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	/*
	* 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 {@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}
	*
	* @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="{@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 {@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;
	}
	}
	}
	}
	}

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