util/concurrent/ConcurrentMap.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.Map;
import java.util.Objects;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Function;

/**
 * A {@link java.util.Map} providing thread safety and atomicity
 * guarantees.
 *
 * <p>To support atomic usages, ConcurrentMaps are expected not to
 * allow {@code null} as a legal value (and to throw exceptions upon
 * attempted insertions). This enables a return value of {@code null}
 * to unambiguously indicate the absence of a mapping. This interface
 * does not strictly forbid implementations that may hold {@code null}
 * values. However, in any that do so, a {@code null} value must bear
 * the same interpretation as the absence of a mapping in order to
 * conform to method atomicity requirements.  Further, any that do so
 * must override all default method implementations.
 *
 * <p>Several methods (for example {@link #putIfAbsent}) inherited
 * from {@link Map} do not have default implementations, and so must
 * be provided by implementations of this interface, even though they
 * have (non-atomic) default implementations in the {@link Map}
 * interface.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code ConcurrentMap} as a key or value
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that object from
 * the {@code ConcurrentMap} in another thread.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
public interface ConcurrentMap<K,V> extends Map<K,V> {

    /**
     * {@inheritDoc}
     *
     * @implNote The default implementation returns the result of
     * {@code get(key)} unless {@code null}, in which case it returns
     * the given defaultValue.
     *
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V getOrDefault(Object key, V defaultValue) {
        V v;
        return ((v = get(key)) != null) ? v : defaultValue;
    }

   /**
     * {@inheritDoc}
     *
     * @implSpec The default implementation is equivalent to, for this
     * {@code map}:
     * <pre> {@code
     * for ((Map.Entry<K,V> entry : map.entrySet())
     *   action.accept(entry.getKey(), entry.getValue());
     * }</pre>
     *
     * @implNote The default implementation assumes that {@code
     * IllegalStateException} thrown by {@code getKey()} or {@code
     * getValue()} indicates that the entry no longer exists.
     * Operation continues for subsequent entries.
     *
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default void forEach(BiConsumer<? super K, ? super V> action) {
        Objects.requireNonNull(action);
        for (Map.Entry<K,V> entry : entrySet()) {
            K k;
            V v;
            try {
                k = entry.getKey();
                v = entry.getValue();
            } catch (IllegalStateException ise) {
                // this usually means the entry is no longer in the map.
                continue;
            }
            action.accept(k, v);
        }
    }

    /**
     * If the specified key is not already associated
     * with a value, associates it with the given value.
     * This is equivalent to
     *  <pre> {@code
     * if (map.containsKey(key))
     *   return map.get(key);
     * else
     *   return map.put(key, value);
     * }</pre>
     *
     * except that the action is performed atomically.
     *
     * @implNote There is no default implementation.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     */
    V putIfAbsent(K key, V value);

    /**
     * Removes the entry for a key only if currently mapped to a given value.
     * This is equivalent to
     *  <pre> {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *   map.remove(key);
     *   return true;
     * } else
     *   return false;
     * }</pre>
     *
     * except that the action is performed atomically.
     *
     * @implNote There is no default implementation.
     *
     * @param key key with which the specified value is associated
     * @param value value expected to be associated with the specified key
     * @return {@code true} if the value was removed
     * @throws UnsupportedOperationException if the {@code remove} operation
     *         is not supported by this map
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (<a href="../Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (<a href="../Collection.html#optional-restrictions">optional</a>)
     */
    boolean remove(Object key, Object value);

    /**
     * Replaces the entry for a key only if currently mapped to a given value.
     * This is equivalent to
     *  <pre> {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
     *   map.put(key, newValue);
     *   return true;
     * } else
     *   return false;
     * }</pre>
     *
     * except that the action is performed atomically.
     *
     * @implNote There is no default implementation.
     *
     * @param key key with which the specified value is associated
     * @param oldValue value expected to be associated with the specified key
     * @param newValue value to be associated with the specified key
     * @return {@code true} if the value was replaced
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     * @throws ClassCastException if the class of a specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if a specified key or value is null,
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of a specified key
     *         or value prevents it from being stored in this map
     */
    boolean replace(K key, V oldValue, V newValue);

    /**
     * Replaces the entry for a key only if currently mapped to some value.
     * This is equivalent to
     *  <pre> {@code
     * if (map.containsKey(key)) {
     *   return map.put(key, value);
     * } else
     *   return null;
     * }</pre>
     *
     * except that the action is performed atomically.
     *
     * @implNote There is no default implementation.
     *
     * @param key key with which the specified value is associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with the specified key, or
     *         {@code null} if there was no mapping for the key.
     *         (A {@code null} return can also indicate that the map
     *         previously associated {@code null} with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     */
    V replace(K key, V value);

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * <p>The default implementation is equivalent to, for this {@code map}:
     * <pre> {@code
     * for ((Map.Entry<K,V> entry : map.entrySet())
     *     do {
     *        K k = entry.getKey();
     *        V v = entry.getValue();
     *     } while (!replace(k, v, function.apply(k, v)));
     * }</pre>
     *
     * The default implementation may retry these steps when multiple
     * threads attempt updates, and may call the function multiple
     * times.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws IllegalArgumentException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        Objects.requireNonNull(function);
        forEach((k,v) -> {
            while (!replace(k, v, function.apply(k, v))) {
                // v changed or k is gone
                if ( (v = get(k)) == null) {
                    // k is no longer in the map.
                    break;
                }
            }
        });
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to the following steps for this
     * {@code map}, then returning the current value or {@code null} if now
     * absent:
     *
     * <pre> {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         return map.putIfAbsent(key, newValue);
     * }
     * }</pre>
     *
     * The default implementation may retry these steps when multiple
     * threads attempt updates, and may call the mapping function
     * multiple times.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V v, newValue;
        return ((v = get(key)) == null &&
                (newValue = mappingFunction.apply(key)) != null &&
                (v = putIfAbsent(key, newValue)) == null) ? newValue : v;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if now absent. :
     *
     * <pre> {@code
     * if (map.get(key) != null) {
     *     V oldValue = map.get(key);
     *     V newValue = remappingFunction.apply(key, oldValue);
     *     if (newValue != null)
     *         map.replace(key, oldValue, newValue);
     *     else
     *         map.remove(key, oldValue);
     * }
     * }</pre>
     *
     * The default implementation may retry these steps when multiple
     * threads attempt updates, and may call the remapping function
     * multiple times.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue;
        while ((oldValue = get(key)) != null) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue != null) {
                if (replace(key, oldValue, newValue))
                    return newValue;
            } else if (remove(key, oldValue))
               return null;
        }
        return oldValue;
    }

    /**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = remappingFunction.apply(key, oldValue);
     * if (oldValue != null ) {
     *    if (newValue != null)
     *       map.replace(key, oldValue, newValue);
     *    else
     *       map.remove(key, oldValue);
     * } else {
     *    if (newValue != null)
     *       map.putIfAbsent(key, newValue);
     *    else
     *       return null;
     * }
     * }</pre>
     *
     * The default implementation may retry these steps when multiple
     * threads attempt updates, and may call the remapping function
     * multiple times.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V compute(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        V oldValue = get(key);
        for (;;) {
            V newValue = remappingFunction.apply(key, oldValue);
            if (newValue == null) {
                // delete mapping
                if (oldValue != null || containsKey(key)) {
                    // something to remove
                    if (remove(key, oldValue)) {
                        // removed the old value as expected
                        return null;
                    }

                    // some other value replaced old value. try again.
                    oldValue = get(key);
                } else {
                    // nothing to do. Leave things as they were.
                    return null;
                }
            } else {
                // add or replace old mapping
                if (oldValue != null) {
                    // replace
                    if (replace(key, oldValue, newValue)) {
                        // replaced as expected.
                        return newValue;
                    }

                    // some other value replaced old value. try again.
                    oldValue = get(key);
                } else {
                    // add (replace if oldValue was null)
                    if ((oldValue = putIfAbsent(key, newValue)) == null) {
                        // replaced
                        return newValue;
                    }

                    // some other value replaced old value. try again.
                }
            }
        }
    }


    /**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * <pre> {@code
     * V oldValue = map.get(key);
     * V newValue = (oldValue == null) ? value :
     *              remappingFunction.apply(oldValue, value);
     * if (newValue == null)
     *     map.remove(key);
     * else
     *     map.put(key, newValue);
     * }</pre>
     *
     * The default implementation may retry these steps when multiple
     * threads attempt updates, and may call the remapping function
     * multiple times.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V merge(K key, V value,
            BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        Objects.requireNonNull(value);
        V oldValue = get(key);
        for (;;) {
            if (oldValue != null) {
                V newValue = remappingFunction.apply(oldValue, value);
                if (newValue != null) {
                    if (replace(key, oldValue, newValue))
                        return newValue;
                } else if (remove(key, oldValue)) {
                    return null;
                }
                oldValue = get(key);
            } else {
                if ((oldValue = putIfAbsent(key, value)) == null) {
                    return value;
                }
            }
        }
    }
}
Revision:	1.48
Committed:	Sat Dec 7 16:57:09 2013 UTC (10 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.47:	+4 -4 lines
Log Message:	improve code sample clarity
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8	import java.util.Map;
9	import java.util.Objects;
10	import java.util.function.BiConsumer;
11	import java.util.function.BiFunction;
12	import java.util.function.Function;
13
14	/**
15	* A {@link java.util.Map} providing thread safety and atomicity
16	* guarantees.
17	*
18	* <p>To support atomic usages, ConcurrentMaps are expected not to
19	* allow {@code null} as a legal value (and to throw exceptions upon
20	* attempted insertions). This enables a return value of {@code null}
21	* to unambiguously indicate the absence of a mapping. This interface
22	* does not strictly forbid implementations that may hold {@code null}
23	* values. However, in any that do so, a {@code null} value must bear
24	* the same interpretation as the absence of a mapping in order to
25	* conform to method atomicity requirements. Further, any that do so
26	* must override all default method implementations.
27	*
28	* <p>Several methods (for example {@link #putIfAbsent}) inherited
29	* from {@link Map} do not have default implementations, and so must
30	* be provided by implementations of this interface, even though they
31	* have (non-atomic) default implementations in the {@link Map}
32	* interface.
33	*
34	* <p>Memory consistency effects: As with other concurrent
35	* collections, actions in a thread prior to placing an object into a
36	* {@code ConcurrentMap} as a key or value
37	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
38	* actions subsequent to the access or removal of that object from
39	* the {@code ConcurrentMap} in another thread.
40	*
41	* <p>This interface is a member of the
42	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
43	* Java Collections Framework</a>.
44	*
45	* @since 1.5
46	* @author Doug Lea
47	* @param <K> the type of keys maintained by this map
48	* @param <V> the type of mapped values
49	*/
50	public interface ConcurrentMap<K,V> extends Map<K,V> {
51
52	/**
53	* {@inheritDoc}
54	*
55	* @implNote The default implementation returns the result of
56	* {@code get(key)} unless {@code null}, in which case it returns
57	* the given defaultValue.
58	*
59	* @throws ClassCastException {@inheritDoc}
60	* @throws NullPointerException {@inheritDoc}
61	* @since 1.8
62	*/
63	@Override
64	default V getOrDefault(Object key, V defaultValue) {
65	V v;
66	return ((v = get(key)) != null) ? v : defaultValue;
67	}
68
69	/**
70	* {@inheritDoc}
71	*
72	* @implSpec The default implementation is equivalent to, for this
73	* {@code map}:
74	* <pre> {@code
75	* for ((Map.Entry<K,V> entry : map.entrySet())
76	* action.accept(entry.getKey(), entry.getValue());
77	* }</pre>
78	*
79	* @implNote The default implementation assumes that {@code
80	* IllegalStateException} thrown by {@code getKey()} or {@code
81	* getValue()} indicates that the entry no longer exists.
82	* Operation continues for subsequent entries.
83	*
84	* @throws NullPointerException {@inheritDoc}
85	* @since 1.8
86	*/
87	@Override
88	default void forEach(BiConsumer<? super K, ? super V> action) {
89	Objects.requireNonNull(action);
90	for (Map.Entry<K,V> entry : entrySet()) {
91	K k;
92	V v;
93	try {
94	k = entry.getKey();
95	v = entry.getValue();
96	} catch (IllegalStateException ise) {
97	// this usually means the entry is no longer in the map.
98	continue;
99	}
100	action.accept(k, v);
101	}
102	}
103
104	/**
105	* If the specified key is not already associated
106	* with a value, associates it with the given value.
107	* This is equivalent to
108	* <pre> {@code
109	* if (map.containsKey(key))
110	* return map.get(key);
111	* else
112	* return map.put(key, value);
113	* }</pre>
114	*
115	* except that the action is performed atomically.
116	*
117	* @implNote There is no default implementation.
118	*
119	* @param key key with which the specified value is to be associated
120	* @param value value to be associated with the specified key
121	* @return the previous value associated with the specified key, or
122	* {@code null} if there was no mapping for the key.
123	* (A {@code null} return can also indicate that the map
124	* previously associated {@code null} with the key,
125	* if the implementation supports null values.)
126	* @throws UnsupportedOperationException if the {@code put} operation
127	* is not supported by this map
128	* @throws ClassCastException if the class of the specified key or value
129	* prevents it from being stored in this map
130	* @throws NullPointerException if the specified key or value is null,
131	* and this map does not permit null keys or values
132	* @throws IllegalArgumentException if some property of the specified key
133	* or value prevents it from being stored in this map
134	*/
135	V putIfAbsent(K key, V value);
136
137	/**
138	* Removes the entry for a key only if currently mapped to a given value.
139	* This is equivalent to
140	* <pre> {@code
141	* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
142	* map.remove(key);
143	* return true;
144	* } else
145	* return false;
146	* }</pre>
147	*
148	* except that the action is performed atomically.
149	*
150	* @implNote There is no default implementation.
151	*
152	* @param key key with which the specified value is associated
153	* @param value value expected to be associated with the specified key
154	* @return {@code true} if the value was removed
155	* @throws UnsupportedOperationException if the {@code remove} operation
156	* is not supported by this map
157	* @throws ClassCastException if the key or value is of an inappropriate
158	* type for this map
159	* (<a href="../Collection.html#optional-restrictions">optional</a>)
160	* @throws NullPointerException if the specified key or value is null,
161	* and this map does not permit null keys or values
162	* (<a href="../Collection.html#optional-restrictions">optional</a>)
163	*/
164	boolean remove(Object key, Object value);
165
166	/**
167	* Replaces the entry for a key only if currently mapped to a given value.
168	* This is equivalent to
169	* <pre> {@code
170	* if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
171	* map.put(key, newValue);
172	* return true;
173	* } else
174	* return false;
175	* }</pre>
176	*
177	* except that the action is performed atomically.
178	*
179	* @implNote There is no default implementation.
180	*
181	* @param key key with which the specified value is associated
182	* @param oldValue value expected to be associated with the specified key
183	* @param newValue value to be associated with the specified key
184	* @return {@code true} if the value was replaced
185	* @throws UnsupportedOperationException if the {@code put} operation
186	* is not supported by this map
187	* @throws ClassCastException if the class of a specified key or value
188	* prevents it from being stored in this map
189	* @throws NullPointerException if a specified key or value is null,
190	* and this map does not permit null keys or values
191	* @throws IllegalArgumentException if some property of a specified key
192	* or value prevents it from being stored in this map
193	*/
194	boolean replace(K key, V oldValue, V newValue);
195
196	/**
197	* Replaces the entry for a key only if currently mapped to some value.
198	* This is equivalent to
199	* <pre> {@code
200	* if (map.containsKey(key)) {
201	* return map.put(key, value);
202	* } else
203	* return null;
204	* }</pre>
205	*
206	* except that the action is performed atomically.
207	*
208	* @implNote There is no default implementation.
209	*
210	* @param key key with which the specified value is associated
211	* @param value value to be associated with the specified key
212	* @return the previous value associated with the specified key, or
213	* {@code null} if there was no mapping for the key.
214	* (A {@code null} return can also indicate that the map
215	* previously associated {@code null} with the key,
216	* if the implementation supports null values.)
217	* @throws UnsupportedOperationException if the {@code put} operation
218	* is not supported by this map
219	* @throws ClassCastException if the class of the specified key or value
220	* prevents it from being stored in this map
221	* @throws NullPointerException if the specified key or value is null,
222	* and this map does not permit null keys or values
223	* @throws IllegalArgumentException if some property of the specified key
224	* or value prevents it from being stored in this map
225	*/
226	V replace(K key, V value);
227
228	/**
229	* {@inheritDoc}
230	*
231	* @implSpec
232	* <p>The default implementation is equivalent to, for this {@code map}:
233	* <pre> {@code
234	* for ((Map.Entry<K,V> entry : map.entrySet())
235	* do {
236	* K k = entry.getKey();
237	* V v = entry.getValue();
238	* } while (!replace(k, v, function.apply(k, v)));
239	* }</pre>
240	*
241	* The default implementation may retry these steps when multiple
242	* threads attempt updates, and may call the function multiple
243	* times.
244	*
245	* @throws UnsupportedOperationException {@inheritDoc}
246	* @throws NullPointerException {@inheritDoc}
247	* @throws ClassCastException {@inheritDoc}
248	* @throws IllegalArgumentException {@inheritDoc}
249	* @since 1.8
250	*/
251	@Override
252	default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
253	Objects.requireNonNull(function);
254	forEach((k,v) -> {
255	while (!replace(k, v, function.apply(k, v))) {
256	// v changed or k is gone
257	if ( (v = get(k)) == null) {
258	// k is no longer in the map.
259	break;
260	}
261	}
262	});
263	}
264
265	/**
266	* {@inheritDoc}
267	*
268	* @implSpec
269	* The default implementation is equivalent to the following steps for this
270	* {@code map}, then returning the current value or {@code null} if now
271	* absent:
272	*
273	* <pre> {@code
274	* if (map.get(key) == null) {
275	* V newValue = mappingFunction.apply(key);
276	* if (newValue != null)
277	* return map.putIfAbsent(key, newValue);
278	* }
279	* }</pre>
280	*
281	* The default implementation may retry these steps when multiple
282	* threads attempt updates, and may call the mapping function
283	* multiple times.
284	*
285	* @throws UnsupportedOperationException {@inheritDoc}
286	* @throws ClassCastException {@inheritDoc}
287	* @throws NullPointerException {@inheritDoc}
288	* @since 1.8
289	*/
290	@Override
291	default V computeIfAbsent(K key,
292	Function<? super K, ? extends V> mappingFunction) {
293	Objects.requireNonNull(mappingFunction);
294	V v, newValue;
295	return ((v = get(key)) == null &&
296	(newValue = mappingFunction.apply(key)) != null &&
297	(v = putIfAbsent(key, newValue)) == null) ? newValue : v;
298	}
299
300	/**
301	* {@inheritDoc}
302	*
303	* @implSpec
304	* The default implementation is equivalent to performing the following
305	* steps for this {@code map}, then returning the current value or
306	* {@code null} if now absent. :
307	*
308	* <pre> {@code
309	* if (map.get(key) != null) {
310	* V oldValue = map.get(key);
311	* V newValue = remappingFunction.apply(key, oldValue);
312	* if (newValue != null)
313	* map.replace(key, oldValue, newValue);
314	* else
315	* map.remove(key, oldValue);
316	* }
317	* }</pre>
318	*
319	* The default implementation may retry these steps when multiple
320	* threads attempt updates, and may call the remapping function
321	* multiple times.
322	*
323	* @throws UnsupportedOperationException {@inheritDoc}
324	* @throws ClassCastException {@inheritDoc}
325	* @throws NullPointerException {@inheritDoc}
326	* @since 1.8
327	*/
328	@Override
329	default V computeIfPresent(K key,
330	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
331	Objects.requireNonNull(remappingFunction);
332	V oldValue;
333	while ((oldValue = get(key)) != null) {
334	V newValue = remappingFunction.apply(key, oldValue);
335	if (newValue != null) {
336	if (replace(key, oldValue, newValue))
337	return newValue;
338	} else if (remove(key, oldValue))
339	return null;
340	}
341	return oldValue;
342	}
343
344	/**
345	* {@inheritDoc}
346	*
347	* @implSpec
348	* The default implementation is equivalent to performing the following
349	* steps for this {@code map}, then returning the current value or
350	* {@code null} if absent:
351	*
352	* <pre> {@code
353	* V oldValue = map.get(key);
354	* V newValue = remappingFunction.apply(key, oldValue);
355	* if (oldValue != null ) {
356	* if (newValue != null)
357	* map.replace(key, oldValue, newValue);
358	* else
359	* map.remove(key, oldValue);
360	* } else {
361	* if (newValue != null)
362	* map.putIfAbsent(key, newValue);
363	* else
364	* return null;
365	* }
366	* }</pre>
367	*
368	* The default implementation may retry these steps when multiple
369	* threads attempt updates, and may call the remapping function
370	* multiple times.
371	*
372	* @throws UnsupportedOperationException {@inheritDoc}
373	* @throws ClassCastException {@inheritDoc}
374	* @throws NullPointerException {@inheritDoc}
375	* @since 1.8
376	*/
377	@Override
378	default V compute(K key,
379	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
380	Objects.requireNonNull(remappingFunction);
381	V oldValue = get(key);
382	for (;;) {
383	V newValue = remappingFunction.apply(key, oldValue);
384	if (newValue == null) {
385	// delete mapping
386	if (oldValue != null \|\| containsKey(key)) {
387	// something to remove
388	if (remove(key, oldValue)) {
389	// removed the old value as expected
390	return null;
391	}
392
393	// some other value replaced old value. try again.
394	oldValue = get(key);
395	} else {
396	// nothing to do. Leave things as they were.
397	return null;
398	}
399	} else {
400	// add or replace old mapping
401	if (oldValue != null) {
402	// replace
403	if (replace(key, oldValue, newValue)) {
404	// replaced as expected.
405	return newValue;
406	}
407
408	// some other value replaced old value. try again.
409	oldValue = get(key);
410	} else {
411	// add (replace if oldValue was null)
412	if ((oldValue = putIfAbsent(key, newValue)) == null) {
413	// replaced
414	return newValue;
415	}
416
417	// some other value replaced old value. try again.
418	}
419	}
420	}
421	}
422
423
424	/**
425	* {@inheritDoc}
426	*
427	* @implSpec
428	* The default implementation is equivalent to performing the following
429	* steps for this {@code map}, then returning the current value or
430	* {@code null} if absent:
431	*
432	* <pre> {@code
433	* V oldValue = map.get(key);
434	* V newValue = (oldValue == null) ? value :
435	* remappingFunction.apply(oldValue, value);
436	* if (newValue == null)
437	* map.remove(key);
438	* else
439	* map.put(key, newValue);
440	* }</pre>
441	*
442	* The default implementation may retry these steps when multiple
443	* threads attempt updates, and may call the remapping function
444	* multiple times.
445	*
446	* @throws UnsupportedOperationException {@inheritDoc}
447	* @throws ClassCastException {@inheritDoc}
448	* @throws NullPointerException {@inheritDoc}
449	* @since 1.8
450	*/
451	@Override
452	default V merge(K key, V value,
453	BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
454	Objects.requireNonNull(remappingFunction);
455	Objects.requireNonNull(value);
456	V oldValue = get(key);
457	for (;;) {
458	if (oldValue != null) {
459	V newValue = remappingFunction.apply(oldValue, value);
460	if (newValue != null) {
461	if (replace(key, oldValue, newValue))
462	return newValue;
463	} else if (remove(key, oldValue)) {
464	return null;
465	}
466	oldValue = get(key);
467	} else {
468	if ((oldValue = putIfAbsent(key, value)) == null) {
469	return value;
470	}
471	}
472	}
473	}
474	}