Java源码解析之LinkedHashMap

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);
    }
}

// 双向链表的头，用作AccessOrder时也是最老的元素
transient LinkedHashMap.Entry<K,V> head;

// 双向链表的尾，用作AccessOrder时也是最新的元素
transient LinkedHashMap.Entry<K,V> tail;

// true则为访问顺序，false则为插入顺序
final boolean accessOrder;

// 双向链表的头，用作AccessOrder时也是最老的元素
transient LinkedHashMap.Entry<K,V> head;

// 双向链表的尾，用作AccessOrder时也是最新的元素
transient LinkedHashMap.Entry<K,V> tail;

// true则为访问顺序，false则为插入顺序
final boolean accessOrder;

// HashMap中的实现
Node<K, V> newNode(int hash, K key, V value, Node<K, V> next) {
    return new Node<>(hash, key, value, next);
}

// LinkedHashMap中的实现
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;
}

// HashMap中的实现
Node<K, V> replacementNode(Node<K, V> p, Node<K, V> next) {
    return new Node<>(p.hash, p.key, p.value, next);
}

// LinkedHashMap中的实现
Node<K,V> replacementNode(Node<K,V> p, Node<K,V> next) {
    LinkedHashMap.Entry<K,V> q = (LinkedHashMap.Entry<K,V>)p;
    LinkedHashMap.Entry<K,V> t =
        new LinkedHashMap.Entry<K,V>(q.hash, q.key, q.value, next);
    transferLinks(q, t);
    return t;
}

// newTreeNode和replacementTreeNode和此类似

// 就是将元素挂在链尾
private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
    LinkedHashMap.Entry<K,V> last = tail;
    tail = p;
    if (last == null)
        head = p;
    else {
        p.before = last;
        last.after = p;
    }
}

// 用dst替换src
private void transferLinks(LinkedHashMap.Entry<K,V> src,
                            LinkedHashMap.Entry<K,V> dst) {  
    LinkedHashMap.Entry<K,V> b = dst.before = src.before;
    LinkedHashMap.Entry<K,V> a = dst.after = src.after;
    if (b == null)
        head = dst;
    else
        b.after = dst;
    if (a == null)
        tail = dst;
    else
        a.before = dst;
}

// evict在HashMap中说过，为false表示是创建阶段
void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry<K,V> first;
    // 不是创建阶段
    if (evict && (first = head) != null && removeEldestEntry(first)) {
        K key = first.key;
        // 自动删除最老的元素，也就是head元素
        removeNode(hash(key), key, null, false, true);
    }
}

protected boolean removeEldestEntry(Map.Entry<K,V> eldest) {
    return false;
}

public V get(Object key) {
    Node<K,V> e;
    if ((e = getNode(hash(key), key)) == null)
        return null;
    if (accessOrder)
        // 数据被访问，需要将其移动到末尾
        afterNodeAccess(e);
    return e.value;
}

void afterNodeAccess(Node<K,V> e) { // move node to last
    LinkedHashMap.Entry<K,V> last;
    // e元素不在末尾
    if (accessOrder && (last = tail) != e) {
        // p是e，b是前一个元素，a是后一个元素
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        // e要放在末尾，所以没有after
        p.after = null;

        // 把e去掉，把b和a接起来
        if (b == null)
            head = a;
        else
            b.after = a;
        if (a != null)
            a.before = b;
        else
            last = b;

        //把e接在末尾
        if (last == null)
            head = p;
        else {
            p.before = last;
            last.after = p;
        }
        tail = p;
        ++modCount;
    }
}