/**

     * HashMap.Node subclass for normal LinkedHashMap entries.

     */

    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);

        }

    }
 还是原HashMap的，多了两个引用before，after，说明这是要用双向链表了

public LinkedHashMap() {

        super();

        accessOrder = false;

    }

    多了 accessOrder = false;

     /**

     * 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;

    accessOrder表示迭代顺序，true表示访问顺序，false表示插入顺序。   

 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);  //这个方法HashMap是空实现，这里是发生hash冲突后，找到有相同key对值进行处理时调用

                return oldValue;

            }

        }

        ++modCount;

        if (++size > threshold)

            resize();

        afterNodeInsertion(evict);   //这个方法HashMap也是空实现，这里是完成新数据put后调用

        return null;

    }

LinkedHashMap具体实现 :

    1.void afterNodeAccess(Node<K,V> e) { // move node to last    注释就说明了是把该元素移到最后

        LinkedHashMap.Entry<K,V> last;

        if (accessOrder && (last = tail) != e) {    //accessOrder用到了，默认false等于不运行，true时是按插入顺序

            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;

        }

    }

    以上代码看出在按插入顺序时，在有相同key时，对当前节点将其移到链表末尾

    2.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);

        }

    }

    removeEldestEntry 方法在jdk1.8中固定返回false，因此不用关注此方法

    说明一下removeElestEntry用于定义删除最老元素的规则。一旦需要删除最老节点，那么将会调用removeNode删除节点。
    举个例子，如果一个链表只能维持100个元素，那么当插入了第101个元素时，以如下方式重写removeEldestEntry的话，那么将会删除最老的一个元素

 /*基于访问排序*/

        LinkedHashMap<String, String> linkedHashMap_accessOrder = new LinkedHashMap<>(10,0.75f,true);

        linkedHashMap_accessOrder.put("name1", "josan1");

        linkedHashMap_accessOrder.put("name2", "josan2");

        linkedHashMap_accessOrder.put("name3", "josan3");

        linkedHashMap_accessOrder.put("name4", "josan4");

        linkedHashMap_accessOrder.put("name5", "josan5");

        linkedHashMap_accessOrder.get("name5");

        linkedHashMap_accessOrder.get("name1");

        linkedHashMap_accessOrder.get("name2");

        linkedHashMap_accessOrder.get("name4");

        linkedHashMap_accessOrder.get("name3");

        //51243顺序

        for (Map.Entry<String, String> entry : linkedHashMap_accessOrder.entrySet()) {

            String key = entry.getKey();

            String value = entry.getValue();

            System.out.println("key:" + key + ",value:" + 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;

    }

 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;

        }

    }