      * (or remains absent if initially absent).  If the remapping function
    1104      * itself throws an (unchecked) exception, the exception is rethrown, and
    1105      * the current mapping is left unchanged.
    1106      *
    1107      * <p>The remapping function should not modify this map during computation.
    1108      *
    1109      * @implSpec
    1110      * The default implementation is equivalent to performing the following
    1111      * steps for this {@code map}, then returning the current value or
    1112      * {@code null} if absent:
    1113      *
    1114      * <pre> {@code
    1115      * V oldValue = map.get(key);
    1116      * V newValue = remappingFunction.apply(key, oldValue);
    1117      * if (oldValue != null) {
    1118      *    if (newValue != null)
    1119      *       map.put(key, newValue);
    1120      *    else
    1121      *       map.remove(key);
    1122      * } else {
    1123      *    if (newValue != null)
    1124      *       map.put(key, newValue);
    1125      *    else
    1126      *       return null;
    1127      * }
    1128      * }</pre>
    1129      *
    1130      * <p>The default implementation makes no guarantees about detecting if the
    1131      * remapping function modifies this map during computation and, if
    1132      * appropriate, reporting an error. Non-concurrent implementations should
    1133      * override this method and, on a best-effort basis, throw a
    1134      * {@code ConcurrentModificationException} if it is detected that the
    1135      * remapping function modifies this map during computation. Concurrent
    1136      * implementations should override this method and, on a best-effort basis,
    1137      * throw an {@code IllegalStateException} if it is detected that the
    1138      * remapping function modifies this map during computation and as a result
    1139      * computation would never complete.
    1140      *
    1141      * <p>The default implementation makes no guarantees about synchronization
    1142      * or atomicity properties of this method. Any implementation providing
    1143      * atomicity guarantees must override this method and document its
    1144      * concurrency properties. In particular, all implementations of
    1145      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
    1146      * whether the remapping function is applied once atomically only if the
    1147      * value is not present.
    1148      *
    1149      * @param key key with which the specified value is to be associated
    1150      * @param remappingFunction the remapping function to compute a value
    1151      * @return the new value associated with the specified key, or null if none
    1152      * @throws NullPointerException if the specified key is null and
    1153      *         this map does not support null keys, or the
    1154      *         remappingFunction is null
    1155      * @throws UnsupportedOperationException if the {@code put} operation
    1156      *         is not supported by this map
    1157      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1158      * @throws ClassCastException if the class of the specified key or value
    1159      *         prevents it from being stored in this map
    1160      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1161      * @throws IllegalArgumentException if some property of the specified key
    1162      *         or value prevents it from being stored in this map
    1163      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1164      * @since 1.8
    1165      */
    1166     default V compute(K key,
    1167             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
    1168         Objects.requireNonNull(remappingFunction);
    1169         V oldValue = get(key);
    1170
    1171         V newValue = remappingFunction.apply(key, oldValue);
    1172         if (newValue == null) {
    1173             // delete mapping
    1174             if (oldValue != null || containsKey(key)) {
    1175                 // something to remove
    1176                 remove(key);
    1177                 return null;
    1178             } else {
    1179                 // nothing to do. Leave things as they were.
    1180                 return null;
    1181             }
    1182         } else {
    1183             // add or replace old mapping
    1184             put(key, newValue);
    1185             return newValue;
    1186         }
    1187     }
    1188
    1189     /**
    1190      * If the specified key is not already associated with a value or is
    1191      * associated with null, associates it with the given non-null value.
    1192      * Otherwise, replaces the associated value with the results of the given
    1193      * remapping function, or removes if the result is {@code null}. This
    1194      * method may be of use when combining multiple mapped values for a key.
    1195      * For example, to either create or append a {@code String msg} to a
    1196      * value mapping:
    1197      *
    1198      * <pre> {@code
    1199      * map.merge(key, msg, String::concat)
    1200      * }</pre>
    1201      *
    1202      * <p>If the remapping function returns {@code null}, the mapping is removed.
    1203      * If the remapping function itself throws an (unchecked) exception, the
    1204      * exception is rethrown, and the current mapping is left unchanged.
    1205      *
    1206      * <p>The remapping function should not modify this map during computation.
    1207      *
    1208      * @implSpec
    1209      * The default implementation is equivalent to performing the following
    1210      * steps for this {@code map}, then returning the current value or
    1211      * {@code null} if absent:
    1212      *
    1213      * <pre> {@code
    1214      * V oldValue = map.get(key);
    1215      * V newValue = (oldValue == null) ? value :
    1216      *              remappingFunction.apply(oldValue, value);
    1217      * if (newValue == null)
    1218      *     map.remove(key);
    1219      * else
    1220      *     map.put(key, newValue);
    1221      * }</pre>
    1222      *
    1223      * <p>The default implementation makes no guarantees about detecting if the
    1224      * remapping function modifies this map during computation and, if
    1225      * appropriate, reporting an error. Non-concurrent implementations should
    1226      * override this method and, on a best-effort basis, throw a
    1227      * {@code ConcurrentModificationException} if it is detected that the
    1228      * remapping function modifies this map during computation. Concurrent
    1229      * implementations should override this method and, on a best-effort basis,
    1230      * throw an {@code IllegalStateException} if it is detected that the
    1231      * remapping function modifies this map during computation and as a result
    1232      * computation would never complete.
    1233      *
    1234      * <p>The default implementation makes no guarantees about synchronization
    1235      * or atomicity properties of this method. Any implementation providing
    1236      * atomicity guarantees must override this method and document its
    1237      * concurrency properties. In particular, all implementations of
    1238      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
    1239      * whether the remapping function is applied once atomically only if the
    1240      * value is not present.
    1241      *
    1242      * @param key key with which the resulting value is to be associated
    1243      * @param value the non-null value to be merged with the existing value
    1244      *        associated with the key or, if no existing value or a null value
    1245      *        is associated with the key, to be associated with the key
    1246      * @param remappingFunction the remapping function to recompute a value if
    1247      *        present
    1248      * @return the new value associated with the specified key, or null if no
    1249      *         value is associated with the key
    1250      * @throws UnsupportedOperationException if the {@code put} operation
    1251      *         is not supported by this map
    1252      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1253      * @throws ClassCastException if the class of the specified key or value
    1254      *         prevents it from being stored in this map
    1255      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1256      * @throws IllegalArgumentException if some property of the specified key
    1257      *         or value prevents it from being stored in this map
    1258      *         (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
    1259      * @throws NullPointerException if the specified key is null and this map
    1260      *         does not support null keys or the value or remappingFunction is
    1261      *         null
    1262      * @since 1.8
    1263      */
    1264     default V merge(K key, V value,
    1265             BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    1266         Objects.requireNonNull(remappingFunction);
    1267         Objects.requireNonNull(value);
    1268         V oldValue = get(key);
    1269         V newValue = (oldValue == null) ? value :
    1270                    remappingFunction.apply(oldValue, value);
    1271         if (newValue == null) {
    1272             remove(key);
    1273         } else {
    1274             put(key, newValue);
    1275         }
    1276         return newValue;
    1277     }
    1278
    1279     /**
    1280      * Returns an unmodifiable map containing zero mappings.
    1281      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1282      *
    1283      * @param <K> the {@code Map}'s key type
    1284      * @param <V> the {@code Map}'s value type
    1285      * @return an empty {@code Map}
    1286      *
    1287      * @since 9
    1288      */
    1289     static <K, V> Map<K, V> of() {
    1290         return ImmutableCollections.Map0.instance();
    1291     }
    1292
    1293     /**
    1294      * Returns an unmodifiable map containing a single mapping.
    1295      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1296      *
    1297      * @param <K> the {@code Map}'s key type
    1298      * @param <V> the {@code Map}'s value type
    1299      * @param k1 the mapping's key
    1300      * @param v1 the mapping's value
    1301      * @return a {@code Map} containing the specified mapping
    1302      * @throws NullPointerException if the key or the value is {@code null}
    1303      *
    1304      * @since 9
    1305      */
    1306     static <K, V> Map<K, V> of(K k1, V v1) {
    1307         return new ImmutableCollections.Map1<>(k1, v1);
    1308     }
    1309
    1310     /**
    1311      * Returns an unmodifiable map containing two mappings.
    1312      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1313      *
    1314      * @param <K> the {@code Map}'s key type
    1315      * @param <V> the {@code Map}'s value type
    1316      * @param k1 the first mapping's key
    1317      * @param v1 the first mapping's value
    1318      * @param k2 the second mapping's key
    1319      * @param v2 the second mapping's value
    1320      * @return a {@code Map} containing the specified mappings
    1321      * @throws IllegalArgumentException if the keys are duplicates
    1322      * @throws NullPointerException if any key or value is {@code null}
    1323      *
    1324      * @since 9
    1325      */
    1326     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) {
    1327         return new ImmutableCollections.MapN<>(k1, v1, k2, v2);
    1328     }
    1329
    1330     /**
    1331      * Returns an unmodifiable map containing three mappings.
    1332      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1333      *
    1334      * @param <K> the {@code Map}'s key type
    1335      * @param <V> the {@code Map}'s value type
    1336      * @param k1 the first mapping's key
    1337      * @param v1 the first mapping's value
    1338      * @param k2 the second mapping's key
    1339      * @param v2 the second mapping's value
    1340      * @param k3 the third mapping's key
    1341      * @param v3 the third mapping's value
    1342      * @return a {@code Map} containing the specified mappings
    1343      * @throws IllegalArgumentException if there are any duplicate keys
    1344      * @throws NullPointerException if any key or value is {@code null}
    1345      *
    1346      * @since 9
    1347      */
    1348     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
    1349         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3);
    1350     }
    1351
    1352     /**
    1353      * Returns an unmodifiable map containing four mappings.
    1354      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1355      *
    1356      * @param <K> the {@code Map}'s key type
    1357      * @param <V> the {@code Map}'s value type
    1358      * @param k1 the first mapping's key
    1359      * @param v1 the first mapping's value
    1360      * @param k2 the second mapping's key
    1361      * @param v2 the second mapping's value
    1362      * @param k3 the third mapping's key
    1363      * @param v3 the third mapping's value
    1364      * @param k4 the fourth mapping's key
    1365      * @param v4 the fourth mapping's value
    1366      * @return a {@code Map} containing the specified mappings
    1367      * @throws IllegalArgumentException if there are any duplicate keys
    1368      * @throws NullPointerException if any key or value is {@code null}
    1369      *
    1370      * @since 9
    1371      */
    1372     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
    1373         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4);
    1374     }
    1375
    1376     /**
    1377      * Returns an unmodifiable map containing five mappings.
    1378      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1379      *
    1380      * @param <K> the {@code Map}'s key type
    1381      * @param <V> the {@code Map}'s value type
    1382      * @param k1 the first mapping's key
    1383      * @param v1 the first mapping's value
    1384      * @param k2 the second mapping's key
    1385      * @param v2 the second mapping's value
    1386      * @param k3 the third mapping's key
    1387      * @param v3 the third mapping's value
    1388      * @param k4 the fourth mapping's key
    1389      * @param v4 the fourth mapping's value
    1390      * @param k5 the fifth mapping's key
    1391      * @param v5 the fifth mapping's value
    1392      * @return a {@code Map} containing the specified mappings
    1393      * @throws IllegalArgumentException if there are any duplicate keys
    1394      * @throws NullPointerException if any key or value is {@code null}
    1395      *
    1396      * @since 9
    1397      */
    1398     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
    1399         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5);
    1400     }
    1401
    1402     /**
    1403      * Returns an unmodifiable map containing six mappings.
    1404      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1405      *
    1406      * @param <K> the {@code Map}'s key type
    1407      * @param <V> the {@code Map}'s value type
    1408      * @param k1 the first mapping's key
    1409      * @param v1 the first mapping's value
    1410      * @param k2 the second mapping's key
    1411      * @param v2 the second mapping's value
    1412      * @param k3 the third mapping's key
    1413      * @param v3 the third mapping's value
    1414      * @param k4 the fourth mapping's key
    1415      * @param v4 the fourth mapping's value
    1416      * @param k5 the fifth mapping's key
    1417      * @param v5 the fifth mapping's value
    1418      * @param k6 the sixth mapping's key
    1419      * @param v6 the sixth mapping's value
    1420      * @return a {@code Map} containing the specified mappings
    1421      * @throws IllegalArgumentException if there are any duplicate keys
    1422      * @throws NullPointerException if any key or value is {@code null}
    1423      *
    1424      * @since 9
    1425      */
    1426     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
    1427                                K k6, V v6) {
    1428         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
    1429                                                k6, v6);
    1430     }
    1431
    1432     /**
    1433      * Returns an unmodifiable map containing seven mappings.
    1434      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1435      *
    1436      * @param <K> the {@code Map}'s key type
    1437      * @param <V> the {@code Map}'s value type
    1438      * @param k1 the first mapping's key
    1439      * @param v1 the first mapping's value
    1440      * @param k2 the second mapping's key
    1441      * @param v2 the second mapping's value
    1442      * @param k3 the third mapping's key
    1443      * @param v3 the third mapping's value
    1444      * @param k4 the fourth mapping's key
    1445      * @param v4 the fourth mapping's value
    1446      * @param k5 the fifth mapping's key
    1447      * @param v5 the fifth mapping's value
    1448      * @param k6 the sixth mapping's key
    1449      * @param v6 the sixth mapping's value
    1450      * @param k7 the seventh mapping's key
    1451      * @param v7 the seventh mapping's value
    1452      * @return a {@code Map} containing the specified mappings
    1453      * @throws IllegalArgumentException if there are any duplicate keys
    1454      * @throws NullPointerException if any key or value is {@code null}
    1455      *
    1456      * @since 9
    1457      */
    1458     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
    1459                                K k6, V v6, K k7, V v7) {
    1460         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
    1461                                                k6, v6, k7, v7);
    1462     }
    1463
    1464     /**
    1465      * Returns an unmodifiable map containing eight mappings.
    1466      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1467      *
    1468      * @param <K> the {@code Map}'s key type
    1469      * @param <V> the {@code Map}'s value type
    1470      * @param k1 the first mapping's key
    1471      * @param v1 the first mapping's value
    1472      * @param k2 the second mapping's key
    1473      * @param v2 the second mapping's value
    1474      * @param k3 the third mapping's key
    1475      * @param v3 the third mapping's value
    1476      * @param k4 the fourth mapping's key
    1477      * @param v4 the fourth mapping's value
    1478      * @param k5 the fifth mapping's key
    1479      * @param v5 the fifth mapping's value
    1480      * @param k6 the sixth mapping's key
    1481      * @param v6 the sixth mapping's value
    1482      * @param k7 the seventh mapping's key
    1483      * @param v7 the seventh mapping's value
    1484      * @param k8 the eighth mapping's key
    1485      * @param v8 the eighth mapping's value
    1486      * @return a {@code Map} containing the specified mappings
    1487      * @throws IllegalArgumentException if there are any duplicate keys
    1488      * @throws NullPointerException if any key or value is {@code null}
    1489      *
    1490      * @since 9
    1491      */
    1492     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
    1493                                K k6, V v6, K k7, V v7, K k8, V v8) {
    1494         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
    1495                                                k6, v6, k7, v7, k8, v8);
    1496     }
    1497
    1498     /**
    1499      * Returns an unmodifiable map containing nine mappings.
    1500      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1501      *
    1502      * @param <K> the {@code Map}'s key type
    1503      * @param <V> the {@code Map}'s value type
    1504      * @param k1 the first mapping's key
    1505      * @param v1 the first mapping's value
    1506      * @param k2 the second mapping's key
    1507      * @param v2 the second mapping's value
    1508      * @param k3 the third mapping's key
    1509      * @param v3 the third mapping's value
    1510      * @param k4 the fourth mapping's key
    1511      * @param v4 the fourth mapping's value
    1512      * @param k5 the fifth mapping's key
    1513      * @param v5 the fifth mapping's value
    1514      * @param k6 the sixth mapping's key
    1515      * @param v6 the sixth mapping's value
    1516      * @param k7 the seventh mapping's key
    1517      * @param v7 the seventh mapping's value
    1518      * @param k8 the eighth mapping's key
    1519      * @param v8 the eighth mapping's value
    1520      * @param k9 the ninth mapping's key
    1521      * @param v9 the ninth mapping's value
    1522      * @return a {@code Map} containing the specified mappings
    1523      * @throws IllegalArgumentException if there are any duplicate keys
    1524      * @throws NullPointerException if any key or value is {@code null}
    1525      *
    1526      * @since 9
    1527      */
    1528     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
    1529                                K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) {
    1530         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
    1531                                                k6, v6, k7, v7, k8, v8, k9, v9);
    1532     }
    1533
    1534     /**
    1535      * Returns an unmodifiable map containing ten mappings.
    1536      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1537      *
    1538      * @param <K> the {@code Map}'s key type
    1539      * @param <V> the {@code Map}'s value type
    1540      * @param k1 the first mapping's key
    1541      * @param v1 the first mapping's value
    1542      * @param k2 the second mapping's key
    1543      * @param v2 the second mapping's value
    1544      * @param k3 the third mapping's key
    1545      * @param v3 the third mapping's value
    1546      * @param k4 the fourth mapping's key
    1547      * @param v4 the fourth mapping's value
    1548      * @param k5 the fifth mapping's key
    1549      * @param v5 the fifth mapping's value
    1550      * @param k6 the sixth mapping's key
    1551      * @param v6 the sixth mapping's value
    1552      * @param k7 the seventh mapping's key
    1553      * @param v7 the seventh mapping's value
    1554      * @param k8 the eighth mapping's key
    1555      * @param v8 the eighth mapping's value
    1556      * @param k9 the ninth mapping's key
    1557      * @param v9 the ninth mapping's value
    1558      * @param k10 the tenth mapping's key
    1559      * @param v10 the tenth mapping's value
    1560      * @return a {@code Map} containing the specified mappings
    1561      * @throws IllegalArgumentException if there are any duplicate keys
    1562      * @throws NullPointerException if any key or value is {@code null}
    1563      *
    1564      * @since 9
    1565      */
    1566     static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
    1567                                K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) {
    1568         return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
    1569                                                k6, v6, k7, v7, k8, v8, k9, v9, k10, v10);
    1570     }
    1571
    1572     /**
    1573      * Returns an unmodifiable map containing keys and values extracted from the given entries.
    1574      * The entries themselves are not stored in the map.
    1575      * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
    1576      *
    1577      * @apiNote
    1578      * It is convenient to create the map entries using the {@link Map#entry Map.entry()} method.
    1579      * For example,
    1580      *
    1581      * <pre>{@code
    1582      *     import static java.util.Map.entry;
    1583      *
    1584      *     Map<Integer,String> map = Map.ofEntries(
    1585      *         entry(1, "a"),
    1586      *         entry(2, "b"),
    1587      *         entry(3, "c"),
    1588      *         ...
    1589      *         entry(26, "z"));
    1590      * }</pre>
    1591      *
    1592      * @param <K> the {@code Map}'s key type
    1593      * @param <V> the {@code Map}'s value type
    1594      * @param entries {@code Map.Entry}s containing the keys and values from which the map is populated
    1595      * @return a {@code Map} containing the specified mappings
    1596      * @throws IllegalArgumentException if there are any duplicate keys
    1597      * @throws NullPointerException if any entry, key, or value is {@code null}, or if
    1598      *         the {@code entries} array is {@code null}
    1599      *
    1600      * @see Map#entry Map.entry()
    1601      * @since 9
    1602      */
    1603     @SafeVarargs
    1604     @SuppressWarnings("varargs")
    1605     static <K, V> Map<K, V> ofEntries(Entry<? extends K, ? extends V>... entries) {
    1606         if (entries.length == 0) { // implicit null check of entries array
    1607             return ImmutableCollections.Map0.instance();
    1608         } else if (entries.length == 1) {
    1609             // implicit null check of the array slot
    1610             return new ImmutableCollections.Map1<>(entries[0].getKey(),
    1611                                                    entries[0].getValue());
    1612         } else {
    1613             Object[] kva = new Object[entries.length << 1];
    1614             int a = 0;
    1615             for (Entry<? extends K, ? extends V> entry : entries) {
    1616                 // implicit null checks of each array slot
    1617                 kva[a++] = entry.getKey();
    1618                 kva[a++] = entry.getValue();
    1619             }
    1620             return new ImmutableCollections.MapN<>(kva);
    1621         }
    1622     }
    1623
    1624     /**
    1625      * Returns an unmodifiable {@link Entry} containing the given key and value.
    1626      * These entries are suitable for populating {@code Map} instances using the
    1627      * {@link Map#ofEntries Map.ofEntries()} method.
    1628      * The {@code Entry} instances created by this method have the following characteristics:
    1629      *
    1630      * <ul>
    1631      * <li>They disallow {@code null} keys and values. Attempts to create them using a {@code null}
    1632      * key or value result in {@code NullPointerException}.
    1633      * <li>They are unmodifiable. Calls to {@link Entry#setValue Entry.setValue()}
    1634      * on a returned {@code Entry} result in {@code UnsupportedOperationException}.
    1635      * <li>They are not serializable.
    1636      * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
    1637      * Callers should make no assumptions about the identity of the returned instances.
    1638      * This method is free to create new instances or reuse existing ones. Therefore,
    1639      * identity-sensitive operations on these instances (reference equality ({@code ==}),
    1640      * identity hash code, and synchronization) are unreliable and should be avoided.
    1641      * </ul>
    1642      *
    1643      * @apiNote
    1644      * For a serializable {@code Entry}, see {@link AbstractMap.SimpleEntry} or
    1645      * {@link AbstractMap.SimpleImmutableEntry}.
    1646      *
    1647      * @param <K> the key's type
    1648      * @param <V> the value's type
    1649      * @param k the key
    1650      * @param v the value
    1651      * @return an {@code Entry} containing the specified key and value
    1652      * @throws NullPointerException if the key or value is {@code null}
    1653      *
    1654      * @see Map#ofEntries Map.ofEntries()
    1655      * @since 9
    1656      */
    1657     static <K, V> Entry<K, V> entry(K k, V v) {
    1658         // KeyValueHolder checks for nulls
    1659         return new KeyValueHolder<>(k, v);
    1660     }
    1661
    1662     /**
    1663      * Returns an <a href="#unmodifiable">unmodifiable Map</a> containing the entries
    1664      * of the given Map. The given Map must not be null, and it must not contain any
    1665      * null keys or values. If the given Map is subsequently modified, the returned
    1666      * Map will not reflect such modifications.
    1667      *
    1668      * @implNote
    1669      * If the given Map is an <a href="#unmodifiable">unmodifiable Map</a>,
    1670      * calling copyOf will generally not create a copy.
    1671      *
    1672      * @param <K> the {@code Map}'s key type
    1673      * @param <V> the {@code Map}'s value type
    1674      * @param map a {@code Map} from which entries are drawn, must be non-null
    1675      * @return a {@code Map} containing the entries of the given {@code Map}
    1676      * @throws NullPointerException if map is null, or if it contains any null keys or values
    1677      * @since 10
    1678      */
    1679     @SuppressWarnings({"rawtypes","unchecked"})
    1680     static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) {
    1681         if (map instanceof ImmutableCollections.AbstractImmutableMap) {
    1682             return (Map<K,V>)map;
    1683         } else {
    1684             return (Map<K,V>)Map.ofEntries(map.entrySet().toArray(new Entry[0]));
    1685         }
    1686     }
    1687 }
    --- EOF ---