Map.LinkedHashMap

#LinkedHashMap

• LinkedHashMap繼承HashMap，put調用HashMap的put，區別在於HashMap單向鏈表，LinkedHashMap雙向鏈表

//hashMap
static class Node<K,V> implements Map.Entry<K,V> {
        final int hash;
        final K key;
        V value;
        Node<K,V> next;
        ......
    }
    //LinkedHashMap 包裝hashMap的Node，多了順序引用
   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);
        }
    }

• add
• hashMap.add - > LinkedHashMap.newNode，構造LinkHashMap.Node雙向鏈表節點包裝HashMap.Node

//持有頭節點
transient LinkedHashMap.Entry<K,V> head;
//尾節點
transient LinkedHashMap.Entry<K,V> tail;
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)
                    //調用子類LinkedHashMap 的newNode
            tab[i] = newNode(hash, key, value, null);
        else {
            
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;
    }
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;
        }
    }

//正常插入，遍歷有序
public V get(Object key) {
        Node<K,V> e;
        //HashMap.getNode
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);
        return e.value;
    }
    //移動節點到到尾，改變鏈表結構，耗性能
    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;
        }
    }