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	/*
	* Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
	* 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
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	* questions.
	*/

	package java.util;

	import java.util.function.BiConsumer;
	import java.util.function.BiFunction;
	import java.util.function.Function;
	import java.io.Serializable;

	/**
	* An object that maps keys to values. A map cannot contain duplicate keys;
	* each key can map to at most one value.
	*
	* <p>This interface takes the place of the <tt>Dictionary</tt> class, which
	* was a totally abstract class rather than an interface.
	*
	* <p>The <tt>Map</tt> interface provides three <i>collection views</i>, which
	* allow a map's contents to be viewed as a set of keys, collection of values,
	* or set of key-value mappings. The <i>order</i> of a map is defined as
	* the order in which the iterators on the map's collection views return their
	* elements. Some map implementations, like the <tt>TreeMap</tt> class, make
	* specific guarantees as to their order; others, like the <tt>HashMap</tt>
	* class, do not.
	*
	* <p>Note: great care must be exercised if mutable objects are used as map
	* keys. The behavior of a map is not specified if the value of an object is
	* changed in a manner that affects <tt>equals</tt> comparisons while the
	* object is a key in the map. A special case of this prohibition is that it
	* is not permissible for a map to contain itself as a key. While it is
	* permissible for a map to contain itself as a value, extreme caution is
	* advised: the <tt>equals</tt> and <tt>hashCode</tt> methods are no longer
	* well defined on such a map.
	*
	* <p>All general-purpose map implementation classes should provide two
	* "standard" constructors: a void (no arguments) constructor which creates an
	* empty map, and a constructor with a single argument of type <tt>Map</tt>,
	* which creates a new map with the same key-value mappings as its argument.
	* In effect, the latter constructor allows the user to copy any map,
	* producing an equivalent map of the desired class. There is no way to
	* enforce this recommendation (as interfaces cannot contain constructors) but
	* all of the general-purpose map implementations in the JDK comply.
	*
	* <p>The "destructive" methods contained in this interface, that is, the
	* methods that modify the map on which they operate, are specified to throw
	* <tt>UnsupportedOperationException</tt> if this map does not support the
	* operation. If this is the case, these methods may, but are not required
	* to, throw an <tt>UnsupportedOperationException</tt> if the invocation would
	* have no effect on the map. For example, invoking the {@link #putAll(Map)}
	* method on an unmodifiable map may, but is not required to, throw the
	* exception if the map whose mappings are to be "superimposed" is empty.
	*
	* <p>Some map implementations have restrictions on the keys and values they
	* may contain. For example, some implementations prohibit null keys and
	* values, and some have restrictions on the types of their keys. Attempting
	* to insert an ineligible key or value throws an unchecked exception,
	* typically <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.
	* Attempting to query the presence of an ineligible key or value may throw an
	* exception, or it may simply return false; some implementations will exhibit
	* the former behavior and some will exhibit the latter. More generally,
	* attempting an operation on an ineligible key or value whose completion
	* would not result in the insertion of an ineligible element into the map may
	* throw an exception or it may succeed, at the option of the implementation.
	* Such exceptions are marked as "optional" in the specification for this
	* interface.
	*
	* <p>Many methods in Collections Framework interfaces are defined
	* in terms of the {@link Object#equals(Object) equals} method. For
	* example, the specification for the {@link #containsKey(Object)
	* containsKey(Object key)} method says: "returns <tt>true</tt> if and
	* only if this map contains a mapping for a key <tt>k</tt> such that
	* <tt>(key==null ? k==null : key.equals(k))</tt>." This specification should
	* <i>not</i> be construed to imply that invoking <tt>Map.containsKey</tt>
	* with a non-null argument <tt>key</tt> will cause <tt>key.equals(k)</tt> to
	* be invoked for any key <tt>k</tt>. Implementations are free to
	* implement optimizations whereby the <tt>equals</tt> invocation is avoided,
	* for example, by first comparing the hash codes of the two keys. (The
	* {@link Object#hashCode()} specification guarantees that two objects with
	* unequal hash codes cannot be equal.) More generally, implementations of
	* the various Collections Framework interfaces are free to take advantage of
	* the specified behavior of underlying {@link Object} methods wherever the
	* implementor deems it appropriate.
	*
	* <p>Some map operations which perform recursive traversal of the map may fail
	* with an exception for self-referential instances where the map directly or
	* indirectly contains itself. This includes the {@code clone()},
	* {@code equals()}, {@code hashCode()} and {@code toString()} methods.
	* Implementations may optionally handle the self-referential scenario, however
	* most current implementations do not do so.
	*
	* <p>This interface is a member of the
	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
	* Java Collections Framework</a>.
	*
	* @param <K> the type of keys maintained by this map
	* @param <V> the type of mapped values
	*
	* @author Josh Bloch
	* @see HashMap
	* @see TreeMap
	* @see Hashtable
	* @see SortedMap
	* @see Collection
	* @see Set
	* @since 1.2
	*/
	public interface Map<K,V> {
	// Query Operations

	/**
	* Returns the number of key-value mappings in this map. If the
	* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
	* <tt>Integer.MAX_VALUE</tt>.
	*
	* @return the number of key-value mappings in this map
	*/
	int size();

	/**
	* Returns <tt>true</tt> if this map contains no key-value mappings.
	*
	* @return <tt>true</tt> if this map contains no key-value mappings
	*/
	boolean isEmpty();

	/**
	* Returns <tt>true</tt> if this map contains a mapping for the specified
	* key. More formally, returns <tt>true</tt> if and only if
	* this map contains a mapping for a key <tt>k</tt> such that
	* <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
	* at most one such mapping.)
	*
	* @param key key whose presence in this map is to be tested
	* @return <tt>true</tt> if this map contains a mapping for the specified
	* key
	* @throws ClassCastException if the key is of an inappropriate type for
	* this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified key is null and this map
	* does not permit null keys
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean containsKey(Object key);

	/**
	* Returns <tt>true</tt> if this map maps one or more keys to the
	* specified value. More formally, returns <tt>true</tt> if and only if
	* this map contains at least one mapping to a value <tt>v</tt> such that
	* <tt>(value==null ? v==null : value.equals(v))</tt>. This operation
	* will probably require time linear in the map size for most
	* implementations of the <tt>Map</tt> interface.
	*
	* @param value value whose presence in this map is to be tested
	* @return <tt>true</tt> if this map maps one or more keys to the
	* specified value
	* @throws ClassCastException if the value is of an inappropriate type for
	* this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified value is null and this
	* map does not permit null values
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	boolean containsValue(Object value);

	/**
	* Returns the value to which the specified key is mapped,
	* or {@code null} if this map contains no mapping for the key.
	*
	* <p>More formally, if this map contains a mapping from a key
	* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
	* key.equals(k))}, then this method returns {@code v}; otherwise
	* it returns {@code null}. (There can be at most one such mapping.)
	*
	* <p>If this map permits null values, then a return value of
	* {@code null} does not <i>necessarily</i> indicate that the map
	* contains no mapping for the key; it's also possible that the map
	* explicitly maps the key to {@code null}. The {@link #containsKey
	* containsKey} operation may be used to distinguish these two cases.
	*
	* @param key the key whose associated value is to be returned
	* @return the value to which the specified key is mapped, or
	* {@code null} if this map contains no mapping for the key
	* @throws ClassCastException if the key is of an inappropriate type for
	* this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified key is null and this map
	* does not permit null keys
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	V get(Object key);

	// Modification Operations

	/**
	* Associates the specified value with the specified key in this map
	* (optional operation). If the map previously contained a mapping for
	* the key, the old value is replaced by the specified value. (A map
	* <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
	* if {@link #containsKey(Object) m.containsKey(k)} would return
	* <tt>true</tt>.)
	*
	* @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 <tt>key</tt>, or
	* <tt>null</tt> if there was no mapping for <tt>key</tt>.
	* (A <tt>null</tt> return can also indicate that the map
	* previously associated <tt>null</tt> with <tt>key</tt>,
	* if the implementation supports <tt>null</tt> values.)
	* @throws UnsupportedOperationException if the <tt>put</tt> 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 put(K key, V value);

	/**
	* Removes the mapping for a key from this map if it is present
	* (optional operation). More formally, if this map contains a mapping
	* from key <tt>k</tt> to value <tt>v</tt> such that
	* <code>(key==null ? k==null : key.equals(k))</code>, that mapping
	* is removed. (The map can contain at most one such mapping.)
	*
	* <p>Returns the value to which this map previously associated the key,
	* or <tt>null</tt> if the map contained no mapping for the key.
	*
	* <p>If this map permits null values, then a return value of
	* <tt>null</tt> does not <i>necessarily</i> indicate that the map
	* contained no mapping for the key; it's also possible that the map
	* explicitly mapped the key to <tt>null</tt>.
	*
	* <p>The map will not contain a mapping for the specified key once the
	* call returns.
	*
	* @param key key whose mapping is to be removed from the map
	* @return the previous value associated with <tt>key</tt>, or
	* <tt>null</tt> if there was no mapping for <tt>key</tt>.
	* @throws UnsupportedOperationException if the <tt>remove</tt> operation
	* is not supported by this map
	* @throws ClassCastException if the key is of an inappropriate type for
	* this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified key is null and this
	* map does not permit null keys
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	*/
	V remove(Object key);


	// Bulk Operations

	/**
	* Copies all of the mappings from the specified map to this map
	* (optional operation). The effect of this call is equivalent to that
	* of calling {@link #put(Object,Object) put(k, v)} on this map once
	* for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
	* specified map. The behavior of this operation is undefined if the
	* specified map is modified while the operation is in progress.
	*
	* @param m mappings to be stored in this map
	* @throws UnsupportedOperationException if the <tt>putAll</tt> operation
	* is not supported by this map
	* @throws ClassCastException if the class of a key or value in the
	* specified map prevents it from being stored in this map
	* @throws NullPointerException if the specified map is null, or if
	* this map does not permit null keys or values, and the
	* specified map contains null keys or values
	* @throws IllegalArgumentException if some property of a key or value in
	* the specified map prevents it from being stored in this map
	*/
	void putAll(Map<? extends K, ? extends V> m);

	/**
	* Removes all of the mappings from this map (optional operation).
	* The map will be empty after this call returns.
	*
	* @throws UnsupportedOperationException if the <tt>clear</tt> operation
	* is not supported by this map
	*/
	void clear();


	// Views

	/**
	* Returns a {@link Set} view of the keys contained in this map.
	* The set is backed by the map, so changes to the map are
	* reflected in the set, and vice-versa. If the map is modified
	* while an iteration over the set is in progress (except through
	* the iterator's own <tt>remove</tt> operation), the results of
	* the iteration are undefined. The set supports element removal,
	* which removes the corresponding mapping from the map, via the
	* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
	* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
	* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
	* operations.
	*
	* @return a set view of the keys contained in this map
	*/
	Set<K> keySet();

	/**
	* Returns a {@link Collection} view of the values contained in this map.
	* The collection is backed by the map, so changes to the map are
	* reflected in the collection, and vice-versa. If the map is
	* modified while an iteration over the collection is in progress
	* (except through the iterator's own <tt>remove</tt> operation),
	* the results of the iteration are undefined. The collection
	* supports element removal, which removes the corresponding
	* mapping from the map, via the <tt>Iterator.remove</tt>,
	* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
	* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
	* support the <tt>add</tt> or <tt>addAll</tt> operations.
	*
	* @return a collection view of the values contained in this map
	*/
	Collection<V> values();

	/**
	* Returns a {@link Set} view of the mappings contained in this map.
	* The set is backed by the map, so changes to the map are
	* reflected in the set, and vice-versa. If the map is modified
	* while an iteration over the set is in progress (except through
	* the iterator's own <tt>remove</tt> operation, or through the
	* <tt>setValue</tt> operation on a map entry returned by the
	* iterator) the results of the iteration are undefined. The set
	* supports element removal, which removes the corresponding
	* mapping from the map, via the <tt>Iterator.remove</tt>,
	* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
	* <tt>clear</tt> operations. It does not support the
	* <tt>add</tt> or <tt>addAll</tt> operations.
	*
	* @return a set view of the mappings contained in this map
	*/
	Set<Map.Entry<K, V>> entrySet();

	/**
	* A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
	* a collection-view of the map, whose elements are of this class. The
	* <i>only</i> way to obtain a reference to a map entry is from the
	* iterator of this collection-view. These <tt>Map.Entry</tt> objects are
	* valid <i>only</i> for the duration of the iteration; more formally,
	* the behavior of a map entry is undefined if the backing map has been
	* modified after the entry was returned by the iterator, except through
	* the <tt>setValue</tt> operation on the map entry.
	*
	* @see Map#entrySet()
	* @since 1.2
	*/
	interface Entry<K,V> {
	/**
	* Returns the key corresponding to this entry.
	*
	* @return the key corresponding to this entry
	* @throws IllegalStateException implementations may, but are not
	* required to, throw this exception if the entry has been
	* removed from the backing map.
	*/
	K getKey();

	/**
	* Returns the value corresponding to this entry. If the mapping
	* has been removed from the backing map (by the iterator's
	* <tt>remove</tt> operation), the results of this call are undefined.
	*
	* @return the value corresponding to this entry
	* @throws IllegalStateException implementations may, but are not
	* required to, throw this exception if the entry has been
	* removed from the backing map.
	*/
	V getValue();

	/**
	* Replaces the value corresponding to this entry with the specified
	* value (optional operation). (Writes through to the map.) The
	* behavior of this call is undefined if the mapping has already been
	* removed from the map (by the iterator's <tt>remove</tt> operation).
	*
	* @param value new value to be stored in this entry
	* @return old value corresponding to the entry
	* @throws UnsupportedOperationException if the <tt>put</tt> operation
	* is not supported by the backing map
	* @throws ClassCastException if the class of the specified value
	* prevents it from being stored in the backing map
	* @throws NullPointerException if the backing map does not permit
	* null values, and the specified value is null
	* @throws IllegalArgumentException if some property of this value
	* prevents it from being stored in the backing map
	* @throws IllegalStateException implementations may, but are not
	* required to, throw this exception if the entry has been
	* removed from the backing map.
	*/
	V setValue(V value);

	/**
	* Compares the specified object with this entry for equality.
	* Returns <tt>true</tt> if the given object is also a map entry and
	* the two entries represent the same mapping. More formally, two
	* entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
	* if<pre>
	* (e1.getKey()==null ?
	* e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
	* (e1.getValue()==null ?
	* e2.getValue()==null : e1.getValue().equals(e2.getValue()))
	* </pre>
	* This ensures that the <tt>equals</tt> method works properly across
	* different implementations of the <tt>Map.Entry</tt> interface.
	*
	* @param o object to be compared for equality with this map entry
	* @return <tt>true</tt> if the specified object is equal to this map
	* entry
	*/
	boolean equals(Object o);

	/**
	* Returns the hash code value for this map entry. The hash code
	* of a map entry <tt>e</tt> is defined to be: <pre>
	* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
	* (e.getValue()==null ? 0 : e.getValue().hashCode())
	* </pre>
	* This ensures that <tt>e1.equals(e2)</tt> implies that
	* <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
	* <tt>e1</tt> and <tt>e2</tt>, as required by the general
	* contract of <tt>Object.hashCode</tt>.
	*
	* @return the hash code value for this map entry
	* @see Object#hashCode()
	* @see Object#equals(Object)
	* @see #equals(Object)
	*/
	int hashCode();

	/**
	* Returns a comparator that compares {@link Map.Entry} in natural order on key.
	*
	* <p>The returned comparator is serializable and throws {@link
	* NullPointerException} when comparing an entry with a null key.
	*
	* @param <K> the {@link Comparable} type of then map keys
	* @param <V> the type of the map values
	* @return a comparator that compares {@link Map.Entry} in natural order on key.
	* @see Comparable
	* @since 1.8
	*/
	public static <K extends Comparable<? super K>, V> Comparator<Map.Entry<K,V>> comparingByKey() {
	return (Comparator<Map.Entry<K, V>> & Serializable)
	(c1, c2) -> c1.getKey().compareTo(c2.getKey());
	}

	/**
	* Returns a comparator that compares {@link Map.Entry} in natural order on value.
	*
	* <p>The returned comparator is serializable and throws {@link
	* NullPointerException} when comparing an entry with null values.
	*
	* @param <K> the type of the map keys
	* @param <V> the {@link Comparable} type of the map values
	* @return a comparator that compares {@link Map.Entry} in natural order on value.
	* @see Comparable
	* @since 1.8
	*/
	public static <K, V extends Comparable<? super V>> Comparator<Map.Entry<K,V>> comparingByValue() {
	return (Comparator<Map.Entry<K, V>> & Serializable)
	(c1, c2) -> c1.getValue().compareTo(c2.getValue());
	}

	/**
	* Returns a comparator that compares {@link Map.Entry} by key using the given
	* {@link Comparator}.
	*
	* <p>The returned comparator is serializable if the specified comparator
	* is also serializable.
	*
	* @param <K> the type of the map keys
	* @param <V> the type of the map values
	* @param cmp the key {@link Comparator}
	* @return a comparator that compares {@link Map.Entry} by the key.
	* @since 1.8
	*/
	public static <K, V> Comparator<Map.Entry<K, V>> comparingByKey(Comparator<? super K> cmp) {
	Objects.requireNonNull(cmp);
	return (Comparator<Map.Entry<K, V>> & Serializable)
	(c1, c2) -> cmp.compare(c1.getKey(), c2.getKey());
	}

	/**
	* Returns a comparator that compares {@link Map.Entry} by value using the given
	* {@link Comparator}.
	*
	* <p>The returned comparator is serializable if the specified comparator
	* is also serializable.
	*
	* @param <K> the type of the map keys
	* @param <V> the type of the map values
	* @param cmp the value {@link Comparator}
	* @return a comparator that compares {@link Map.Entry} by the value.
	* @since 1.8
	*/
	public static <K, V> Comparator<Map.Entry<K, V>> comparingByValue(Comparator<? super V> cmp) {
	Objects.requireNonNull(cmp);
	return (Comparator<Map.Entry<K, V>> & Serializable)
	(c1, c2) -> cmp.compare(c1.getValue(), c2.getValue());
	}
	}

	// Comparison and hashing

	/**
	* Compares the specified object with this map for equality. Returns
	* <tt>true</tt> if the given object is also a map and the two maps
	* represent the same mappings. More formally, two maps <tt>m1</tt> and
	* <tt>m2</tt> represent the same mappings if
	* <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
	* <tt>equals</tt> method works properly across different implementations
	* of the <tt>Map</tt> interface.
	*
	* @param o object to be compared for equality with this map
	* @return <tt>true</tt> if the specified object is equal to this map
	*/
	boolean equals(Object o);

	/**
	* Returns the hash code value for this map. The hash code of a map is
	* defined to be the sum of the hash codes of each entry in the map's
	* <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
	* implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
	* <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
	* {@link Object#hashCode}.
	*
	* @return the hash code value for this map
	* @see Map.Entry#hashCode()
	* @see Object#equals(Object)
	* @see #equals(Object)
	*/
	int hashCode();

	// Defaultable methods

	/**
	* Returns the value to which the specified key is mapped, or
	* {@code defaultValue} if this map contains no mapping for the key.
	*
	* @implSpec
	* The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @param key the key whose associated value is to be returned
	* @param defaultValue the default mapping of the key
	* @return the value to which the specified key is mapped, or
	* {@code defaultValue} if this map contains no mapping for the key
	* @throws ClassCastException if the key is of an inappropriate type for
	* this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified key is null and this map
	* does not permit null keys
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default V getOrDefault(Object key, V defaultValue) {
	V v;
	return (((v = get(key)) != null) \|\| containsKey(key))
	? v
	: defaultValue;
	}

	/**
	* Performs the given action for each entry in this map until all entries
	* have been processed or the action throws an exception. Unless
	* otherwise specified by the implementing class, actions are performed in
	* the order of entry set iteration (if an iteration order is specified.)
	* Exceptions thrown by the action are relayed to the caller.
	*
	* @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>
	*
	* The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @param action The action to be performed for each entry
	* @throws NullPointerException if the specified action is null
	* @throws ConcurrentModificationException if an entry is found to be
	* removed during iteration
	* @since 1.8
	*/
	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.
	throw new ConcurrentModificationException(ise);
	}
	action.accept(k, v);
	}
	}

	/**
	* Replaces each entry's value with the result of invoking the given
	* function on that entry until all entries have been processed or the
	* function throws an exception. Exceptions thrown by the function are
	* relayed to the caller.
	*
	* @implSpec
	* <p>The default implementation is equivalent to, for this {@code map}:
	* <pre> {@code
	* for (Map.Entry<K, V> entry : map.entrySet())
	* entry.setValue(function.apply(entry.getKey(), entry.getValue()));
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @param function the function to apply to each entry
	* @throws UnsupportedOperationException if the {@code set} operation
	* is not supported by this map's entry set iterator.
	* @throws ClassCastException if the class of a replacement value
	* prevents it from being stored in this map
	* @throws NullPointerException if the specified function is null, or the
	* specified replacement value is null, and this map does not permit null
	* values
	* @throws ClassCastException if a replacement value is of an inappropriate
	* type for this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if function or a replacement value is null,
	* and this map does not permit null keys or values
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws IllegalArgumentException if some property of a replacement value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ConcurrentModificationException if an entry is found to be
	* removed during iteration
	* @since 1.8
	*/
	default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
	Objects.requireNonNull(function);
	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.
	throw new ConcurrentModificationException(ise);
	}

	// ise thrown from function is not a cme.
	v = function.apply(k, v);

	try {
	entry.setValue(v);
	} catch(IllegalStateException ise) {
	// this usually means the entry is no longer in the map.
	throw new ConcurrentModificationException(ise);
	}
	}
	}

	/**
	* If the specified key is not already associated with a value (or is mapped
	* to {@code null}) associates it with the given value and returns
	* {@code null}, else returns the current value.
	*
	* @implSpec
	* The default implementation is equivalent to, for this {@code
	* map}:
	*
	* <pre> {@code
	* V v = map.get(key);
	* if (v == null)
	* v = map.put(key, value);
	*
	* return v;
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @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
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the key or value is of an inappropriate
	* type for this map
	* (<a href="{@docRoot}/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/util/Collection.html#optional-restrictions">optional</a>)
	* @throws IllegalArgumentException if some property of the specified key
	* or value prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default V putIfAbsent(K key, V value) {
	V v = get(key);
	if (v == null) {
	v = put(key, value);
	}

	return v;
	}

	/**
	* Removes the entry for the specified key only if it is currently
	* mapped to the specified value.
	*
	* @implSpec
	* The default implementation 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>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @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
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the key or value is of an inappropriate
	* type for this map
	* (<a href="{@docRoot}/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/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default boolean remove(Object key, Object value) {
	Object curValue = get(key);
	if (!Objects.equals(curValue, value) \|\|
	(curValue == null && !containsKey(key))) {
	return false;
	}
	remove(key);
	return true;
	}

	/**
	* Replaces the entry for the specified key only if currently
	* mapped to the specified value.
	*
	* @implSpec
	* The default implementation is equivalent to, for this {@code map}:
	*
	* <pre> {@code
	* if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
	* map.put(key, newValue);
	* return true;
	* } else
	* return false;
	* }</pre>
	*
	* The default implementation does not throw NullPointerException
	* for maps that do not support null values if oldValue is null unless
	* newValue is also null.
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @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
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @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 newValue is null,
	* and this map does not permit null keys or values
	* @throws NullPointerException if oldValue is null and this map does not
	* permit null values
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws IllegalArgumentException if some property of a specified key
	* or value prevents it from being stored in this map
	* @since 1.8
	*/
	default boolean replace(K key, V oldValue, V newValue) {
	Object curValue = get(key);
	if (!Objects.equals(curValue, oldValue) \|\|
	(curValue == null && !containsKey(key))) {
	return false;
	}
	put(key, newValue);
	return true;
	}

	/**
	* Replaces the entry for the specified key only if it is
	* currently mapped to some value.
	*
	* @implSpec
	* The default implementation is equivalent to, for this {@code map}:
	*
	* <pre> {@code
	* if (map.containsKey(key)) {
	* return map.put(key, value);
	* } else
	* return null;
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties.
	*
	* @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
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the class of the specified key or value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/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
	* @throws IllegalArgumentException if some property of the specified key
	* or value prevents it from being stored in this map
	* @since 1.8
	*/
	default V replace(K key, V value) {
	V curValue;
	if (((curValue = get(key)) != null) \|\| containsKey(key)) {
	curValue = put(key, value);
	}
	return curValue;
	}

	/**
	* If the specified key is not already associated with a value (or is mapped
	* to {@code null}), attempts to compute its value using the given mapping
	* function and enters it into this map unless {@code null}.
	*
	* <p>If the function returns {@code null} no mapping is recorded. If
	* the function itself throws an (unchecked) exception, the
	* exception is rethrown, and no mapping is recorded. The most
	* common usage is to construct a new object serving as an initial
	* mapped value or memoized result, as in:
	*
	* <pre> {@code
	* map.computeIfAbsent(key, k -> new Value(f(k)));
	* }</pre>
	*
	* <p>Or to implement a multi-value map, {@code Map<K,Collection<V>>},
	* supporting multiple values per key:
	*
	* <pre> {@code
	* map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);
	* }</pre>
	*
	*
	* @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)
	* map.put(key, newValue);
	* }
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties. In particular, all implementations of
	* subinterface {@link java.util.concurrent.ConcurrentMap} must document
	* whether the function is applied once atomically only if the value is not
	* present.
	*
	* @param key key with which the specified value is to be associated
	* @param mappingFunction the function to compute a value
	* @return the current (existing or computed) value associated with
	* the specified key, or null if the computed value is null
	* @throws NullPointerException if the specified key is null and
	* this map does not support null keys, or the mappingFunction
	* is null
	* @throws UnsupportedOperationException if the {@code put} operation
	* is not supported by this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the class of the specified key or value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default V computeIfAbsent(K key,
	Function<? super K, ? extends V> mappingFunction) {
	Objects.requireNonNull(mappingFunction);
	V v;
	if ((v = get(key)) == null) {
	V newValue;
	if ((newValue = mappingFunction.apply(key)) != null) {
	put(key, newValue);
	return newValue;
	}
	}

	return v;
	}

	/**
	* If the value for the specified key is present and non-null, attempts to
	* compute a new mapping given the key and its current mapped value.
	*
	* <p>If the function returns {@code null}, the mapping is removed. If the
	* function itself throws an (unchecked) exception, the exception is
	* rethrown, and the current mapping is left unchanged.
	*
	* @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.put(key, newValue);
	* else
	* map.remove(key);
	* }
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties. In particular, all implementations of
	* subinterface {@link java.util.concurrent.ConcurrentMap} must document
	* whether the function is applied once atomically only if the value is not
	* present.
	*
	* @param key key with which the specified value is to be associated
	* @param remappingFunction the function to compute a value
	* @return the new value associated with the specified key, or null if none
	* @throws NullPointerException if the specified key is null and
	* this map does not support null keys, or the
	* remappingFunction is null
	* @throws UnsupportedOperationException if the {@code put} operation
	* is not supported by this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the class of the specified key or value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default V computeIfPresent(K key,
	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
	Objects.requireNonNull(remappingFunction);
	V oldValue;
	if ((oldValue = get(key)) != null) {
	V newValue = remappingFunction.apply(key, oldValue);
	if (newValue != null) {
	put(key, newValue);
	return newValue;
	} else {
	remove(key);
	return null;
	}
	} else {
	return null;
	}
	}

	/**
	* Attempts to compute a mapping for the specified key and its current
	* mapped value (or {@code null} if there is no current mapping). For
	* example, to either create or append a {@code String} msg to a value
	* mapping:
	*
	* <pre> {@code
	* map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}</pre>
	* (Method {@link #merge merge()} is often simpler to use for such purposes.)
	*
	* <p>If the function returns {@code null}, the mapping is removed (or
	* remains absent if initially absent). If the function itself throws an
	* (unchecked) exception, the exception is rethrown, and the current mapping
	* is left unchanged.
	*
	* @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.put(key, newValue);
	* else
	* map.remove(key);
	* } else {
	* if (newValue != null)
	* map.put(key, newValue);
	* else
	* return null;
	* }
	* }</pre>
	*
	* <p>The default implementation makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties. In particular, all implementations of
	* subinterface {@link java.util.concurrent.ConcurrentMap} must document
	* whether the function is applied once atomically only if the value is not
	* present.
	*
	* @param key key with which the specified value is to be associated
	* @param remappingFunction the function to compute a value
	* @return the new value associated with the specified key, or null if none
	* @throws NullPointerException if the specified key is null and
	* this map does not support null keys, or the
	* remappingFunction is null
	* @throws UnsupportedOperationException if the {@code put} operation
	* is not supported by this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the class of the specified key or value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @since 1.8
	*/
	default V compute(K key,
	BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
	Objects.requireNonNull(remappingFunction);
	V oldValue = get(key);

	V newValue = remappingFunction.apply(key, oldValue);
	if (newValue == null) {
	// delete mapping
	if (oldValue != null \|\| containsKey(key)) {
	// something to remove
	remove(key);
	return null;
	} else {
	// nothing to do. Leave things as they were.
	return null;
	}
	} else {
	// add or replace old mapping
	put(key, newValue);
	return newValue;
	}
	}

	/**
	* If the specified key is not already associated with a value or is
	* associated with null, associates it with the given non-null value.
	* Otherwise, replaces the associated value with the results of the given
	* remapping function, or removes if the result is {@code null}. This
	* method may be of use when combining multiple mapped values for a key.
	* For example, to either create or append a {@code String msg} to a
	* value mapping:
	*
	* <pre> {@code
	* map.merge(key, msg, String::concat)
	* }</pre>
	*
	* <p>If the function returns {@code null} the mapping is removed. If the
	* function itself throws an (unchecked) exception, the exception is
	* rethrown, and the current mapping is left unchanged.
	*
	* @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 makes no guarantees about synchronization
	* or atomicity properties of this method. Any implementation providing
	* atomicity guarantees must override this method and document its
	* concurrency properties. In particular, all implementations of
	* subinterface {@link java.util.concurrent.ConcurrentMap} must document
	* whether the function is applied once atomically only if the value is not
	* present.
	*
	* @param key key with which the resulting value is to be associated
	* @param value the non-null value to be merged with the existing value
	* associated with the key or, if no existing value or a null value
	* is associated with the key, to be associated with the key
	* @param remappingFunction the function to recompute a value if present
	* @return the new value associated with the specified key, or null if no
	* value is associated with the key
	* @throws UnsupportedOperationException if the {@code put} operation
	* is not supported by this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws ClassCastException if the class of the specified key or value
	* prevents it from being stored in this map
	* (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
	* @throws NullPointerException if the specified key is null and this map
	* does not support null keys or the value or remappingFunction is
	* null
	* @since 1.8
	*/
	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);
	V newValue = (oldValue == null) ? value :
	remappingFunction.apply(oldValue, value);
	if(newValue == null) {
	remove(key);
	} else {
	put(key, newValue);
	}
	return newValue;
	}
	}

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