LinkedHashMap

LinkedHashMap

JDK1.8

之后的版本可能有些不同

//继承HashMap，增加了前后指针
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);
        }
    }

//域
transient LinkedHashMap.Entry<K,V> head;
transient LinkedHashMap.Entry<K,V> tail;
//true是访问顺序，false是插入顺序
final boolean accessOrder;

//初始化散列表和双向链表
void reinitialize() {
        super.reinitialize();
        head = tail = null;
    }

//在构建新节点时，构建的是LinkedHashMap.Entry 不再是Node
//印证底层为散列表和双向链表
Node<K,V> newNode(int hash, K key, V value, Node<K,V> e) {
    //创建一个普通的entry，将entry插入到双向链表的末尾，最后返回entry
        LinkedHashMap.Entry<K,V> p =
            new LinkedHashMap.Entry<>(hash, key, value, e);
        linkNodeLast(p);
        return p;
    }

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;
        }
    }

put

//继承HashMap方法
public V put(K key, V value) {
        return putVal(hash(key), key, value, false, true);
    }

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)
        //在创建节点的时候，调用的是LinkedHashMap的
            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);
                return oldValue;
            }
        }
        ++modCount;
        if (++size > threshold)
            resize();
        afterNodeInsertion(evict);
        return null;
    }

get

//get方法相比HashMap仅仅多了一个判断访问顺序
public V get(Object key) {
        Node<K,V> e;
    //调用HashMap的方法
        if ((e = getNode(hash(key), key)) == null)
            return null;
    //如果是访问顺序，把节点放到链表最后面
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }

//判断访问顺序
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;
        }
    }

remove

调用父类的remove()方法

在LinkedHashMap重写了afterNodeRemoval(Node<K,V> e)，remove()中调用了它

void afterNodeRemoval(Node<K,V> e) { // unlink
        LinkedHashMap.Entry<K,V> p =
            (LinkedHashMap.Entry<K,V>)e, b = p.before, a = p.after;
        p.before = p.after = null;
        if (b == null)
            head = a;
        else
            b.after = a;
        if (a == null)
            tail = b;
        else
            a.before = b;
    }

根据表的结构：可以得出以下结论

初始容量对遍历没有影响，因为它遍历的是LinkedHashMap内部维护的一个双向链表，而不是散列表（都来源于散列表）

1. 最常用的元素放在最后边
2. 可以通过重写方法removeEldestEntry(Map.Entry<K,V> eldest)来删除老元素
3. 可以通过方法afterNodeInsertion(boolean evict)来判断新增元素时是否需要删除老元素