LinkedHashMap详解

    // LinkedEntry就是一个双向链表。除了保存当前对象的引用外，还保存了其上一个元素 before 和下一个元素 after 的引用
    static class LinkedEntry<K, V> extends HashMapEntry<K, V> {
        LinkedEntry<K, V> nxt;
        LinkedEntry<K, V> prv;

        /** Create the header entry */
        LinkedEntry() {
            super(null, null, 0, null);
            nxt = prv = this;
        }

        /** Create a normal entry */
        LinkedEntry(K key, V value, int hash, HashMapEntry<K, V> next,
                    LinkedEntry<K, V> nxt, LinkedEntry<K, V> prv) {
            super(key, value, hash, next);
            this.nxt = nxt;
            this.prv = prv;
        }
    }

public class LinkedHashMap<K, V> extends HashMap<K, V> {
    // 如果用transient声明一个实例变量，当对象存储时，它的值不需要维持。换句话来说就是，用transient关键字标记的成员变量不参与序列化过程。
    transient LinkedEntry<K, V> header;

    /**
     * True if access ordered, false if insertion ordered.
     */
    private final boolean accessOrder;

    //添加到header的prv位置，作为新的tail
    @Override 
    void addNewEntry(K key, V value, int hash, int index) {
        LinkedEntry<K, V> header = this.header;

        // Remove eldest entry if instructed to do so.
        LinkedEntry<K, V> eldest = header.nxt;
        if (eldest != header && removeEldestEntry(eldest)) {
            remove(eldest.key);
        }

        // Create new entry, link it on to list, and put it into table
        LinkedEntry<K, V> oldTail = header.prv;
        LinkedEntry<K, V> newTail = new LinkedEntry<K,V>(
                key, value, hash, table[index], header, oldTail);
        table[index] = oldTail.nxt = header.prv = newTail;
    }


    //这个方法和HashMap很类似，唯一需要注意的就是accessOrder那一句
    @Override 
    public V get(Object key) {
        /*
         * This method is overridden to eliminate the need for a polymorphic
         * invocation in superclass at the expense of code duplication.
         */
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            if (e == null)
                return null;
            if (accessOrder)
                makeTail((LinkedEntry<K, V>) e);
            return e.value;
        }

        // Replace with Collections.secondaryHash when the VM is fast enough (http://b/8290590).
        int hash = secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
                e != null; e = e.next) {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                //如果按照读取顺序，那么要调整e节点的位置
                if (accessOrder)
                    makeTail((LinkedEntry<K, V>) e);
                return e.value;
            }
        }
        return null;
    }

    //取出e节点插入到header前面一个节点
    private void makeTail(LinkedEntry<K, V> e) {
        // 把e节点取出来
        e.prv.nxt = e.nxt;
        e.nxt.prv = e.prv;

        // 把e节点插入到header前面一个节点中
        LinkedEntry<K, V> header = this.header;
        LinkedEntry<K, V> oldTail = header.prv;
        e.nxt = header;
        e.prv = oldTail;
        oldTail.nxt = header.prv = e;
        modCount++;
    }
}