LinkedHashMap.java

/*
 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.util;

import java.io.IOException;
import java.util.function.BiConsumer;
import java.util.function.BiFunction;
import java.util.function.Consumer;

/**
 * <p>Hash table and linked list implementation of the {@code Map} interface,
 * with predictable iteration order.  This implementation differs from
 * {@code HashMap} in that it maintains a doubly-linked list running through
 * all of its entries.  This linked list defines the iteration ordering,
 * which is normally the order in which keys were inserted into the map
 * (<i>insertion-order</i>).  Note that insertion order is not affected
 * if a key is <i>re-inserted</i> into the map.  (A key {@code k} is
 * reinserted into a map {@code m} if {@code m.put(k, v)} is invoked when
 * {@code m.containsKey(k)} would return {@code true} immediately prior to
 * the invocation.)
 *
 * <p>This implementation spares its clients from the unspecified, generally
 * chaotic ordering provided by {@link HashMap} (and {@link Hashtable}),
 * without incurring the increased cost associated with {@link TreeMap}.  It
 * can be used to produce a copy of a map that has the same order as the
 * original, regardless of the original map's implementation:
 * <pre>
 *     void foo(Map m) {
 *         Map copy = new LinkedHashMap(m);
 *         ...
 *     }
 * </pre>
 * This technique is particularly useful if a module takes a map on input,
 * copies it, and later returns results whose order is determined by that of
 * the copy.  (Clients generally appreciate having things returned in the same
 * order they were presented.)
 *
 * <p>A special {@link #LinkedHashMap(int,float,boolean) constructor} is
 * provided to create a linked hash map whose order of iteration is the order
 * in which its entries were last accessed, from least-recently accessed to
 * most-recently (<i>access-order</i>).  This kind of map is well-suited to
 * building LRU caches.  Invoking the {@code put}, {@code putIfAbsent},
 * {@code get}, {@code getOrDefault}, {@code compute}, {@code computeIfAbsent},
 * {@code computeIfPresent}, or {@code merge} methods results
 * in an access to the corresponding entry (assuming it exists after the
 * invocation completes). The {@code replace} methods only result in an access
 * of the entry if the value is replaced.  The {@code putAll} method generates one
 * entry access for each mapping in the specified map, in the order that
 * key-value mappings are provided by the specified map's entry set iterator.
 * <i>No other methods generate entry accesses.</i>  In particular, operations
 * on collection-views do <i>not</i> affect the order of iteration of the
 * backing map.
 *
 * <p>The {@link #removeEldestEntry(Map.Entry)} method may be overridden to
 * impose a policy for removing stale mappings automatically when new mappings
 * are added to the map.
 *
 * <p>This class provides all of the optional {@code Map} operations, and
 * permits null elements.  Like {@code HashMap}, it provides constant-time
 * performance for the basic operations ({@code add}, {@code contains} and
 * {@code remove}), assuming the hash function disperses elements
 * properly among the buckets.  Performance is likely to be just slightly
 * below that of {@code HashMap}, due to the added expense of maintaining the
 * linked list, with one exception: Iteration over the collection-views
 * of a {@code LinkedHashMap} requires time proportional to the <i>size</i>
 * of the map, regardless of its capacity.  Iteration over a {@code HashMap}
 * is likely to be more expensive, requiring time proportional to its
 * <i>capacity</i>.
 *
 * <p>A linked hash map has two parameters that affect its performance:
 * <i>initial capacity</i> and <i>load factor</i>.  They are defined precisely
 * as for {@code HashMap}.  Note, however, that the penalty for choosing an
 * excessively high value for initial capacity is less severe for this class
 * than for {@code HashMap}, as iteration times for this class are unaffected
 * by capacity.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access a linked hash map concurrently, and at least
 * one of the threads modifies the map structurally, it <em>must</em> be
 * synchronized externally.  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the map.
 *
 * If no such object exists, the map should be "wrapped" using the
 * {@link Collections#synchronizedMap Collections.synchronizedMap}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the map:<pre>
 *   Map m = Collections.synchronizedMap(new LinkedHashMap(...));</pre>
 *
 * A structural modification is any operation that adds or deletes one or more
 * mappings or, in the case of access-ordered linked hash maps, affects
 * iteration order.  In insertion-ordered linked hash maps, merely changing
 * the value associated with a key that is already contained in the map is not
 * a structural modification.  <strong>In access-ordered linked hash maps,
 * merely querying the map with {@code get} is a structural modification.
 * </strong>)
 *
 * <p>The iterators returned by the {@code iterator} method of the collections
 * returned by all of this class's collection view methods are
 * <em>fail-fast</em>: if the map is structurally modified at any time after
 * the iterator is created, in any way except through the iterator's own
 * {@code remove} method, the iterator will throw a {@link
 * ConcurrentModificationException}.  Thus, in the face of concurrent
 * modification, the iterator fails quickly and cleanly, rather than risking
 * arbitrary, non-deterministic behavior at an undetermined time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:   <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>The spliterators returned by the spliterator method of the collections
 * returned by all of this class's collection view methods are
 * <em><a href="Spliterator.html#binding">late-binding</a></em>,
 * <em>fail-fast</em>, and additionally report {@link Spliterator#ORDERED}.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @implNote
 * The spliterators returned by the spliterator method of the collections
 * returned by all of this class's collection view methods are created from
 * the iterators of the corresponding collections.
 *
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 *
 * @author  Josh Bloch
 * @see     Object#hashCode()
 * @see     Collection
 * @see     Map
 * @see     HashMap
 * @see     TreeMap
 * @see     Hashtable
 * @since   1.4
 */

/*
 * LinkedHashMap是有序Map，也是HashMap的子类，其基础结构与HashMap一致
 * 这里有序的含义是说其遍历顺序与元素加入的顺序有关
 * 该Map中的元素按其加入的顺序，为其额外建立了前后链接
 *
 * 普通情况下，LinkedHashMap的遍历操作中，元素顺序就是其加入到Map时的顺序
 * 如果开启了accessOrder标记，那么元素顺序与每个元素被访问的频率也有关
 */
public class LinkedHashMap<K, V> extends HashMap<K, V> implements Map<K, V> {
    
    /*
     * Implementation note.  A previous version of this class was
     * internally structured a little differently. Because superclass
     * HashMap now uses trees for some of its nodes, class
     * LinkedHashMap.Entry is now treated as intermediary node class
     * that can also be converted to tree form. The name of this
     * class, LinkedHashMap.Entry, is confusing in several ways in its
     * current context, but cannot be changed.  Otherwise, even though
     * it is not exported outside this package, some existing source
     * code is known to have relied on a symbol resolution corner case
     * rule in calls to removeEldestEntry that suppressed compilation
     * errors due to ambiguous usages. So, we keep the name to
     * preserve unmodified compilability.
     *
     * The changes in node classes also require using two fields
     * (head, tail) rather than a pointer to a header node to maintain
     * the doubly-linked before/after list. This class also
     * previously used a different style of callback methods upon
     * access, insertion, and removal.
     */
    
    private static final long serialVersionUID = 3801124242820219131L;
    
    /**
     * The iteration ordering method for this linked hash map: {@code true}
     * for access-order, {@code false} for insertion-order.
     *
     * @serial
     */
    // 是否开启afterNodeAccess()功能
    final boolean accessOrder;
    
    /**
     * The head (eldest) of the doubly linked list.
     */
    // 将加入的结点串成一个链表，head指向表头
    transient LinkedHashMap.Entry<K, V> head;
    
    /**
     * The tail (youngest) of the doubly linked list.
     */
    // 将加入的结点串成一个链表，tail指向表尾
    transient LinkedHashMap.Entry<K, V> tail;
    
    
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Constructs an empty insertion-ordered {@code LinkedHashMap} instance
     * with the default initial capacity (16) and load factor (0.75).
     */
    public LinkedHashMap() {
        super();
        accessOrder = false;
    }
    
    /**
     * Constructs an empty insertion-ordered {@code LinkedHashMap} instance
     * with the specified initial capacity and a default load factor (0.75).
     *
     * @param initialCapacity the initial capacity
     *
     * @throws IllegalArgumentException if the initial capacity is negative
     */
    public LinkedHashMap(int initialCapacity) {
        super(initialCapacity);
        accessOrder = false;
    }
    
    /**
     * Constructs an empty insertion-ordered {@code LinkedHashMap} instance
     * with the specified initial capacity and load factor.
     *
     * @param initialCapacity the initial capacity
     * @param loadFactor      the load factor
     *
     * @throws IllegalArgumentException if the initial capacity is negative
     *                                  or the load factor is nonpositive
     */
    public LinkedHashMap(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);
        accessOrder = false;
    }
    
    /**
     * Constructs an empty {@code LinkedHashMap} instance with the
     * specified initial capacity, load factor and ordering mode.
     *
     * @param initialCapacity the initial capacity
     * @param loadFactor      the load factor
     * @param accessOrder     the ordering mode - {@code true} for
     *                        access-order, {@code false} for insertion-order
     *
     * @throws IllegalArgumentException if the initial capacity is negative
     *                                  or the load factor is nonpositive
     */
    public LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }
    
    /**
     * Constructs an insertion-ordered {@code LinkedHashMap} instance with
     * the same mappings as the specified map.  The {@code LinkedHashMap}
     * instance is created with a default load factor (0.75) and an initial
     * capacity sufficient to hold the mappings in the specified map.
     *
     * @param m the map whose mappings are to be placed in this map
     *
     * @throws NullPointerException if the specified map is null
     */
    public LinkedHashMap(Map<? extends K, ? extends V> map) {
        super();
        accessOrder = false;
        // 将指定LinkedHashMap中的元素存入到当前LinkedHashMap（允许覆盖）
        putMapEntries(map, false);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 取值 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code (key==null ? k==null :
     * key.equals(k))}, then this method returns {@code v}; otherwise
     * it returns {@code null}.  (There can be at most one such mapping.)
     *
     * <p>A return value of {@code null} does not <i>necessarily</i>
     * indicate that the map contains no mapping for the key; it's also
     * possible that the map explicitly maps the key to {@code null}.
     * The {@link #containsKey containsKey} operation may be used to
     * distinguish these two cases.
     */
    // 根据指定的key获取对应的value，如果不存在，则返回null
    public V get(Object key) {
        // 根据给定的key和hash（由key计算而来）查找对应的（同位）元素，如果找不到，则返回null
        Node<K, V> e = getNode(hash(key), key);
        
        if(e == null) {
            return null;
        }
        
        if(accessOrder) {
            afterNodeAccess(e);
        }
        
        return e.value;
    }
    
    /**
     * {@inheritDoc}
     */
    // 根据指定的key获取对应的value，如果不存在，则返回指定的默认值defaultValue
    public V getOrDefault(Object key, V defaultValue) {
        // 根据给定的key和hash（由key计算而来）查找对应的（同位）元素，如果找不到，则返回null
        Node<K, V> e = getNode(hash(key), key);
        
        if(e == null) {
            return defaultValue;
        }
        
        if(accessOrder) {
            afterNodeAccess(e);
        }
        
        return e.value;
    }
    
    /*▲ 取值 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 替换 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 替换当前HashMap中的所有元素，替换策略由function决定，function的入参是元素的key和value，出参作为新值
    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        if(function == null) {
            throw new NullPointerException();
        }
        
        int mc = modCount;
        
        for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after) {
            e.value = function.apply(e.key, e.value);
        }
        
        if(modCount != mc) {
            throw new ConcurrentModificationException();
        }
    }
    
    /*▲ 替换 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 包含查询 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if this map maps one or more keys to the
     * specified value.
     *
     * @param value value whose presence in this map is to be tested
     *
     * @return {@code true} if this map maps one or more keys to the
     * specified value
     */
    // 判断LinkedHashMap中是否存在指定value的元素
    public boolean containsValue(Object value) {
        for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after) {
            V v = e.value;
            if(v == value || (value != null && value.equals(v))) {
                return true;
            }
        }
        return false;
    }
    
    /*▲ 包含查询 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator's own {@code remove} operation), the results of
     * the iteration are undefined.  The set supports element removal,
     * which removes the corresponding mapping from the map, via the
     * {@code Iterator.remove}, {@code Set.remove},
     * {@code removeAll}, {@code retainAll}, and {@code clear}
     * operations.  It does not support the {@code add} or {@code addAll}
     * operations.
     * Its {@link Spliterator} typically provides faster sequential
     * performance but much poorer parallel performance than that of
     * {@code HashMap}.
     *
     * @return a set view of the keys contained in this map
     */
    // 获取LinkedHashMap中key的集合
    public Set<K> keySet() {
        Set<K> ks = keySet;
        if(ks == null) {
            ks = new LinkedKeySet();
            keySet = ks;
        }
        return ks;
    }
    
    /**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  If the map is
     * modified while an iteration over the collection is in progress
     * (except through the iterator's own {@code remove} operation),
     * the results of the iteration are undefined.  The collection
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Collection.remove}, {@code removeAll},
     * {@code retainAll} and {@code clear} operations.  It does not
     * support the {@code add} or {@code addAll} operations.
     * Its {@link Spliterator} typically provides faster sequential
     * performance but much poorer parallel performance than that of
     * {@code HashMap}.
     *
     * @return a view of the values contained in this map
     */
    // 获取LinkedHashMap中value的集合
    public Collection<V> values() {
        Collection<V> vs = values;
        if(vs == null) {
            vs = new LinkedValues();
            values = vs;
        }
        return vs;
    }
    
    /**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  If the map is modified
     * while an iteration over the set is in progress (except through
     * the iterator's own {@code remove} operation, or through the
     * {@code setValue} operation on a map entry returned by the
     * iterator) the results of the iteration are undefined.  The set
     * supports element removal, which removes the corresponding
     * mapping from the map, via the {@code Iterator.remove},
     * {@code Set.remove}, {@code removeAll}, {@code retainAll} and
     * {@code clear} operations.  It does not support the
     * {@code add} or {@code addAll} operations.
     * Its {@link Spliterator} typically provides faster sequential
     * performance but much poorer parallel performance than that of
     * {@code HashMap}.
     *
     * @return a set view of the mappings contained in this map
     */
    // 获取LinkedHashMap中key-value对的集合
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> es;
        return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
    }
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 遍历 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 遍历LinkedHashMap中的元素，并对其应用action操作，action的入参是元素的key和value
    public void forEach(BiConsumer<? super K, ? super V> action) {
        if(action == null) {
            throw new NullPointerException();
        }
        
        int mc = modCount;
        
        for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after) {
            action.accept(e.key, e.value);
        }
        
        if(modCount != mc) {
            throw new ConcurrentModificationException();
        }
    }
    
    /*▲ 遍历 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 杂项 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns {@code true} if this map should remove its eldest entry.
     * This method is invoked by {@code put} and {@code putAll} after
     * inserting a new entry into the map.  It provides the implementor
     * with the opportunity to remove the eldest entry each time a new one
     * is added.  This is useful if the map represents a cache: it allows
     * the map to reduce memory consumption by deleting stale entries.
     *
     * <p>Sample use: this override will allow the map to grow up to 100
     * entries and then delete the eldest entry each time a new entry is
     * added, maintaining a steady state of 100 entries.
     * <pre>
     *     private static final int MAX_ENTRIES = 100;
     *
     *     protected boolean removeEldestEntry(Map.Entry eldest) {
     *        return size() &gt; MAX_ENTRIES;
     *     }
     * </pre>
     *
     * <p>This method typically does not modify the map in any way,
     * instead allowing the map to modify itself as directed by its
     * return value.  It <i>is</i> permitted for this method to modify
     * the map directly, but if it does so, it <i>must</i> return
     * {@code false} (indicating that the map should not attempt any
     * further modification).  The effects of returning {@code true}
     * after modifying the map from within this method are unspecified.
     *
     * <p>This implementation merely returns {@code false} (so that this
     * map acts like a normal map - the eldest element is never removed).
     *
     * @param eldest The least recently inserted entry in the map, or if
     *               this is an access-ordered map, the least recently accessed
     *               entry.  This is the entry that will be removed it this
     *               method returns {@code true}.  If the map was empty prior
     *               to the {@code put} or {@code putAll} invocation resulting
     *               in this invocation, this will be the entry that was just
     *               inserted; in other words, if the map contains a single
     *               entry, the eldest entry is also the newest.
     *
     * @return {@code true} if the eldest entry should be removed
     * from the map; {@code false} if it should be retained.
     */
    // 移除LinkedHashMap中最老的结点eldest，返回值代表是否成功移除
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return false;
    }
    
    
    /**
     * {@inheritDoc}
     */
    // 删除HashMap中所有元素
    public void clear() {
        super.clear();
        head = tail = null;
    }
    
    /*▲ 杂项 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ LinkedHashMap ████████████████████████████████████████████████████████████████████████████████┓ */
    
    // 创建一个普通Node
    Node<K, V> newNode(int hash, K key, V value, Node<K, V> e) {
        LinkedHashMap.Entry<K, V> p = new LinkedHashMap.Entry<>(hash, key, value, e);
        // 将结点p链接在已有的结点之后，在插入新结点时会用到
        linkNodeLast(p);
        return p;
    }
    
    // 创建一个红黑树的TreeNode
    TreeNode<K, V> newTreeNode(int hash, K key, V value, Node<K, V> next) {
        TreeNode<K, V> p = new TreeNode<>(hash, key, value, next);
        // 将结点p链接在已有的结点之后，在插入新结点时会用到
        linkNodeLast(p);
        return p;
    }
    
    // 从红黑树的TreeNode转换为一个普通Node
    Node<K, V> replacementNode(Node<K, V> p, Node<K, V> next) {
        LinkedHashMap.Entry<K, V> src = (LinkedHashMap.Entry<K, V>) p;
        LinkedHashMap.Entry<K, V> dst = new LinkedHashMap.Entry<>(src.hash, src.key, src.value, next);
        // 用dst替换src，在替换结点时候会用到
        transferLinks(src, dst);
        return dst;
    }
    
    // 从普通Node转换为一个红黑树的TreeNode
    TreeNode<K, V> replacementTreeNode(Node<K, V> p, Node<K, V> next) {
        LinkedHashMap.Entry<K, V> src = (LinkedHashMap.Entry<K, V>) p;
        TreeNode<K, V> dst = new TreeNode<>(src.hash, src.key, src.value, next);
        // 用dst替换src，在替换结点时候会用到
        transferLinks(src, dst);
        return dst;
    }
    
    // 在插入新结点的同时，移除LinkedHashMap中最老的结点（满足一定条件之后才移除）
    void afterNodeInsertion(boolean evict) { // possibly remove eldest
        LinkedHashMap.Entry<K, V> first;
        
        if(evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;
            removeNode(hash(key), key, null, false, true);
        }
    }
    
    // 从Map中移除结点e之后，也要解除其在链表上的链接
    void afterNodeRemoval(Node<K, V> e) { // unlink
        LinkedHashMap.Entry<K, V> p = (LinkedHashMap.Entry<K, V>) e, b = p.before, a = p.after;
        
        p.before = p.after = null;
        
        if(b == null) {
            head = a;
        } else {
            b.after = a;
        }
        
        if(a == null) {
            tail = b;
        } else {
            a.before = b;
        }
    }
    
    // 访问结点e之后，如果结点e不在表尾，则会将其移动到表尾（该项功能默认是关闭的，由accessOrder负责开启）
    void afterNodeAccess(Node<K, V> e) { // move node to last
        LinkedHashMap.Entry<K, V> last;
        
        if(accessOrder && (last = tail) != e) {
            LinkedHashMap.Entry<K, V> p = (LinkedHashMap.Entry<K, V>) e, b = p.before, a = p.after;
            
            p.after = null;
            
            if(b == null) {
                head = a;
            } else {
                b.after = a;
            }
            
            if(a != null) {
                a.before = b;
            } else {
                last = b;
            }
            
            if(last == null) {
                head = p;
            } else {
                p.before = last;
                last.after = p;
            }
            
            tail = p;
            
            ++modCount;
        }
    }
    
    // 重置当前LinkedHashMap，清空一切参数
    void reinitialize() {
        super.reinitialize();
        head = tail = null;
    }
    
    // 用于序列化过程
    void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
        for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after) {
            s.writeObject(e.key);
            s.writeObject(e.value);
        }
    }
    
    
    // 将结点p链接在已有的结点之后
    private void linkNodeLast(LinkedHashMap.Entry<K, V> p) {
        LinkedHashMap.Entry<K, V> last = tail;
        tail = p;
        if(last == null) {
            head = p;
        } else {
            p.before = last;
            last.after = p;
        }
    }
    
    // 用dst替换src
    private void transferLinks(LinkedHashMap.Entry<K, V> src, LinkedHashMap.Entry<K, V> dst) {
        LinkedHashMap.Entry<K, V> b = dst.before = src.before;
        LinkedHashMap.Entry<K, V> a = dst.after = src.after;
        
        if(b == null) {
            head = dst;
        } else {
            b.after = dst;
        }
        
        if(a == null) {
            tail = dst;
        } else {
            a.before = dst;
        }
    }
    
    /*▲ LinkedHashMap ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    
    
    
    /**
     * HashMap.Node subclass for normal LinkedHashMap entries.
     */
    // LinkedHashMap中的结点信息
    static class Entry<K, V> extends HashMap.Node<K, V> {
        // 将加入的结点串成一个链表，before指向前一个元素，after指向后一个元素
        Entry<K, V> before, after;
        
        Entry(int hash, K key, V value, Node<K, V> next) {
            super(hash, key, value, next);
        }
    }
    
    
    
    // LinkedHashMap中key的集合
    final class LinkedKeySet extends AbstractSet<K> {
        public final int size() {
            return size;
        }
        
        public final void clear() {
            LinkedHashMap.this.clear();
        }
        
        public final boolean contains(Object o) {
            return containsKey(o);
        }
        
        public final boolean remove(Object key) {
            return removeNode(hash(key), key, null, false, true) != null;
        }
        
        public final Iterator<K> iterator() {
            return new LinkedKeyIterator();
        }
        
        public final Spliterator<K> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED | Spliterator.DISTINCT);
        }
        
        public final void forEach(Consumer<? super K> action) {
            if(action == null)
                throw new NullPointerException();
            int mc = modCount;
            for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after)
                action.accept(e.key);
            if(modCount != mc)
                throw new ConcurrentModificationException();
        }
    }
    
    // LinkedHashMap中value的集合
    final class LinkedValues extends AbstractCollection<V> {
        public final int size() {
            return size;
        }
        
        public final void clear() {
            LinkedHashMap.this.clear();
        }
        
        public final boolean contains(Object o) {
            return containsValue(o);
        }
        
        public final Iterator<V> iterator() {
            return new LinkedValueIterator();
        }
        
        public final Spliterator<V> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED);
        }
        
        public final void forEach(Consumer<? super V> action) {
            if(action == null)
                throw new NullPointerException();
            int mc = modCount;
            for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after)
                action.accept(e.value);
            if(modCount != mc)
                throw new ConcurrentModificationException();
        }
    }
    
    // LinkedHashMap中key-value的集合，Entry的本质就是Node
    final class LinkedEntrySet extends AbstractSet<Map.Entry<K, V>> {
        public final int size() {
            return size;
        }
        
        public final void clear() {
            LinkedHashMap.this.clear();
        }
        
        public final boolean contains(Object o) {
            if(!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            Object key = e.getKey();
            
            // 根据给定的key和hash（由key计算而来）查找对应的（同位）元素，如果找不到，则返回null
            Node<K, V> candidate = getNode(hash(key), key);
            
            return candidate != null && candidate.equals(e);
        }
        
        public final boolean remove(Object o) {
            if(o instanceof Map.Entry) {
                Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
                Object key = e.getKey();
                Object value = e.getValue();
                return removeNode(hash(key), key, value, true, true) != null;
            }
            return false;
        }
        
        public final Iterator<Map.Entry<K, V>> iterator() {
            return new LinkedEntryIterator();
        }
        
        public final Spliterator<Map.Entry<K, V>> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED | Spliterator.DISTINCT);
        }
        
        public final void forEach(Consumer<? super Map.Entry<K, V>> action) {
            if(action == null) {
                throw new NullPointerException();
            }
            
            int mc = modCount;
            
            for(LinkedHashMap.Entry<K, V> e = head; e != null; e = e.after) {
                action.accept(e);
            }
            
            if(modCount != mc) {
                throw new ConcurrentModificationException();
            }
        }
    }
    
    
    
    // LinkedHashMap迭代器
    abstract class LinkedHashIterator {
        LinkedHashMap.Entry<K, V> next;
        LinkedHashMap.Entry<K, V> current;
        int expectedModCount;
        
        LinkedHashIterator() {
            next = head;
            expectedModCount = modCount;
            current = null;
        }
        
        public final boolean hasNext() {
            return next != null;
        }
        
        final LinkedHashMap.Entry<K, V> nextNode() {
            LinkedHashMap.Entry<K, V> e = next;
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            
            if(e == null) {
                throw new NoSuchElementException();
            }
            
            current = e;
            next = e.after;
            return e;
        }
        
        public final void remove() {
            Node<K, V> p = current;
            
            if(p == null) {
                throw new IllegalStateException();
            }
            
            if(modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            
            current = null;
            
            removeNode(p.hash, p.key, null, false, false);
            
            expectedModCount = modCount;
        }
    }
    
    // key的迭代器
    final class LinkedKeyIterator extends LinkedHashIterator implements Iterator<K> {
        public final K next() {
            return nextNode().getKey();
        }
    }
    
    // value的迭代器
    final class LinkedValueIterator extends LinkedHashIterator implements Iterator<V> {
        public final V next() {
            return nextNode().value;
        }
    }
    
    // key-value对的迭代器
    final class LinkedEntryIterator extends LinkedHashIterator implements Iterator<Map.Entry<K, V>> {
        public final Map.Entry<K, V> next() {
            return nextNode();
        }
    }
}