【java源码阅读】Map接口

/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.

 */

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.
 * 将键映射到值的对象，一个映射不能包含重复的键，每一个键只能映射最多一个值。

 * 
This interface takes the place of the Dictionary class, which
 * was a totally abstract class rather than an interface.
 *这个接口取代了字典类，他是一个完全抽象的类而不是一个接口。

 * 
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.
 *这个映射接口提供了三种集合视图，他允许一个映射内容视为一组键，一组值，或者键值映射集。
  映射的次序被定为映射集合视图上迭代器返回元素的值。一些映射的实现，对他们的顺序做了具体的保证，
  像TreeMap，而其他的像HashMap类，却没有。

 * 
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.
 注意：如果将可变对象用作映射的键，则必须小心。当对象是映射的键时，如果对象的值通过equals比较
 进行改变，则不能确定确定映射的行为。这个禁令的一个特殊的例子就是不允许将map自己当做键。虽然可以 
 将map本身当做值，特别要小心的建议是：equals 和 hashcode方法已经没有了明确的定义在这样的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
 */
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
     */
    int size();

    /**
     * Returns true if this map contains no key-value mappings.
     *
     * @return true if this map contains no key-value mappings
     */
    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)
     */
    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)
     */
    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)
     */
    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
     */
    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)
     */
    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
     */
    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
     */
    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
     */
    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
     */
    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
     */
    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
     */
    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.
         */
        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.
         */
        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.
         */
        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)
         */
        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;
    }
}