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20   *
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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 non-null value.
1114      * Otherwise, replaces the associated 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.  If the
1125      * function itself throws an (unchecked) exception, the exception is
1126      * rethrown, and the current mapping is left unchanged.
1127      *
1128      * @implSpec
1129      * The default implementation is equivalent to performing the following
1130      * steps for this {@code map}, then returning the current value or
1131      * {@code null} if absent:
1132      *
1133      * <pre> {@code
1134      * V oldValue = map.get(key);
1135      * V newValue = (oldValue == null) ? value :
1136      *              remappingFunction.apply(oldValue, value);
1137      * if (newValue == null)
1138      *     map.remove(key);
1139      * else
1140      *     map.put(key, newValue);
1141      * }</pre>
1142      *
1143      * <p>The default implementation makes no guarantees about synchronization
1144      * or atomicity properties of this method. Any implementation providing
1145      * atomicity guarantees must override this method and document its
1146      * concurrency properties. In particular, all implementations of
1147      * subinterface {@link java.util.concurrent.ConcurrentMap} must document
1148      * whether the function is applied once atomically only if the value is not
1149      * present.
1150      *
1151      * @param key key with which the resulting value is to be associated
1152      * @param value the non-null value to be merged with the existing value
1153      *        associated with the key or, if no existing value or a null value
1154      *        is associated with the key, to be associated with the key
1155      * @param remappingFunction the function to recompute a value if present
1156      * @return the new value associated with the specified key, or null if no
1157      *         value is associated with the key
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 this map
1165      *         does not support null keys or the value or remappingFunction is
1166      *         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         Objects.requireNonNull(value);
1173         V oldValue = get(key);
1174         V newValue = (oldValue == null) ? value :
1175                    remappingFunction.apply(oldValue, value);
1176         if(newValue == null) {
1177             remove(key);
1178         } else {
1179             put(key, newValue);
1180         }
1181         return newValue;
1182     }
1183 }