content: /*
   * 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;
  
  // BEGIN android-note
  // removed link to collections framework docs
  // fixed framework docs link to "Collection#optional"
  // END android-note
  
  /**
   * 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.
   *
   * @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 (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()) {
       *   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}
       *
       * @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}
       *
       * @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}
       *
       * @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}
       *
       * @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;
                  }
              }
          }
      }
  }
  