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