• LinkedHashMap继承于HashMap
• LinkedHashMap是hashmap和链表的结合体，通过链表来记录元素的顺序和链接关系，通过HashMap来存储数据,它可以控制元素的被遍历时候输出的顺序（按照最近访问顺序来排序，还是按插入顺序）。元素被保存在一个双向链表中，默认的遍历顺序是按插入顺序来被遍历的。通过构造函数的accessOrder来控制是否按照访问顺序来遍历。
• 首先 LinkHashMap是继承自HashMap的一个类，对于HashMap来说它只是多定义了两个变量如下，当然也是通过这两个变量以及重写HashMap的部分方法来实现，LRU的功能。

构造函数

    /**
     * Constructs an empty insertion-ordered <tt>LinkedHashMap</tt> instance
     * with the default initial capacity (16) and load factor (0.75).
     * 默认的构造函数 
     */
    public LinkedHashMap() {
        super();
        accessOrder = false; // 插入顺序访问
    }

    /**
     * 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 存储容量
     * @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 specified initial capacity and load factor.
     *
     * @param  initialCapacity the initial capacity 初始容量
     * @param  loadFactor      the load factor  附在因子
     * @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 <tt>LinkedHashMap</tt> instance with the
     * specified initial capacity, load factor and ordering mode.
     *
     * @param  initialCapacity the initial capacity 初始化存储容量
     * @param  loadFactor      the load factor 负载因子 
     * @param  accessOrder     the ordering mode - <tt>true</tt> for
     *         access-order, <tt>false</tt> for insertion-order 访问顺序默认存储顺序
     * @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;
    }