/*
 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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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 {@code Dictionary} class, which
 * was a totally abstract class rather than an interface.
 *
 * <p>The {@code Map} 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 {@code TreeMap} class, make
 * specific guarantees as to their order; others, like the {@code HashMap}
 * 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 {@code equals} 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 {@code equals} and {@code hashCode} 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 {@code Map},
 * 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
 * {@code UnsupportedOperationException} if this map does not support the
 * operation. If this is the case, these methods may, but are not required
 * to, throw an {@code UnsupportedOperationException} 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 {@code NullPointerException} or {@code ClassCastException}.
 * 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 {@code true} if and
 * only if this map contains a mapping for a key {@code k} such that
 * {@code (key==null ? k==null : key.equals(k))}." This specification should
 * <i>not</i> be construed to imply that invoking {@code Map.containsKey}
 * with a non-null argument {@code key} will cause {@code key.equals(k)} to
 * be invoked for any key {@code k}. Implementations are free to
 * implement optimizations whereby the {@code equals} 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.
 *
 * <h2><a id="unmodifiable">Unmodifiable Maps</a></h2>
 * <p>The {@link Map#of() Map.of},
 * {@link Map#ofEntries(Map.Entry...) Map.ofEntries}, and
 * {@link Map#copyOf Map.copyOf}
 * static factory methods provide a convenient way to create unmodifiable maps.
 * The {@code Map}
 * instances created by these methods have the following characteristics:
 *
 * <ul>
 * <li>They are <a href="Collection.html#unmodifiable"><i>unmodifiable</i></a>. Keys and values
 * cannot be added, removed, or updated. Calling any mutator method on the Map
 * will always cause {@code UnsupportedOperationException} to be thrown.
 * However, if the contained keys or values are themselves mutable, this may cause the
 * Map to behave inconsistently or its contents to appear to change.
 * <li>They disallow {@code null} keys and values. Attempts to create them with
 * {@code null} keys or values result in {@code NullPointerException}.
 * <li>They are serializable if all keys and values are serializable.
 * <li>They reject duplicate keys at creation time. Duplicate keys
 * passed to a static factory method result in {@code IllegalArgumentException}.
 * <li>The iteration order of mappings is unspecified and is subject to change.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * Callers should make no assumptions about the identity of the returned instances.
 * Factories are free to create new instances or reuse existing ones. Therefore,
 * identity-sensitive operations on these instances (reference equality ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 * <li>They are serialized as specified on the
 * <a href="{@docRoot}/serialized-form.html#java.util.CollSer">Serialized Form</a>
 * page.
 * </ul>
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * 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 {@code Integer.MAX_VALUE} elements, returns
 * {@code Integer.MAX_VALUE}.
 *
 * @return the number of key-value mappings in this map
 */
 int size();

 /**
 * Returns {@code true} if this map contains no key-value mappings.
 *
 * @return {@code true} if this map contains no key-value mappings
 */
 boolean isEmpty();

 /**
 * Returns {@code true} if this map contains a mapping for the specified
 * key. More formally, returns {@code true} if and only if
 * this map contains a mapping for a key {@code k} such that
 * {@code Objects.equals(key, k)}. (There can be
 * at most one such mapping.)
 *
 * @param key key whose presence in this map is to be tested
 * @return {@code true} 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.base/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.base/java/util/Collection.html#optional-restrictions">optional</a>)
 */
 boolean containsKey(Object key);

 /**
 * Returns {@code true} if this map maps one or more keys to the
 * specified value. More formally, returns {@code true} if and only if
 * this map contains at least one mapping to a value {@code v} such that
 * {@code Objects.equals(value, v)}. This operation
 * will probably require time linear in the map size for most
 * implementations of the {@code Map} interface.
 *
 * @param value value whose presence in this map is to be tested
 * @return {@code true} 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.base/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.base/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 Objects.equals(key, 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.base/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.base/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
 * {@code m} is said to contain a mapping for a key {@code k} if and only
 * if {@link #containsKey(Object) m.containsKey(k)} would return
 * {@code true}.)
 *
 * @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 {@code key}, or
 * {@code null} if there was no mapping for {@code key}.
 * (A {@code null} return can also indicate that the map
 * previously associated {@code null} with {@code key},
 * if the implementation supports {@code 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 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 {@code k} to value {@code v} such that
 * {@code Objects.equals(key, k)}, 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 {@code null} if the map contained no mapping for the key.
 *
 * <p>If this map permits null values, then a return value of
 * {@code null} 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 {@code null}.
 *
 * <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 {@code key}, or
 * {@code null} if there was no mapping for {@code key}.
 * @throws UnsupportedOperationException if the {@code remove} operation
 * is not supported by this map
 * @throws ClassCastException if the key 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 is null and this
 * map does not permit null keys
 * (<a href="{@docRoot}/java.base/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 {@code k} to value {@code v} 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 {@code putAll} 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 {@code clear} 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 {@code remove} operation), the results of
 * the iteration are undefined. The set supports element removal,
 * which removes the corresponding mapping from the map, via the
 * {@code Iterator.remove}, {@code Set.remove},
 * {@code removeAll}, {@code retainAll}, and {@code clear}
 * operations. It does not support the {@code add} or {@code addAll}
 * 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 {@code remove} operation),
 * the results of the iteration are undefined. The collection
 * supports element removal, which removes the corresponding
 * mapping from the map, via the {@code Iterator.remove},
 * {@code Collection.remove}, {@code removeAll},
 * {@code retainAll} and {@code clear} operations. It does not
 * support the {@code add} or {@code addAll} 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 {@code remove} operation, or through the
 * {@code setValue} 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 {@code Iterator.remove},
 * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
 * {@code clear} operations. It does not support the
 * {@code add} or {@code addAll} 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 {@code Map.entrySet} 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 {@code Map.Entry} 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 {@code setValue} 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
 * {@code remove} 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 {@code remove} operation).
 *
 * @param value new value to be stored in this entry
 * @return old value corresponding to the entry
 * @throws UnsupportedOperationException if the {@code put} 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 {@code true} if the given object is also a map entry and
 * the two entries represent the same mapping. More formally, two
 * entries {@code e1} and {@code e2} represent the same mapping
 * if<pre>
 * (e1.getKey()==null ?
 * e2.getKey()==null : e1.getKey().equals(e2.getKey())) &amp;&amp;
 * (e1.getValue()==null ?
 * e2.getValue()==null : e1.getValue().equals(e2.getValue()))
 * </pre>
 * This ensures that the {@code equals} method works properly across
 * different implementations of the {@code Map.Entry} interface.
 *
 * @param o object to be compared for equality with this map entry
 * @return {@code true} 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 {@code e} is defined to be: <pre>
 * (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
 * (e.getValue()==null ? 0 : e.getValue().hashCode())
 * </pre>
 * This ensures that {@code e1.equals(e2)} implies that
 * {@code e1.hashCode()==e2.hashCode()} for any two Entries
 * {@code e1} and {@code e2}, as required by the general
 * contract of {@code Object.hashCode}.
 *
 * @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
 * {@code true} if the given object is also a map and the two maps
 * represent the same mappings. More formally, two maps {@code m1} and
 * {@code m2} represent the same mappings if
 * {@code m1.entrySet().equals(m2.entrySet())}. This ensures that the
 * {@code equals} method works properly across different implementations
 * of the {@code Map} interface.
 *
 * @param o object to be compared for equality with this map
 * @return {@code true} 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
 * {@code entrySet()} view. This ensures that {@code m1.equals(m2)}
 * implies that {@code m1.hashCode()==m2.hashCode()} for any two maps
 * {@code m1} and {@code m2}, 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.base/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.base/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.base/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.base/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.base/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.base/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.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>)
 * @throws IllegalArgumentException if some property of the specified key
 * or value prevents it from being stored in this map
 * (<a href="{@docRoot}/java.base/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.base/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.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>)
 * @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.base/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.base/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.base/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.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
 * @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 mapping function returns {@code null}, no mapping is recorded.
 * If the mapping 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>
 *
 * <p>The mapping function should not modify this map during computation.
 *
 * @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 detecting if the
 * mapping function modifies this map during computation and, if
 * appropriate, reporting an error. Non-concurrent implementations should
 * override this method and, on a best-effort basis, throw a
 * {@code ConcurrentModificationException} if it is detected that the
 * mapping function modifies this map during computation. Concurrent
 * implementations should override this method and, on a best-effort basis,
 * throw an {@code IllegalStateException} if it is detected that the
 * mapping function modifies this map during computation and as a result
 * computation would never complete.
 *
 * <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 mapping 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 mapping 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.base/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.base/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.base/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 remapping function returns {@code null}, the mapping is removed.
 * If the remapping function itself throws an (unchecked) exception, the
 * exception is rethrown, and the current mapping is left unchanged.
 *
 * <p>The remapping function should not modify this map during computation.
 *
 * @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 detecting if the
 * remapping function modifies this map during computation and, if
 * appropriate, reporting an error. Non-concurrent implementations should
 * override this method and, on a best-effort basis, throw a
 * {@code ConcurrentModificationException} if it is detected that the
 * remapping function modifies this map during computation. Concurrent
 * implementations should override this method and, on a best-effort basis,
 * throw an {@code IllegalStateException} if it is detected that the
 * remapping function modifies this map during computation and as a result
 * computation would never complete.
 *
 * <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 remapping 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 remapping 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.base/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.base/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.base/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 remapping function returns {@code null}, the mapping is removed
 * (or remains absent if initially absent). If the remapping function
 * itself throws an (unchecked) exception, the exception is rethrown, and
 * the current mapping is left unchanged.
 *
 * <p>The remapping function should not modify this map during computation.
 *
 * @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 detecting if the
 * remapping function modifies this map during computation and, if
 * appropriate, reporting an error. Non-concurrent implementations should
 * override this method and, on a best-effort basis, throw a
 * {@code ConcurrentModificationException} if it is detected that the
 * remapping function modifies this map during computation. Concurrent
 * implementations should override this method and, on a best-effort basis,
 * throw an {@code IllegalStateException} if it is detected that the
 * remapping function modifies this map during computation and as a result
 * computation would never complete.
 *
 * <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 remapping 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 remapping 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.base/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.base/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.base/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 remapping function returns {@code null}, the mapping is removed.
 * If the remapping function itself throws an (unchecked) exception, the
 * exception is rethrown, and the current mapping is left unchanged.
 *
 * <p>The remapping function should not modify this map during computation.
 *
 * @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 detecting if the
 * remapping function modifies this map during computation and, if
 * appropriate, reporting an error. Non-concurrent implementations should
 * override this method and, on a best-effort basis, throw a
 * {@code ConcurrentModificationException} if it is detected that the
 * remapping function modifies this map during computation. Concurrent
 * implementations should override this method and, on a best-effort basis,
 * throw an {@code IllegalStateException} if it is detected that the
 * remapping function modifies this map during computation and as a result
 * computation would never complete.
 *
 * <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 remapping 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 remapping 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.base/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.base/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.base/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;
 }

 /**
 * Returns an unmodifiable map containing zero mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @return an empty {@code Map}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of() {
 return ImmutableCollections.emptyMap();
 }

 /**
 * Returns an unmodifiable map containing a single mapping.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the mapping's key
 * @param v1 the mapping's value
 * @return a {@code Map} containing the specified mapping
 * @throws NullPointerException if the key or the value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1) {
 return new ImmutableCollections.Map1<>(k1, v1);
 }

 /**
 * Returns an unmodifiable map containing two mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if the keys are duplicates
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2);
 }

 /**
 * Returns an unmodifiable map containing three mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3);
 }

 /**
 * Returns an unmodifiable map containing four mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4);
 }

 /**
 * Returns an unmodifiable map containing five mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5);
 }

 /**
 * Returns an unmodifiable map containing six mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @param k6 the sixth mapping's key
 * @param v6 the sixth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
 K k6, V v6) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
 k6, v6);
 }

 /**
 * Returns an unmodifiable map containing seven mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @param k6 the sixth mapping's key
 * @param v6 the sixth mapping's value
 * @param k7 the seventh mapping's key
 * @param v7 the seventh mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
 K k6, V v6, K k7, V v7) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
 k6, v6, k7, v7);
 }

 /**
 * Returns an unmodifiable map containing eight mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @param k6 the sixth mapping's key
 * @param v6 the sixth mapping's value
 * @param k7 the seventh mapping's key
 * @param v7 the seventh mapping's value
 * @param k8 the eighth mapping's key
 * @param v8 the eighth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
 K k6, V v6, K k7, V v7, K k8, V v8) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
 k6, v6, k7, v7, k8, v8);
 }

 /**
 * Returns an unmodifiable map containing nine mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @param k6 the sixth mapping's key
 * @param v6 the sixth mapping's value
 * @param k7 the seventh mapping's key
 * @param v7 the seventh mapping's value
 * @param k8 the eighth mapping's key
 * @param v8 the eighth mapping's value
 * @param k9 the ninth mapping's key
 * @param v9 the ninth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
 K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
 k6, v6, k7, v7, k8, v8, k9, v9);
 }

 /**
 * Returns an unmodifiable map containing ten mappings.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param k1 the first mapping's key
 * @param v1 the first mapping's value
 * @param k2 the second mapping's key
 * @param v2 the second mapping's value
 * @param k3 the third mapping's key
 * @param v3 the third mapping's value
 * @param k4 the fourth mapping's key
 * @param v4 the fourth mapping's value
 * @param k5 the fifth mapping's key
 * @param v5 the fifth mapping's value
 * @param k6 the sixth mapping's key
 * @param v6 the sixth mapping's value
 * @param k7 the seventh mapping's key
 * @param v7 the seventh mapping's value
 * @param k8 the eighth mapping's key
 * @param v8 the eighth mapping's value
 * @param k9 the ninth mapping's key
 * @param v9 the ninth mapping's value
 * @param k10 the tenth mapping's key
 * @param v10 the tenth mapping's value
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any key or value is {@code null}
 *
 * @since 9
 */
 static <K, V> Map<K, V> of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5,
 K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10) {
 return new ImmutableCollections.MapN<>(k1, v1, k2, v2, k3, v3, k4, v4, k5, v5,
 k6, v6, k7, v7, k8, v8, k9, v9, k10, v10);
 }

 /**
 * Returns an unmodifiable map containing keys and values extracted from the given entries.
 * The entries themselves are not stored in the map.
 * See <a href="#unmodifiable">Unmodifiable Maps</a> for details.
 *
 * @apiNote
 * It is convenient to create the map entries using the {@link Map#entry Map.entry()} method.
 * For example,
 *
 * <pre>{@code
 * import static java.util.Map.entry;
 *
 * Map<Integer,String> map = Map.ofEntries(
 * entry(1, "a"),
 * entry(2, "b"),
 * entry(3, "c"),
 * ...
 * entry(26, "z"));
 * }</pre>
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param entries {@code Map.Entry}s containing the keys and values from which the map is populated
 * @return a {@code Map} containing the specified mappings
 * @throws IllegalArgumentException if there are any duplicate keys
 * @throws NullPointerException if any entry, key, or value is {@code null}, or if
 * the {@code entries} array is {@code null}
 *
 * @see Map#entry Map.entry()
 * @since 9
 */
 @SafeVarargs
 @SuppressWarnings("varargs")
 static <K, V> Map<K, V> ofEntries(Entry<? extends K, ? extends V>... entries) {
 if (entries.length == 0) { // implicit null check of entries array
 return ImmutableCollections.emptyMap();
 } else if (entries.length == 1) {
 // implicit null check of the array slot
 return new ImmutableCollections.Map1<>(entries[0].getKey(),
 entries[0].getValue());
 } else {
 Object[] kva = new Object[entries.length << 1];
 int a = 0;
 for (Entry<? extends K, ? extends V> entry : entries) {
 // implicit null checks of each array slot
 kva[a++] = entry.getKey();
 kva[a++] = entry.getValue();
 }
 return new ImmutableCollections.MapN<>(kva);
 }
 }

 /**
 * Returns an unmodifiable {@link Entry} containing the given key and value.
 * These entries are suitable for populating {@code Map} instances using the
 * {@link Map#ofEntries Map.ofEntries()} method.
 * The {@code Entry} instances created by this method have the following characteristics:
 *
 * <ul>
 * <li>They disallow {@code null} keys and values. Attempts to create them using a {@code null}
 * key or value result in {@code NullPointerException}.
 * <li>They are unmodifiable. Calls to {@link Entry#setValue Entry.setValue()}
 * on a returned {@code Entry} result in {@code UnsupportedOperationException}.
 * <li>They are not serializable.
 * <li>They are <a href="../lang/doc-files/ValueBased.html">value-based</a>.
 * Callers should make no assumptions about the identity of the returned instances.
 * This method is free to create new instances or reuse existing ones. Therefore,
 * identity-sensitive operations on these instances (reference equality ({@code ==}),
 * identity hash code, and synchronization) are unreliable and should be avoided.
 * </ul>
 *
 * @apiNote
 * For a serializable {@code Entry}, see {@link AbstractMap.SimpleEntry} or
 * {@link AbstractMap.SimpleImmutableEntry}.
 *
 * @param <K> the key's type
 * @param <V> the value's type
 * @param k the key
 * @param v the value
 * @return an {@code Entry} containing the specified key and value
 * @throws NullPointerException if the key or value is {@code null}
 *
 * @see Map#ofEntries Map.ofEntries()
 * @since 9
 */
 static <K, V> Entry<K, V> entry(K k, V v) {
 // KeyValueHolder checks for nulls
 return new KeyValueHolder<>(k, v);
 }

 /**
 * Returns an <a href="#unmodifiable">unmodifiable Map</a> containing the entries
 * of the given Map. The given Map must not be null, and it must not contain any
 * null keys or values. If the given Map is subsequently modified, the returned
 * Map will not reflect such modifications.
 *
 * @implNote
 * If the given Map is an <a href="#unmodifiable">unmodifiable Map</a>,
 * calling copyOf will generally not create a copy.
 *
 * @param <K> the {@code Map}'s key type
 * @param <V> the {@code Map}'s value type
 * @param map a {@code Map} from which entries are drawn, must be non-null
 * @return a {@code Map} containing the entries of the given {@code Map}
 * @throws NullPointerException if map is null, or if it contains any null keys or values
 * @since 10
 */
 @SuppressWarnings({"rawtypes","unchecked"})
 static <K, V> Map<K, V> copyOf(Map<? extends K, ? extends V> map) {
 if (map instanceof ImmutableCollections.AbstractImmutableMap) {
 return (Map<K,V>)map;
 } else {
 return (Map<K,V>)Map.ofEntries(map.entrySet().toArray(new Entry[0]));
 }
 }
}