1 /*
   2  * Copyright (c) 1997, 2013, 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 <tt>Dictionary</tt> class, which
  38  * was a totally abstract class rather than an interface.
  39  *
  40  * <p>The <tt>Map</tt> 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 <tt>TreeMap</tt> class, make
  45  * specific guarantees as to their order; others, like the <tt>HashMap</tt>
  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 <tt>equals</tt> 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 <tt>equals</tt> and <tt>hashCode</tt> 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 <tt>Map</tt>,
  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  * <tt>UnsupportedOperationException</tt> 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 <tt>UnsupportedOperationException</tt> 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 <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
  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>This interface is a member of the
  90  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  91  * Java Collections Framework</a>.
  92  *
  93  * <p>Many methods in Collections Framework interfaces are defined
  94  * in terms of the {@link Object#equals(Object) equals} method.  For
  95  * example, the specification for the {@link #containsKey(Object)
  96  * containsKey(Object key)} method says: "returns <tt>true</tt> if and
  97  * only if this map contains a mapping for a key <tt>k</tt> such that
  98  * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
  99  * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
 100  * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
 101  * be invoked for any key <tt>k</tt>.  Implementations are free to
 102  * implement optimizations whereby the <tt>equals</tt> invocation is avoided,
 103  * for example, by first comparing the hash codes of the two keys.  (The
 104  * {@link Object#hashCode()} specification guarantees that two objects with
 105  * unequal hash codes cannot be equal.)  More generally, implementations of
 106  * the various Collections Framework interfaces are free to take advantage of
 107  * the specified behavior of underlying {@link Object} methods wherever the
 108  * implementor deems it appropriate.
 109  *
 110  * @param <K> the type of keys maintained by this map
 111  * @param <V> the type of mapped values
 112  *
 113  * @author  Josh Bloch
 114  * @see HashMap
 115  * @see TreeMap
 116  * @see Hashtable
 117  * @see SortedMap
 118  * @see Collection
 119  * @see Set
 120  * @since 1.2
 121  */
 122 public interface Map<K,V> {
 123     // Query Operations
 124 
 125     /**
 126      * Returns the number of key-value mappings in this map.  If the
 127      * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 128      * <tt>Integer.MAX_VALUE</tt>.
 129      *
 130      * @return the number of key-value mappings in this map
 131      */
 132     int size();
 133 
 134     /**
 135      * Returns <tt>true</tt> if this map contains no key-value mappings.
 136      *
 137      * @return <tt>true</tt> if this map contains no key-value mappings
 138      */
 139     boolean isEmpty();
 140 
 141     /**
 142      * Returns <tt>true</tt> if this map contains a mapping for the specified
 143      * key.  More formally, returns <tt>true</tt> if and only if
 144      * this map contains a mapping for a key <tt>k</tt> such that
 145      * <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be
 146      * at most one such mapping.)
 147      *
 148      * @param key key whose presence in this map is to be tested
 149      * @return <tt>true</tt> if this map contains a mapping for the specified
 150      *         key
 151      * @throws ClassCastException if the key is of an inappropriate type for
 152      *         this map
 153      * (<a href="Collection.html#optional-restrictions">optional</a>)
 154      * @throws NullPointerException if the specified key is null and this map
 155      *         does not permit null keys
 156      * (<a href="Collection.html#optional-restrictions">optional</a>)
 157      */
 158     boolean containsKey(Object key);
 159 
 160     /**
 161      * Returns <tt>true</tt> if this map maps one or more keys to the
 162      * specified value.  More formally, returns <tt>true</tt> if and only if
 163      * this map contains at least one mapping to a value <tt>v</tt> such that
 164      * <tt>(value==null ? v==null : value.equals(v))</tt>.  This operation
 165      * will probably require time linear in the map size for most
 166      * implementations of the <tt>Map</tt> interface.
 167      *
 168      * @param value value whose presence in this map is to be tested
 169      * @return <tt>true</tt> if this map maps one or more keys to the
 170      *         specified value
 171      * @throws ClassCastException if the value is of an inappropriate type for
 172      *         this map
 173      * (<a href="Collection.html#optional-restrictions">optional</a>)
 174      * @throws NullPointerException if the specified value is null and this
 175      *         map does not permit null values
 176      * (<a href="Collection.html#optional-restrictions">optional</a>)
 177      */
 178     boolean containsValue(Object value);
 179 
 180     /**
 181      * Returns the value to which the specified key is mapped,
 182      * or {@code null} if this map contains no mapping for the key.
 183      *
 184      * <p>More formally, if this map contains a mapping from a key
 185      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
 186      * key.equals(k))}, then this method returns {@code v}; otherwise
 187      * it returns {@code null}.  (There can be at most one such mapping.)
 188      *
 189      * <p>If this map permits null values, then a return value of
 190      * {@code null} does not <i>necessarily</i> indicate that the map
 191      * contains no mapping for the key; it's also possible that the map
 192      * explicitly maps the key to {@code null}.  The {@link #containsKey
 193      * containsKey} operation may be used to distinguish these two cases.
 194      *
 195      * @param key the key whose associated value is to be returned
 196      * @return the value to which the specified key is mapped, or
 197      *         {@code null} if this map contains no mapping for the key
 198      * @throws ClassCastException if the key is of an inappropriate type for
 199      *         this map
 200      * (<a href="Collection.html#optional-restrictions">optional</a>)
 201      * @throws NullPointerException if the specified key is null and this map
 202      *         does not permit null keys
 203      * (<a href="Collection.html#optional-restrictions">optional</a>)
 204      */
 205     V get(Object key);
 206 
 207     // Modification Operations
 208 
 209     /**
 210      * Associates the specified value with the specified key in this map
 211      * (optional operation).  If the map previously contained a mapping for
 212      * the key, the old value is replaced by the specified value.  (A map
 213      * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
 214      * if {@link #containsKey(Object) m.containsKey(k)} would return
 215      * <tt>true</tt>.)
 216      *
 217      * @param key key with which the specified value is to be associated
 218      * @param value value to be associated with the specified key
 219      * @return the previous value associated with <tt>key</tt>, or
 220      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 221      *         (A <tt>null</tt> return can also indicate that the map
 222      *         previously associated <tt>null</tt> with <tt>key</tt>,
 223      *         if the implementation supports <tt>null</tt> values.)
 224      * @throws UnsupportedOperationException if the <tt>put</tt> operation
 225      *         is not supported by this map
 226      * @throws ClassCastException if the class of the specified key or value
 227      *         prevents it from being stored in this map
 228      * @throws NullPointerException if the specified key or value is null
 229      *         and this map does not permit null keys or values
 230      * @throws IllegalArgumentException if some property of the specified key
 231      *         or value prevents it from being stored in this map
 232      */
 233     V put(K key, V value);
 234 
 235     /**
 236      * Removes the mapping for a key from this map if it is present
 237      * (optional operation).   More formally, if this map contains a mapping
 238      * from key <tt>k</tt> to value <tt>v</tt> such that
 239      * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
 240      * is removed.  (The map can contain at most one such mapping.)
 241      *
 242      * <p>Returns the value to which this map previously associated the key,
 243      * or <tt>null</tt> if the map contained no mapping for the key.
 244      *
 245      * <p>If this map permits null values, then a return value of
 246      * <tt>null</tt> does not <i>necessarily</i> indicate that the map
 247      * contained no mapping for the key; it's also possible that the map
 248      * explicitly mapped the key to <tt>null</tt>.
 249      *
 250      * <p>The map will not contain a mapping for the specified key once the
 251      * call returns.
 252      *
 253      * @param key key whose mapping is to be removed from the map
 254      * @return the previous value associated with <tt>key</tt>, or
 255      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 256      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 257      *         is not supported by this map
 258      * @throws ClassCastException if the key is of an inappropriate type for
 259      *         this map
 260      * (<a href="Collection.html#optional-restrictions">optional</a>)
 261      * @throws NullPointerException if the specified key is null and this
 262      *         map does not permit null keys
 263      * (<a href="Collection.html#optional-restrictions">optional</a>)
 264      */
 265     V remove(Object key);
 266 
 267 
 268     // Bulk Operations
 269 
 270     /**
 271      * Copies all of the mappings from the specified map to this map
 272      * (optional operation).  The effect of this call is equivalent to that
 273      * of calling {@link #put(Object,Object) put(k, v)} on this map once
 274      * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
 275      * specified map.  The behavior of this operation is undefined if the
 276      * specified map is modified while the operation is in progress.
 277      *
 278      * @param m mappings to be stored in this map
 279      * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
 280      *         is not supported by this map
 281      * @throws ClassCastException if the class of a key or value in the
 282      *         specified map prevents it from being stored in this map
 283      * @throws NullPointerException if the specified map is null, or if
 284      *         this map does not permit null keys or values, and the
 285      *         specified map contains null keys or values
 286      * @throws IllegalArgumentException if some property of a key or value in
 287      *         the specified map prevents it from being stored in this map
 288      */
 289     void putAll(Map<? extends K, ? extends V> m);
 290 
 291     /**
 292      * Removes all of the mappings from this map (optional operation).
 293      * The map will be empty after this call returns.
 294      *
 295      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 296      *         is not supported by this map
 297      */
 298     void clear();
 299 
 300 
 301     // Views
 302 
 303     /**
 304      * Returns a {@link Set} view of the keys contained in this map.
 305      * The set is backed by the map, so changes to the map are
 306      * reflected in the set, and vice-versa.  If the map is modified
 307      * while an iteration over the set is in progress (except through
 308      * the iterator's own <tt>remove</tt> operation), the results of
 309      * the iteration are undefined.  The set supports element removal,
 310      * which removes the corresponding mapping from the map, via the
 311      * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
 312      * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
 313      * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
 314      * operations.
 315      *
 316      * @return a set view of the keys contained in this map
 317      */
 318     Set<K> keySet();
 319 
 320     /**
 321      * Returns a {@link Collection} view of the values contained in this map.
 322      * The collection is backed by the map, so changes to the map are
 323      * reflected in the collection, and vice-versa.  If the map is
 324      * modified while an iteration over the collection is in progress
 325      * (except through the iterator's own <tt>remove</tt> operation),
 326      * the results of the iteration are undefined.  The collection
 327      * supports element removal, which removes the corresponding
 328      * mapping from the map, via the <tt>Iterator.remove</tt>,
 329      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
 330      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
 331      * support the <tt>add</tt> or <tt>addAll</tt> operations.
 332      *
 333      * @return a collection view of the values contained in this map
 334      */
 335     Collection<V> values();
 336 
 337     /**
 338      * Returns a {@link Set} view of the mappings contained in this map.
 339      * The set is backed by the map, so changes to the map are
 340      * reflected in the set, and vice-versa.  If the map is modified
 341      * while an iteration over the set is in progress (except through
 342      * the iterator's own <tt>remove</tt> operation, or through the
 343      * <tt>setValue</tt> operation on a map entry returned by the
 344      * iterator) the results of the iteration are undefined.  The set
 345      * supports element removal, which removes the corresponding
 346      * mapping from the map, via the <tt>Iterator.remove</tt>,
 347      * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
 348      * <tt>clear</tt> operations.  It does not support the
 349      * <tt>add</tt> or <tt>addAll</tt> operations.
 350      *
 351      * @return a set view of the mappings contained in this map
 352      */
 353     Set<Map.Entry<K, V>> entrySet();
 354 
 355     /**
 356      * A map entry (key-value pair).  The <tt>Map.entrySet</tt> method returns
 357      * a collection-view of the map, whose elements are of this class.  The
 358      * <i>only</i> way to obtain a reference to a map entry is from the
 359      * iterator of this collection-view.  These <tt>Map.Entry</tt> objects are
 360      * valid <i>only</i> for the duration of the iteration; more formally,
 361      * the behavior of a map entry is undefined if the backing map has been
 362      * modified after the entry was returned by the iterator, except through
 363      * the <tt>setValue</tt> operation on the map entry.
 364      *
 365      * @see Map#entrySet()
 366      * @since 1.2
 367      */
 368     interface Entry<K,V> {
 369         /**
 370          * Returns the key corresponding to this entry.
 371          *
 372          * @return the key corresponding to this entry
 373          * @throws IllegalStateException implementations may, but are not
 374          *         required to, throw this exception if the entry has been
 375          *         removed from the backing map.
 376          */
 377         K getKey();
 378 
 379         /**
 380          * Returns the value corresponding to this entry.  If the mapping
 381          * has been removed from the backing map (by the iterator's
 382          * <tt>remove</tt> operation), the results of this call are undefined.
 383          *
 384          * @return the value corresponding to this entry
 385          * @throws IllegalStateException implementations may, but are not
 386          *         required to, throw this exception if the entry has been
 387          *         removed from the backing map.
 388          */
 389         V getValue();
 390 
 391         /**
 392          * Replaces the value corresponding to this entry with the specified
 393          * value (optional operation).  (Writes through to the map.)  The
 394          * behavior of this call is undefined if the mapping has already been
 395          * removed from the map (by the iterator's <tt>remove</tt> operation).
 396          *
 397          * @param value new value to be stored in this entry
 398          * @return old value corresponding to the entry
 399          * @throws UnsupportedOperationException if the <tt>put</tt> operation
 400          *         is not supported by the backing map
 401          * @throws ClassCastException if the class of the specified value
 402          *         prevents it from being stored in the backing map
 403          * @throws NullPointerException if the backing map does not permit
 404          *         null values, and the specified value is null
 405          * @throws IllegalArgumentException if some property of this value
 406          *         prevents it from being stored in the backing map
 407          * @throws IllegalStateException implementations may, but are not
 408          *         required to, throw this exception if the entry has been
 409          *         removed from the backing map.
 410          */
 411         V setValue(V value);
 412 
 413         /**
 414          * Compares the specified object with this entry for equality.
 415          * Returns <tt>true</tt> if the given object is also a map entry and
 416          * the two entries represent the same mapping.  More formally, two
 417          * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
 418          * if<pre>
 419          *     (e1.getKey()==null ?
 420          *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
 421          *     (e1.getValue()==null ?
 422          *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
 423          * </pre>
 424          * This ensures that the <tt>equals</tt> method works properly across
 425          * different implementations of the <tt>Map.Entry</tt> interface.
 426          *
 427          * @param o object to be compared for equality with this map entry
 428          * @return <tt>true</tt> if the specified object is equal to this map
 429          *         entry
 430          */
 431         boolean equals(Object o);
 432 
 433         /**
 434          * Returns the hash code value for this map entry.  The hash code
 435          * of a map entry <tt>e</tt> is defined to be: <pre>
 436          *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
 437          *     (e.getValue()==null ? 0 : e.getValue().hashCode())
 438          * </pre>
 439          * This ensures that <tt>e1.equals(e2)</tt> implies that
 440          * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
 441          * <tt>e1</tt> and <tt>e2</tt>, as required by the general
 442          * contract of <tt>Object.hashCode</tt>.
 443          *
 444          * @return the hash code value for this map entry
 445          * @see Object#hashCode()
 446          * @see Object#equals(Object)
 447          * @see #equals(Object)
 448          */
 449         int hashCode();
 450 
 451         /**
 452          * Returns a comparator that compares {@link Map.Entry} in natural order on key.
 453          *
 454          * <p>The returned comparator is serializable and throws {@link
 455          * NullPointerException} when comparing a map with a null key.
 456          *
 457          * @param  <K> {@link Comparable} key type
 458          * @param  <V> value type
 459          * @return A comparator that compares {@link Map.Entry} in natural order on key.
 460          * @see Comparable
 461          */
 462         public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
 463             return (Comparator<Map.Entry<K, V>> & Serializable)
 464                 (c1, c2) -> c1.getKey().compareTo(c2.getKey());
 465         }
 466 
 467         /**
 468          * Returns a comparator that compares {@link Map.Entry} in natural order on value.
 469          *
 470          * <p>The returned comparator is serializable and throws {@link
 471          * NullPointerException} when comparing a map with null values.
 472          *
 473          * @param <K> key type
 474          * @param <V> {@link Comparable} value type
 475          * @return A comparator that compares {@link Map.Entry} in natural order on value.
 476          * @see Comparable
 477          */
 478         public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
 479             return (Comparator<Map.Entry<K, V>> & Serializable)
 480                 (c1, c2) -> c1.getValue().compareTo(c2.getValue());
 481         }
 482 
 483         /**
 484          * Returns a comparator that compares {@link Map.Entry} by key using the given
 485          * {@link Comparator}.
 486          *
 487          * <p>The returned comparator is serializable if the specified comparator
 488          * is also serializable.
 489          *
 490          * @param  <K> key type
 491          * @param  <V> value type
 492          * @param  cmp the key {@link Comparator}
 493          * @return A comparator that compares {@link Map.Entry} by the key.
 494          */
 495         public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
 496             Objects.requireNonNull(cmp);
 497             return (Comparator<Map.Entry<K, V>> & Serializable)
 498                 (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
 499         }
 500 
 501         /**
 502          * Returns a comparator that compares {@link Map.Entry} by value using the given
 503          * {@link Comparator}.
 504          *
 505          * <p>The returned comparator is serializable if the specified comparator
 506          * is also serializable.
 507          *
 508          * @param  <K> key type
 509          * @param  <V> value type
 510          * @param  cmp the value {@link Comparator}
 511          * @return A comparator that compares {@link Map.Entry} by the value.
 512          */
 513         public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
 514             Objects.requireNonNull(cmp);
 515             return (Comparator<Map.Entry<K, V>> & Serializable)
 516                 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
 517         }
 518     }
 519 
 520     // Comparison and hashing
 521 
 522     /**
 523      * Compares the specified object with this map for equality.  Returns
 524      * <tt>true</tt> if the given object is also a map and the two maps
 525      * represent the same mappings.  More formally, two maps <tt>m1</tt> and
 526      * <tt>m2</tt> represent the same mappings if
 527      * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the
 528      * <tt>equals</tt> method works properly across different implementations
 529      * of the <tt>Map</tt> interface.
 530      *
 531      * @param o object to be compared for equality with this map
 532      * @return <tt>true</tt> if the specified object is equal to this map
 533      */
 534     boolean equals(Object o);
 535 
 536     /**
 537      * Returns the hash code value for this map.  The hash code of a map is
 538      * defined to be the sum of the hash codes of each entry in the map's
 539      * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
 540      * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
 541      * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
 542      * {@link Object#hashCode}.
 543      *
 544      * @return the hash code value for this map
 545      * @see Map.Entry#hashCode()
 546      * @see Object#equals(Object)
 547      * @see #equals(Object)
 548      */
 549     int hashCode();
 550 
 551     // Defaultable methods
 552 
 553     /**
 554     *  Returns the value to which the specified key is mapped,
 555     *  or {@code defaultValue} if this map contains no mapping
 556     *  for the key.
 557     *
 558     * <p>The default implementation makes no guarantees about synchronization
 559     * or atomicity properties of this method. Any implementation providing
 560     * atomicity guarantees must override this method and document its
 561     * concurrency properties.
 562     *
 563     * @param key the key whose associated value is to be returned
 564     * @return the value to which the specified key is mapped, or
 565     * {@code defaultValue} if this map contains no mapping for the key
 566     * @throws ClassCastException if the key is of an inappropriate type for
 567     * this map
 568     * (<a href="Collection.html#optional-restrictions">optional</a>)
 569     * @throws NullPointerException if the specified key is null and this map
 570     * does not permit null keys
 571     * (<a href="Collection.html#optional-restrictions">optional</a>)
 572     */
 573     default V getOrDefault(Object key, V defaultValue) {
 574         V v;
 575         return (((v = get(key)) != null) || containsKey(key))
 576             ? v
 577             : defaultValue;
 578     }
 579 
 580     /**
 581      * Performs the given action on each entry in this map, in the order entries
 582      * are returned by an entry set iterator (which may be unspecified), until
 583      * all entries have been processed or the action throws an {@code Exception}.
 584      * Exceptions thrown by the action are relayed to the caller.
 585      *
 586      * <p>The default implementation should be overridden by implementations if
 587      * they can provide a more performant implementation than an iterator-based
 588      * one.
 589      *
 590      * <p>The default implementation makes no guarantees about synchronization
 591      * or atomicity properties of this method. Any implementation providing
 592      * atomicity guarantees must override this method and document its
 593      * concurrency properties.
 594      *
 595      * @implSpec The default implementation is equivalent to, for this
 596      * {@code map}:
 597      * <pre> {@code
 598      * for ((Map.Entry<K, V> entry : map.entrySet())
 599      *     action.accept(entry.getKey(), entry.getValue());
 600      * }</pre>
 601      *
 602      * @param action The action to be performed for each entry
 603      * @throws NullPointerException if the specified action is null
 604      * @throws ConcurrentModificationException if an entry is found to be
 605      * removed during iteration
 606      * @since 1.8
 607      */
 608     default void forEach(BiConsumer<? super K, ? super V> action) {
 609         Objects.requireNonNull(action);
 610         for (Map.Entry<K, V> entry : entrySet()) {
 611             K k;
 612             V v;
 613             try {
 614                 k = entry.getKey();
 615                 v = entry.getValue();
 616             } catch(IllegalStateException ise) {
 617                 // this usually means the entry is no longer in the map.
 618                 throw new ConcurrentModificationException(ise);
 619             }
 620             action.accept(k, v);
 621         }
 622     }
 623 
 624     /**
 625      * Replaces each entry's value with the result of invoking the given
 626      * function on that entry, in the order entries are returned by an entry
 627      * set iterator, until all entries have been processed or the function
 628      * throws an exception.
 629      *
 630      * <p>The default implementation makes no guarantees about synchronization
 631      * or atomicity properties of this method. Any implementation providing
 632      * atomicity guarantees must override this method and document its
 633      * concurrency properties.
 634      *
 635      * @implSpec
 636      * <p>The default implementation is equivalent to, for this {@code map}:
 637      * <pre> {@code
 638      * for ((Map.Entry<K, V> entry : map.entrySet())
 639      *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
 640      * }</pre>
 641      *
 642      * @param function the function to apply to each entry
 643      * @throws UnsupportedOperationException if the {@code set} operation
 644      * is not supported by this map's entry set iterator.
 645      * @throws ClassCastException if the class of a replacement value
 646      * prevents it from being stored in this map
 647      * @throws NullPointerException if the specified function is null, or the
 648      * specified replacement value is null, and this map does not permit null
 649      * values
 650      * @throws ClassCastException if a replacement value is of an inappropriate
 651      *         type for this map
 652      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 653      * @throws NullPointerException if function or a replacement value is null,
 654      *         and this map does not permit null keys or values
 655      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 656      * @throws IllegalArgumentException if some property of a replacement value
 657      *         prevents it from being stored in this map
 658      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 659      * @throws ConcurrentModificationException if an entry is found to be
 660      * removed during iteration
 661      * @since 1.8
 662      */
 663     default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
 664         Objects.requireNonNull(function);
 665         for (Map.Entry<K, V> entry : entrySet()) {
 666             K k;
 667             V v;
 668             try {
 669                 k = entry.getKey();
 670                 v = entry.getValue();
 671             } catch(IllegalStateException ise) {
 672                 // this usually means the entry is no longer in the map.
 673                 throw new ConcurrentModificationException(ise);
 674             }
 675 
 676             // ise thrown from function is not a cme.
 677             v = function.apply(k, v);
 678 
 679             try {
 680                 entry.setValue(v);
 681             } catch(IllegalStateException ise) {
 682                 // this usually means the entry is no longer in the map.
 683                 throw new ConcurrentModificationException(ise);
 684             }
 685         }
 686     }
 687 
 688     /**
 689      * If the specified key is not already associated with a value (or is mapped
 690      * to {@code null}) associates it with the given value and returns
 691      * {@code null}, else returns the current value.
 692      *
 693      * <p>The default implementation makes no guarantees about synchronization
 694      * or atomicity properties of this method. Any implementation providing
 695      * atomicity guarantees must override this method and document its
 696      * concurrency properties.
 697      *
 698      * @implSpec
 699      * The default implementation is equivalent to, for this {@code
 700      * map}:
 701      *
 702      * <pre> {@code
 703      * if (map.get(key) == null)
 704      *     return map.put(key, value);
 705      * else
 706      *     return map.get(key);
 707      * }</pre>
 708      *
 709      * @param key key with which the specified value is to be associated
 710      * @param value value to be associated with the specified key
 711      * @return the previous value associated with the specified key, or
 712      *         {@code null} if there was no mapping for the key.
 713      *         (A {@code null} return can also indicate that the map
 714      *         previously associated {@code null} with the key,
 715      *         if the implementation supports null values.)
 716      * @throws UnsupportedOperationException if the {@code put} operation
 717      *         is not supported by this map
 718      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 719      * @throws ClassCastException if the key or value is of an inappropriate
 720      *         type for this map
 721      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 722      * @throws NullPointerException if the specified key or value is null,
 723      *         and this map does not permit null keys or values
 724      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 725      * @throws IllegalArgumentException if some property of the specified key
 726      *         or value prevents it from being stored in this map
 727      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 728      * @throws ConcurrentModificationException if a modification of the map is
 729      * detected during insertion of the value.
 730      * @since 1.8
 731      */
 732     default V putIfAbsent(K key, V value) {
 733         V v = get(key);
 734         if (v == null) {
 735             if (put(key, value) != null) {
 736                 throw new ConcurrentModificationException();
 737             }
 738         }
 739 
 740         return v;
 741     }
 742 
 743     /**
 744      * Removes the entry for the specified key only if it is currently
 745      * mapped to the specified value.
 746      *
 747      * <p>The default implementation makes no guarantees about synchronization
 748      * or atomicity properties of this method. Any implementation providing
 749      * atomicity guarantees must override this method and document its
 750      * concurrency properties.
 751      *
 752      * @implSpec
 753      * The default implementation is equivalent to, for this {@code map}:
 754      *
 755      * <pre> {@code
 756      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 757      *     map.remove(key);
 758      *     return true;
 759      * } else
 760      *     return false;
 761      * }</pre>
 762      *
 763      * @param key key with which the specified value is associated
 764      * @param value value expected to be associated with the specified key
 765      * @return {@code true} if the value was removed
 766      * @throws UnsupportedOperationException if the {@code remove} operation
 767      *         is not supported by this map
 768      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 769      * @throws ClassCastException if the key or value is of an inappropriate
 770      *         type for this map
 771      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 772      * @throws NullPointerException if the specified key or value is null,
 773      *         and this map does not permit null keys or values
 774      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 775      * @since 1.8
 776      */
 777     default boolean remove(Object key, Object value) {
 778         Object curValue = get(key);
 779         if (!Objects.equals(curValue, value) ||
 780             (curValue == null && !containsKey(key))) {
 781             return false;
 782         }
 783         remove(key);
 784         return true;
 785     }
 786 
 787     /**
 788      * Replaces the entry for the specified key only if currently
 789      * mapped to the specified value.
 790      *
 791      * <p>The default implementation makes no guarantees about synchronization
 792      * or atomicity properties of this method. Any implementation providing
 793      * atomicity guarantees must override this method and document its
 794      * concurrency properties.
 795     *
 796      * @implSpec
 797      * The default implementation is equivalent to, for this {@code map}:
 798      *
 799      * <pre> {@code
 800      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 801      *     map.put(key, newValue);
 802      *     return true;
 803      * } else
 804      *     return false;
 805      * }</pre>
 806      *
 807      * @param key key with which the specified value is associated
 808      * @param oldValue value expected to be associated with the specified key
 809      * @param newValue value to be associated with the specified key
 810      * @return {@code true} if the value was replaced
 811      * @throws UnsupportedOperationException if the {@code put} operation
 812      *         is not supported by this map
 813      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 814      * @throws ClassCastException if the class of a specified key or value
 815      *         prevents it from being stored in this map
 816      * @throws NullPointerException if a specified key or value is null,
 817      *         and this map does not permit null keys or values
 818      * @throws IllegalArgumentException if some property of a specified key
 819      *         or value prevents it from being stored in this map
 820      * @since 1.8
 821      */
 822     default boolean replace(K key, V oldValue, V newValue) {
 823         Object curValue = get(key);
 824         if (!Objects.equals(curValue, oldValue) ||
 825             (curValue == null && !containsKey(key))) {
 826             return false;
 827         }
 828         put(key, newValue);
 829         return true;
 830     }
 831 
 832     /**
 833      * Replaces the entry for the specified key only if it is
 834      * currently mapped to some value.
 835      *
 836      * <p>The default implementation makes no guarantees about synchronization
 837      * or atomicity properties of this method. Any implementation providing
 838      * atomicity guarantees must override this method and document its
 839      * concurrency properties.
 840      *
 841      * @implSpec
 842      * The default implementation is equivalent to, for this {@code map}:
 843      *
 844      * <pre> {@code
 845      * if (map.containsKey(key)) {
 846      *     return map.put(key, value);
 847      * } else
 848      *     return null;
 849      * }</pre>
 850      *
 851      * @param key key with which the specified value is associated
 852      * @param value value to be associated with the specified key
 853      * @return the previous value associated with the specified key, or
 854      *         {@code null} if there was no mapping for the key.
 855      *         (A {@code null} return can also indicate that the map
 856      *         previously associated {@code null} with the key,
 857      *         if the implementation supports null values.)
 858      * @throws UnsupportedOperationException if the {@code put} operation
 859      *         is not supported by this map
 860      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 861      * @throws ClassCastException if the class of the specified key or value
 862      *         prevents it from being stored in this map
 863      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 864      * @throws NullPointerException if the specified key or value is null,
 865      *         and this map does not permit null keys or values
 866      * @throws IllegalArgumentException if some property of the specified key
 867      *         or value prevents it from being stored in this map
 868      * @since 1.8
 869      */
 870     default V replace(K key, V value) {
 871         return containsKey(key) ? put(key, value) : null;
 872     }
 873 
 874     /**
 875      * If the specified key is not already associated with a value (or
 876      * is mapped to {@code null}), attempts to compute its value using
 877      * the given mapping function and enters it into this map unless
 878      * {@code null}.
 879      *
 880      * <p>If the function returns {@code null} no mapping is recorded. If
 881      * the function itself throws an (unchecked) exception, the
 882      * exception is rethrown, and no mapping is recorded.  The most
 883      * common usage is to construct a new object serving as an initial
 884      * mapped value or memoized result, as in:
 885      *
 886      * <pre> {@code
 887      * map.computeIfAbsent(key, k -> new Value(f(k)));
 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. In particular, all implementations of
 894      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 895      * whether the function is applied once atomically only if the value is not
 896      * present.  Any class that permits null values must document
 897      * whether and how this method distinguishes absence from null mappings.
 898      *
 899      * @implSpec
 900      * The default implementation is equivalent to the following
 901      * steps for this {@code map}, then returning the current value or
 902      * {@code null} if now absent:
 903      *
 904      * <pre> {@code
 905      * if (map.get(key) == null) {
 906      *     V newValue = mappingFunction.apply(key);
 907      *     if (newValue != null)
 908      *         map.putIfAbsent(key, newValue);
 909      * }
 910      * }</pre>
 911      *
 912      * @param key key with which the specified value is to be associated
 913      * @param mappingFunction the function to compute a value
 914      * @return the current (existing or computed) value associated with
 915      *         the specified key, or null if the computed value is null
 916      * @throws NullPointerException if the specified key is null and
 917      *         this map does not support null keys, or the
 918      *         mappingFunction is null
 919      * @throws UnsupportedOperationException if the {@code put} operation
 920      *         is not supported by this map
 921      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 922      * @throws ClassCastException if the class of the specified key or value
 923      *         prevents it from being stored in this map
 924      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 925      * @since 1.8
 926      */
 927     default V computeIfAbsent(K key,
 928             Function<? super K, ? extends V> mappingFunction) {
 929         V v, newValue;
 930         return ((v = get(key)) == null &&
 931                 (newValue = mappingFunction.apply(key)) != null &&
 932                 (v = putIfAbsent(key, newValue)) == null) ? newValue : v;
 933     }
 934 
 935     /**
 936      * If the value for the specified key is present and non-null, attempts to
 937      * compute a new mapping given the key and its current mapped value.
 938      *
 939      * <p>If the function returns {@code null}, the mapping is removed.  If the
 940      * function itself throws an (unchecked) exception, the exception is
 941      * rethrown, and the current mapping is left unchanged.
 942      *
 943      * <p>The default implementation makes no guarantees about synchronization
 944      * or atomicity properties of this method. Any implementation providing
 945      * atomicity guarantees must override this method and document its
 946      * concurrency properties. In particular, all implementations of
 947      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 948      * whether the function is applied once atomically only if the value is not
 949      * present.  Any class that permits null values must document
 950      * whether and how this method distinguishes absence from null mappings.
 951      *
 952      * @implSpec
 953      * The default implementation is equivalent to performing the
 954      * following steps for this {@code map}, then returning the
 955      * current value or {@code null} if now absent:
 956      *
 957      * <pre> {@code
 958      * if (map.get(key) != null) {
 959      *     V oldValue = map.get(key);
 960      *     V newValue = remappingFunction.apply(key, oldValue);
 961      *     if (newValue != null)
 962      *         map.replace(key, oldValue, newValue);
 963      *     else
 964      *         map.remove(key, oldValue);
 965      * }
 966      * }</pre>
 967      *
 968      * In concurrent contexts, the default implementation may retry
 969      * these steps when multiple threads attempt updates.
 970      *
 971      * @param key key with which the specified value is to be associated
 972      * @param remappingFunction the function to compute a value
 973      * @return the new value associated with the specified key, or null if none
 974      * @throws NullPointerException if the specified key is null and
 975      *         this map does not support null keys, or the
 976      *         remappingFunction is null
 977      * @throws UnsupportedOperationException if the {@code put} operation
 978      *         is not supported by this map
 979      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 980      * @throws ClassCastException if the class of the specified key or value
 981      *         prevents it from being stored in this map
 982      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 983      * @since 1.8
 984      */
 985     default V computeIfPresent(K key,
 986             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
 987         V oldValue;
 988         while ((oldValue = get(key)) != null) {
 989             V newValue = remappingFunction.apply(key, oldValue);
 990             if (newValue != null) {
 991                 if (replace(key, oldValue, newValue))
 992                     return newValue;
 993             } else if (remove(key, oldValue))
 994                 return null;
 995         }
 996         return oldValue;
 997     }
 998 
 999     /**
1000      * Attempts to compute a mapping for the specified key and its
1001      * current mapped value (or {@code null} if there is no current
1002      * mapping). For example, to either create or append a {@code
1003      * String msg} to a value mapping:
1004      *
1005      * <pre> {@code
1006      * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
1007      * (Method {@link #merge merge()} is often simpler to use for such purposes.)
1008      *
1009      * <p>If the function returns {@code null}, the mapping is removed (or
1010      * remains absent if initially absent).  If the function itself throws an
1011      * (unchecked) exception, the exception is rethrown, and the current mapping
1012      * is left unchanged.
1013      *
1014      * <p>The default implementation makes no guarantees about synchronization
1015      * or atomicity properties of this method. Any implementation providing
1016      * atomicity guarantees must override this method and document its
1017      * concurrency properties. In particular, all implementations of
1018      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1019      * whether the function is applied once atomically only if the value is not
1020      * present.  Any class that permits null values must document
1021      * whether and how this method distinguishes absence from null mappings.
1022      *
1023      * @implSpec
1024      * The default implementation is equivalent to performing the following
1025      * steps for this {@code map}, then returning the current value or
1026      * {@code null} if absent:
1027      *
1028      * <pre> {@code
1029      * V oldValue = map.get(key);
1030      * V newValue = remappingFunction.apply(key, oldValue);
1031      * if (oldValue != null ) {
1032      *    if (newValue != null)
1033      *       map.replace(key, oldValue, newValue);
1034      *    else
1035      *       map.remove(key, oldValue);
1036      * } else {
1037      *    if (newValue != null)
1038      *       map.putIfAbsent(key, newValue);
1039      *    else
1040      *       return null;
1041      * }
1042      * }</pre>
1043      *
1044      * In concurrent contexts, the default implementation may retry
1045      * these steps when multiple threads attempt updates.
1046      *
1047      * @param key key with which the specified value is to be associated
1048      * @param remappingFunction the function to compute a value
1049      * @return the new value associated with the specified key, or null if none
1050      * @throws NullPointerException if the specified key is null and
1051      *         this map does not support null keys, or the
1052      *         remappingFunction is null
1053      * @throws UnsupportedOperationException if the {@code put} operation
1054      *         is not supported by this map
1055      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1056      * @throws ClassCastException if the class of the specified key or value
1057      *         prevents it from being stored in this map
1058      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1059      * @since 1.8
1060      */
1061     default V compute(K key,
1062             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1063         V oldValue = get(key);
1064         for (;;) {
1065             V newValue = remappingFunction.apply(key, oldValue);
1066             if (newValue == null) {
1067                 // delete mapping
1068                 if(oldValue != null || containsKey(key)) {
1069                     // something to remove
1070                     if (remove(key, oldValue)) {
1071                         // removed the old value as expected
1072                         return null;
1073                     }
1074 
1075                     // some other value replaced old value. try again.
1076                     oldValue = get(key);
1077                 } else {
1078                     // nothing to do. Leave things as they were.
1079                     return null;
1080                 }
1081             } else {
1082                 // add or replace old mapping
1083                 if (oldValue != null) {
1084                     // replace
1085                     if (replace(key, oldValue, newValue)) {
1086                         // replaced as expected.
1087                         return newValue;
1088                     }
1089 
1090                     // some other value replaced old value. try again.
1091                     oldValue = get(key);
1092                 } else {
1093                     // add (replace if oldValue was null)
1094                     if ((oldValue = putIfAbsent(key, newValue)) == null) {
1095                         // replaced
1096                         return newValue;
1097                     }
1098 
1099                     // some other value replaced old value. try again.
1100                 }
1101             }
1102         }
1103     }
1104 
1105     /**
1106      * If the specified key is not already associated with a value or is
1107      * associated with null, associates it with the given value.
1108      * Otherwise, replaces the value with the results of the given
1109      * remapping function, or removes if the result is {@code null}. This
1110      * method may be of use when combining multiple mapped values for a key.
1111      * For example, to either create or append a {@code String msg} to a
1112      * value mapping:
1113      *
1114      * <pre> {@code
1115      * map.merge(key, msg, String::concat)
1116      * }</pre>
1117      *
1118      * <p>If the function returns {@code null}, the mapping is removed (or
1119      * remains absent if initially absent).  If the function itself throws an
1120      * (unchecked) exception, the exception is rethrown, and the current mapping
1121      * is left unchanged.
1122      *
1123      * <p>The default implementation makes no guarantees about synchronization
1124      * or atomicity properties of this method. Any implementation providing
1125      * atomicity guarantees must override this method and document its
1126      * concurrency properties. In particular, all implementations of
1127      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1128      * whether the function is applied once atomically only if the value is not
1129      * present.  Any class that permits null values must document
1130      * whether and how this method distinguishes absence from null mappings.
1131      *
1132      * @implSpec
1133      * The default implementation is equivalent to performing the
1134      * following steps for this {@code map}, then returning the
1135      * current value or {@code null} if absent:
1136      *
1137      * <pre> {@code
1138      * V oldValue = map.get(key);
1139      * V newValue = (oldValue == null) ? value :
1140      *              remappingFunction.apply(oldValue, value);
1141      * if (newValue == null)
1142      *     map.remove(key, oldValue);
1143      * else if (oldValue == null)
1144      *     map.putIfAbsent(key, newValue);
1145      * else
1146      *     map.replace(key, oldValue, newValue);
1147      * }</pre>
1148      *
1149      * In concurrent contexts, the default implementation may retry
1150      * these steps when multiple threads attempt updates.
1151      *
1152      * @param key key with which the specified value is to be associated
1153      * @param value the value to use if absent
1154      * @param remappingFunction the function to recompute a value if present
1155      * @return the new value associated with the specified key, or null if none
1156      * @throws UnsupportedOperationException if the {@code put} operation
1157      *         is not supported by this map
1158      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1159      * @throws ClassCastException if the class of the specified key or value
1160      *         prevents it from being stored in this map
1161      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1162      * @throws NullPointerException if the specified key is null and
1163      *         this map does not support null keys, or the
1164      *         remappingFunction is null
1165      * @since 1.8
1166      */
1167     default V merge(K key, V value,
1168             BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
1169         V oldValue = get(key);
1170         for (;;) {
1171             if (oldValue != null) {
1172                 V newValue = remappingFunction.apply(oldValue, value);
1173                 if (newValue != null) {
1174                     if (replace(key, oldValue, newValue))
1175                         return newValue;
1176                 } else if (remove(key, oldValue)) {
1177                     return null;
1178                 }
1179                 oldValue = get(key);
1180             } else {
1181                 if (value == null) {
1182                     return null;
1183                 }
1184 
1185                 if ((oldValue = putIfAbsent(key, value)) == null) {
1186                     return value;
1187                 }
1188             }
1189         }
1190     }
1191 }