LinkedHashMap 源代码

扩容机制等可以看父类HashMap文章：HashMap 源代码

该类主要实现了用链表来实现HashMap的实现方式

/* * 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.Consumer;import java.util.function.BiConsumer;import java.util.function.BiFunction;import java.io.IOException;/** * <p>Hash table and linked list implementation of the <tt>Map</tt> interface, * with predictable iteration order.  This implementation differs from * <tt>HashMap</tt> 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 <tt>k</tt> is * reinserted into a map <tt>m</tt> if <tt>m.put(k, v)</tt> is invoked when * <tt>m.containsKey(k)</tt> would return <tt>true</tt> 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 <tt>Map</tt> operations, and * permits null elements.  Like <tt>HashMap</tt>, it provides constant-time * performance for the basic operations (<tt>add</tt>, <tt>contains</tt> and * <tt>remove</tt>), assuming the hash function disperses elements * properly among the buckets.  Performance is likely to be just slightly * below that of <tt>HashMap</tt>, due to the added expense of maintaining the * linked list, with one exception: Iteration over the collection-views * of a <tt>LinkedHashMap</tt> requires time proportional to the <i>size</i> * of the map, regardless of its capacity.  Iteration over a <tt>HashMap</tt> * 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 <tt>HashMap</tt>.  Note, however, that the penalty for choosing an * excessively high value for initial capacity is less severe for this class * than for <tt>HashMap</tt>, 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 <tt>get</tt> is a structural modification. * </strong>) * * <p>The iterators returned by the <tt>iterator</tt> 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 * <tt>remove</tt> 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 <tt>ConcurrentModificationException</tt> 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}/../technotes/guides/collections/index.html"> * 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 *//*Map 接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现与 HashMap 的不同之处在于，后者维护着一个运行于所有条目的双重链接列表。此链接列表定义了迭代顺序，该迭代顺序通常就是将键插入到映射中的顺序（插入顺序）。注意，如果在映射中重新插入键，则插入顺序不受影响。（如果在调用 m.put(k, v) 前 m.containsKey(k) 返回了 true，则调用时会将键 k 重新插入到映射 m 中。）此实现可以让客户避免未指定的、由 HashMap（及 Hashtable）所提供的通常为杂乱无章的排序工作，同时无需增加与 TreeMap 相关的成本。使用它可以生成一个与原来顺序相同的映射副本，而与原映射的实现无关：     void foo(Map m) {         Map copy = new LinkedHashMap(m);         ...     } 如果模块通过输入得到一个映射，复制这个映射，然后返回由此副本确定其顺序的结果，这种情况下这项技术特别有用。 （客户通常期望返回的内容与其出现的顺序相同。）提供特殊的构造方法来创建链接哈希映射，该哈希映射的迭代顺序就是最后访问其条目的顺序，从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建 LRU 缓存。调用 put 或 get 方法将会访问相应的条目（假定调用完成后它还存在）。putAll 方法以指定映射的条目集迭代器提供的键-值映射关系的顺序，为指定映射的每个映射关系生成一个条目访问。任何其他方法均不生成条目访问。特别是，collection 视图上的操作不 影响底层映射的迭代顺序。可以重写 removeEldestEntry(Map.Entry) 方法来实施策略，以便在将新映射关系添加到映射时自动移除旧的映射关系。此类提供所有可选的 Map 操作，并且允许 null 元素。与 HashMap 一样，它可以为基本操作（add、contains 和 remove）提供稳定的性能，假定哈希函数将元素正确分布到桶中。由于增加了维护链接列表的开支，其性能很可能比 HashMap 稍逊一筹，不过这一点例外：LinkedHashMap 的 collection 视图迭代所需时间与映射的大小 成比例。HashMap 迭代时间很可能开支较大，因为它所需要的时间与其容量 成比例。链接的哈希映射具有两个影响其性能的参数：初始容量和加载因子。它们的定义与 HashMap 极其相似。要注意，为初始容量选择非常高的值对此类的影响比对 HashMap 要小，因为此类的迭代时间不受容量的影响。注意，此实现不是同步的。如果多个线程同时访问链接的哈希映射，而其中至少一个线程从结构上修改了该映射，则它必须 保持外部同步。这一般通过对自然封装该映射的对象进行同步操作来完成。如果不存在这样的对象，则应该使用 Collections.synchronizedMap 方法来“包装”该映射。最好在创建时完成这一操作，以防止对映射的意外的非同步访问：    Map m = Collections.synchronizedMap(new LinkedHashMap(...));结构修改是指添加或删除一个或多个映射关系，    或者在按访问顺序链接的哈希映射中影响迭代顺序的任何操作。在按插入顺序链接的哈希映射中，    仅更改与映射中已包含键关联的值不是结构修改。在按访问顺序链接的哈希映射中，仅利用 get 查询映射不是结构修改。）Collection（由此类的所有 collection 视图方法所返回）的 iterator 方法返回的迭代器都是快速失败 的：在迭代器创建之后，如果从结构上对映射进行修改，除非通过迭代器自身的 remove 方法，其他任何时间任何方式的修改，迭代器都将抛出 ConcurrentModificationException。因此，面对并发的修改，迭代器很快就会完全失败，而不冒将来不确定的时间任意发生不确定行为的风险。注意，迭代器的快速失败行为无法得到保证，因为一般来说，不可能对是否出现不同步并发修改做出任何硬性保证。快速失败迭代器会尽最大努力抛出 ConcurrentModificationException。因此，编写依赖于此异常的程序的方式是错误的，正确做法是：迭代器的快速失败行为应该仅用于检测程序错误。 */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.     */    /**     * HashMap.Node subclass for normal LinkedHashMap entries.     */    //LinkedHashMap中专用的键值对，添加了before和after，也就是双链表    static class Entry<K,V> extends HashMap.Node<K,V> {        Entry<K,V> before, after;        Entry(int hash, K key, V value, Node<K,V> next) {            super(hash, key, value, next);        }    }    private static final long serialVersionUID = 3801124242820219131L;    /**     * The head (eldest) of the doubly linked list.     */    //指向头指针    transient LinkedHashMap.Entry<K,V> head;    /**     * The tail (youngest) of the doubly linked list.     */    //指向尾指针    transient LinkedHashMap.Entry<K,V> tail;    /**     * The iteration ordering method for this linked hash map: <tt>true</tt>     * for access-order, <tt>false</tt> for insertion-order.     *     * @serial     */    //生成迭代器时的顺序    // false为插入顺序，true为访问顺序（通过访问次数）    final boolean accessOrder;    // internal utilities    // link at the end of list    //在链表末尾添加结点p    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;        }    }    // apply src's links to dst    //将dst指向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;    }    // overrides of HashMap hook methods    //重新初始化    void reinitialize() {        super.reinitialize();        head = tail = null;    }    //创建节点，并链接到尾端    Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {        LinkedHashMap.Entry<K,V> p =            new LinkedHashMap.Entry<K,V>(hash, key, value, e);        linkNodeLast(p);        return p;    }    //替换节点    Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {        LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;        LinkedHashMap.Entry<K,V> t =            new LinkedHashMap.Entry<K,V>(q.hash, q.key, q.value, next);        transferLinks(q, t);        return t;    }    //创建新的树节点    TreeNode<K,V> newTreeNode(int hash, K key, V value, Node<K,V> next) {        TreeNode<K,V> p = new TreeNode<K,V>(hash, key, value, next);        linkNodeLast(p);        return p;    }    //替换树节点    TreeNode<K,V> replacementTreeNode(Node<K,V> p, Node<K,V> next) {        LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;        TreeNode<K,V> t = new TreeNode<K,V>(q.hash, q.key, q.value, next);        transferLinks(q, t);        return t;    }    //将e结点remove掉    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;    }    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);        }    }    //将e移到最后一个    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;            //将e先移除            if (b == null)                head = a;            else                b.after = a;            if (a != null)                a.before = b;            else                last = b;            //将e移到最后            if (last == null)                head = p;            else {                p.before = last;                last.after = p;            }            tail = p;            ++modCount;        }    }    //用于序列化时的写键值对    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);        }    }    /**     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> 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 insertion-ordered <tt>LinkedHashMap</tt> 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 <tt>LinkedHashMap</tt> instance     * with the default initial capacity (16) and load factor (0.75).     */    public LinkedHashMap() {        super();        accessOrder = false;    }    /**     * Constructs an insertion-ordered <tt>LinkedHashMap</tt> instance with     * the same mappings as the specified map.  The <tt>LinkedHashMap</tt>     * 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> m) {        super();        accessOrder = false;        putMapEntries(m, false);    }    /**     * Constructs an empty <tt>LinkedHashMap</tt> 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 - <tt>true</tt> for     *         access-order, <tt>false</tt> 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;    }    /**     * Returns <tt>true</tt> 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 <tt>true</tt> if this map maps one or more keys to the     *         specified value     */    public boolean containsValue(Object value) {        //遍历链表        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;    }    /**     * 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.     */    //获取键为key的value    public V get(Object key) {        Node<K,V> e;        if ((e = getNode(hash(key), key)) == null)            return null;        if (accessOrder)            afterNodeAccess(e);        return e.value;    }    /**     * {@inheritDoc}     */    public V getOrDefault(Object key, V defaultValue) {       Node<K,V> e;       if ((e = getNode(hash(key), key)) == null)           return defaultValue;       if (accessOrder)           afterNodeAccess(e);       return e.value;   }    /**     * {@inheritDoc}     */    public void clear() {        super.clear();        head = tail = null;    }    /**     * Returns <tt>true</tt> if this map should remove its eldest entry.     * This method is invoked by <tt>put</tt> and <tt>putAll</tt> 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() > 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     * <tt>false</tt> (indicating that the map should not attempt any     * further modification).  The effects of returning <tt>true</tt>     * after modifying the map from within this method are unspecified.     *     * <p>This implementation merely returns <tt>false</tt> (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 <tt>true</tt>.  If the map was empty prior     *           to the <tt>put</tt> or <tt>putAll</tt> 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   <tt>true</tt> if the eldest entry should be removed     *           from the map; <tt>false</tt> if it should be retained.     */    protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {        return false;    }    /**     * 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 <tt>remove</tt> operation), the results of     * the iteration are undefined.  The set supports element removal,     * which removes the corresponding mapping from the map, via the     * <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>     * operations.  It does not support the <tt>add</tt> or <tt>addAll</tt>     * 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     */    public Set<K> keySet() {        Set<K> ks;        return (ks = keySet) == null ? (keySet = new LinkedKeySet()) : ks;    }    final class LinkedKeySet extends AbstractSet<K> {        public final int size()                 { return size; }        public final void clear()               { LinkedHashMap.this.clear(); }        public final Iterator<K> iterator() {            return new LinkedKeyIterator();        }        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 Spliterator<K> spliterator()  {            return Spliterators.spliterator(this, Spliterator.SIZED |                                            Spliterator.ORDERED |                                            Spliterator.DISTINCT);        }        //遍历每个键值对，传入到action        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();        }    }    /**     * 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 <tt>remove</tt> operation),     * the results of the iteration are undefined.  The collection     * supports element removal, which removes the corresponding     * mapping from the map, via the <tt>Iterator.remove</tt>,     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,     * <tt>retainAll</tt> and <tt>clear</tt> operations.  It does not     * support the <tt>add</tt> or <tt>addAll</tt> 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     */    public Collection<V> values() {        Collection<V> vs;        return (vs = values) == null ? (values = new LinkedValues()) : vs;    }    final class LinkedValues extends AbstractCollection<V> {        public final int size()                 { return size; }        public final void clear()               { LinkedHashMap.this.clear(); }        public final Iterator<V> iterator() {            return new LinkedValueIterator();        }        public final boolean contains(Object o) { return containsValue(o); }        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();        }    }    /**     * 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 <tt>remove</tt> operation, or through the     * <tt>setValue</tt> 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 <tt>Iterator.remove</tt>,     * <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and     * <tt>clear</tt> operations.  It does not support the     * <tt>add</tt> or <tt>addAll</tt> 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     */    public Set<Map.Entry<K,V>> entrySet() {        Set<Map.Entry<K,V>> es;        return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;    }    final class LinkedEntrySet extends AbstractSet<Map.Entry<K,V>> {        public final int size()                 { return size; }        public final void clear()               { LinkedHashMap.this.clear(); }        public final Iterator<Map.Entry<K,V>> iterator() {            return new LinkedEntryIterator();        }        public final boolean contains(Object o) {            if (!(o instanceof Map.Entry))                return false;            Map.Entry<?,?> e = (Map.Entry<?,?>) o;            Object key = e.getKey();            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 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();        }    }    // Map overrides    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();    }    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();    }    // Iterators    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;            K key = p.key;            removeNode(hash(key), key, null, false, false);            expectedModCount = modCount;        }    }    final class LinkedKeyIterator extends LinkedHashIterator        implements Iterator<K> {        public final K next() { return nextNode().getKey(); }    }    final class LinkedValueIterator extends LinkedHashIterator        implements Iterator<V> {        public final V next() { return nextNode().value; }    }    final class LinkedEntryIterator extends LinkedHashIterator        implements Iterator<Map.Entry<K,V>> {        public final Map.Entry<K,V> next() { return nextNode(); }    }}