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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

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 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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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>.

 * Programmers should treat instances that are {@linkplain #equals(Object) equal}

 * as interchangeable and should not use them for synchronization, or

 * unpredictable behavior may occur. For example, in a future release,

 * synchronization may fail. Callers should make no assumptions

 * about the identity of the returned instances. Factories are free to

 * create new instances or reuse existing ones.

 * <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 Entry may be unmodifiable, or the

     * value may be modifiable if the optional {@code setValue} method is

     * implemented. The Entry may be independent of any map, or it may represent

     * an entry of the entry-set view of a map.

     * <p>

     * Instances of the {@code Map.Entry} interface may be obtained by iterating

     * the entry-set view of a map. These instances maintain a connection to the

     * original, backing map. This connection to the backing map is valid

     * <i>only</i> for the duration of iteration over the entry-set view.

     * During iteration of the entry-set view, if supported by the backing map,

     * a change to a {@code Map.Entry}'s value via the

     * {@link Map.Entry#setValue setValue} method will be visible in the backing map.

     * The behavior of such a {@code Map.Entry} instance is undefined outside of

     * iteration of the map's entry-set view. It is also undefined if the backing

     * map has been modified after the {@code Map.Entry} was returned by the

     * iterator, except through the {@code Map.Entry.setValue} method. In particular,

     * a change to the value of a mapping in the backing map might or might not be

     * visible in the corresponding {@code Map.Entry} element of the entry-set view.

     *

     * @apiNote

     * It is possible to create a {@code Map.Entry} instance that is disconnected

     * from a backing map by using the {@link Map.Entry#copyOf copyOf} method. For example,

     * the following creates a snapshot of a map's entries that is guaranteed not to

     * change even if the original map is modified:

     * <pre> {@code

     * var entries = map.entrySet().stream().map(Map.Entry::copyOf).toList()

     * }</pre>

     *

     * @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()))  &&

         *     (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());

        }

        /**

         * Returns a copy of the given {@code Map.Entry}. The returned instance is not

         * associated with any map. The returned instance has the same characteristics

         * as instances returned by the {@link Map#entry Map::entry} method.

         *

         * @apiNote

         * An instance obtained from a map's entry-set view has a connection to that map.

         * The {@code copyOf}  method may be used to create a {@code Map.Entry} instance,

         * containing the same key and value, that is independent of any map.

         *

         * @implNote

         * If the given entry was obtained from a call to {@code copyOf} or {@code Map::entry},

         * calling {@code copyOf} will generally not create another copy.

         *

         * @param <K> the type of the key

         * @param <V> the type of the value

         * @param e the entry to be copied

         * @return a map entry equal to the given entry

         * @throws NullPointerException if e is null or if either of its key or value is null

         * @since 17

         */

        @SuppressWarnings("unchecked")

        public static <K, V> Map.Entry<K, V> copyOf(Map.Entry<? extends K, ? extends V> e) {

            Objects.requireNonNull(e);

            if (e instanceof KeyValueHolder) {

                return (Map.Entry<K, V>) e;

            } else {

                return Map.entry(e.getKey(), e.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), oldValue)) {

     *     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}:

     *

     * <pre> {@code

     * V oldValue = map.get(key);

     * V newValue = remappingFunction.apply(key, oldValue);

     * if (newValue != null) {

     *     map.put(key, newValue);

     * } else if (oldValue != null || map.containsKey(key)) {

     *     map.remove(key);

     * }

     * return 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