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 maintain the specified guarantees, default implementations of
 * methods including {@link #putIfAbsent} inherited from {@link Map}
 * must be overridden by implementations of this interface. Similarly,
 * implementations of the collections returned by methods {@link
 * #keySet}, {@link #values}, and {@link #entrySet} must override
 * methods such as {@code removeIf} when necessary to
 * preserve atomicity guarantees.
 *
 * <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 This implementation assumes that the ConcurrentMap cannot
     * contain null values and {@code get()} returning null unambiguously means
     * the key is absent. Implementations which support null values
     * <strong>must</strong> override this default implementation.
     *
     * @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 has been removed and cannot
     * be processed. 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, associate it with the given value.
     * This is equivalent to
     *  <pre> {@code
     * if (!map.containsKey(key))
     *   return map.put(key, value);
     * else
     *   return map.get(key);
     * }</pre>
     *
     * except that the action is performed atomically.
     *
     * @implNote This implementation intentionally re-abstracts the
     * inappropriate default provided in {@code Map}.
     *
     * @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 This implementation intentionally re-abstracts the
     * inappropriate default provided in {@code Map}.
     *
     * @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 This implementation intentionally re-abstracts the
     * inappropriate default provided in {@code Map}.
     *
     * @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 This implementation intentionally re-abstracts the
     * inappropriate default provided in {@code Map}.
     *
     * @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 including potentially calling the function
     * repeatedly for a given key.
     *
     * <p>This implementation assumes that the ConcurrentMap cannot contain null
     * values and {@code get()} returning null unambiguously means the key is
     * absent. Implementations which support null values <strong>must</strong>
     * override this default implementation.
     *
     * @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 including potentially calling the mapping
     * function multiple times.
     *
     * <p>This implementation assumes that the ConcurrentMap cannot contain null
     * values and {@code get()} returning null unambiguously means the key is
     * absent. Implementations which support null values <strong>must</strong>
     * override this default implementation.
     *
     * @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 including potentially calling the remapping function
     * multiple times.
     *
     * <p>This implementation assumes that the ConcurrentMap cannot contain null
     * values and {@code get()} returning null unambiguously means the key is
     * absent. Implementations which support null values <strong>must</strong>
     * override this default implementation.
     *
     * @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 including potentially calling the remapping
     * function multiple times.
     *
     * <p>This implementation assumes that the ConcurrentMap cannot contain null
     * values and {@code get()} returning null unambiguously means the key is
     * absent. Implementations which support null values <strong>must</strong>
     * override this default implementation.
     *
     * @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>
     *
     * <p>The default implementation may retry these steps when multiple
     * threads attempt updates including potentially calling the remapping
     * function multiple times.
     *
     * <p>This implementation assumes that the ConcurrentMap cannot contain null
     * values and {@code get()} returning null unambiguously means the key is
     * absent. Implementations which support null values <strong>must</strong>
     * override this default implementation.
     *
     * @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:	1.56
Committed:	Wed Apr 29 18:01:41 2015 UTC (9 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.55:	+13 -13 lines
Log Message:	jsr166 coding 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
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 maintain the specified guarantees, default implementations of
20	* methods including {@link #putIfAbsent} inherited from {@link Map}
21	* must be overridden by implementations of this interface. Similarly,
22	* implementations of the collections returned by methods {@link
23	* #keySet}, {@link #values}, and {@link #entrySet} must override
24	* methods such as {@code removeIf} when necessary to
25	* preserve atomicity guarantees.
26	*
27	* <p>Memory consistency effects: As with other concurrent
28	* collections, actions in a thread prior to placing an object into a
29	* {@code ConcurrentMap} as a key or value
30	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
31	* actions subsequent to the access or removal of that object from
32	* the {@code ConcurrentMap} in another thread.
33	*
34	* <p>This interface is a member of the
35	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
36	* Java Collections Framework</a>.
37	*
38	* @since 1.5
39	* @author Doug Lea
40	* @param <K> the type of keys maintained by this map
41	* @param <V> the type of mapped values
42	*/
43	public interface ConcurrentMap<K,V> extends Map<K,V> {
44
45	/**
46	* {@inheritDoc}
47	*
48	* @implNote This implementation assumes that the ConcurrentMap cannot
49	* contain null values and {@code get()} returning null unambiguously means
50	* the key is absent. Implementations which support null values
51	* <strong>must</strong> override this default implementation.
52	*
53	* @throws ClassCastException {@inheritDoc}
54	* @throws NullPointerException {@inheritDoc}
55	* @since 1.8
56	*/
57	@Override
58	default V getOrDefault(Object key, V defaultValue) {
59	V v;
60	return ((v = get(key)) != null) ? v : defaultValue;
61	}
62
63	/**
64	* {@inheritDoc}
65	*
66	* @implSpec The default implementation is equivalent to, for this
67	* {@code map}:
68	* <pre> {@code
69	* for (Map.Entry<K,V> entry : map.entrySet())
70	* action.accept(entry.getKey(), entry.getValue());
71	* }</pre>
72	*
73	* @implNote The default implementation assumes that
74	* {@code IllegalStateException} thrown by {@code getKey()} or
75	* {@code getValue()} indicates that the entry has been removed and cannot
76	* be processed. Operation continues for subsequent entries.
77	*
78	* @throws NullPointerException {@inheritDoc}
79	* @since 1.8
80	*/
81	@Override
82	default void forEach(BiConsumer<? super K, ? super V> action) {
83	Objects.requireNonNull(action);
84	for (Map.Entry<K,V> entry : entrySet()) {
85	K k;
86	V v;
87	try {
88	k = entry.getKey();
89	v = entry.getValue();
90	} catch (IllegalStateException ise) {
91	// this usually means the entry is no longer in the map.
92	continue;
93	}
94	action.accept(k, v);
95	}
96	}
97
98	/**
99	* If the specified key is not already associated
100	* with a value, associate it with the given value.
101	* This is equivalent to
102	* <pre> {@code
103	* if (!map.containsKey(key))
104	* return map.put(key, value);
105	* else
106	* return map.get(key);
107	* }</pre>
108	*
109	* except that the action is performed atomically.
110	*
111	* @implNote This implementation intentionally re-abstracts the
112	* inappropriate default provided in {@code Map}.
113	*
114	* @param key key with which the specified value is to be associated
115	* @param value value to be associated with the specified key
116	* @return the previous value associated with the specified key, or
117	* {@code null} if there was no mapping for the key.
118	* (A {@code null} return can also indicate that the map
119	* previously associated {@code null} with the key,
120	* if the implementation supports null values.)
121	* @throws UnsupportedOperationException if the {@code put} operation
122	* is not supported by this map
123	* @throws ClassCastException if the class of the specified key or value
124	* prevents it from being stored in this map
125	* @throws NullPointerException if the specified key or value is null,
126	* and this map does not permit null keys or values
127	* @throws IllegalArgumentException if some property of the specified key
128	* or value prevents it from being stored in this map
129	*/
130	V putIfAbsent(K key, V value);
131
132	/**
133	* Removes the entry for a key only if currently mapped to a given value.
134	* This is equivalent to
135	* <pre> {@code
136	* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
137	* map.remove(key);
138	* return true;
139	* } else
140	* return false;
141	* }</pre>
142	*
143	* except that the action is performed atomically.
144	*
145	* @implNote This implementation intentionally re-abstracts the
146	* inappropriate default provided in {@code Map}.
147	*
148	* @param key key with which the specified value is associated
149	* @param value value expected to be associated with the specified key
150	* @return {@code true} if the value was removed
151	* @throws UnsupportedOperationException if the {@code remove} operation
152	* is not supported by this map
153	* @throws ClassCastException if the key or value is of an inappropriate
154	* type for this map
155	* (<a href="../Collection.html#optional-restrictions">optional</a>)
156	* @throws NullPointerException if the specified key or value is null,
157	* and this map does not permit null keys or values
158	* (<a href="../Collection.html#optional-restrictions">optional</a>)
159	*/
160	boolean remove(Object key, Object value);
161
162	/**
163	* Replaces the entry for a key only if currently mapped to a given value.
164	* This is equivalent to
165	* <pre> {@code
166	* if (map.containsKey(key) && Objects.equals(map.get(key), oldValue)) {
167	* map.put(key, newValue);
168	* return true;
169	* } else
170	* return false;
171	* }</pre>
172	*
173	* except that the action is performed atomically.
174	*
175	* @implNote This implementation intentionally re-abstracts the
176	* inappropriate default provided in {@code Map}.
177	*
178	* @param key key with which the specified value is associated
179	* @param oldValue value expected to be associated with the specified key
180	* @param newValue value to be associated with the specified key
181	* @return {@code true} if the value was replaced
182	* @throws UnsupportedOperationException if the {@code put} operation
183	* is not supported by this map
184	* @throws ClassCastException if the class of a specified key or value
185	* prevents it from being stored in this map
186	* @throws NullPointerException if a specified key or value is null,
187	* and this map does not permit null keys or values
188	* @throws IllegalArgumentException if some property of a specified key
189	* or value prevents it from being stored in this map
190	*/
191	boolean replace(K key, V oldValue, V newValue);
192
193	/**
194	* Replaces the entry for a key only if currently mapped to some value.
195	* This is equivalent to
196	* <pre> {@code
197	* if (map.containsKey(key)) {
198	* return map.put(key, value);
199	* } else
200	* return null;
201	* }</pre>
202	*
203	* except that the action is performed atomically.
204	*
205	* @implNote This implementation intentionally re-abstracts the
206	* inappropriate default provided in {@code Map}.
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	* do {
234	* K k = entry.getKey();
235	* V v = entry.getValue();
236	* } while (!replace(k, v, function.apply(k, v)));
237	* }</pre>
238	*
239	* The default implementation may retry these steps when multiple
240	* threads attempt updates including potentially calling the function
241	* repeatedly for a given key.
242	*
243	* <p>This implementation assumes that the ConcurrentMap cannot contain null
244	* values and {@code get()} returning null unambiguously means the key is
245	* absent. Implementations which support null values <strong>must</strong>
246	* override this default implementation.
247	*
248	* @throws UnsupportedOperationException {@inheritDoc}
249	* @throws NullPointerException {@inheritDoc}
250	* @throws ClassCastException {@inheritDoc}
251	* @throws IllegalArgumentException {@inheritDoc}
252	* @since 1.8
253	*/
254	@Override
255	default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
256	Objects.requireNonNull(function);
257	forEach((k,v) -> {
258	while (!replace(k, v, function.apply(k, v))) {
259	// v changed or k is gone
260	if ( (v = get(k)) == null) {
261	// k is no longer in the map.
262	break;
263	}
264	}
265	});
266	}
267
268	/**
269	* {@inheritDoc}
270	*
271	* @implSpec
272	* The default implementation is equivalent to the following steps for this
273	* {@code map}, then returning the current value or {@code null} if now
274	* absent:
275	*
276	* <pre> {@code
277	* if (map.get(key) == null) {
278	* V newValue = mappingFunction.apply(key);
279	* if (newValue != null)
280	* return map.putIfAbsent(key, newValue);
281	* }
282	* }</pre>
283	*
284	* The default implementation may retry these steps when multiple
285	* threads attempt updates including potentially calling the mapping
286	* function multiple times.
287	*
288	* <p>This implementation assumes that the ConcurrentMap cannot contain null
289	* values and {@code get()} returning null unambiguously means the key is
290	* absent. Implementations which support null values <strong>must</strong>
291	* override this default implementation.
292	*
293	* @throws UnsupportedOperationException {@inheritDoc}
294	* @throws ClassCastException {@inheritDoc}
295	* @throws NullPointerException {@inheritDoc}
296	* @since 1.8
297	*/
298	@Override
299	default V computeIfAbsent(K key,
300	Function<? super K, ? extends V> mappingFunction) {
301	Objects.requireNonNull(mappingFunction);
302	V v, newValue;
303	return ((v = get(key)) == null &&
304	(newValue = mappingFunction.apply(key)) != null &&
305	(v = putIfAbsent(key, newValue)) == null) ? newValue : v;
306	}
307
308	/**
309	* {@inheritDoc}
310	*
311	* @implSpec
312	* The default implementation is equivalent to performing the following
313	* steps for this {@code map}, then returning the current value or
314	* {@code null} if now absent. :
315	*
316	* <pre> {@code
317	* if (map.get(key) != null) {
318	* V oldValue = map.get(key);
319	* V newValue = remappingFunction.apply(key, oldValue);
320	* if (newValue != null)
321	* map.replace(key, oldValue, newValue);
322	* else
323	* map.remove(key, oldValue);
324	* }
325	* }</pre>
326	*
327	* The default implementation may retry these steps when multiple threads
328	* attempt updates including potentially calling the remapping function
329	* multiple times.
330	*
331	* <p>This implementation assumes that the ConcurrentMap cannot contain null
332	* values and {@code get()} returning null unambiguously means the key is
333	* absent. Implementations which support null values <strong>must</strong>
334	* override this default implementation.
335	*
336	* @throws UnsupportedOperationException {@inheritDoc}
337	* @throws ClassCastException {@inheritDoc}
338	* @throws NullPointerException {@inheritDoc}
339	* @since 1.8
340	*/
341	@Override
342	default V computeIfPresent(K key,
343	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
344	Objects.requireNonNull(remappingFunction);
345	V oldValue;
346	while ((oldValue = get(key)) != null) {
347	V newValue = remappingFunction.apply(key, oldValue);
348	if (newValue != null) {
349	if (replace(key, oldValue, newValue))
350	return newValue;
351	} else if (remove(key, oldValue))
352	return null;
353	}
354	return oldValue;
355	}
356
357	/**
358	* {@inheritDoc}
359	*
360	* @implSpec
361	* The default implementation is equivalent to performing the following
362	* steps for this {@code map}, then returning the current value or
363	* {@code null} if absent:
364	*
365	* <pre> {@code
366	* V oldValue = map.get(key);
367	* V newValue = remappingFunction.apply(key, oldValue);
368	* if (oldValue != null ) {
369	* if (newValue != null)
370	* map.replace(key, oldValue, newValue);
371	* else
372	* map.remove(key, oldValue);
373	* } else {
374	* if (newValue != null)
375	* map.putIfAbsent(key, newValue);
376	* else
377	* return null;
378	* }
379	* }</pre>
380	*
381	* The default implementation may retry these steps when multiple
382	* threads attempt updates including potentially calling the remapping
383	* function multiple times.
384	*
385	* <p>This implementation assumes that the ConcurrentMap cannot contain null
386	* values and {@code get()} returning null unambiguously means the key is
387	* absent. Implementations which support null values <strong>must</strong>
388	* override this default implementation.
389	*
390	* @throws UnsupportedOperationException {@inheritDoc}
391	* @throws ClassCastException {@inheritDoc}
392	* @throws NullPointerException {@inheritDoc}
393	* @since 1.8
394	*/
395	@Override
396	default V compute(K key,
397	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
398	Objects.requireNonNull(remappingFunction);
399	V oldValue = get(key);
400	for (;;) {
401	V newValue = remappingFunction.apply(key, oldValue);
402	if (newValue == null) {
403	// delete mapping
404	if (oldValue != null \|\| containsKey(key)) {
405	// something to remove
406	if (remove(key, oldValue)) {
407	// removed the old value as expected
408	return null;
409	}
410
411	// some other value replaced old value. try again.
412	oldValue = get(key);
413	} else {
414	// nothing to do. Leave things as they were.
415	return null;
416	}
417	} else {
418	// add or replace old mapping
419	if (oldValue != null) {
420	// replace
421	if (replace(key, oldValue, newValue)) {
422	// replaced as expected.
423	return newValue;
424	}
425
426	// some other value replaced old value. try again.
427	oldValue = get(key);
428	} else {
429	// add (replace if oldValue was null)
430	if ((oldValue = putIfAbsent(key, newValue)) == null) {
431	// replaced
432	return newValue;
433	}
434
435	// some other value replaced old value. try again.
436	}
437	}
438	}
439	}
440
441
442	/**
443	* {@inheritDoc}
444	*
445	* @implSpec
446	* The default implementation is equivalent to performing the following
447	* steps for this {@code map}, then returning the current value or
448	* {@code null} if absent:
449	*
450	* <pre> {@code
451	* V oldValue = map.get(key);
452	* V newValue = (oldValue == null) ? value :
453	* remappingFunction.apply(oldValue, value);
454	* if (newValue == null)
455	* map.remove(key);
456	* else
457	* map.put(key, newValue);
458	* }</pre>
459	*
460	* <p>The default implementation may retry these steps when multiple
461	* threads attempt updates including potentially calling the remapping
462	* function multiple times.
463	*
464	* <p>This implementation assumes that the ConcurrentMap cannot contain null
465	* values and {@code get()} returning null unambiguously means the key is
466	* absent. Implementations which support null values <strong>must</strong>
467	* override this default implementation.
468	*
469	* @throws UnsupportedOperationException {@inheritDoc}
470	* @throws ClassCastException {@inheritDoc}
471	* @throws NullPointerException {@inheritDoc}
472	* @since 1.8
473	*/
474	@Override
475	default V merge(K key, V value,
476	BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
477	Objects.requireNonNull(remappingFunction);
478	Objects.requireNonNull(value);
479	V oldValue = get(key);
480	for (;;) {
481	if (oldValue != null) {
482	V newValue = remappingFunction.apply(oldValue, value);
483	if (newValue != null) {
484	if (replace(key, oldValue, newValue))
485	return newValue;
486	} else if (remove(key, oldValue)) {
487	return null;
488	}
489	oldValue = get(key);
490	} else {
491	if ((oldValue = putIfAbsent(key, value)) == null) {
492	return value;
493	}
494	}
495	}
496	}
497	}