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          * @since 1.8
 469          */
 470         public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
 471             return (Comparator<Map.Entry<K, V>> & Serializable)
 472                 (c1, c2) -> c1.getKey().compareTo(c2.getKey());
 473         }
 474 
 475         /**
 476          * Returns a comparator that compares {@link Map.Entry} in natural order on value.
 477          *
 478          * <p>The returned comparator is serializable and throws {@link
 479          * NullPointerException} when comparing an entry with null values.
 480          *
 481          * @param <K> the type of the map keys
 482          * @param <V> the {@link Comparable} type of the map values
 483          * @return a comparator that compares {@link Map.Entry} in natural order on value.
 484          * @see Comparable
 485          * @since 1.8
 486          */
 487         public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
 488             return (Comparator<Map.Entry<K, V>> & Serializable)
 489                 (c1, c2) -> c1.getValue().compareTo(c2.getValue());
 490         }
 491 
 492         /**
 493          * Returns a comparator that compares {@link Map.Entry} by key using the given
 494          * {@link Comparator}.
 495          *
 496          * <p>The returned comparator is serializable if the specified comparator
 497          * is also serializable.
 498          *
 499          * @param  <K> the type of the map keys
 500          * @param  <V> the type of the map values
 501          * @param  cmp the key {@link Comparator}
 502          * @return a comparator that compares {@link Map.Entry} by the key.
 503          * @since 1.8
 504          */
 505         public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
 506             Objects.requireNonNull(cmp);
 507             return (Comparator<Map.Entry<K, V>> & Serializable)
 508                 (c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
 509         }
 510 
 511         /**
 512          * Returns a comparator that compares {@link Map.Entry} by value using the given
 513          * {@link Comparator}.
 514          *
 515          * <p>The returned comparator is serializable if the specified comparator
 516          * is also serializable.
 517          *
 518          * @param  <K> the type of the map keys
 519          * @param  <V> the type of the map values
 520          * @param  cmp the value {@link Comparator}
 521          * @return a comparator that compares {@link Map.Entry} by the value.
 522          * @since 1.8
 523          */
 524         public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
 525             Objects.requireNonNull(cmp);
 526             return (Comparator<Map.Entry<K, V>> & Serializable)
 527                 (c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
 528         }
 529     }
 530 
 531     // Comparison and hashing
 532 
 533     /**
 534      * Compares the specified object with this map for equality.  Returns
 535      * <tt>true</tt> if the given object is also a map and the two maps
 536      * represent the same mappings.  More formally, two maps <tt>m1</tt> and
 537      * <tt>m2</tt> represent the same mappings if
 538      * <tt>m1.entrySet().equals(m2.entrySet())</tt>.  This ensures that the
 539      * <tt>equals</tt> method works properly across different implementations
 540      * of the <tt>Map</tt> interface.
 541      *
 542      * @param o object to be compared for equality with this map
 543      * @return <tt>true</tt> if the specified object is equal to this map
 544      */
 545     boolean equals(Object o);
 546 
 547     /**
 548      * Returns the hash code value for this map.  The hash code of a map is
 549      * defined to be the sum of the hash codes of each entry in the map's
 550      * <tt>entrySet()</tt> view.  This ensures that <tt>m1.equals(m2)</tt>
 551      * implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
 552      * <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
 553      * {@link Object#hashCode}.
 554      *
 555      * @return the hash code value for this map
 556      * @see Map.Entry#hashCode()
 557      * @see Object#equals(Object)
 558      * @see #equals(Object)
 559      */
 560     int hashCode();
 561 
 562     // Defaultable methods
 563 
 564     /**
 565      * Returns the value to which the specified key is mapped, or
 566      * {@code defaultValue} if this map contains no mapping for the key.
 567      *
 568      * @implSpec
 569      * The default implementation makes no guarantees about synchronization
 570      * or atomicity properties of this method. Any implementation providing
 571      * atomicity guarantees must override this method and document its
 572      * concurrency properties.
 573      *
 574      * @param key the key whose associated value is to be returned
 575      * @param defaultValue the default mapping of the key
 576      * @return the value to which the specified key is mapped, or
 577      * {@code defaultValue} if this map contains no mapping for the key
 578      * @throws ClassCastException if the key is of an inappropriate type for
 579      * this map
 580      * (<a href="Collection.html#optional-restrictions">optional</a>)
 581      * @throws NullPointerException if the specified key is null and this map
 582      * does not permit null keys
 583      * (<a href="Collection.html#optional-restrictions">optional</a>)
 584      * @since 1.8
 585      */
 586     default V getOrDefault(Object key, V defaultValue) {
 587         V v;
 588         return (((v = get(key)) != null) || containsKey(key))
 589             ? v
 590             : defaultValue;
 591     }
 592 
 593     /**
 594      * Performs the given action for each entry in this map until all entries
 595      * have been processed or the action throws an exception.   Unless
 596      * otherwise specified by the implementing class, actions are performed in
 597      * the order of entry set iteration (if an iteration order is specified.)
 598      * Exceptions thrown by the action are relayed to the caller.
 599      *
 600      * @implSpec
 601      * The default implementation is equivalent to, for this {@code map}:
 602      * <pre> {@code
 603      * for ((Map.Entry<K, V> entry : map.entrySet())
 604      *     action.accept(entry.getKey(), entry.getValue());
 605      * }</pre>
 606      *
 607      * The default implementation makes no guarantees about synchronization
 608      * or atomicity properties of this method. Any implementation providing
 609      * atomicity guarantees must override this method and document its
 610      * concurrency properties.
 611      *
 612      * @param action The action to be performed for each entry
 613      * @throws NullPointerException if the specified action is null
 614      * @throws ConcurrentModificationException if an entry is found to be
 615      * removed during iteration
 616      * @since 1.8
 617      */
 618     default void forEach(BiConsumer<? super K, ? super V> action) {
 619         Objects.requireNonNull(action);
 620         for (Map.Entry<K, V> entry : entrySet()) {
 621             K k;
 622             V v;
 623             try {
 624                 k = entry.getKey();
 625                 v = entry.getValue();
 626             } catch(IllegalStateException ise) {
 627                 // this usually means the entry is no longer in the map.
 628                 throw new ConcurrentModificationException(ise);
 629             }
 630             action.accept(k, v);
 631         }
 632     }
 633 
 634     /**
 635      * Replaces each entry's value with the result of invoking the given
 636      * function on that entry until all entries have been processed or the
 637      * function throws an exception.  Exceptions thrown by the function are
 638      * relayed to the caller.
 639      *
 640      * @implSpec
 641      * <p>The default implementation is equivalent to, for this {@code map}:
 642      * <pre> {@code
 643      * for ((Map.Entry<K, V> entry : map.entrySet())
 644      *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
 645      * }</pre>
 646      *
 647      * <p>The default implementation makes no guarantees about synchronization
 648      * or atomicity properties of this method. Any implementation providing
 649      * atomicity guarantees must override this method and document its
 650      * concurrency properties.
 651      *
 652      * @param function the function to apply to each entry
 653      * @throws UnsupportedOperationException if the {@code set} operation
 654      * is not supported by this map's entry set iterator.
 655      * @throws ClassCastException if the class of a replacement value
 656      * prevents it from being stored in this map
 657      * @throws NullPointerException if the specified function is null, or the
 658      * specified replacement value is null, and this map does not permit null
 659      * values
 660      * @throws ClassCastException if a replacement value is of an inappropriate
 661      *         type for this map
 662      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 663      * @throws NullPointerException if function or a replacement value is null,
 664      *         and this map does not permit null keys or values
 665      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 666      * @throws IllegalArgumentException if some property of a replacement value
 667      *         prevents it from being stored in this map
 668      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 669      * @throws ConcurrentModificationException if an entry is found to be
 670      * removed during iteration
 671      * @since 1.8
 672      */
 673     default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
 674         Objects.requireNonNull(function);
 675         for (Map.Entry<K, V> entry : entrySet()) {
 676             K k;
 677             V v;
 678             try {
 679                 k = entry.getKey();
 680                 v = entry.getValue();
 681             } catch(IllegalStateException ise) {
 682                 // this usually means the entry is no longer in the map.
 683                 throw new ConcurrentModificationException(ise);
 684             }
 685 
 686             // ise thrown from function is not a cme.
 687             v = function.apply(k, v);
 688 
 689             try {
 690                 entry.setValue(v);
 691             } catch(IllegalStateException ise) {
 692                 // this usually means the entry is no longer in the map.
 693                 throw new ConcurrentModificationException(ise);
 694             }
 695         }
 696     }
 697 
 698     /**
 699      * If the specified key is not already associated with a value (or is mapped
 700      * to {@code null}) associates it with the given value and returns
 701      * {@code null}, else returns the current value.
 702      *
 703      * @implSpec
 704      * The default implementation is equivalent to, for this {@code
 705      * map}:
 706      *
 707      * <pre> {@code
 708      * V v = map.get(key);
 709      * if (v == null)
 710      *     v = map.put(key, value);
 711      *
 712      * return v;
 713      * }</pre>
 714      *
 715      * <p>The default implementation makes no guarantees about synchronization
 716      * or atomicity properties of this method. Any implementation providing
 717      * atomicity guarantees must override this method and document its
 718      * concurrency properties.
 719      *
 720      * @param key key with which the specified value is to be associated
 721      * @param value value to be associated with the specified key
 722      * @return the previous value associated with the specified key, or
 723      *         {@code null} if there was no mapping for the key.
 724      *         (A {@code null} return can also indicate that the map
 725      *         previously associated {@code null} with the key,
 726      *         if the implementation supports null values.)
 727      * @throws UnsupportedOperationException if the {@code put} operation
 728      *         is not supported by this map
 729      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 730      * @throws ClassCastException if the key or value is of an inappropriate
 731      *         type for this map
 732      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 733      * @throws NullPointerException if the specified key or value is null,
 734      *         and this map does not permit null keys or values
 735      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 736      * @throws IllegalArgumentException if some property of the specified key
 737      *         or value prevents it from being stored in this map
 738      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 739      * @since 1.8
 740      */
 741     default V putIfAbsent(K key, V value) {
 742         V v = get(key);
 743         if (v == null) {
 744             v = put(key, value);
 745         }
 746 
 747         return v;
 748     }
 749 
 750     /**
 751      * Removes the entry for the specified key only if it is currently
 752      * mapped to the specified value.
 753      *
 754      * @implSpec
 755      * The default implementation is equivalent to, for this {@code map}:
 756      *
 757      * <pre> {@code
 758      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 759      *     map.remove(key);
 760      *     return true;
 761      * } else
 762      *     return false;
 763      * }</pre>
 764      *
 765      * <p>The default implementation makes no guarantees about synchronization
 766      * or atomicity properties of this method. Any implementation providing
 767      * atomicity guarantees must override this method and document its
 768      * concurrency properties.
 769      *
 770      * @param key key with which the specified value is associated
 771      * @param value value expected to be associated with the specified key
 772      * @return {@code true} if the value was removed
 773      * @throws UnsupportedOperationException if the {@code remove} operation
 774      *         is not supported by this map
 775      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 776      * @throws ClassCastException if the key or value is of an inappropriate
 777      *         type for this map
 778      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 779      * @throws NullPointerException if the specified key or value is null,
 780      *         and this map does not permit null keys or values
 781      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 782      * @since 1.8
 783      */
 784     default boolean remove(Object key, Object value) {
 785         Object curValue = get(key);
 786         if (!Objects.equals(curValue, value) ||
 787             (curValue == null && !containsKey(key))) {
 788             return false;
 789         }
 790         remove(key);
 791         return true;
 792     }
 793 
 794     /**
 795      * Replaces the entry for the specified key only if currently
 796      * mapped to the specified value.
 797      *
 798      * @implSpec
 799      * The default implementation is equivalent to, for this {@code map}:
 800      *
 801      * <pre> {@code
 802      * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
 803      *     map.put(key, newValue);
 804      *     return true;
 805      * } else
 806      *     return false;
 807      * }</pre>
 808      *
 809      * The default implementation does not throw NullPointerException
 810      * for maps that do not support null values if oldValue is null unless
 811      * newValue is also null.
 812      *
 813      * <p>The default implementation makes no guarantees about synchronization
 814      * or atomicity properties of this method. Any implementation providing
 815      * atomicity guarantees must override this method and document its
 816      * concurrency properties.
 817      *
 818      * @param key key with which the specified value is associated
 819      * @param oldValue value expected to be associated with the specified key
 820      * @param newValue value to be associated with the specified key
 821      * @return {@code true} if the value was replaced
 822      * @throws UnsupportedOperationException if the {@code put} operation
 823      *         is not supported by this map
 824      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 825      * @throws ClassCastException if the class of a specified key or value
 826      *         prevents it from being stored in this map
 827      * @throws NullPointerException if a specified key or newValue is null,
 828      *         and this map does not permit null keys or values
 829      * @throws NullPointerException if oldValue is null and this map does not
 830      *         permit null values
 831      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 832      * @throws IllegalArgumentException if some property of a specified key
 833      *         or value prevents it from being stored in this map
 834      * @since 1.8
 835      */
 836     default boolean replace(K key, V oldValue, V newValue) {
 837         Object curValue = get(key);
 838         if (!Objects.equals(curValue, oldValue) ||
 839             (curValue == null && !containsKey(key))) {
 840             return false;
 841         }
 842         put(key, newValue);
 843         return true;
 844     }
 845 
 846     /**
 847      * Replaces the entry for the specified key only if it is
 848      * currently mapped to some value.
 849      *
 850      * @implSpec
 851      * The default implementation is equivalent to, for this {@code map}:
 852      *
 853      * <pre> {@code
 854      * if (map.containsKey(key)) {
 855      *     return map.put(key, value);
 856      * } else
 857      *     return null;
 858      * }</pre>
 859      *
 860      * <p>The default implementation makes no guarantees about synchronization
 861      * or atomicity properties of this method. Any implementation providing
 862      * atomicity guarantees must override this method and document its
 863      * concurrency properties.
 864       *
 865      * @param key key with which the specified value is associated
 866      * @param value value to be associated with the specified key
 867      * @return the previous value associated with the specified key, or
 868      *         {@code null} if there was no mapping for the key.
 869      *         (A {@code null} return can also indicate that the map
 870      *         previously associated {@code null} with the key,
 871      *         if the implementation supports null values.)
 872      * @throws UnsupportedOperationException if the {@code put} operation
 873      *         is not supported by this map
 874      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 875      * @throws ClassCastException if the class of the specified key or value
 876      *         prevents it from being stored in this map
 877      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 878      * @throws NullPointerException if the specified key or value is null,
 879      *         and this map does not permit null keys or values
 880      * @throws IllegalArgumentException if some property of the specified key
 881      *         or value prevents it from being stored in this map
 882      * @since 1.8
 883      */
 884     default V replace(K key, V value) {
 885         V curValue;
 886         if (((curValue = get(key)) != null) || containsKey(key)) {
 887             curValue = put(key, value);
 888         }
 889         return curValue;
 890     }
 891 
 892     /**
 893      * If the specified key is not already associated with a value (or is mapped
 894      * to {@code null}), attempts to compute its value using the given mapping
 895      * function and enters it into this map unless {@code null}.
 896      *
 897      * <p>If the function returns {@code null} no mapping is recorded. If
 898      * the function itself throws an (unchecked) exception, the
 899      * exception is rethrown, and no mapping is recorded.  The most
 900      * common usage is to construct a new object serving as an initial
 901      * mapped value or memoized result, as in:
 902      *
 903      * <pre> {@code
 904      * map.computeIfAbsent(key, k -> new Value(f(k)));
 905      * }</pre>
 906      *
 907      * <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
 908      * supporting multiple values per key:
 909      *
 910      * <pre> {@code
 911      * map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
 912      * }</pre>
 913      *
 914      *
 915      * @implSpec
 916      * The default implementation is equivalent to the following steps for this
 917      * {@code map}, then returning the current value or {@code null} if now
 918      * absent:
 919      *
 920      * <pre> {@code
 921      * if (map.get(key) == null) {
 922      *     V newValue = mappingFunction.apply(key);
 923      *     if (newValue != null)
 924      *         map.put(key, newValue);
 925      * }
 926      * }</pre>
 927      *
 928      * <p>The default implementation makes no guarantees about synchronization
 929      * or atomicity properties of this method. Any implementation providing
 930      * atomicity guarantees must override this method and document its
 931      * concurrency properties. In particular, all implementations of
 932      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 933      * whether the function is applied once atomically only if the value is not
 934      * present.
 935      *
 936      * @param key key with which the specified value is to be associated
 937      * @param mappingFunction the function to compute a value
 938      * @return the current (existing or computed) value associated with
 939      *         the specified key, or null if the computed value is null
 940      * @throws NullPointerException if the specified key is null and
 941      *         this map does not support null keys, or the mappingFunction
 942      *         is null
 943      * @throws UnsupportedOperationException if the {@code put} operation
 944      *         is not supported by this map
 945      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 946      * @throws ClassCastException if the class of the specified key or value
 947      *         prevents it from being stored in this map
 948      *         (<a href="Collection.html#optional-restrictions">optional</a>)
 949      * @since 1.8
 950      */
 951     default V computeIfAbsent(K key,
 952             Function<? super K, ? extends V> mappingFunction) {
 953         Objects.requireNonNull(mappingFunction);
 954         V v;
 955         if ((v = get(key)) == null) {
 956             V newValue;
 957             if ((newValue = mappingFunction.apply(key)) != null) {
 958                 put(key, newValue);
 959                 return newValue;
 960             }
 961         }
 962 
 963         return v;
 964     }
 965 
 966     /**
 967      * If the value for the specified key is present and non-null, attempts to
 968      * compute a new mapping given the key and its current mapped value.
 969      *
 970      * <p>If the function returns {@code null}, the mapping is removed.  If the
 971      * function itself throws an (unchecked) exception, the exception is
 972      * rethrown, and the current mapping is left unchanged.
 973     *
 974      * @implSpec
 975      * The default implementation is equivalent to performing the following
 976      * steps for this {@code map}, then returning the current value or
 977      * {@code null} if now absent:
 978      *
 979      * <pre> {@code
 980      * if (map.get(key) != null) {
 981      *     V oldValue = map.get(key);
 982      *     V newValue = remappingFunction.apply(key, oldValue);
 983      *     if (newValue != null)
 984      *         map.put(key, newValue);
 985      *     else
 986      *         map.remove(key);
 987      * }
 988      * }</pre>
 989      *
 990      * <p>The default implementation makes no guarantees about synchronization
 991      * or atomicity properties of this method. Any implementation providing
 992      * atomicity guarantees must override this method and document its
 993      * concurrency properties. In particular, all implementations of
 994      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
 995      * whether the function is applied once atomically only if the value is not
 996      * present.
 997      *
 998      * @param key key with which the specified value is to be associated
 999      * @param remappingFunction the function to compute a value
1000      * @return the new value associated with the specified key, or null if none
1001      * @throws NullPointerException if the specified key is null and
1002      *         this map does not support null keys, or the
1003      *         remappingFunction is null
1004      * @throws UnsupportedOperationException if the {@code put} operation
1005      *         is not supported by this map
1006      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1007      * @throws ClassCastException if the class of the specified key or value
1008      *         prevents it from being stored in this map
1009      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1010      * @since 1.8
1011      */
1012     default V computeIfPresent(K key,
1013             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1014         Objects.requireNonNull(remappingFunction);
1015         V oldValue;
1016         if ((oldValue = get(key)) != null) {
1017             V newValue = remappingFunction.apply(key, oldValue);
1018             if (newValue != null) {
1019                 put(key, newValue);
1020                 return newValue;
1021             } else {
1022                 remove(key);
1023                 return null;
1024             }
1025         } else {
1026             return null;
1027         }
1028     }
1029 
1030     /**
1031      * Attempts to compute a mapping for the specified key and its current
1032      * mapped value (or {@code null} if there is no current mapping). For
1033      * example, to either create or append a {@code String} msg to a value
1034      * mapping:
1035      *
1036      * <pre> {@code
1037      * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
1038      * (Method {@link #merge merge()} is often simpler to use for such purposes.)
1039      *
1040      * <p>If the function returns {@code null}, the mapping is removed (or
1041      * remains absent if initially absent).  If the function itself throws an
1042      * (unchecked) exception, the exception is rethrown, and the current mapping
1043      * is left unchanged.
1044      *
1045      * @implSpec
1046      * The default implementation is equivalent to performing the following
1047      * steps for this {@code map}, then returning the current value or
1048      * {@code null} if absent:
1049      *
1050      * <pre> {@code
1051      * V oldValue = map.get(key);
1052      * V newValue = remappingFunction.apply(key, oldValue);
1053      * if (oldValue != null ) {
1054      *    if (newValue != null)
1055      *       map.put(key, newValue);
1056      *    else
1057      *       map.remove(key);
1058      * } else {
1059      *    if (newValue != null)
1060      *       map.put(key, newValue);
1061      *    else
1062      *       return null;
1063      * }
1064      * }</pre>
1065      *
1066      * <p>The default implementation makes no guarantees about synchronization
1067      * or atomicity properties of this method. Any implementation providing
1068      * atomicity guarantees must override this method and document its
1069      * concurrency properties. In particular, all implementations of
1070      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1071      * whether the function is applied once atomically only if the value is not
1072      * present.
1073      *
1074      * @param key key with which the specified value is to be associated
1075      * @param remappingFunction the function to compute a value
1076      * @return the new value associated with the specified key, or null if none
1077      * @throws NullPointerException if the specified key is null and
1078      *         this map does not support null keys, or the
1079      *         remappingFunction is null
1080      * @throws UnsupportedOperationException if the {@code put} operation
1081      *         is not supported by this map
1082      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1083      * @throws ClassCastException if the class of the specified key or value
1084      *         prevents it from being stored in this map
1085      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1086      * @since 1.8
1087      */
1088     default V compute(K key,
1089             BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
1090         Objects.requireNonNull(remappingFunction);
1091         V oldValue = get(key);
1092 
1093         V newValue = remappingFunction.apply(key, oldValue);
1094         if (newValue == null) {
1095             // delete mapping
1096             if (oldValue != null || containsKey(key)) {
1097                 // something to remove
1098                 remove(key);
1099                 return null;
1100             } else {
1101                 // nothing to do. Leave things as they were.
1102                 return null;
1103             }
1104         } else {
1105             // add or replace old mapping
1106             put(key, newValue);
1107             return newValue;
1108         }
1109     }
1110 
1111     /**
1112      * If the specified key is not already associated with a value or is
1113      * associated with null, associates it with the given value.
1114      * Otherwise, replaces the value with the results of the given
1115      * remapping function, or removes if the result is {@code null}. This
1116      * method may be of use when combining multiple mapped values for a key.
1117      * For example, to either create or append a {@code String msg} to a
1118      * value mapping:
1119      *
1120      * <pre> {@code
1121      * map.merge(key, msg, String::concat)
1122      * }</pre>
1123      *
1124      * <p>If the function returns {@code null}, the mapping is removed (or
1125      * remains absent if initially absent).  If the function itself throws an
1126      * (unchecked) exception, the exception is rethrown, and the current mapping
1127      * is left unchanged.
1128      *
1129      * @implSpec
1130      * The default implementation is equivalent to performing the
1131      * following steps for this {@code map}, then returning the
1132      * current value or {@code null} if absent:
1133      *
1134      * <pre> {@code
1135      * V oldValue = map.get(key);
1136      * V newValue = (oldValue == null) ? value :
1137      *              remappingFunction.apply(oldValue, value);
1138      * if (newValue == null)
1139      *     map.remove(key);
1140      * else if (oldValue == null)
1141      *     map.remove(key);
1142      * else
1143      *     map.put(key, newValue);
1144      * }</pre>
1145      *
1146      * <p>The default implementation makes no guarantees about synchronization
1147      * or atomicity properties of this method. Any implementation providing
1148      * atomicity guarantees must override this method and document its
1149      * concurrency properties. In particular, all implementations of
1150      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1151      * whether the function is applied once atomically only if the value is not
1152      * present.
1153      *
1154      * @param key key with which the specified value is to be associated
1155      * @param value the value to use if absent
1156      * @param remappingFunction the function to recompute a value if present
1157      * @return the new value associated with the specified key, or null if none
1158      * @throws UnsupportedOperationException if the {@code put} operation
1159      *         is not supported by this map
1160      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1161      * @throws ClassCastException if the class of the specified key or value
1162      *         prevents it from being stored in this map
1163      *         (<a href="Collection.html#optional-restrictions">optional</a>)
1164      * @throws NullPointerException if the specified key is null and
1165      *         this map does not support null keys, or the remappingFunction
1166      *         is null
1167      * @since 1.8
1168      */
1169     default V merge(K key, V value,
1170             BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
1171         Objects.requireNonNull(remappingFunction);
1172         V oldValue = get(key);
1173         if (oldValue != null) {
1174             V newValue = remappingFunction.apply(oldValue, value);
1175             if (newValue != null) {
1176                 put(key, newValue);
1177                 return newValue;
1178             } else {
1179                 remove(key);
1180                 return null;
1181             }
1182         } else {
1183             if (value == null) {
1184                 remove(key);
1185                 return null;
1186             } else {
1187                 put(key, value);
1188                 return value;
1189             }
1190         }
1191     }
1192 }