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, for this {@code map}:
     * <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, for this {@code map}:
     * <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, for this {@code map}:
     * <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, for this {@code map}:
     * <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()) {
     *   K k;
     *   V v;
     *   do {
     *     k = entry.getKey();
     *     v = entry.getValue();
     *   } while (!map.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:	1.52
Committed:	Mon Dec 9 17:00:23 2013 UTC (10 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.51:	+20 -24 lines
Log Message:	use the one true code snippet style
#	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());}</pre>
77	*
78	* @implNote The default implementation assumes that {@code
79	* IllegalStateException} thrown by {@code getKey()} or {@code
80	* getValue()} indicates that the entry no longer exists.
81	* Operation continues for subsequent entries.
82	*
83	* @throws NullPointerException {@inheritDoc}
84	* @since 1.8
85	*/
86	@Override
87	default void forEach(BiConsumer<? super K, ? super V> action) {
88	Objects.requireNonNull(action);
89	for (Map.Entry<K,V> entry : entrySet()) {
90	K k;
91	V v;
92	try {
93	k = entry.getKey();
94	v = entry.getValue();
95	} catch (IllegalStateException ise) {
96	// this usually means the entry is no longer in the map.
97	continue;
98	}
99	action.accept(k, v);
100	}
101	}
102
103	/**
104	* If the specified key is not already associated
105	* with a value, associates it with the given value.
106	* This is equivalent to, for this {@code map}:
107	* <pre> {@code
108	* if (map.containsKey(key))
109	* return map.get(key);
110	* else
111	* return map.put(key, value);}</pre>
112	*
113	* except that the action is performed atomically.
114	*
115	* @implNote There is no default implementation.
116	*
117	* @param key key with which the specified value is to be associated
118	* @param value value to be associated with the specified key
119	* @return the previous value associated with the specified key, or
120	* {@code null} if there was no mapping for the key.
121	* (A {@code null} return can also indicate that the map
122	* previously associated {@code null} with the key,
123	* if the implementation supports null values.)
124	* @throws UnsupportedOperationException if the {@code put} operation
125	* is not supported by this map
126	* @throws ClassCastException if the class of the specified key or value
127	* prevents it from being stored in this map
128	* @throws NullPointerException if the specified key or value is null,
129	* and this map does not permit null keys or values
130	* @throws IllegalArgumentException if some property of the specified key
131	* or value prevents it from being stored in this map
132	*/
133	V putIfAbsent(K key, V value);
134
135	/**
136	* Removes the entry for a key only if currently mapped to a given value.
137	* This is equivalent to, for this {@code map}:
138	* <pre> {@code
139	* if (map.containsKey(key)
140	* && Objects.equals(map.get(key), value)) {
141	* map.remove(key);
142	* return true;
143	* } else
144	* return false;}</pre>
145	*
146	* except that the action is performed atomically.
147	*
148	* @implNote There is no default implementation.
149	*
150	* @param key key with which the specified value is associated
151	* @param value value expected to be associated with the specified key
152	* @return {@code true} if the value was removed
153	* @throws UnsupportedOperationException if the {@code remove} operation
154	* is not supported by this map
155	* @throws ClassCastException if the key or value is of an inappropriate
156	* type for this map
157	* (<a href="../Collection.html#optional-restrictions">optional</a>)
158	* @throws NullPointerException if the specified key or value is null,
159	* and this map does not permit null keys or values
160	* (<a href="../Collection.html#optional-restrictions">optional</a>)
161	*/
162	boolean remove(Object key, Object value);
163
164	/**
165	* Replaces the entry for a key only if currently mapped to a given value.
166	* This is equivalent to, for this {@code map}:
167	* <pre> {@code
168	* if (map.containsKey(key)
169	* && Objects.equals(map.get(key), oldValue)) {
170	* map.put(key, newValue);
171	* return true;
172	* } else
173	* return false;}</pre>
174	*
175	* except that the action is performed atomically.
176	*
177	* @implNote There is no default implementation.
178	*
179	* @param key key with which the specified value is associated
180	* @param oldValue value expected to be associated with the specified key
181	* @param newValue value to be associated with the specified key
182	* @return {@code true} if the value was replaced
183	* @throws UnsupportedOperationException if the {@code put} operation
184	* is not supported by this map
185	* @throws ClassCastException if the class of a specified key or value
186	* prevents it from being stored in this map
187	* @throws NullPointerException if a specified key or value is null,
188	* and this map does not permit null keys or values
189	* @throws IllegalArgumentException if some property of a specified key
190	* or value prevents it from being stored in this map
191	*/
192	boolean replace(K key, V oldValue, V newValue);
193
194	/**
195	* Replaces the entry for a key only if currently mapped to some value.
196	* This is equivalent to, for this {@code map}:
197	* <pre> {@code
198	* if (map.containsKey(key))
199	* return map.put(key, value);
200	* else
201	* return null;}</pre>
202	*
203	* except that the action is performed atomically.
204	*
205	* @implNote There is no default implementation.
206	*
207	* @param key key with which the specified value is associated
208	* @param value value to be associated with the specified key
209	* @return the previous value associated with the specified key, or
210	* {@code null} if there was no mapping for the key.
211	* (A {@code null} return can also indicate that the map
212	* previously associated {@code null} with the key,
213	* if the implementation supports null values.)
214	* @throws UnsupportedOperationException if the {@code put} operation
215	* is not supported by this map
216	* @throws ClassCastException if the class of the specified key or value
217	* prevents it from being stored in this map
218	* @throws NullPointerException if the specified key or value is null,
219	* and this map does not permit null keys or values
220	* @throws IllegalArgumentException if some property of the specified key
221	* or value prevents it from being stored in this map
222	*/
223	V replace(K key, V value);
224
225	/**
226	* {@inheritDoc}
227	*
228	* @implSpec
229	* <p>The default implementation is equivalent to, for this {@code map}:
230	* <pre> {@code
231	* for (Map.Entry<K,V> entry : map.entrySet()) {
232	* K k;
233	* V v;
234	* do {
235	* k = entry.getKey();
236	* v = entry.getValue();
237	* } while (!map.replace(k, v, function.apply(k, v)));
238	* }}</pre>
239	*
240	* The default implementation may retry these steps when multiple
241	* threads attempt updates, and may call the function multiple
242	* times.
243	*
244	* @throws UnsupportedOperationException {@inheritDoc}
245	* @throws NullPointerException {@inheritDoc}
246	* @throws ClassCastException {@inheritDoc}
247	* @throws IllegalArgumentException {@inheritDoc}
248	* @since 1.8
249	*/
250	@Override
251	default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
252	Objects.requireNonNull(function);
253	forEach((k,v) -> {
254	while (!replace(k, v, function.apply(k, v))) {
255	// v changed or k is gone
256	if ( (v = get(k)) == null) {
257	// k is no longer in the map.
258	break;
259	}
260	}
261	});
262	}
263
264	/**
265	* {@inheritDoc}
266	*
267	* @implSpec
268	* The default implementation is equivalent to the following steps for this
269	* {@code map}, then returning the current value or {@code null} if now
270	* absent:
271	*
272	* <pre> {@code
273	* if (map.get(key) == null) {
274	* V newValue = mappingFunction.apply(key);
275	* if (newValue != null)
276	* return map.putIfAbsent(key, newValue);
277	* }}</pre>
278	*
279	* The default implementation may retry these steps when multiple
280	* threads attempt updates, and may call the mapping function
281	* multiple times.
282	*
283	* @throws UnsupportedOperationException {@inheritDoc}
284	* @throws ClassCastException {@inheritDoc}
285	* @throws NullPointerException {@inheritDoc}
286	* @since 1.8
287	*/
288	@Override
289	default V computeIfAbsent(K key,
290	Function<? super K, ? extends V> mappingFunction) {
291	Objects.requireNonNull(mappingFunction);
292	V v, newValue;
293	return ((v = get(key)) == null &&
294	(newValue = mappingFunction.apply(key)) != null &&
295	(v = putIfAbsent(key, newValue)) == null) ? newValue : v;
296	}
297
298	/**
299	* {@inheritDoc}
300	*
301	* @implSpec
302	* The default implementation is equivalent to performing the following
303	* steps for this {@code map}, then returning the current value or
304	* {@code null} if now absent:
305	*
306	* <pre> {@code
307	* if (map.get(key) != null) {
308	* V oldValue = map.get(key);
309	* V newValue = remappingFunction.apply(key, oldValue);
310	* if (newValue != null)
311	* map.replace(key, oldValue, newValue);
312	* else
313	* map.remove(key, oldValue);
314	* }}</pre>
315	*
316	* The default implementation may retry these steps when multiple
317	* threads attempt updates, and may call the remapping function
318	* multiple times.
319	*
320	* @throws UnsupportedOperationException {@inheritDoc}
321	* @throws ClassCastException {@inheritDoc}
322	* @throws NullPointerException {@inheritDoc}
323	* @since 1.8
324	*/
325	@Override
326	default V computeIfPresent(K key,
327	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
328	Objects.requireNonNull(remappingFunction);
329	V oldValue;
330	while ((oldValue = get(key)) != null) {
331	V newValue = remappingFunction.apply(key, oldValue);
332	if (newValue != null) {
333	if (replace(key, oldValue, newValue))
334	return newValue;
335	} else if (remove(key, oldValue))
336	return null;
337	}
338	return oldValue;
339	}
340
341	/**
342	* {@inheritDoc}
343	*
344	* @implSpec
345	* The default implementation is equivalent to performing the following
346	* steps for this {@code map}, then returning the current value or
347	* {@code null} if absent:
348	*
349	* <pre> {@code
350	* V oldValue = map.get(key);
351	* V newValue = remappingFunction.apply(key, oldValue);
352	* if (oldValue != null ) {
353	* if (newValue != null)
354	* map.replace(key, oldValue, newValue);
355	* else
356	* map.remove(key, oldValue);
357	* } else {
358	* if (newValue != null)
359	* map.putIfAbsent(key, newValue);
360	* else
361	* return null;
362	* }}</pre>
363	*
364	* The default implementation may retry these steps when multiple
365	* threads attempt updates, and may call the remapping function
366	* multiple times.
367	*
368	* @throws UnsupportedOperationException {@inheritDoc}
369	* @throws ClassCastException {@inheritDoc}
370	* @throws NullPointerException {@inheritDoc}
371	* @since 1.8
372	*/
373	@Override
374	default V compute(K key,
375	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
376	Objects.requireNonNull(remappingFunction);
377	V oldValue = get(key);
378	for (;;) {
379	V newValue = remappingFunction.apply(key, oldValue);
380	if (newValue == null) {
381	// delete mapping
382	if (oldValue != null \|\| containsKey(key)) {
383	// something to remove
384	if (remove(key, oldValue)) {
385	// removed the old value as expected
386	return null;
387	}
388
389	// some other value replaced old value. try again.
390	oldValue = get(key);
391	} else {
392	// nothing to do. Leave things as they were.
393	return null;
394	}
395	} else {
396	// add or replace old mapping
397	if (oldValue != null) {
398	// replace
399	if (replace(key, oldValue, newValue)) {
400	// replaced as expected.
401	return newValue;
402	}
403
404	// some other value replaced old value. try again.
405	oldValue = get(key);
406	} else {
407	// add (replace if oldValue was null)
408	if ((oldValue = putIfAbsent(key, newValue)) == null) {
409	// replaced
410	return newValue;
411	}
412
413	// some other value replaced old value. try again.
414	}
415	}
416	}
417	}
418
419	/**
420	* {@inheritDoc}
421	*
422	* @implSpec
423	* The default implementation is equivalent to performing the following
424	* steps for this {@code map}, then returning the current value or
425	* {@code null} if absent:
426	*
427	* <pre> {@code
428	* V oldValue = map.get(key);
429	* V newValue = (oldValue == null) ? value :
430	* remappingFunction.apply(oldValue, value);
431	* if (newValue == null)
432	* map.remove(key);
433	* else
434	* map.put(key, newValue);}</pre>
435	*
436	* The default implementation may retry these steps when multiple
437	* threads attempt updates, and may call the remapping function
438	* multiple times.
439	*
440	* @throws UnsupportedOperationException {@inheritDoc}
441	* @throws ClassCastException {@inheritDoc}
442	* @throws NullPointerException {@inheritDoc}
443	* @since 1.8
444	*/
445	@Override
446	default V merge(K key, V value,
447	BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
448	Objects.requireNonNull(remappingFunction);
449	Objects.requireNonNull(value);
450	V oldValue = get(key);
451	for (;;) {
452	if (oldValue != null) {
453	V newValue = remappingFunction.apply(oldValue, value);
454	if (newValue != null) {
455	if (replace(key, oldValue, newValue))
456	return newValue;
457	} else if (remove(key, oldValue)) {
458	return null;
459	}
460	oldValue = get(key);
461	} else {
462	if ((oldValue = putIfAbsent(key, value)) == null) {
463	return value;
464	}
465	}
466	}
467	}
468	}