雙鏈迴環迴圈連結串列，名字相當唬人。首尾相接才算迴圈，但1.8這個沒發現有首尾相接，LinkedList也沒有

LinkedHashMap,有兩種排序，一種是順序儲存排序，像佇列，先放入的元素在隊頭，後放入的元素在隊尾，移除操作在隊頭進行。另一種是訪問排序，被訪問的元素放在隊尾，沒被訪問的放隊頭，put get這些操作都算是訪問

1.順序儲存排序

LinkedHashMap繼承了HashMap，比其多了兩個屬性head 和tail，加了一種連結串列的資料

public class LinkedHashMap<K,V>
    extends HashMap<K,V>
    implements Map<K,V>
{

    static class Entry<K,V> extends HashMap.Node<K,V> { //繼承了HashMap內部節點類，加了兩個屬性before上個節點，after和下個節點
        Entry<K,V> before, after;
        Entry(int hash, K key, V value, Node<K,V> next) {
            super(hash, key, value, next);
        }
    }

    transient LinkedHashMap.Entry<K,V> head;//頭節點

    transient LinkedHashMap.Entry<K,V> tail;//尾節點

    /**
     * The iteration ordering method for this linked hash map: <tt>true</tt>
     * for access-order, <tt>false</tt> for insertion-order.
     *
     * @serial
     */
    final boolean accessOrder; //如果是false，則用儲存排序，true用訪問排序，LinkedHashMap的構造方法預設都是false


    private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {  //把新節點放在變成尾節點
        LinkedHashMap.Entry<K,V> last = tail;
        tail = p;
        if (last == null)
            head = p;
        else {
            p.before = last;
            last.after = p;
        }
    }
 

    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;
    }

LinkedHashMap其實沒有重寫HashMap的put方法，它並有放棄HashMap的雜湊連結串列，只是在這個基礎上加上了自己的連結串列。 來看HashMap的原始碼，put方法呼叫了newNode方法，LinkedHashMap只需要重寫此方法就可以在put時把元素也放在自己的連結串列中，新節點變成尾節點

    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)
            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;
                afterNodeAccess(e);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }   

/* ------------------------------------------------------------ */原始碼特地註釋了 以下是支援LinekdHashMap的
    // LinkedHashMap support


    /*
     * The following package-protected methods are designed to be
     * overridden by LinkedHashMap, but not by any other subclass.
     * Nearly all other internal methods are also package-protected
     * but are declared final, so can be used by LinkedHashMap, view
     * classes, and HashSet.
     */

    // Create a regular (non-tree) node
    Node<K,V> newNode(int hash, K key, V value, Node<K,V> next) {
        return new Node<>(hash, key, value, next);
    }

LinkedHashMap put先呼叫HashMap的put ，在put中呼叫了LinkedHashMap的newNode方法（把元素插入雙向連結串列中）

來看看LinkedHashMap的remove方法：先呼叫HashMap的remove方法，此方法中呼叫了一個afterNodeRemoval方法，HashMap的afterNodeRemoval是空方法，LinkedHashMap重寫了此方法，把元素從雙向連結串列中移除

HashMap：    

final Node<K,V> removeNode(int hash, Object key, Object value,
                               boolean matchValue, boolean movable) {

....省略
if (node != null && (!matchValue || (v = node.value) == value ||
                                 (value != null && value.equals(v)))) {
                if (node instanceof TreeNode)
                    ((TreeNode<K,V>)node).removeTreeNode(this, tab, movable);
                else if (node == p)
                    tab[index] = node.next;
                else
                    p.next = node.next;
                ++modCount;
                --size;
                afterNodeRemoval(node);
                return node;
            }
}

    // Callbacks to allow LinkedHashMap post-actions
    void afterNodeRemoval(Node<K,V> p) { }

LinkeHashMap的afterNodeRemoval把元素移除

    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;
    }

2.訪問排序

想使用訪問排序，需要在初始化LinkedHashMap時設定accessOrder為true

看看get方法：

    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;
    }

    void afterNodeAccess(Node<K,V> e) { // move node to last  把被訪問的元素 放到隊尾
        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;
        }
    }