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>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 <tt>true</tt> if and
  93  * only if this map contains a mapping for a key <tt>k</tt> such that
  94  * <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
  95  * <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
  96  * with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
  97  * be invoked for any key <tt>k</tt>.  Implementations are free to
  98  * implement optimizations whereby the <tt>equals</tt> 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  * <p>This interface is a member of the
 114  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 115  * Java Collections Framework</a>.
 116  *
 117  * @param <K> the type of keys maintained by this map
 118  * @param <V> the type of mapped values
 119  *
 120  * @author  Josh Bloch
 121  * @see HashMap
 122  * @see TreeMap
 123  * @see Hashtable
 124  * @see SortedMap
 125  * @see Collection
 126  * @see Set
 127  * @since 1.2
 128  */
 129 public interface Map<K,V> {
 130     // Query Operations
 131 
 132     /**
 133      * Returns the number of key-value mappings in this map.  If the
 134      * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
 135      * <tt>Integer.MAX_VALUE</tt>.
 136      *
 137      * @return the number of key-value mappings in this map
 138      */
 139     int size();
 140 
 141     /**
 142      * Returns <tt>true</tt> if this map contains no key-value mappings.
 143      *
 144      * @return <tt>true</tt> if this map contains no key-value mappings
 145      */
 146     boolean isEmpty();
 147 
 148     /**
 149      * Returns <tt>true</tt> if this map contains a mapping for the specified
 150      * key.  More formally, returns <tt>true</tt> if and only if
 151      * this map contains a mapping for a key <tt>k</tt> such that
 152      * <tt>(key==null ? k==null : key.equals(k))</tt>.  (There can be
 153      * at most one such mapping.)
 154      *
 155      * @param key key whose presence in this map is to be tested
 156      * @return <tt>true</tt> if this map contains a mapping for the specified
 157      *         key
 158      * @throws ClassCastException if the key is of an inappropriate type for
 159      *         this map
 160      * (<a href="Collection.html#optional-restrictions">optional</a>)
 161      * @throws NullPointerException if the specified key is null and this map
 162      *         does not permit null keys
 163      * (<a href="Collection.html#optional-restrictions">optional</a>)
 164      */
 165     boolean containsKey(Object key);
 166 
 167     /**
 168      * Returns <tt>true</tt> if this map maps one or more keys to the
 169      * specified value.  More formally, returns <tt>true</tt> if and only if
 170      * this map contains at least one mapping to a value <tt>v</tt> such that
 171      * <tt>(value==null ? v==null : value.equals(v))</tt>.  This operation
 172      * will probably require time linear in the map size for most
 173      * implementations of the <tt>Map</tt> interface.
 174      *
 175      * @param value value whose presence in this map is to be tested
 176      * @return <tt>true</tt> if this map maps one or more keys to the
 177      *         specified value
 178      * @throws ClassCastException if the value is of an inappropriate type for
 179      *         this map
 180      * (<a href="Collection.html#optional-restrictions">optional</a>)
 181      * @throws NullPointerException if the specified value is null and this
 182      *         map does not permit null values
 183      * (<a href="Collection.html#optional-restrictions">optional</a>)
 184      */
 185     boolean containsValue(Object value);
 186 
 187     /**
 188      * Returns the value to which the specified key is mapped,
 189      * or {@code null} if this map contains no mapping for the key.
 190      *
 191      * <p>More formally, if this map contains a mapping from a key
 192      * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
 193      * key.equals(k))}, then this method returns {@code v}; otherwise
 194      * it returns {@code null}.  (There can be at most one such mapping.)
 195      *
 196      * <p>If this map permits null values, then a return value of
 197      * {@code null} does not <i>necessarily</i> indicate that the map
 198      * contains no mapping for the key; it's also possible that the map
 199      * explicitly maps the key to {@code null}.  The {@link #containsKey
 200      * containsKey} operation may be used to distinguish these two cases.
 201      *
 202      * @param key the key whose associated value is to be returned
 203      * @return the value to which the specified key is mapped, or
 204      *         {@code null} if this map contains no mapping for the key
 205      * @throws ClassCastException if the key is of an inappropriate type for
 206      *         this map
 207      * (<a href="Collection.html#optional-restrictions">optional</a>)
 208      * @throws NullPointerException if the specified key is null and this map
 209      *         does not permit null keys
 210      * (<a href="Collection.html#optional-restrictions">optional</a>)
 211      */
 212     V get(Object key);
 213 
 214     // Modification Operations
 215 
 216     /**
 217      * Associates the specified value with the specified key in this map
 218      * (optional operation).  If the map previously contained a mapping for
 219      * the key, the old value is replaced by the specified value.  (A map
 220      * <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
 221      * if {@link #containsKey(Object) m.containsKey(k)} would return
 222      * <tt>true</tt>.)
 223      *
 224      * @param key key with which the specified value is to be associated
 225      * @param value value to be associated with the specified key
 226      * @return the previous value associated with <tt>key</tt>, or
 227      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 228      *         (A <tt>null</tt> return can also indicate that the map
 229      *         previously associated <tt>null</tt> with <tt>key</tt>,
 230      *         if the implementation supports <tt>null</tt> values.)
 231      * @throws UnsupportedOperationException if the <tt>put</tt> operation
 232      *         is not supported by this map
 233      * @throws ClassCastException if the class of the specified key or value
 234      *         prevents it from being stored in this map
 235      * @throws NullPointerException if the specified key or value is null
 236      *         and this map does not permit null keys or values
 237      * @throws IllegalArgumentException if some property of the specified key
 238      *         or value prevents it from being stored in this map
 239      */
 240     V put(K key, V value);
 241 
 242     /**
 243      * Removes the mapping for a key from this map if it is present
 244      * (optional operation).   More formally, if this map contains a mapping
 245      * from key <tt>k</tt> to value <tt>v</tt> such that
 246      * <code>(key==null ?  k==null : key.equals(k))</code>, that mapping
 247      * is removed.  (The map can contain at most one such mapping.)
 248      *
 249      * <p>Returns the value to which this map previously associated the key,
 250      * or <tt>null</tt> if the map contained no mapping for the key.
 251      *
 252      * <p>If this map permits null values, then a return value of
 253      * <tt>null</tt> does not <i>necessarily</i> indicate that the map
 254      * contained no mapping for the key; it's also possible that the map
 255      * explicitly mapped the key to <tt>null</tt>.
 256      *
 257      * <p>The map will not contain a mapping for the specified key once the
 258      * call returns.
 259      *
 260      * @param key key whose mapping is to be removed from the map
 261      * @return the previous value associated with <tt>key</tt>, or
 262      *         <tt>null</tt> if there was no mapping for <tt>key</tt>.
 263      * @throws UnsupportedOperationException if the <tt>remove</tt> operation
 264      *         is not supported by this map
 265      * @throws ClassCastException if the key is of an inappropriate type for
 266      *         this map
 267      * (<a href="Collection.html#optional-restrictions">optional</a>)
 268      * @throws NullPointerException if the specified key is null and this
 269      *         map does not permit null keys
 270      * (<a href="Collection.html#optional-restrictions">optional</a>)
 271      */
 272     V remove(Object key);
 273 
 274 
 275     // Bulk Operations
 276 
 277     /**
 278      * Copies all of the mappings from the specified map to this map
 279      * (optional operation).  The effect of this call is equivalent to that
 280      * of calling {@link #put(Object,Object) put(k, v)} on this map once
 281      * for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
 282      * specified map.  The behavior of this operation is undefined if the
 283      * specified map is modified while the operation is in progress.
 284      *
 285      * @param m mappings to be stored in this map
 286      * @throws UnsupportedOperationException if the <tt>putAll</tt> operation
 287      *         is not supported by this map
 288      * @throws ClassCastException if the class of a key or value in the
 289      *         specified map prevents it from being stored in this map
 290      * @throws NullPointerException if the specified map is null, or if
 291      *         this map does not permit null keys or values, and the
 292      *         specified map contains null keys or values
 293      * @throws IllegalArgumentException if some property of a key or value in
 294      *         the specified map prevents it from being stored in this map
 295      */
 296     void putAll(Map<? extends K, ? extends V> m);
 297 
 298     /**
 299      * Removes all of the mappings from this map (optional operation).
 300      * The map will be empty after this call returns.
 301      *
 302      * @throws UnsupportedOperationException if the <tt>clear</tt> operation
 303      *         is not supported by this map
 304      */
 305     void clear();
 306 
 307 
 308     // Views
 309 
 310     /**
 311      * Returns a {@link Set} view of the keys contained in this map.
 312      * The set is backed by the map, so changes to the map are
 313      * reflected in the set, and vice-versa.  If the map is modified
 314      * while an iteration over the set is in progress (except through
 315      * the iterator's own <tt>remove</tt> operation), the results of
 316      * the iteration are undefined.  The set supports element removal,
 317      * which removes the corresponding mapping from the map, via the
 318      * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
 319      * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
 320      * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>
 321      * operations.
 322      *
 323      * @return a set view of the keys contained in this map
 324      */
 325     Set<K> keySet();
 326 
 327     /**
 328      * Returns a {@link Collection} view of the values contained in this map.
 329      * The collection is backed by the map, so changes to the map are
 330      * reflected in the collection, and vice-versa.  If the map is
 331      * modified while an iteration over the collection is in progress
 332      * (except through the iterator's own <tt>remove</tt> operation),
 333      * the results of the iteration are undefined.  The collection
 334      * supports element removal, which removes the corresponding
 335      * mapping from the map, via the <tt>Iterator.remove</tt>,
 336      * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
 337      * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not
 338      * support the <tt>add</tt> or <tt>addAll</tt> operations.
 339      *
 340      * @return a collection view of the values contained in this map
 341      */
 342     Collection<V> values();
 343 
 344     /**
 345      * Returns a {@link Set} view of the mappings contained in this map.
 346      * The set is backed by the map, so changes to the map are
 347      * reflected in the set, and vice-versa.  If the map is modified
 348      * while an iteration over the set is in progress (except through
 349      * the iterator's own <tt>remove</tt> operation, or through the
 350      * <tt>setValue</tt> operation on a map entry returned by the
 351      * iterator) the results of the iteration are undefined.  The set
 352      * supports element removal, which removes the corresponding
 353      * mapping from the map, via the <tt>Iterator.remove</tt>,
 354      * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
 355      * <tt>clear</tt> operations.  It does not support the
 356      * <tt>add</tt> or <tt>addAll</tt> operations.
 357      *
 358      * @return a set view of the mappings contained in this map
 359      */
 360     Set<Map.Entry<K, V>> entrySet();
 361 
 362     /**
 363      * A map entry (key-value pair).  The <tt>Map.entrySet</tt> method returns
 364      * a collection-view of the map, whose elements are of this class.  The
 365      * <i>only</i> way to obtain a reference to a map entry is from the
 366      * iterator of this collection-view.  These <tt>Map.Entry</tt> objects are
 367      * valid <i>only</i> for the duration of the iteration; more formally,
 368      * the behavior of a map entry is undefined if the backing map has been
 369      * modified after the entry was returned by the iterator, except through
 370      * the <tt>setValue</tt> operation on the map entry.
 371      *
 372      * @see Map#entrySet()
 373      * @since 1.2
 374      */
 375     interface Entry<K,V> {
 376         /**
 377          * Returns the key corresponding to this entry.
 378          *
 379          * @return the key corresponding to this entry
 380          * @throws IllegalStateException implementations may, but are not
 381          *         required to, throw this exception if the entry has been
 382          *         removed from the backing map.
 383          */
 384         K getKey();
 385 
 386         /**
 387          * Returns the value corresponding to this entry.  If the mapping
 388          * has been removed from the backing map (by the iterator's
 389          * <tt>remove</tt> operation), the results of this call are undefined.
 390          *
 391          * @return the value corresponding to this entry
 392          * @throws IllegalStateException implementations may, but are not
 393          *         required to, throw this exception if the entry has been
 394          *         removed from the backing map.
 395          */
 396         V getValue();
 397 
 398         /**
 399          * Replaces the value corresponding to this entry with the specified
 400          * value (optional operation).  (Writes through to the map.)  The
 401          * behavior of this call is undefined if the mapping has already been
 402          * removed from the map (by the iterator's <tt>remove</tt> operation).
 403          *
 404          * @param value new value to be stored in this entry
 405          * @return old value corresponding to the entry
 406          * @throws UnsupportedOperationException if the <tt>put</tt> operation
 407          *         is not supported by the backing map
 408          * @throws ClassCastException if the class of the specified value
 409          *         prevents it from being stored in the backing map
 410          * @throws NullPointerException if the backing map does not permit
 411          *         null values, and the specified value is null
 412          * @throws IllegalArgumentException if some property of this value
 413          *         prevents it from being stored in the backing map
 414          * @throws IllegalStateException implementations may, but are not
 415          *         required to, throw this exception if the entry has been
 416          *         removed from the backing map.
 417          */
 418         V setValue(V value);
 419 
 420         /**
 421          * Compares the specified object with this entry for equality.
 422          * Returns <tt>true</tt> if the given object is also a map entry and
 423          * the two entries represent the same mapping.  More formally, two
 424          * entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
 425          * if<pre>
 426          *     (e1.getKey()==null ?
 427          *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &amp;&amp;
 428          *     (e1.getValue()==null ?
 429          *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
 430          * </pre>
 431          * This ensures that the <tt>equals</tt> method works properly across
 432          * different implementations of the <tt>Map.Entry</tt> interface.
 433          *
 434          * @param o object to be compared for equality with this map entry
 435          * @return <tt>true</tt> if the specified object is equal to this map
 436          *         entry
 437          */
 438         boolean equals(Object o);
 439 
 440         /**
 441          * Returns the hash code value for this map entry.  The hash code
 442          * of a map entry <tt>e</tt> is defined to be: <pre>
 443          *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
 444          *     (e.getValue()==null ? 0 : e.getValue().hashCode())
 445          * </pre>
 446          * This ensures that <tt>e1.equals(e2)</tt> implies that
 447          * <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
 448          * <tt>e1</tt> and <tt>e2</tt>, as required by the general
 449          * contract of <tt>Object.hashCode</tt>.
 450          *
 451          * @return the hash code value for this map entry
 452          * @see Object#hashCode()
 453          * @see Object#equals(Object)
 454          * @see #equals(Object)
 455          */
 456         int hashCode();
 457 
 458         /**
 459          * Returns a comparator that compares {@link Map.Entry} in natural order on key.
 460          *
 461          * <p>The returned comparator is serializable and throws {@link
 462          * NullPointerException} when comparing an entry with a null key.
 463          *
 464          * @param  <K> the {@link Comparable} type of then map keys
 465          * @param  <V> the type of the map values
 466          * @return a comparator that compares {@link Map.Entry} in natural order on key.
 467          * @see Comparable
 468          */
 469         public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
 470             return (Comparator<Map.Entry<K, V>> & Serializable)
 471                 (c1, c2) -> c1.getKey().compareTo(c2.getKey());
 472         }
 473 
 474         /**
 475          * Returns a comparator that compares {@link Map.Entry} in natural order on value.
 476          *
 477          * <p>The returned comparator is serializable and throws {@link
 478          * NullPointerException} when comparing an entry with null values.
 479          *
 480          * @param <K> the type of the map keys
 481          * @param <V> the {@link Comparable} type of the map values
 482          * @return a comparator that compares {@link Map.Entry} in natural order on value.
 483          * @see Comparable
 484          */
 485         public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
 486             return (Comparator<Map.Entry<K, V>> & Serializable)
 487                 (c1, c2) -> c1.getValue().compareTo(c2.getValue());
 488         }
 489 
 490         /**
 491          * Returns a comparator that compares {@link Map.Entry} by key using the given
 492          * {@link Comparator}.
 493          *
 494          * <p>The returned comparator is serializable if the specified comparator
 495          * is also serializable.
 496          *
 497          * @param  <K> the type of the map keys
 498          * @param  <V> the type of the map values
 499          * @param  cmp the key {@link Comparator}
 500          * @return a comparator that compares {@link Map.Entry} by the key.
 501          */
 502         public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
 503             Objects.requireNonNull(cmp);
 504             return (Comparator<Map.Entry<K, V>> & Serializable)
 505                 (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
 506         }
 507 
 508         /**
 509          * Returns a comparator that compares {@link Map.Entry} by value using the given
 510          * {@link Comparator}.
 511          *
 512          * <p>The returned comparator is serializable if the specified comparator
 513          * is also serializable.
 514          *
 515          * @param  <K> the type of the map keys
 516          * @param  <V> the type of the map values
 517          * @param  cmp the value {@link Comparator}
 518          * @return a comparator that compares {@link Map.Entry} by the value.
 519          */
 520         public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
 521             Objects.requireNonNull(cmp);
 522             return (Comparator<Map.Entry<K, V>> & Serializable)
 523                 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
 524         }
 525     }
 526 
 527     // Comparison and hashing
 528 
 529     /**
 530      * Compares the specified object with this map for equality.  Returns
 531      * <tt>true</tt> if the given object is also a map and the two maps
 532      * represent the same mappings.  More formally, two maps <tt>m1</tt> and
 533      * <tt>m2</tt> represent the same mappings if
 534      * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the
 535      * <tt>equals</tt> method works properly across different implementations
 536      * of the <tt>Map</tt> interface.
 537      *
 538      * @param o object to be compared for equality with this map
 539      * @return <tt>true</tt> if the specified object is equal to this map
 540      */
 541     boolean equals(Object o);
 542 
 543     /**
 544      * Returns the hash code value for this map.  The hash code of a map is
 545      * defined to be the sum of the hash codes of each entry in the map's
 546      * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
 547      * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
 548      * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
 549      * {@link Object#hashCode}.
 550      *
 551      * @return the hash code value for this map
 552      * @see Map.Entry#hashCode()
 553      * @see Object#equals(Object)
 554      * @see #equals(Object)
 555      */
 556     int hashCode();
 557 
 558     // Defaultable methods
 559 
 560     /**
 561     *  Returns the value to which the specified key is mapped,
 562     *  or {@code defaultValue} if this map contains no mapping
 563     *  for the key.
 564     *
 565     * <p>The default implementation makes no guarantees about synchronization
 566     * or atomicity properties of this method. Any implementation providing
 567     * atomicity guarantees must override this method and document its
 568     * concurrency properties.
 569     *
 570     * @param key the key whose associated value is to be returned
 571     * @param defaultValue the default mapping of the key
 572     * @return the value to which the specified key is mapped, or
 573     * {@code defaultValue} if this map contains no mapping for the key
 574     * @throws ClassCastException if the key is of an inappropriate type for
 575     * this map
 576     * (<a href="Collection.html#optional-restrictions">optional</a>)
 577     * @throws NullPointerException if the specified key is null and this map
 578     * does not permit null keys
 579     * (<a href="Collection.html#optional-restrictions">optional</a>)
 580     */
 581     default V getOrDefault(Object key, V defaultValue) {
 582         V v;
 583         return (((v = get(key)) != null) || containsKey(key))
 584             ? v
 585             : defaultValue;
 586     }
 587 
 588     /**
 589      * Performs the given action on each entry in this map, in the order entries
 590      * are returned by an entry set iterator (which may be unspecified), until
 591      * all entries have been processed or the action throws an {@code Exception}.
 592      * Exceptions thrown by the action are relayed to the caller.
 593      *
 594      * <p>The default implementation should be overridden by implementations if
 595      * they can provide a more performant implementation than an iterator-based
 596      * one.
 597      *
 598      * <p>The default implementation makes no guarantees about synchronization
 599      * or atomicity properties of this method. Any implementation providing
 600      * atomicity guarantees must override this method and document its
 601      * concurrency properties.
 602      *
 603      * @implSpec The default implementation is equivalent to, for this
 604      * {@code map}:
 605      * <pre> {@code
 606      * for ((Map.Entry<K, V> entry : map.entrySet())
 607      *     action.accept(entry.getKey(), entry.getValue());
 608      * }</pre>
 609      *
 610      * @param action The action to be performed for each entry
 611      * @throws NullPointerException if the specified action is null
 612      * @throws ConcurrentModificationException if an entry is found to be
 613      * removed during iteration
 614      * @since 1.8
 615      */
 616     default void forEach(BiConsumer<? super K, ? super V> action) {
 617         Objects.requireNonNull(action);
 618         for (Map.Entry<K, V> entry : entrySet()) {
 619             K k;
 620             V v;
 621             try {
 622                 k = entry.getKey();
 623                 v = entry.getValue();
 624             } catch(IllegalStateException ise) {
 625                 // this usually means the entry is no longer in the map.
 626                 throw new ConcurrentModificationException(ise);
 627             }
 628             action.accept(k, v);
 629         }
 630     }
 631 
 632     /**
 633      * Replaces each entry's value with the result of invoking the given
 634      * function on that entry, in the order entries are returned by an entry
 635      * set iterator, until all entries have been processed or the function
 636      * throws an exception.
 637      *
 638      * <p>The default implementation makes no guarantees about synchronization
 639      * or atomicity properties of this method. Any implementation providing
 640      * atomicity guarantees must override this method and document its
 641      * concurrency properties.
 642      *
 643      * @implSpec
 644      * <p>The default implementation is equivalent to, for this {@code map}:
 645      * <pre> {@code
 646      * for ((Map.Entry<K, V> entry : map.entrySet())
 647      *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
 648      * }</pre>
 649      *
 650      * @param function the function to apply to each entry
 651      * @throws UnsupportedOperationException if the {@code set} operation
 652      * is not supported by this map's entry set iterator.
 653      * @throws ClassCastException if the class of a replacement value
 654      * prevents it from being stored in this map
 655      * @throws NullPointerException if the specified function is null, or the
 656      * specified replacement value is null, and this map does not permit null
 657      * values
 658      * @throws ClassCastException if a replacement value is of an inappropriate
 659      *         type for this map
 660      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 661      * @throws NullPointerException if function or a replacement value is null,
 662      *         and this map does not permit null keys or values
 663      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 664      * @throws IllegalArgumentException if some property of a replacement value
 665      *         prevents it from being stored in this map
 666      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 667      * @throws ConcurrentModificationException if an entry is found to be
 668      * removed during iteration
 669      * @since 1.8
 670      */
 671     default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
 672         Objects.requireNonNull(function);
 673         for (Map.Entry<K, V> entry : entrySet()) {
 674             K k;
 675             V v;
 676             try {
 677                 k = entry.getKey();
 678                 v = entry.getValue();
 679             } catch(IllegalStateException ise) {
 680                 // this usually means the entry is no longer in the map.
 681                 throw new ConcurrentModificationException(ise);
 682             }
 683 
 684             // ise thrown from function is not a cme.
 685             v = function.apply(k, v);
 686 
 687             try {
 688                 entry.setValue(v);
 689             } catch(IllegalStateException ise) {
 690                 // this usually means the entry is no longer in the map.
 691                 throw new ConcurrentModificationException(ise);
 692             }
 693         }
 694     }
 695 
 696     /**
 697      * If the specified key is not already associated with a value (or is mapped
 698      * to {@code null}) associates it with the given value and returns
 699      * {@code null}, else returns the current value.
 700      *
 701      * <p>The default implementation makes no guarantees about synchronization
 702      * or atomicity properties of this method. Any implementation providing
 703      * atomicity guarantees must override this method and document its
 704      * concurrency properties.
 705      *
 706      * @implSpec
 707      * The default implementation is equivalent to, for this {@code
 708      * map}:
 709      *
 710      * <pre> {@code
 711      * if (map.get(key) == null)
 712      *     return map.put(key, value);
 713      * else
 714      *     return map.get(key);
 715      * }</pre>
 716      *
 717      * @param key key with which the specified value is to be associated
 718      * @param value value to be associated with the specified key
 719      * @return the previous value associated with the specified key, or
 720      *         {@code null} if there was no mapping for the key.
 721      *         (A {@code null} return can also indicate that the map
 722      *         previously associated {@code null} with the key,
 723      *         if the implementation supports null values.)
 724      * @throws UnsupportedOperationException if the {@code put} operation
 725      *         is not supported by this map
 726      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 727      * @throws ClassCastException if the key or value is of an inappropriate
 728      *         type for this map
 729      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 730      * @throws NullPointerException if the specified key or value is null,
 731      *         and this map does not permit null keys or values
 732      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 733      * @throws IllegalArgumentException if some property of the specified key
 734      *         or value prevents it from being stored in this map
 735      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 736      * @throws ConcurrentModificationException if a modification of the map is
 737      * detected during insertion of the value.
 738      * @since 1.8
 739      */
 740     default V putIfAbsent(K key, V value) {
 741         V v = get(key);
 742         if (v == null) {
 743             if (put(key, value) != null) {
 744                 throw new ConcurrentModificationException();
 745             }
 746         }
 747 
 748         return v;
 749     }
 750 
 751     /**
 752      * Removes the entry for the specified key only if it is currently
 753      * mapped to the specified value.
 754      *
 755      * <p>The default implementation makes no guarantees about synchronization
 756      * or atomicity properties of this method. Any implementation providing
 757      * atomicity guarantees must override this method and document its
 758      * concurrency properties.
 759      *
 760      * @implSpec
 761      * The default implementation is equivalent to, for this {@code map}:
 762      *
 763      * <pre> {@code
 764      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 765      *     map.remove(key);
 766      *     return true;
 767      * } else
 768      *     return false;
 769      * }</pre>
 770      *
 771      * @param key key with which the specified value is associated
 772      * @param value value expected to be associated with the specified key
 773      * @return {@code true} if the value was removed
 774      * @throws UnsupportedOperationException if the {@code remove} operation
 775      *         is not supported by this map
 776      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 777      * @throws ClassCastException if the key or value is of an inappropriate
 778      *         type for this map
 779      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 780      * @throws NullPointerException if the specified key or value is null,
 781      *         and this map does not permit null keys or values
 782      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 783      * @since 1.8
 784      */
 785     default boolean remove(Object key, Object value) {
 786         Object curValue = get(key);
 787         if (!Objects.equals(curValue, value) ||
 788             (curValue == null && !containsKey(key))) {
 789             return false;
 790         }
 791         remove(key);
 792         return true;
 793     }
 794 
 795     /**
 796      * Replaces the entry for the specified key only if currently
 797      * mapped to the specified value.
 798      *
 799      * <p>The default implementation makes no guarantees about synchronization
 800      * or atomicity properties of this method. Any implementation providing
 801      * atomicity guarantees must override this method and document its
 802      * concurrency properties.
 803     *
 804      * @implSpec
 805      * The default implementation is equivalent to, for this {@code map}:
 806      *
 807      * <pre> {@code
 808      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 809      *     map.put(key, newValue);
 810      *     return true;
 811      * } else
 812      *     return false;
 813      * }</pre>
 814      *
 815      * The default implementation does not throw NullPointerException
 816      * for maps that do not support null values if oldValue is null unless
 817      * newValue is also null.
 818      *
 819      * @param key key with which the specified value is associated
 820      * @param oldValue value expected to be associated with the specified key
 821      * @param newValue value to be associated with the specified key
 822      * @return {@code true} if the value was replaced
 823      * @throws UnsupportedOperationException if the {@code put} operation
 824      *         is not supported by this map
 825      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 826      * @throws ClassCastException if the class of a specified key or value
 827      *         prevents it from being stored in this map
 828      * @throws NullPointerException if a specified key or newValue is null,
 829      *         and this map does not permit null keys or values
 830      * @throws NullPointerException if oldValue is null and this map does not
 831      *         permit null values
 832      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 833      * @throws IllegalArgumentException if some property of a specified key
 834      *         or value prevents it from being stored in this map
 835      * @since 1.8
 836      */
 837     default boolean replace(K key, V oldValue, V newValue) {
 838         Object curValue = get(key);
 839         if (!Objects.equals(curValue, oldValue) ||
 840             (curValue == null && !containsKey(key))) {
 841             return false;
 842         }
 843         put(key, newValue);
 844         return true;
 845     }
 846 
 847     /**
 848      * Replaces the entry for the specified key only if it is
 849      * currently mapped to some value.
 850      *
 851      * <p>The default implementation makes no guarantees about synchronization
 852      * or atomicity properties of this method. Any implementation providing
 853      * atomicity guarantees must override this method and document its
 854      * concurrency properties.
 855      *
 856      * @implSpec
 857      * The default implementation is equivalent to, for this {@code map}:
 858      *
 859      * <pre> {@code
 860      * if (map.containsKey(key)) {
 861      *     return map.put(key, value);
 862      * } else
 863      *     return null;
 864      * }</pre>
 865      *
 866      * @param key key with which the specified value is associated
 867      * @param value value to be associated with the specified key
 868      * @return the previous value associated with the specified key, or
 869      *         {@code null} if there was no mapping for the key.
 870      *         (A {@code null} return can also indicate that the map
 871      *         previously associated {@code null} with the key,
 872      *         if the implementation supports null values.)
 873      * @throws UnsupportedOperationException if the {@code put} operation
 874      *         is not supported by this map
 875      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 876      * @throws ClassCastException if the class of the specified key or value
 877      *         prevents it from being stored in this map
 878      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 879      * @throws NullPointerException if the specified key or value is null,
 880      *         and this map does not permit null keys or values
 881      * @throws IllegalArgumentException if some property of the specified key
 882      *         or value prevents it from being stored in this map
 883      * @since 1.8
 884      */
 885     default V replace(K key, V value) {
 886         return containsKey(key) ? put(key, value) : null;
 887     }
 888 
 889     /**
 890      * If the specified key is not already associated with a value (or
 891      * is mapped to {@code null}), attempts to compute its value using
 892      * the given mapping function and enters it into this map unless
 893      * {@code null}.
 894      *
 895      * <p>If the function returns {@code null} no mapping is recorded. If
 896      * the function itself throws an (unchecked) exception, the
 897      * exception is rethrown, and no mapping is recorded.  The most
 898      * common usage is to construct a new object serving as an initial
 899      * mapped value or memoized result, as in:
 900      *
 901      * <pre> {@code
 902      * map.computeIfAbsent(key, k -> new Value(f(k)));
 903      * }</pre>
 904      *
 905      * <p>The default implementation makes no guarantees about synchronization
 906      * or atomicity properties of this method. Any implementation providing
 907      * atomicity guarantees must override this method and document its
 908      * concurrency properties. In particular, all implementations of
 909      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 910      * whether the function is applied once atomically only if the value is not
 911      * present.  Any class that permits null values must document
 912      * whether and how this method distinguishes absence from null mappings.
 913      *
 914      * @implSpec
 915      * The default implementation is equivalent to the following
 916      * steps for this {@code map}, then returning the current value or
 917      * {@code null} if now absent:
 918      *
 919      * <pre> {@code
 920      * if (map.get(key) == null) {
 921      *     V newValue = mappingFunction.apply(key);
 922      *     if (newValue != null)
 923      *         map.putIfAbsent(key, newValue);
 924      * }
 925      * }</pre>
 926      *
 927      * @param key key with which the specified value is to be associated
 928      * @param mappingFunction the function to compute a value
 929      * @return the current (existing or computed) value associated with
 930      *         the specified key, or null if the computed value is null
 931      * @throws NullPointerException if the specified key is null and
 932      *         this map does not support null keys, or the
 933      *         mappingFunction is null
 934      * @throws UnsupportedOperationException if the {@code put} operation
 935      *         is not supported by this map
 936      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 937      * @throws ClassCastException if the class of the specified key or value
 938      *         prevents it from being stored in this map
 939      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 940      * @since 1.8
 941      */
 942     default V computeIfAbsent(K key,
 943             Function<? super K, ? extends V> mappingFunction) {
 944         Objects.requireNonNull(mappingFunction);
 945         V v, newValue;
 946         return ((v = get(key)) == null &&
 947                 (newValue = mappingFunction.apply(key)) != null &&
 948                 (v = putIfAbsent(key, newValue)) == null) ? newValue : v;
 949     }
 950 
 951     /**
 952      * If the value for the specified key is present and non-null, attempts to
 953      * compute a new mapping given the key and its current mapped value.
 954      *
 955      * <p>If the function returns {@code null}, the mapping is removed.  If the
 956      * function itself throws an (unchecked) exception, the exception is
 957      * rethrown, and the current mapping is left unchanged.
 958      *
 959      * <p>The default implementation makes no guarantees about synchronization
 960      * or atomicity properties of this method. Any implementation providing
 961      * atomicity guarantees must override this method and document its
 962      * concurrency properties. In particular, all implementations of
 963      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 964      * whether the function is applied once atomically only if the value is not
 965      * present.  Any class that permits null values must document
 966      * whether and how this method distinguishes absence from null mappings.
 967      *
 968      * @implSpec
 969      * The default implementation is equivalent to performing the
 970      * following steps for this {@code map}, then returning the
 971      * current value or {@code null} if now absent:
 972      *
 973      * <pre> {@code
 974      * if (map.get(key) != null) {
 975      *     V oldValue = map.get(key);
 976      *     V newValue = remappingFunction.apply(key, oldValue);
 977      *     if (newValue != null)
 978      *         map.replace(key, oldValue, newValue);
 979      *     else
 980      *         map.remove(key, oldValue);
 981      * }
 982      * }</pre>
 983      *
 984      * In concurrent contexts, the default implementation may retry
 985      * these steps when multiple threads attempt updates.
 986      *
 987      * @param key key with which the specified value is to be associated
 988      * @param remappingFunction the function to compute a value
 989      * @return the new value associated with the specified key, or null if none
 990      * @throws NullPointerException if the specified key is null and
 991      *         this map does not support null keys, or the
 992      *         remappingFunction is null
 993      * @throws UnsupportedOperationException if the {@code put} operation
 994      *         is not supported by this map
 995      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 996      * @throws ClassCastException if the class of the specified key or value
 997      *         prevents it from being stored in this map
 998      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 999      * @since 1.8
1000      */
1001     default V computeIfPresent(K key,
1002             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1003         Objects.requireNonNull(remappingFunction);
1004         V oldValue;
1005         while ((oldValue = get(key)) != null) {
1006             V newValue = remappingFunction.apply(key, oldValue);
1007             if (newValue != null) {
1008                 if (replace(key, oldValue, newValue))
1009                     return newValue;
1010             } else if (remove(key, oldValue))
1011                 return null;
1012         }
1013         return oldValue;
1014     }
1015 
1016     /**
1017      * Attempts to compute a mapping for the specified key and its
1018      * current mapped value (or {@code null} if there is no current
1019      * mapping). For example, to either create or append a {@code
1020      * String msg} to a value mapping:
1021      *
1022      * <pre> {@code
1023      * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
1024      * (Method {@link #merge merge()} is often simpler to use for such purposes.)
1025      *
1026      * <p>If the function returns {@code null}, the mapping is removed (or
1027      * remains absent if initially absent).  If the function itself throws an
1028      * (unchecked) exception, the exception is rethrown, and the current mapping
1029      * is left unchanged.
1030      *
1031      * <p>The default implementation makes no guarantees about synchronization
1032      * or atomicity properties of this method. Any implementation providing
1033      * atomicity guarantees must override this method and document its
1034      * concurrency properties. In particular, all implementations of
1035      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1036      * whether the function is applied once atomically only if the value is not
1037      * present.  Any class that permits null values must document
1038      * whether and how this method distinguishes absence from null mappings.
1039      *
1040      * @implSpec
1041      * The default implementation is equivalent to performing the following
1042      * steps for this {@code map}, then returning the current value or
1043      * {@code null} if absent:
1044      *
1045      * <pre> {@code
1046      * V oldValue = map.get(key);
1047      * V newValue = remappingFunction.apply(key, oldValue);
1048      * if (oldValue != null ) {
1049      *    if (newValue != null)
1050      *       map.replace(key, oldValue, newValue);
1051      *    else
1052      *       map.remove(key, oldValue);
1053      * } else {
1054      *    if (newValue != null)
1055      *       map.putIfAbsent(key, newValue);
1056      *    else
1057      *       return null;
1058      * }
1059      * }</pre>
1060      *
1061      * In concurrent contexts, the default implementation may retry
1062      * these steps when multiple threads attempt updates.
1063      *
1064      * @param key key with which the specified value is to be associated
1065      * @param remappingFunction the function to compute a value
1066      * @return the new value associated with the specified key, or null if none
1067      * @throws NullPointerException if the specified key is null and
1068      *         this map does not support null keys, or the
1069      *         remappingFunction is null
1070      * @throws UnsupportedOperationException if the {@code put} operation
1071      *         is not supported by this map
1072      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1073      * @throws ClassCastException if the class of the specified key or value
1074      *         prevents it from being stored in this map
1075      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1076      * @since 1.8
1077      */
1078     default V compute(K key,
1079             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1080         Objects.requireNonNull(remappingFunction);
1081         V oldValue = get(key);
1082         for (;;) {
1083             V newValue = remappingFunction.apply(key, oldValue);
1084             if (newValue == null) {
1085                 // delete mapping
1086                 if(oldValue != null || containsKey(key)) {
1087                     // something to remove
1088                     if (remove(key, oldValue)) {
1089                         // removed the old value as expected
1090                         return null;
1091                     }
1092 
1093                     // some other value replaced old value. try again.
1094                     oldValue = get(key);
1095                 } else {
1096                     // nothing to do. Leave things as they were.
1097                     return null;
1098                 }
1099             } else {
1100                 // add or replace old mapping
1101                 if (oldValue != null) {
1102                     // replace
1103                     if (replace(key, oldValue, newValue)) {
1104                         // replaced as expected.
1105                         return newValue;
1106                     }
1107 
1108                     // some other value replaced old value. try again.
1109                     oldValue = get(key);
1110                 } else {
1111                     // add (replace if oldValue was null)
1112                     if ((oldValue = putIfAbsent(key, newValue)) == null) {
1113                         // replaced
1114                         return newValue;
1115                     }
1116 
1117                     // some other value replaced old value. try again.
1118                 }
1119             }
1120         }
1121     }
1122 
1123     /**
1124      * If the specified key is not already associated with a value or is
1125      * associated with null, associates it with the given value.
1126      * Otherwise, replaces the value with the results of the given
1127      * remapping function, or removes if the result is {@code null}. This
1128      * method may be of use when combining multiple mapped values for a key.
1129      * For example, to either create or append a {@code String msg} to a
1130      * value mapping:
1131      *
1132      * <pre> {@code
1133      * map.merge(key, msg, String::concat)
1134      * }</pre>
1135      *
1136      * <p>If the function returns {@code null}, the mapping is removed (or
1137      * remains absent if initially absent).  If the function itself throws an
1138      * (unchecked) exception, the exception is rethrown, and the current mapping
1139      * is left unchanged.
1140      *
1141      * <p>The default implementation makes no guarantees about synchronization
1142      * or atomicity properties of this method. Any implementation providing
1143      * atomicity guarantees must override this method and document its
1144      * concurrency properties. In particular, all implementations of
1145      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1146      * whether the function is applied once atomically only if the value is not
1147      * present.  Any class that permits null values must document
1148      * whether and how this method distinguishes absence from null mappings.
1149      *
1150      * @implSpec
1151      * The default implementation is equivalent to performing the
1152      * following steps for this {@code map}, then returning the
1153      * current value or {@code null} if absent:
1154      *
1155      * <pre> {@code
1156      * V oldValue = map.get(key);
1157      * V newValue = (oldValue == null) ? value :
1158      *              remappingFunction.apply(oldValue, value);
1159      * if (newValue == null)
1160      *     map.remove(key, oldValue);
1161      * else if (oldValue == null)
1162      *     map.putIfAbsent(key, newValue);
1163      * else
1164      *     map.replace(key, oldValue, newValue);
1165      * }</pre>
1166      *
1167      * In concurrent contexts, the default implementation may retry
1168      * these steps when multiple threads attempt updates.
1169      *
1170      * @param key key with which the specified value is to be associated
1171      * @param value the value to use if absent
1172      * @param remappingFunction the function to recompute a value if present
1173      * @return the new value associated with the specified key, or null if none
1174      * @throws UnsupportedOperationException if the {@code put} operation
1175      *         is not supported by this map
1176      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1177      * @throws ClassCastException if the class of the specified key or value
1178      *         prevents it from being stored in this map
1179      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1180      * @throws NullPointerException if the specified key is null and
1181      *         this map does not support null keys, or the
1182      *         remappingFunction is null
1183      * @since 1.8
1184      */
1185     default V merge(K key, V value,
1186             BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
1187         Objects.requireNonNull(remappingFunction);
1188         V oldValue = get(key);
1189         for (;;) {
1190             if (oldValue != null) {
1191                 V newValue = remappingFunction.apply(oldValue, value);
1192                 if (newValue != null) {
1193                     if (replace(key, oldValue, newValue))
1194                         return newValue;
1195                 } else if (remove(key, oldValue)) {
1196                     return null;
1197                 }
1198                 oldValue = get(key);
1199             } else {
1200                 if (value == null) {
1201                     return null;
1202                 }
1203 
1204                 if ((oldValue = putIfAbsent(key, value)) == null) {
1205                     return value;
1206                 }
1207             }
1208         }
1209     }
1210 }