LinkedHashMap

1、简介

1.1 介绍

哈希表和链表实现的Map接口，具有可预测的迭代次序。 这种实现不同于HashMap，它维持于所有条目的运行双向链表。 此链接列表定义迭代排序，通常是将键插入到Map（插入顺序 ）中的顺序 。

1.2 构造

//构造具有默认初始容量（16）和负载因子（0.75）LinkedHashMap()//构造具有指定初始容量和默认负载因子（0.75）LinkedHashMap(int initialCapacity)//构造具有指定初始容量和负载因子的LinkedHashMap实例。LinkedHashMap(int initialCapacity, float loadFactor)//带订购模式的构造LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)//带子map的构造LinkedHashMap(Map<? extends K,? extends V> m)

1.3 继承关系

java.lang.Object    java.util.AbstractMap<K,V>        java.util.HashMap<K,V>            java.util.LinkedHashMap<K,V>

2、源代码

public class LinkedHashMap<K,V>    extends HashMap<K,V>    implements Map<K,V>{    //Entry继承自HashMap.Node    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);        }    }    //链表的头尾    transient LinkedHashMap.Entry<K,V> head;    transient LinkedHashMap.Entry<K,V> tail;    //false： 基于插入顺序     true：  基于访问顺序     final boolean accessOrder;    //构造    public LinkedHashMap(int initialCapacity, float loadFactor) {        super(initialCapacity, loadFactor);        accessOrder = false;    }    public LinkedHashMap(int initialCapacity) {        super(initialCapacity);        accessOrder = false;    }    public LinkedHashMap() {        super();        accessOrder = false;    }    public LinkedHashMap(Map<? extends K, ? extends V> m) {        super();        accessOrder = false;        putMapEntries(m, false);    }    public LinkedHashMap(int initialCapacity,                         float loadFactor,                         boolean accessOrder) {        super(initialCapacity, loadFactor);        this.accessOrder = accessOrder;    }

2.1.2 主要看get方法

    public V get(Object key) {        Node<K,V> e;        //调用HashMap的getNode        if ((e = getNode(hash(key), key)) == null)            return null;        //如果accessOrder为true        if (accessOrder)            afterNodeAccess(e);        return e.value;    }    //操作元素后改变LinkedHashMap维护的双向链表    void afterNodeAccess(Node<K,V> e) { // move node to last        LinkedHashMap.Entry<K,V> last;        //如果末尾不是传入的参数        if (accessOrder && (last = tail) != e) {            //构建双向循环链表            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;            }            //最近使用的将被插入链尾            tail = p;            ++modCount;        }    }

每一次增删改查都会操作LinkedHashMap的循环双向链表，将最新修改的节点赋为尾节点，最少使用、最久未使用的节点将变为头节点

2.1.3 LinkedHashMap调用的是HashMap的put方法

    final V putVal(int hash, K key, V value, boolean onlyIfAbsent,                   boolean evict) {        Node<K,V>[] tab; Node<K,V> p; int n, i;        if ((tab = table) == null || (n = tab.length) == 0)            n = (tab = resize()).length;        if ((p = tab[i = (n - 1) & hash]) == null)            //这里的newNode是LinkedHashMap的newNode            tab[i] = newNode(hash, key, value, null);        else {            Node<K,V> e; K k;            if (p.hash == hash &&                ((k = p.key) == key || (key != null && key.equals(k))))                e = p;            else if (p instanceof TreeNode)                e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);            else {                for (int binCount = 0; ; ++binCount) {                    if ((e = p.next) == null) {                        p.next = newNode(hash, key, value, null);                        if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st                            treeifyBin(tab, hash);                        break;                    }                    if (e.hash == hash &&                        ((k = e.key) == key || (key != null && key.equals(k))))                        break;                    p = e;                }            }            if (e != null) { // existing mapping for key                V oldValue = e.value;                if (!onlyIfAbsent || oldValue == null)                    e.value = value;                //如果操作了元素，将调用afterNodeAccess调整双向循环链表                afterNodeAccess(e);                return oldValue;            }        }        ++modCount;        if (++size > threshold)            resize();        //插入后，根据条件操作头节点。        afterNodeInsertion(evict);        return null;    }    Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {        //新建节点        LinkedHashMap.Entry<K,V> p =            new LinkedHashMap.Entry<K,V>(hash, key, value, e);        //放入链表尾        linkNodeLast(p);        return 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;        }    }

LinkedHashMap使用HashMap的数据结构，只是对关键的方法进行重写，如newNode

遍历操作

    public static void main(String[] args) throws Exception {        LinkedHashMap linkedHashMap = new LinkedHashMap(16, (float) 0.75,true);        linkedHashMap.put("1", "1");        linkedHashMap.put("2", "2");        linkedHashMap.put("3", "3");        linkedHashMap.get("1");        Iterator<Map.Entry> iterator= linkedHashMap.entrySet().iterator();        while(iterator.hasNext()) {            Map.Entry entry = iterator.next();            System.out.println(entry.getKey()+":"+entry.getValue());        }    }

源码部分

    public Set<Map.Entry<K,V>> entrySet() {        Set<Map.Entry<K,V>> es;        //返回LinkedEntrySet对象        return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;    }    final class LinkedEntrySet extends AbstractSet<Map.Entry<K,V>> {        public final Iterator<Map.Entry<K,V>> iterator() {            //返回LinkedEntryIterator对象            return new LinkedEntryIterator();        }    }    //LinkedEntryIterator继承LinkedHashIterator    final class LinkedEntryIterator extends LinkedHashIterator        implements Iterator<Map.Entry<K,V>> {        public final Map.Entry<K,V> next() { return nextNode(); }    }    //最终的迭代操作    abstract class LinkedHashIterator {        LinkedHashMap.Entry<K,V> next;        LinkedHashMap.Entry<K,V> current;        int expectedModCount;        LinkedHashIterator() {            //在构造中 将head赋给next            next = head;            expectedModCount = modCount;            current = null;        }        public final boolean hasNext() {            return next != null;        }        final LinkedHashMap.Entry<K,V> nextNode() {            //直接返回next            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;        }    }

迭代器是从双向循环链表头开始遍历。

    //将元素p插入双链表尾部    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;        }    }    //将src替换为dst    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;    }    //新建节点    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;    }}

总结：
LinkedHashMap主体使用HashMap的数据结构，通过重写一些重要的方法如newNode，轻松将HashMap的Entry转换为LinkedHashMap的，体现了继承的优势

LinkedHashMap保持HashMap的数据结构，但是自己也维护一个双向循环链表。

LinkedHashMap内部有accessOrder标记，为false时，双向链表按插入的顺序排列。因为新插入的元素都是尾插法

accessOrder=true时，每次对元素进行增删改查，都会将该元素放到链表尾部。使这个链表将最新操作的元素放入链表尾，长时间未使用的放入头部。这种即是LRU算法。