Map 接口 源代码

只是简单的介绍了Map中的Java8之前的代码，Java8新特性的代码还没了解，以后会补上

/*
 * Copyright (c) 1997, 2013, 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.
 *
 * 
This interface takes the place of the Dictionary class, which
 * was a totally abstract class rather than an interface.
 *
 * 
The Map interface provides three collection views, which
 * allow a map's contents to be viewed as a set of keys, collection of values,
 * or set of key-value mappings.  The order 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 TreeMap class, make
 * specific guarantees as to their order; others, like the HashMap
 * class, do not.
 *
 * 
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 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 equals and hashCode methods are no longer
 * well defined on such a map.
 *
 * 
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 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.
 *
 * 
The "destructive" methods contained in this interface, that is, the
 * methods that modify the map on which they operate, are specified to throw
 * UnsupportedOperationException if this map does not support the
 * operation.  If this is the case, these methods may, but are not required
 * to, throw an 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.
 *
 * 
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 NullPointerException or 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.
 *
 * 
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 true if and
 * only if this map contains a mapping for a key k such that
 * (key==null ? k==null : key.equals(k))." This specification should
 * not be construed to imply that invoking Map.containsKey
 * with a non-null argument key will cause key.equals(k) to
 * be invoked for any key k.  Implementations are free to
 * implement optimizations whereby the 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.
 *
 * 
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.
 *
 * 
This interface is a member of the
 * 
 * Java Collections Framework.
 *
 * @param  the type of keys maintained by this map
 * @param  the type of mapped values
 *
 * @author  Josh Bloch
 * @see HashMap
 * @see TreeMap
 * @see Hashtable
 * @see SortedMap
 * @see Collection
 * @see Set
 * @since 1.2
 */
/*
将键映射到值的对象。一个映射不能包含重复的键；每个键最多只能映射到一个值。

此接口取代 Dictionary 类，后者完全是一个抽象类，而不是一个接口。

Map 接口提供三种collection 视图，允许以键集、值集或键-值映射关系集的形式查看某个映射的内容。
映射顺序定义为迭代器在映射的 collection 视图上返回其元素的顺序。某些映射实现可明确保证其顺序，
如 TreeMap 类；另一些映射实现则不保证顺序，如 HashMap 类。

注：将可变对象用作映射键时必须格外小心。当对象是映射中某个键时，如果以影响 equals 比较的方式更改了对象的值，
则映射的行为将是不确定的。此项禁止的一种特殊情况是不允许某个映射将自身作为一个键包含。
虽然允许某个映射将自身作为值包含，但请格外小心：在这样的映射上 equals 和 hashCode 方法的定义将不再是明确的。

所有通用的映射实现类应该提供两个“标准的”构造方法：一个 void（无参数）构造方法，用于创建空映射；
一个是带有单个 Map 类型参数的构造方法，用于创建一个与其参数具有相同键-值映射关系的新映射。
实际上，后一个构造方法允许用户复制任意映射，生成所需类的一个等价映射。尽管无法强制执行此建议
（因为接口不能包含构造方法），但是 JDK 中所有通用的映射实现都遵从它。

此接口中包含的“破坏”方法可修改其操作的映射，如果此映射不支持该操作，这些方法将抛出
UnsupportedOperationException。如果是这样，那么在调用对映射无效时，这些方法可以
（但不要求）抛出 UnsupportedOperationException。例如，如果某个不可修改的映射（
其映射关系是“重叠”的）为空，则对该映射调用 putAll(Map) 方法时，可以（但不要求）抛出异常。

某些映射实现对可能包含的键和值有所限制。例如，某些实现禁止 null 键和值，另一些则对其键的类型有限制。
尝试插入不合格的键或值将抛出一个未经检查的异常，通常是 NullPointerException 或 ClassCastException。
试图查询是否存在不合格的键或值可能抛出异常，或者返回 false；某些实现将表现出前一种行为，
而另一些则表现后一种。一般来说，试图对不合格的键或值执行操作且该操作的完成不会导致不合格的元素被插入映射中时，
将可能抛出一个异常，也可能操作成功，这取决于实现本身。这样的异常在此接口的规范中标记为“可选”。

此接口是 Java Collections Framework 的成员。

Collections Framework 接口中的很多方法是根据 equals 方法定义的。例如，containsKey(Object key) 方法的规范中写道：
“当且仅当此映射包含针对满足 (key==null ? k==null : key.equals(k)) 的键 k 的映射关系时，返回 true”。不
应将此规范解释为：调用具有非空参数 key 的 Map.containsKey 将导致对任意的键 k 调用 key.equals(k)。实现可随意进行优化，
以避免调用 equals，例如，可首先比较两个键的哈希码（Object.hashCode() 规范保证哈希码不相等的两个对象不会相等）。
一般来说，只要实现者认为合适，各种 Collections Framework 接口的实现可随意利用底层 Object 方法的指定行为。

 */
public interface Map {
    // Query Operations

    /**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than Integer.MAX_VALUE elements, returns
     * Integer.MAX_VALUE.
     *
     * @return the number of key-value mappings in this map
     */
    //返回Map的大小，最大为Integer.MAX_VALUE
    int size();

    /**
     * Returns true if this map contains no key-value mappings.
     *
     * @return true if this map contains no key-value mappings
     */
    //判断是否为空
    boolean isEmpty();

    /**
     * Returns true if this map contains a mapping for the specified
     * key.  More formally, returns true if and only if
     * this map contains a mapping for a key k such that
     * (key==null ? k==null : key.equals(k)).  (There can be
     * at most one such mapping.)
     *
     * @param key key whose presence in this map is to be tested
     * @return true if this map contains a mapping for the specified
     *         key
     * @throws ClassCastException if the key is of an inappropriate type for
     *         this map
     * (optional)
     * @throws NullPointerException if the specified key is null and this map
     *         does not permit null keys
     * (optional)
     */
    //判断键中是否包含key
    boolean containsKey(Object key);

    /**
     * Returns true if this map maps one or more keys to the
     * specified value.  More formally, returns true if and only if
     * this map contains at least one mapping to a value v such that
     * (value==null ? v==null : value.equals(v)).  This operation
     * will probably require time linear in the map size for most
     * implementations of the Map interface.
     *
     * @param value value whose presence in this map is to be tested
     * @return 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
     * (optional)
     * @throws NullPointerException if the specified value is null and this
     *         map does not permit null values
     * (optional)
     */
    //判断值是否包含value
    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.
     *
     * 
More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * 
If this map permits null values, then a return value of
     * {@code null} does not necessarily 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
     * (optional)
     * @throws NullPointerException if the specified key is null and this map
     *         does not permit null keys
     * (optional)
     */
    //返回Map中键为key的值
    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
     * m is said to contain a mapping for a key k if and only
     * if {@link #containsKey(Object) m.containsKey(k)} would return
     * 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 key, or
     *         null if there was no mapping for key.
     *         (A null return can also indicate that the map
     *         previously associated null with key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the 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
     */
    //添加键为key,值为value的元素
    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 k to value v such that
     * (key==null ?  k==null : key.equals(k)), that mapping
     * is removed.  (The map can contain at most one such mapping.)
     *
     * 
Returns the value to which this map previously associated the key,
     * or null if the map contained no mapping for the key.
     *
     * 
If this map permits null values, then a return value of
     * null does not necessarily indicate that the map
     * contained no mapping for the key; it's also possible that the map
     * explicitly mapped the key to null.
     *
     * 
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 key, or
     *         null if there was no mapping for key.
     * @throws UnsupportedOperationException if the remove operation
     *         is not supported by this map
     * @throws ClassCastException if the key is of an inappropriate type for
     *         this map
     * (optional)
     * @throws NullPointerException if the specified key is null and this
     *         map does not permit null keys
     * (optional)
     */
    //移除掉键为key的元素
    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 k to value 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 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
     */
    //将m中的键值对添加到本Map中
    void putAll(Map m);

    /**
     * Removes all of the mappings from this map (optional operation).
     * The map will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the clear operation
     *         is not supported by this map
     */
    //清除掉键值对
    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 remove operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * Iterator.remove, Set.remove,
     * removeAll, retainAll, and clear
     * operations.  It does not support the add or addAll
     * operations.
     *
     * @return a set view of the keys contained in this map
     */
    //将Map中的所有key放到Set中并返回
    Set 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 remove operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the Iterator.remove,
     * Collection.remove, removeAll,
     * retainAll and clear operations.  It does not
     * support the add or addAll operations.
     *
     * @return a collection view of the values contained in this map
     */
    //将Map中的所有value转为放到容器中并返回
    Collection 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 remove operation, or through the
     * 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 Iterator.remove,
     * Set.remove, removeAll, retainAll and
     * clear operations.  It does not support the
     * add or addAll operations.
     *
     * @return a set view of the mappings contained in this map
     */
    //将Map中的所有键值对添加到Set中并返回
    Set> entrySet();

    /**
     * A map entry (key-value pair).  The Map.entrySet method returns
     * a collection-view of the map, whose elements are of this class.  The
     * only way to obtain a reference to a map entry is from the
     * iterator of this collection-view.  These Map.Entry objects are
     * valid only 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 setValue operation on the map entry.
     *
     * @see Map#entrySet()
     * @since 1.2
     */
    /*
    映射项（键-值对）。Map.entrySet 方法返回映射的 collection 视图，其中的元素属于此类。
    获得映射项引用的唯一 方法是通过此 collection 视图的迭代器来实现。这些 Map.Entry 对象仅
    在迭代期间有效；更确切地讲，如果在迭代器返回项之后修改了底层映射，则某些映射项的行为是不确定的，
    除了通过 setValue 在映射项上执行操作之外
     */
    interface Entry {
        /**
         * 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.
         */
        //获取键
        K getKey();

        /**
         * Returns the value corresponding to this entry.  If the mapping
         * has been removed from the backing map (by the iterator's
         * 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.
         */
        //获取值
        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 remove operation).
         *
         * @param value new value to be stored in this entry
         * @return old value corresponding to the entry
         * @throws UnsupportedOperationException if the 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.
         */
        //设置值
        V setValue(V value);

        /**
         * Compares the specified object with this entry for equality.
         * Returns true if the given object is also a map entry and
         * the two entries represent the same mapping.  More formally, two
         * entries e1 and e2 represent the same mapping
         * if
         *     (e1.getKey()==null ?
         *      e2.getKey()==null : e1.getKey().equals(e2.getKey()))  &&
         *     (e1.getValue()==null ?
         *      e2.getValue()==null : e1.getValue().equals(e2.getValue()))
         * 
         * This ensures that the equals method works properly across
         * different implementations of the Map.Entry interface.
         *
         * @param o object to be compared for equality with this map entry
         * @return 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 e is defined to be: 
         *     (e.getKey()==null   ? 0 : e.getKey().hashCode()) ^
         *     (e.getValue()==null ? 0 : e.getValue().hashCode())
         * 
         * This ensures that e1.equals(e2) implies that
         * e1.hashCode()==e2.hashCode() for any two Entries
         * e1 and e2, as required by the general
         * contract of Object.hashCode.
         *
         * @return the hash code value for this map entry
         * @see Object#hashCode()
         * @see Object#equals(Object)
         * @see #equals(Object)
         */
        //计算hash值
        int hashCode();

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

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

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

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

    // Comparison and hashing

    /**
     * Compares the specified object with this map for equality.  Returns
     * true if the given object is also a map and the two maps
     * represent the same mappings.  More formally, two maps m1 and
     * m2 represent the same mappings if
     * m1.entrySet().equals(m2.entrySet()).  This ensures that the
     * equals method works properly across different implementations
     * of the Map interface.
     *
     * @param o object to be compared for equality with this map
     * @return 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
     * entrySet() view.  This ensures that m1.equals(m2)
     * implies that m1.hashCode()==m2.hashCode() for any two maps
     * m1 and 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
     * (optional)
     * @throws NullPointerException if the specified key is null and this map
     * does not permit null keys
     * (optional)
     * @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}:
     * 
 {@code
     * for (Map.Entry entry : map.entrySet())
     *     action.accept(entry.getKey(), entry.getValue());
     * }
     *
     * 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 action) {
        Objects.requireNonNull(action);
        for (Map.Entry 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
     * 
The default implementation is equivalent to, for this {@code map}:
     * 
 {@code
     * for (Map.Entry entry : map.entrySet())
     *     entry.setValue(function.apply(entry.getKey(), entry.getValue()));
     * }
     *
     * 
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
     *         (optional)
     * @throws NullPointerException if function or a replacement value is null,
     *         and this map does not permit null keys or values
     *         (optional)
     * @throws IllegalArgumentException if some property of a replacement value
     *         prevents it from being stored in this map
     *         (optional)
     * @throws ConcurrentModificationException if an entry is found to be
     * removed during iteration
     * @since 1.8
     */
    default void replaceAll(BiFunction function) {
        Objects.requireNonNull(function);
        for (Map.Entry 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}:
     *
     * 
 {@code
     * V v = map.get(key);
     * if (v == null)
     *     v = map.put(key, value);
     *
     * return v;
     * }
     *
     * 
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
     *         (optional)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (optional)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (optional)
     * @throws IllegalArgumentException if some property of the specified key
     *         or value prevents it from being stored in this map
     *         (optional)
     * @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}:
     *
     * 
 {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.remove(key);
     *     return true;
     * } else
     *     return false;
     * }
     *
     * 
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
     *         (optional)
     * @throws ClassCastException if the key or value is of an inappropriate
     *         type for this map
     *         (optional)
     * @throws NullPointerException if the specified key or value is null,
     *         and this map does not permit null keys or values
     *         (optional)
     * @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}:
     *
     * 
 {@code
     * if (map.containsKey(key) && Objects.equals(map.get(key), value)) {
     *     map.put(key, newValue);
     *     return true;
     * } else
     *     return false;
     * }
     *
     * The default implementation does not throw NullPointerException
     * for maps that do not support null values if oldValue is null unless
     * newValue is also null.
     *
     * 
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
     *         (optional)
     * @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
     *         (optional)
     * @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}:
     *
     * 
 {@code
     * if (map.containsKey(key)) {
     *     return map.put(key, value);
     * } else
     *     return null;
     * }
     *
     * 
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
     *         (optional)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (optional)
     * @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}.
     *
     * 
If the function returns {@code null} no mapping is recorded. If
     * the function itself throws an (unchecked) exception, the
     * exception is rethrown, and no mapping is recorded.  The most
     * common usage is to construct a new object serving as an initial
     * mapped value or memoized result, as in:
     *
     * 
 {@code
     * map.computeIfAbsent(key, k -> new Value(f(k)));
     * }
     *
     * 
Or to implement a multi-value map, {@code Map>},
     * supporting multiple values per key:
     *
     * 
 {@code
     * map.computeIfAbsent(key, k -> new HashSet()).add(v);
     * }
     *
     *
     * @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:
     *
     * 
 {@code
     * if (map.get(key) == null) {
     *     V newValue = mappingFunction.apply(key);
     *     if (newValue != null)
     *         map.put(key, newValue);
     * }
     * }
     *
     * 
The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param mappingFunction the function to compute a value
     * @return the current (existing or computed) value associated with
     *         the specified key, or null if the computed value is null
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the mappingFunction
     *         is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (optional)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (optional)
     * @since 1.8
     */
    default V computeIfAbsent(K key,
            Function 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.
     *
     * 
If the function returns {@code null}, the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
    *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if now absent:
     *
     * 
 {@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);
     * }
     * }
     *
     * 
The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (optional)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (optional)
     * @since 1.8
     */
    default V computeIfPresent(K key,
            BiFunction 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:
     *
     * 
 {@code
     * map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))}
     * (Method {@link #merge merge()} is often simpler to use for such purposes.)
     *
     * 
If the function returns {@code null}, the mapping is removed (or
     * remains absent if initially absent).  If the function itself throws an
     * (unchecked) exception, the exception is rethrown, and the current mapping
     * is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * 
 {@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;
     * }
     * }
     *
     * 
The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the specified value is to be associated
     * @param remappingFunction the function to compute a value
     * @return the new value associated with the specified key, or null if none
     * @throws NullPointerException if the specified key is null and
     *         this map does not support null keys, or the
     *         remappingFunction is null
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (optional)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (optional)
     * @since 1.8
     */
    default V compute(K key,
            BiFunction 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:
     *
     * 
 {@code
     * map.merge(key, msg, String::concat)
     * }
     *
     * 
If the function returns {@code null} the mapping is removed.  If the
     * function itself throws an (unchecked) exception, the exception is
     * rethrown, and the current mapping is left unchanged.
     *
     * @implSpec
     * The default implementation is equivalent to performing the following
     * steps for this {@code map}, then returning the current value or
     * {@code null} if absent:
     *
     * 
 {@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);
     * }
     *
     * 
The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties. In particular, all implementations of
     * subinterface {@link java.util.concurrent.ConcurrentMap} must document
     * whether the function is applied once atomically only if the value is not
     * present.
     *
     * @param key key with which the resulting value is to be associated
     * @param value the non-null value to be merged with the existing value
     *        associated with the key or, if no existing value or a null value
     *        is associated with the key, to be associated with the key
     * @param remappingFunction the function to recompute a value if present
     * @return the new value associated with the specified key, or null if no
     *         value is associated with the key
     * @throws UnsupportedOperationException if the {@code put} operation
     *         is not supported by this map
     *         (optional)
     * @throws ClassCastException if the class of the specified key or value
     *         prevents it from being stored in this map
     *         (optional)
     * @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 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;
    }
}