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, associates it with the given value.
     * This is equivalent to, for this {@code map}:
     * <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, 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 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, 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 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, 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 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:	1.61
Committed:	Thu Apr 30 18:50:22 2015 UTC (9 years, 1 month ago) by jsr166
Branch:	MAIN
Changes since 1.60:	+4 -2 lines
Log Message:	whitespace
#	User	Rev	Content
1	dl	1.2	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3	dl	1.17	* Expert Group and released to the public domain, as explained at
4	jsr166	1.31	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.2	*/
6
7	tim	1.1	package java.util.concurrent;
8	jsr166	1.53
9	tim	1.1	import java.util.Map;
10	dl	1.46	import java.util.Objects;
11			import java.util.function.BiConsumer;
12	dl	1.44	import java.util.function.BiFunction;
13	dl	1.46	import java.util.function.Function;
14	tim	1.1
15			/**
16	jsr166	1.43	* A {@link java.util.Map} providing thread safety and atomicity
17	dl	1.42	* guarantees.
18	dl	1.19	*
19	dl	1.55	* <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	dl	1.46	*
27	jsr166	1.29	* <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	dl	1.19	* <p>This interface is a member of the
35	jsr166	1.30	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
36	jsr166	1.47	* Java Collections Framework</a>.
37	jsr166	1.21	*
38	dl	1.4	* @since 1.5
39			* @author Doug Lea
40	dl	1.13	* @param <K> the type of keys maintained by this map
41	jsr166	1.21	* @param <V> the type of mapped values
42	dl	1.4	*/
43	jsr166	1.56	public interface ConcurrentMap<K,V> extends Map<K,V> {
44	dl	1.41
45			/**
46			* {@inheritDoc}
47			*
48	dl	1.55	* @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	dl	1.46	*
53			* @throws ClassCastException {@inheritDoc}
54			* @throws NullPointerException {@inheritDoc}
55			* @since 1.8
56	dl	1.41	*/
57			@Override
58			default V getOrDefault(Object key, V defaultValue) {
59			V v;
60			return ((v = get(key)) != null) ? v : defaultValue;
61			}
62
63	dl	1.46	/**
64			* {@inheritDoc}
65			*
66			* @implSpec The default implementation is equivalent to, for this
67			* {@code map}:
68	jsr166	1.52	* <pre> {@code
69	jsr166	1.57	* for (Map.Entry<K,V> entry : map.entrySet()) {
70	jsr166	1.56	* action.accept(entry.getKey(), entry.getValue());
71	jsr166	1.57	* }}</pre>
72	dl	1.55	*
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	dl	1.46	*
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	jsr166	1.56	for (Map.Entry<K,V> entry : entrySet()) {
85	dl	1.46	K k;
86			V v;
87			try {
88			k = entry.getKey();
89			v = entry.getValue();
90	jsr166	1.56	} catch (IllegalStateException ise) {
91	dl	1.46	// this usually means the entry is no longer in the map.
92			continue;
93			}
94			action.accept(k, v);
95			}
96			}
97
98	tim	1.1	/**
99			* If the specified key is not already associated
100	jsr166	1.60	* with a value, associates it with the given value.
101			* This is equivalent to, for this {@code map}:
102	jsr166	1.59	* <pre> {@code
103	dl	1.55	* if (!map.containsKey(key))
104			* return map.put(key, value);
105			* else
106	jsr166	1.59	* return map.get(key);}</pre>
107	jsr166	1.36	*
108	jsr166	1.21	* except that the action is performed atomically.
109			*
110	dl	1.55	* @implNote This implementation intentionally re-abstracts the
111			* inappropriate default provided in {@code Map}.
112	dl	1.46	*
113	jsr166	1.24	* @param key key with which the specified value is to be associated
114			* @param value value to be associated with the specified key
115	jsr166	1.25	* @return the previous value associated with the specified key, or
116	jsr166	1.38	* {@code null} if there was no mapping for the key.
117			* (A {@code null} return can also indicate that the map
118			* previously associated {@code null} with the key,
119	jsr166	1.25	* if the implementation supports null values.)
120	jsr166	1.38	* @throws UnsupportedOperationException if the {@code put} operation
121	jsr166	1.24	* is not supported by this map
122	tim	1.1	* @throws ClassCastException if the class of the specified key or value
123	jsr166	1.24	* prevents it from being stored in this map
124			* @throws NullPointerException if the specified key or value is null,
125			* and this map does not permit null keys or values
126			* @throws IllegalArgumentException if some property of the specified key
127			* or value prevents it from being stored in this map
128	dl	1.18	*/
129	jsr166	1.57	V putIfAbsent(K key, V value);
130	dl	1.6
131			/**
132	jsr166	1.21	* Removes the entry for a key only if currently mapped to a given value.
133	jsr166	1.60	* This is equivalent to, for this {@code map}:
134	jsr166	1.59	* <pre> {@code
135	jsr166	1.61	* if (map.containsKey(key)
136			* && Objects.equals(map.get(key), value)) {
137	jsr166	1.36	* map.remove(key);
138			* return true;
139	jsr166	1.59	* } else {
140	dl	1.55	* return false;
141	jsr166	1.59	* }}</pre>
142	jsr166	1.36	*
143	dl	1.6	* except that the action is performed atomically.
144	jsr166	1.21	*
145	dl	1.55	* @implNote This implementation intentionally re-abstracts the
146			* inappropriate default provided in {@code Map}.
147	dl	1.46	*
148	jsr166	1.24	* @param key key with which the specified value is associated
149	jsr166	1.25	* @param value value expected to be associated with the specified key
150	jsr166	1.38	* @return {@code true} if the value was removed
151			* @throws UnsupportedOperationException if the {@code remove} operation
152	jsr166	1.24	* is not supported by this map
153	jsr166	1.25	* @throws ClassCastException if the key or value is of an inappropriate
154	jsr166	1.35	* type for this map
155	dl	1.33	* (<a href="../Collection.html#optional-restrictions">optional</a>)
156	jsr166	1.24	* @throws NullPointerException if the specified key or value is null,
157	dl	1.32	* and this map does not permit null keys or values
158	dl	1.33	* (<a href="../Collection.html#optional-restrictions">optional</a>)
159	dl	1.6	*/
160	dl	1.7	boolean remove(Object key, Object value);
161	dl	1.14
162			/**
163	jsr166	1.21	* Replaces the entry for a key only if currently mapped to a given value.
164	jsr166	1.60	* This is equivalent to, for this {@code map}:
165	jsr166	1.59	* <pre> {@code
166	jsr166	1.61	* if (map.containsKey(key)
167			* && Objects.equals(map.get(key), oldValue)) {
168	jsr166	1.36	* map.put(key, newValue);
169			* return true;
170	jsr166	1.59	* } else {
171	dl	1.55	* return false;
172	jsr166	1.59	* }}</pre>
173	jsr166	1.36	*
174	dl	1.14	* except that the action is performed atomically.
175	jsr166	1.21	*
176	dl	1.55	* @implNote This implementation intentionally re-abstracts the
177			* inappropriate default provided in {@code Map}.
178	dl	1.46	*
179	jsr166	1.24	* @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	jsr166	1.38	* @return {@code true} if the value was replaced
183			* @throws UnsupportedOperationException if the {@code put} operation
184	jsr166	1.24	* is not supported by this map
185	jsr166	1.25	* @throws ClassCastException if the class of a specified key or value
186			* prevents it from being stored in this map
187	jsr166	1.24	* @throws NullPointerException if a specified key or value is null,
188			* and this map does not permit null keys or values
189	jsr166	1.25	* @throws IllegalArgumentException if some property of a specified key
190			* or value prevents it from being stored in this map
191	dl	1.14	*/
192			boolean replace(K key, V oldValue, V newValue);
193	dl	1.6
194	dl	1.15	/**
195	jsr166	1.21	* Replaces the entry for a key only if currently mapped to some value.
196	jsr166	1.60	* This is equivalent to, for this {@code map}:
197	jsr166	1.59	* <pre> {@code
198	dl	1.55	* if (map.containsKey(key)) {
199	jsr166	1.36	* return map.put(key, value);
200	jsr166	1.59	* } else {
201	dl	1.55	* return null;
202	jsr166	1.59	* }}</pre>
203	jsr166	1.36	*
204	dl	1.15	* except that the action is performed atomically.
205	jsr166	1.21	*
206	dl	1.55	* @implNote This implementation intentionally re-abstracts the
207			* inappropriate default provided in {@code Map}.
208	dl	1.46	*
209	jsr166	1.24	* @param key key with which the specified value is associated
210			* @param value value to be associated with the specified key
211	jsr166	1.25	* @return the previous value associated with the specified key, or
212	jsr166	1.38	* {@code null} if there was no mapping for the key.
213			* (A {@code null} return can also indicate that the map
214			* previously associated {@code null} with the key,
215	jsr166	1.25	* if the implementation supports null values.)
216	jsr166	1.38	* @throws UnsupportedOperationException if the {@code put} operation
217	jsr166	1.24	* is not supported by this map
218	jsr166	1.25	* @throws ClassCastException if the class of the specified key or value
219			* prevents it from being stored in this map
220	jsr166	1.24	* @throws NullPointerException if the specified key or value is null,
221			* and this map does not permit null keys or values
222	jsr166	1.25	* @throws IllegalArgumentException if some property of the specified key
223			* or value prevents it from being stored in this map
224	dl	1.15	*/
225	dl	1.16	V replace(K key, V value);
226	dl	1.44
227			/**
228			* {@inheritDoc}
229			*
230	dl	1.46	* @implSpec
231			* <p>The default implementation is equivalent to, for this {@code map}:
232	jsr166	1.52	* <pre> {@code
233	jsr166	1.57	* for (Map.Entry<K,V> entry : map.entrySet()) {
234	jsr166	1.56	* do {
235			* K k = entry.getKey();
236			* V v = entry.getValue();
237			* } while (!replace(k, v, function.apply(k, v)));
238	jsr166	1.57	* }}</pre>
239	dl	1.46	*
240			* The default implementation may retry these steps when multiple
241	dl	1.55	* threads attempt updates including potentially calling the function
242			* repeatedly for a given key.
243			*
244			* <p>This implementation assumes that the ConcurrentMap cannot contain null
245			* values and {@code get()} returning null unambiguously means the key is
246			* absent. Implementations which support null values <strong>must</strong>
247			* override this default implementation.
248	dl	1.46	*
249			* @throws UnsupportedOperationException {@inheritDoc}
250			* @throws NullPointerException {@inheritDoc}
251			* @throws ClassCastException {@inheritDoc}
252			* @throws IllegalArgumentException {@inheritDoc}
253			* @since 1.8
254	dl	1.44	*/
255			@Override
256			default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
257	dl	1.46	Objects.requireNonNull(function);
258			forEach((k,v) -> {
259	jsr166	1.56	while (!replace(k, v, function.apply(k, v))) {
260	dl	1.46	// v changed or k is gone
261			if ( (v = get(k)) == null) {
262			// k is no longer in the map.
263			break;
264			}
265	dl	1.44	}
266	dl	1.46	});
267			}
268
269			/**
270			* {@inheritDoc}
271			*
272			* @implSpec
273			* The default implementation is equivalent to the following steps for this
274			* {@code map}, then returning the current value or {@code null} if now
275			* absent:
276			*
277	jsr166	1.52	* <pre> {@code
278	dl	1.46	* if (map.get(key) == null) {
279	jsr166	1.59	* V newValue = mappingFunction.apply(key);
280			* if (newValue != null)
281			* return map.putIfAbsent(key, newValue);
282			* }}</pre>
283	dl	1.46	*
284			* The default implementation may retry these steps when multiple
285	dl	1.55	* 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	dl	1.46	*
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	dl	1.44
308	dl	1.46	/**
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	jsr166	1.59	* {@code null} if now absent:
315	dl	1.46	*
316	jsr166	1.52	* <pre> {@code
317	dl	1.46	* if (map.get(key) != null) {
318	jsr166	1.59	* 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			* }}</pre>
325	dl	1.46	*
326	dl	1.55	* The default implementation may retry these steps when multiple threads
327			* attempt updates including potentially calling the remapping function
328	dl	1.46	* multiple times.
329			*
330	dl	1.55	* <p>This implementation assumes that the ConcurrentMap cannot contain null
331			* values and {@code get()} returning null unambiguously means the key is
332			* absent. Implementations which support null values <strong>must</strong>
333			* override this default implementation.
334			*
335	dl	1.46	* @throws UnsupportedOperationException {@inheritDoc}
336			* @throws ClassCastException {@inheritDoc}
337			* @throws NullPointerException {@inheritDoc}
338			* @since 1.8
339			*/
340			@Override
341			default V computeIfPresent(K key,
342			BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
343			Objects.requireNonNull(remappingFunction);
344			V oldValue;
345	jsr166	1.56	while ((oldValue = get(key)) != null) {
346	dl	1.46	V newValue = remappingFunction.apply(key, oldValue);
347			if (newValue != null) {
348			if (replace(key, oldValue, newValue))
349			return newValue;
350			} else if (remove(key, oldValue))
351	jsr166	1.57	return null;
352	dl	1.46	}
353			return oldValue;
354			}
355
356			/**
357			* {@inheritDoc}
358			*
359			* @implSpec
360			* The default implementation is equivalent to performing the following
361			* steps for this {@code map}, then returning the current value or
362			* {@code null} if absent:
363			*
364	jsr166	1.52	* <pre> {@code
365	dl	1.46	* V oldValue = map.get(key);
366			* V newValue = remappingFunction.apply(key, oldValue);
367			* if (oldValue != null ) {
368	jsr166	1.59	* if (newValue != null)
369			* map.replace(key, oldValue, newValue);
370			* else
371			* map.remove(key, oldValue);
372	dl	1.46	* } else {
373	jsr166	1.59	* if (newValue != null)
374			* map.putIfAbsent(key, newValue);
375			* else
376			* return null;
377			* }}</pre>
378	dl	1.46	*
379			* The default implementation may retry these steps when multiple
380	dl	1.55	* threads attempt updates including potentially calling the remapping
381			* function multiple times.
382			*
383			* <p>This implementation assumes that the ConcurrentMap cannot contain null
384			* values and {@code get()} returning null unambiguously means the key is
385			* absent. Implementations which support null values <strong>must</strong>
386			* override this default implementation.
387	dl	1.46	*
388			* @throws UnsupportedOperationException {@inheritDoc}
389			* @throws ClassCastException {@inheritDoc}
390			* @throws NullPointerException {@inheritDoc}
391			* @since 1.8
392			*/
393			@Override
394			default V compute(K key,
395			BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
396			Objects.requireNonNull(remappingFunction);
397			V oldValue = get(key);
398	jsr166	1.56	for (;;) {
399	dl	1.46	V newValue = remappingFunction.apply(key, oldValue);
400			if (newValue == null) {
401			// delete mapping
402			if (oldValue != null \|\| containsKey(key)) {
403			// something to remove
404			if (remove(key, oldValue)) {
405			// removed the old value as expected
406			return null;
407			}
408
409			// some other value replaced old value. try again.
410			oldValue = get(key);
411			} else {
412			// nothing to do. Leave things as they were.
413			return null;
414			}
415			} else {
416			// add or replace old mapping
417			if (oldValue != null) {
418			// replace
419			if (replace(key, oldValue, newValue)) {
420			// replaced as expected.
421			return newValue;
422			}
423
424			// some other value replaced old value. try again.
425			oldValue = get(key);
426			} else {
427			// add (replace if oldValue was null)
428			if ((oldValue = putIfAbsent(key, newValue)) == null) {
429			// replaced
430			return newValue;
431			}
432
433			// some other value replaced old value. try again.
434			}
435	dl	1.44	}
436			}
437			}
438
439	dl	1.46	/**
440			* {@inheritDoc}
441			*
442			* @implSpec
443			* The default implementation is equivalent to performing the following
444			* steps for this {@code map}, then returning the current value or
445			* {@code null} if absent:
446			*
447	jsr166	1.52	* <pre> {@code
448	dl	1.46	* V oldValue = map.get(key);
449			* V newValue = (oldValue == null) ? value :
450	jsr166	1.59	* remappingFunction.apply(oldValue, value);
451	dl	1.46	* if (newValue == null)
452	jsr166	1.59	* map.remove(key);
453	dl	1.46	* else
454	jsr166	1.59	* map.put(key, newValue);}</pre>
455	dl	1.46	*
456	dl	1.55	* <p>The default implementation may retry these steps when multiple
457			* threads attempt updates including potentially calling the remapping
458			* function multiple times.
459			*
460			* <p>This implementation assumes that the ConcurrentMap cannot contain null
461			* values and {@code get()} returning null unambiguously means the key is
462			* absent. Implementations which support null values <strong>must</strong>
463			* override this default implementation.
464	dl	1.46	*
465			* @throws UnsupportedOperationException {@inheritDoc}
466			* @throws ClassCastException {@inheritDoc}
467			* @throws NullPointerException {@inheritDoc}
468			* @since 1.8
469			*/
470			@Override
471			default V merge(K key, V value,
472			BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
473			Objects.requireNonNull(remappingFunction);
474			Objects.requireNonNull(value);
475			V oldValue = get(key);
476			for (;;) {
477			if (oldValue != null) {
478			V newValue = remappingFunction.apply(oldValue, value);
479			if (newValue != null) {
480			if (replace(key, oldValue, newValue))
481			return newValue;
482			} else if (remove(key, oldValue)) {
483			return null;
484			}
485			oldValue = get(key);
486			} else {
487			if ((oldValue = putIfAbsent(key, value)) == null) {
488			return value;
489			}
490			}
491			}
492			}
493	tim	1.1	}