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:	1.53
Committed:	Wed Dec 31 07:54:13 2014 UTC (9 years, 5 months ago) by jsr166
Branch:	MAIN
Changes since 1.52:	+1 -0 lines
Log Message:	standardize import statement order
#	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
9	import java.util.Map;
10	import java.util.Objects;
11	import java.util.function.BiConsumer;
12	import java.util.function.BiFunction;
13	import java.util.function.Function;
14
15	/**
16	* A {@link java.util.Map} providing thread safety and atomicity
17	* guarantees.
18	*
19	* <p>To support atomic usages, ConcurrentMaps are expected not to
20	* allow {@code null} as a legal value (and to throw exceptions upon
21	* attempted insertions). This enables a return value of {@code null}
22	* to unambiguously indicate the absence of a mapping. This interface
23	* does not strictly forbid implementations that may hold {@code null}
24	* values. However, in any that do so, a {@code null} value must bear
25	* the same interpretation as the absence of a mapping in order to
26	* conform to method atomicity requirements. Further, any that do so
27	* must override all default method implementations.
28	*
29	* <p>Several methods (for example {@link #putIfAbsent}) inherited
30	* from {@link Map} do not have default implementations, and so must
31	* be provided by implementations of this interface, even though they
32	* have (non-atomic) default implementations in the {@link Map}
33	* interface.
34	*
35	* <p>Memory consistency effects: As with other concurrent
36	* collections, actions in a thread prior to placing an object into a
37	* {@code ConcurrentMap} as a key or value
38	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
39	* actions subsequent to the access or removal of that object from
40	* the {@code ConcurrentMap} in another thread.
41	*
42	* <p>This interface is a member of the
43	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
44	* Java Collections Framework</a>.
45	*
46	* @since 1.5
47	* @author Doug Lea
48	* @param <K> the type of keys maintained by this map
49	* @param <V> the type of mapped values
50	*/
51	public interface ConcurrentMap<K,V> extends Map<K,V> {
52
53	/**
54	* {@inheritDoc}
55	*
56	* @implNote The default implementation returns the result of
57	* {@code get(key)} unless {@code null}, in which case it returns
58	* the given defaultValue.
59	*
60	* @throws ClassCastException {@inheritDoc}
61	* @throws NullPointerException {@inheritDoc}
62	* @since 1.8
63	*/
64	@Override
65	default V getOrDefault(Object key, V defaultValue) {
66	V v;
67	return ((v = get(key)) != null) ? v : defaultValue;
68	}
69
70	/**
71	* {@inheritDoc}
72	*
73	* @implSpec The default implementation is equivalent to, for this
74	* {@code map}:
75	* <pre> {@code
76	* for (Map.Entry<K,V> entry : map.entrySet())
77	* action.accept(entry.getKey(), entry.getValue());}</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, for this {@code map}:
108	* <pre> {@code
109	* if (map.containsKey(key))
110	* return map.get(key);
111	* else
112	* return map.put(key, value);}</pre>
113	*
114	* except that the action is performed atomically.
115	*
116	* @implNote There is no default implementation.
117	*
118	* @param key key with which the specified value is to be associated
119	* @param value value to be associated with the specified key
120	* @return the previous value associated with the specified key, or
121	* {@code null} if there was no mapping for the key.
122	* (A {@code null} return can also indicate that the map
123	* previously associated {@code null} with the key,
124	* if the implementation supports null values.)
125	* @throws UnsupportedOperationException if the {@code put} operation
126	* is not supported by this map
127	* @throws ClassCastException if the class of the specified key or value
128	* prevents it from being stored in this map
129	* @throws NullPointerException if the specified key or value is null,
130	* and this map does not permit null keys or values
131	* @throws IllegalArgumentException if some property of the specified key
132	* or value prevents it from being stored in this map
133	*/
134	V putIfAbsent(K key, V value);
135
136	/**
137	* Removes the entry for a key only if currently mapped to a given value.
138	* This is equivalent to, for this {@code map}:
139	* <pre> {@code
140	* if (map.containsKey(key)
141	* && Objects.equals(map.get(key), value)) {
142	* map.remove(key);
143	* return true;
144	* } else
145	* return false;}</pre>
146	*
147	* except that the action is performed atomically.
148	*
149	* @implNote There is no default implementation.
150	*
151	* @param key key with which the specified value is associated
152	* @param value value expected to be associated with the specified key
153	* @return {@code true} if the value was removed
154	* @throws UnsupportedOperationException if the {@code remove} operation
155	* is not supported by this map
156	* @throws ClassCastException if the key or value is of an inappropriate
157	* type for this map
158	* (<a href="../Collection.html#optional-restrictions">optional</a>)
159	* @throws NullPointerException if the specified key or value is null,
160	* and this map does not permit null keys or values
161	* (<a href="../Collection.html#optional-restrictions">optional</a>)
162	*/
163	boolean remove(Object key, Object value);
164
165	/**
166	* Replaces the entry for a key only if currently mapped to a given value.
167	* This is equivalent to, for this {@code map}:
168	* <pre> {@code
169	* if (map.containsKey(key)
170	* && Objects.equals(map.get(key), oldValue)) {
171	* map.put(key, newValue);
172	* return true;
173	* } else
174	* return false;}</pre>
175	*
176	* except that the action is performed atomically.
177	*
178	* @implNote There is no default implementation.
179	*
180	* @param key key with which the specified value is associated
181	* @param oldValue value expected to be associated with the specified key
182	* @param newValue value to be associated with the specified key
183	* @return {@code true} if the value was replaced
184	* @throws UnsupportedOperationException if the {@code put} operation
185	* is not supported by this map
186	* @throws ClassCastException if the class of a specified key or value
187	* prevents it from being stored in this map
188	* @throws NullPointerException if a specified key or value is null,
189	* and this map does not permit null keys or values
190	* @throws IllegalArgumentException if some property of a specified key
191	* or value prevents it from being stored in this map
192	*/
193	boolean replace(K key, V oldValue, V newValue);
194
195	/**
196	* Replaces the entry for a key only if currently mapped to some value.
197	* This is equivalent to, for this {@code map}:
198	* <pre> {@code
199	* if (map.containsKey(key))
200	* return map.put(key, value);
201	* else
202	* return null;}</pre>
203	*
204	* except that the action is performed atomically.
205	*
206	* @implNote There is no default implementation.
207	*
208	* @param key key with which the specified value is associated
209	* @param value value to be associated with the specified key
210	* @return the previous value associated with the specified key, or
211	* {@code null} if there was no mapping for the key.
212	* (A {@code null} return can also indicate that the map
213	* previously associated {@code null} with the key,
214	* if the implementation supports null values.)
215	* @throws UnsupportedOperationException if the {@code put} operation
216	* is not supported by this map
217	* @throws ClassCastException if the class of the specified key or value
218	* prevents it from being stored in this map
219	* @throws NullPointerException if the specified key or value is null,
220	* and this map does not permit null keys or values
221	* @throws IllegalArgumentException if some property of the specified key
222	* or value prevents it from being stored in this map
223	*/
224	V replace(K key, V value);
225
226	/**
227	* {@inheritDoc}
228	*
229	* @implSpec
230	* <p>The default implementation is equivalent to, for this {@code map}:
231	* <pre> {@code
232	* for (Map.Entry<K,V> entry : map.entrySet()) {
233	* K k;
234	* V v;
235	* do {
236	* k = entry.getKey();
237	* v = entry.getValue();
238	* } while (!map.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	* }}</pre>
279	*
280	* The default implementation may retry these steps when multiple
281	* threads attempt updates, and may call the mapping function
282	* multiple times.
283	*
284	* @throws UnsupportedOperationException {@inheritDoc}
285	* @throws ClassCastException {@inheritDoc}
286	* @throws NullPointerException {@inheritDoc}
287	* @since 1.8
288	*/
289	@Override
290	default V computeIfAbsent(K key,
291	Function<? super K, ? extends V> mappingFunction) {
292	Objects.requireNonNull(mappingFunction);
293	V v, newValue;
294	return ((v = get(key)) == null &&
295	(newValue = mappingFunction.apply(key)) != null &&
296	(v = putIfAbsent(key, newValue)) == null) ? newValue : v;
297	}
298
299	/**
300	* {@inheritDoc}
301	*
302	* @implSpec
303	* The default implementation is equivalent to performing the following
304	* steps for this {@code map}, then returning the current value or
305	* {@code null} if now absent:
306	*
307	* <pre> {@code
308	* if (map.get(key) != null) {
309	* V oldValue = map.get(key);
310	* V newValue = remappingFunction.apply(key, oldValue);
311	* if (newValue != null)
312	* map.replace(key, oldValue, newValue);
313	* else
314	* map.remove(key, oldValue);
315	* }}</pre>
316	*
317	* The default implementation may retry these steps when multiple
318	* threads attempt updates, and may call the remapping function
319	* multiple times.
320	*
321	* @throws UnsupportedOperationException {@inheritDoc}
322	* @throws ClassCastException {@inheritDoc}
323	* @throws NullPointerException {@inheritDoc}
324	* @since 1.8
325	*/
326	@Override
327	default V computeIfPresent(K key,
328	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
329	Objects.requireNonNull(remappingFunction);
330	V oldValue;
331	while ((oldValue = get(key)) != null) {
332	V newValue = remappingFunction.apply(key, oldValue);
333	if (newValue != null) {
334	if (replace(key, oldValue, newValue))
335	return newValue;
336	} else if (remove(key, oldValue))
337	return null;
338	}
339	return oldValue;
340	}
341
342	/**
343	* {@inheritDoc}
344	*
345	* @implSpec
346	* The default implementation is equivalent to performing the following
347	* steps for this {@code map}, then returning the current value or
348	* {@code null} if absent:
349	*
350	* <pre> {@code
351	* V oldValue = map.get(key);
352	* V newValue = remappingFunction.apply(key, oldValue);
353	* if (oldValue != null ) {
354	* if (newValue != null)
355	* map.replace(key, oldValue, newValue);
356	* else
357	* map.remove(key, oldValue);
358	* } else {
359	* if (newValue != null)
360	* map.putIfAbsent(key, newValue);
361	* else
362	* return null;
363	* }}</pre>
364	*
365	* The default implementation may retry these steps when multiple
366	* threads attempt updates, and may call the remapping function
367	* multiple times.
368	*
369	* @throws UnsupportedOperationException {@inheritDoc}
370	* @throws ClassCastException {@inheritDoc}
371	* @throws NullPointerException {@inheritDoc}
372	* @since 1.8
373	*/
374	@Override
375	default V compute(K key,
376	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
377	Objects.requireNonNull(remappingFunction);
378	V oldValue = get(key);
379	for (;;) {
380	V newValue = remappingFunction.apply(key, oldValue);
381	if (newValue == null) {
382	// delete mapping
383	if (oldValue != null \|\| containsKey(key)) {
384	// something to remove
385	if (remove(key, oldValue)) {
386	// removed the old value as expected
387	return null;
388	}
389
390	// some other value replaced old value. try again.
391	oldValue = get(key);
392	} else {
393	// nothing to do. Leave things as they were.
394	return null;
395	}
396	} else {
397	// add or replace old mapping
398	if (oldValue != null) {
399	// replace
400	if (replace(key, oldValue, newValue)) {
401	// replaced as expected.
402	return newValue;
403	}
404
405	// some other value replaced old value. try again.
406	oldValue = get(key);
407	} else {
408	// add (replace if oldValue was null)
409	if ((oldValue = putIfAbsent(key, newValue)) == null) {
410	// replaced
411	return newValue;
412	}
413
414	// some other value replaced old value. try again.
415	}
416	}
417	}
418	}
419
420	/**
421	* {@inheritDoc}
422	*
423	* @implSpec
424	* The default implementation is equivalent to performing the following
425	* steps for this {@code map}, then returning the current value or
426	* {@code null} if absent:
427	*
428	* <pre> {@code
429	* V oldValue = map.get(key);
430	* V newValue = (oldValue == null) ? value :
431	* remappingFunction.apply(oldValue, value);
432	* if (newValue == null)
433	* map.remove(key);
434	* else
435	* map.put(key, newValue);}</pre>
436	*
437	* The default implementation may retry these steps when multiple
438	* threads attempt updates, and may call the remapping function
439	* multiple times.
440	*
441	* @throws UnsupportedOperationException {@inheritDoc}
442	* @throws ClassCastException {@inheritDoc}
443	* @throws NullPointerException {@inheritDoc}
444	* @since 1.8
445	*/
446	@Override
447	default V merge(K key, V value,
448	BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
449	Objects.requireNonNull(remappingFunction);
450	Objects.requireNonNull(value);
451	V oldValue = get(key);
452	for (;;) {
453	if (oldValue != null) {
454	V newValue = remappingFunction.apply(oldValue, value);
455	if (newValue != null) {
456	if (replace(key, oldValue, newValue))
457	return newValue;
458	} else if (remove(key, oldValue)) {
459	return null;
460	}
461	oldValue = get(key);
462	} else {
463	if ((oldValue = putIfAbsent(key, value)) == null) {
464	return value;
465	}
466	}
467	}
468	}
469	}