https://openjdk.java.net/ 
/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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 Map providing thread safety and atomicity guarantees. 
To maintain the specified guarantees, default implementations of methods including putIfAbsent inherited from Map must be overridden by implementations of this interface. Similarly, implementations of the collections returned by methods Map.keySet, Map.values, and Map.entrySet must override methods such as removeIf when necessary to preserve atomicity guarantees. 
Memory consistency effects: As with other concurrent collections, actions in a thread prior to placing an object into a ConcurrentMap as a key or value happen-before actions subsequent to the access or removal of that object from the ConcurrentMap in another thread. 
This interface is a member of the

Java Collections Framework.

Author:Doug Lea
Type parameters:
  • <K> – the type of keys maintained by this map
  • <V> – the type of mapped values
Since:1.5
/**
 * A {@link 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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}

Throws:
Implementation Note:This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.
Since:1.8
/**
     * {@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}

Throws:
Implementation Requirements:The default implementation is equivalent to, for this map: 
 
for (Map.Entry<K,V> entry : map.entrySet()) {
  action.accept(entry.getKey(), entry.getValue());
 }
Implementation Note:The default implementation assumes that IllegalStateException thrown by getKey() or getValue() indicates that the entry has been removed and cannot be processed. Operation continues for subsequent entries.
Since:1.8
/**
     * {@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 map: 
 
if (!map.containsKey(key))
  return map.put(key, value);
else
  return map.get(key);

except that the action is performed atomically.

Params:
  • key – key with which the specified value is to be associated
  • value – value to be associated with the specified key
Throws:
Implementation Note:This implementation intentionally re-abstracts the inappropriate default provided in Map.
Returns:the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
/**
     * 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 map: 
 
if (map.containsKey(key)
    && Objects.equals(map.get(key), value)) {
  map.remove(key);
  return true;
 } else {
  return false;
 }

except that the action is performed atomically.

Params:
  • key – key with which the specified value is associated
  • value – value expected to be associated with the specified key
Throws:
Implementation Note:This implementation intentionally re-abstracts the inappropriate default provided in Map.
Returns:true if the value was removed
/**
     * 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="{@docRoot}/java.base/java/util/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="{@docRoot}/java.base/java/util/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 map: 
 
if (map.containsKey(key)
    && Objects.equals(map.get(key), oldValue)) {
  map.put(key, newValue);
  return true;
 } else {
  return false;
 }

except that the action is performed atomically.

Params:
  • key – key with which the specified value is associated
  • oldValue – value expected to be associated with the specified key
  • newValue – value to be associated with the specified key
Throws:
Implementation Note:This implementation intentionally re-abstracts the inappropriate default provided in Map.
Returns:true if the value was replaced
/**
     * 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 map: 
 
if (map.containsKey(key))
  return map.put(key, value);
else
  return null;

except that the action is performed atomically.

Params:
  • key – key with which the specified value is associated
  • value – value to be associated with the specified key
Throws:
Implementation Note:This implementation intentionally re-abstracts the inappropriate default provided in Map.
Returns:the previous value associated with the specified key, or null if there was no mapping for the key. (A null return can also indicate that the map previously associated null with the key, if the implementation supports null values.)
/**
     * 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}

Throws:
Implementation Requirements:

The default implementation is equivalent to, for this map: 
 
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)));
 }

The default implementation may retry these steps when multiple
threads attempt updates including potentially calling the function
repeatedly for a given key.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must
override this default implementation.
Since:1.8
/**
     * {@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 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}

Throws:
Implementation Requirements: The default implementation is equivalent to the following steps for this map: 
 
V oldValue, newValue;
return ((oldValue = map.get(key)) == null
        && (newValue = mappingFunction.apply(key)) != null
        && (oldValue = map.putIfAbsent(key, newValue)) == null)
  ? newValue
  : oldValue;


This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must
override this default implementation.
Since:1.8
/**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to the following steps for this
     * {@code map}:
     *
     * <pre> {@code
     * V oldValue, newValue;
     * return ((oldValue = map.get(key)) == null
     *         && (newValue = mappingFunction.apply(key)) != null
     *         && (oldValue = map.putIfAbsent(key, newValue)) == null)
     *   ? newValue
     *   : oldValue;}</pre>
     *
     * <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}
     * @throws IllegalArgumentException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
        Objects.requireNonNull(mappingFunction);
        V oldValue, newValue;
        return ((oldValue = get(key)) == null
                && (newValue = mappingFunction.apply(key)) != null
                && (oldValue = putIfAbsent(key, newValue)) == null)
            ? newValue
            : oldValue;
    }

    
{@inheritDoc}

Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map: 
 
for (V oldValue; (oldValue = map.get(key)) != null; ) {
  V newValue = remappingFunction.apply(key, oldValue);
  if ((newValue == null)
      ? map.remove(key, oldValue)
      : map.replace(key, oldValue, newValue))
    return newValue;
 }
return null;

When multiple threads attempt updates, map operations and the
remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must
override this default implementation.
Since:1.8
/**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}:
     *
     * <pre> {@code
     * for (V oldValue; (oldValue = map.get(key)) != null; ) {
     *   V newValue = remappingFunction.apply(key, oldValue);
     *   if ((newValue == null)
     *       ? map.remove(key, oldValue)
     *       : map.replace(key, oldValue, newValue))
     *     return newValue;
     * }
     * return null;}</pre>
     * When multiple threads attempt updates, map operations and the
     * remapping function may be called 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}
     * @throws IllegalArgumentException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V computeIfPresent(K key,
            BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        Objects.requireNonNull(remappingFunction);
        for (V oldValue; (oldValue = get(key)) != null; ) {
            V newValue = remappingFunction.apply(key, oldValue);
            if ((newValue == null)
                ? remove(key, oldValue)
                : replace(key, oldValue, newValue))
                return newValue;
        }
        return null;
    }

    
{@inheritDoc}

Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map: 
 
for (;;) {
  V oldValue = map.get(key);
  V newValue = remappingFunction.apply(key, oldValue);
  if (newValue != null) {
    if ((oldValue != null)
      ? map.replace(key, oldValue, newValue)
      : map.putIfAbsent(key, newValue) == null)
      return newValue;
  } else if (oldValue == null || map.remove(key, oldValue)) {
    return null;
  }
 }

When multiple threads attempt updates, map operations and the
remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must
override this default implementation.
Since:1.8
/**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}:
     *
     * <pre> {@code
     * for (;;) {
     *   V oldValue = map.get(key);
     *   V newValue = remappingFunction.apply(key, oldValue);
     *   if (newValue != null) {
     *     if ((oldValue != null)
     *       ? map.replace(key, oldValue, newValue)
     *       : map.putIfAbsent(key, newValue) == null)
     *       return newValue;
     *   } else if (oldValue == null || map.remove(key, oldValue)) {
     *     return null;
     *   }
     * }}</pre>
     * When multiple threads attempt updates, map operations and the
     * remapping function may be called 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}
     * @throws IllegalArgumentException {@inheritDoc}
     * @since 1.8
     */
    @Override
    default V compute(K key,
                      BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
        retry: for (;;) {
            V oldValue = get(key);
            // if putIfAbsent fails, opportunistically use its return value
            haveOldValue: for (;;) {
                V newValue = remappingFunction.apply(key, oldValue);
                if (newValue != null) {
                    if (oldValue != null) {
                        if (replace(key, oldValue, newValue))
                            return newValue;
                    }
                    else if ((oldValue = putIfAbsent(key, newValue)) == null)
                        return newValue;
                    else continue haveOldValue;
                } else if (oldValue == null || remove(key, oldValue)) {
                    return null;
                }
                continue retry;
            }
        }
    }

    
{@inheritDoc}

Throws:
Implementation Requirements: The default implementation is equivalent to performing the following steps for this map: 
 
for (;;) {
  V oldValue = map.get(key);
  if (oldValue != null) {
    V newValue = remappingFunction.apply(oldValue, value);
    if (newValue != null) {
      if (map.replace(key, oldValue, newValue))
        return newValue;
    } else if (map.remove(key, oldValue)) {
      return null;
    }
  } else if (map.putIfAbsent(key, value) == null) {
    return value;
  }
 }

When multiple threads attempt updates, map operations and the
remapping function may be called multiple times.

This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must
override this default implementation.
Since:1.8
/**
     * {@inheritDoc}
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}:
     *
     * <pre> {@code
     * for (;;) {
     *   V oldValue = map.get(key);
     *   if (oldValue != null) {
     *     V newValue = remappingFunction.apply(oldValue, value);
     *     if (newValue != null) {
     *       if (map.replace(key, oldValue, newValue))
     *         return newValue;
     *     } else if (map.remove(key, oldValue)) {
     *       return null;
     *     }
     *   } else if (map.putIfAbsent(key, value) == null) {
     *     return value;
     *   }
     * }}</pre>
     * When multiple threads attempt updates, map operations and the
     * remapping function may be called 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}
     * @throws IllegalArgumentException {@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);
        retry: for (;;) {
            V oldValue = get(key);
            // if putIfAbsent fails, opportunistically use its return value
            haveOldValue: 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;
                    }
                    continue retry;
                } else {
                    if ((oldValue = putIfAbsent(key, value)) == null)
                        return value;
                    continue haveOldValue;
                }
            }
        }
    }
}