LinkedHashMap

LinkedHashMap是Map接口的哈希表和链接列表实现，具有可预知的迭代顺序。此实现提供所有可选的映射操作，并允许使用null值和null键。此类不保证映射的顺序，特别是它不保证该顺序恒久不变。
LinkedHashMap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，该迭代顺序可以是插入顺序或者是访问顺序。
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。

以下是linkedhashmap的entity 主要是比hash map的entity增加了两个指向前后的指针。

 private static class LinkedHashMapEntry<K,V> extends HashMapEntry<K,V> {
        // These fields comprise the doubly linked list used for iteration.
        LinkedHashMapEntry<K,V> before, after;

        LinkedHashMapEntry(int hash, K key, V value, HashMapEntry<K,V> next) {
            super(hash, key, value, next);
        }

        /**
         * Removes this entry from the linked list.
         */
        private void remove() {
            before.after = after;
            after.before = before;
        }

        /**
         * Inserts this entry before the specified existing entry in the list.
         */
        private void addBefore(LinkedHashMapEntry<K,V> existingEntry) {
            after  = existingEntry;
            before = existingEntry.before;
            before.after = this;
            after.before = this;
        }

        /**
         * This method is invoked by the superclass whenever the value
         * of a pre-existing entry is read by Map.get or modified by Map.set.
         * If the enclosing Map is access-ordered, it moves the entry
         * to the end of the list; otherwise, it does nothing.
         */
        void recordAccess(HashMap<K,V> m) {
            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }

        void recordRemoval(HashMap<K,V> m) {
            remove();
        }
    }

初始化时增加boolean变量accessOrder 来确认链表是根据插入顺序还是读取顺序排序

  public LinkedHashMap(int initialCapacity,
                         float loadFactor,
                         boolean accessOrder) {
        super(initialCapacity, loadFactor);
        this.accessOrder = accessOrder;
    }

当模式为读取排序时，读取元素后会将该元素从原有链表中移除然后放到链表顶部

  public V get(Object key) {
        LinkedHashMapEntry<K,V> e = (LinkedHashMapEntry<K,V>)getEntry(key);
        if (e == null)
            return null;
        e.recordAccess(this);
        return e.value;
    }

   void recordAccess(HashMap<K,V> m) {
            LinkedHashMap<K,V> lm = (LinkedHashMap<K,V>)m;
            if (lm.accessOrder) {
                lm.modCount++;
                remove();
                addBefore(lm.header);
            }
        }