JDK源码学习笔记——LinkedHashMap

HashMap有一个问题，就是迭代HashMap的顺序并不是HashMap放置的顺序，也就是无序。

LinkedHashMap保证了元素迭代的顺序。该迭代顺序可以是插入顺序或者是访问顺序。通过维护一个双向链表实现。

需要在理解HashMap实现原理的基础上学习LinkedHashMap，JDK源码学习笔记——HashMap

一、数据结构

实际上就是在HashMap的基础上加了LinkedList

（图片来自Java集合之LinkedHashMap）

LinkedHashMap.Entry继承了HashMap.Node，并扩展了before ，after属性

    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);
        }
    }

二、类

继承了HashMap

public class LinkedHashMap<K,V> extends HashMap<K,V> implements Map<K,V>

三、属性

    transient LinkedHashMap.Entry<K,V> head;
    transient LinkedHashMap.Entry<K,V> tail;
    final boolean accessOrder;//true表示按照访问顺序迭代(最近访问在后)，false时表示按照插入顺序(先插入在前，默认)

四、主要方法

put

    /**
     * 直接使用HashMap的put方法
     * 重写了newNode()方法：维护双向链表
     * 重写了afterNodeAccess(e)方法：如果按访问顺序排序，把node移动到双链表的尾端
     */
    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;
        }
    }

    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;
        }
    }

get

    /**
     * 重写了get方法
     * 如果按访问顺序排序，把node移动到双链表的尾端
     */
    public V get(Object key) {
        Node<K,V> e;
        if ((e = getNode(hash(key), key)) == null)
            return null;
        if (accessOrder)
            afterNodeAccess(e);// 如果按访问顺序排序，把node移动到双链表的尾端
        return e.value;
    }

remove

    /**
     * 直接使用HashMap的remove方法
     * 重写了afterNodeRemoval()方法：维护双端链表
     */
    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;
    }

遍历

    /**
     * 主要看nextNode()方法
     * 直接遍历的双向链表
     */
    final class LinkedKeyIterator extends LinkedHashIterator implements Iterator<K> {
        public final K next() {
            return nextNode().getKey();
        }
    }

    final class LinkedValueIterator extends LinkedHashIterator implements Iterator<V> {
        public final V next() {
            return nextNode().value;
        }
    }

    final class LinkedEntryIterator extends LinkedHashIterator implements Iterator<Map.Entry<K, V>> {
        public final Map.Entry<K, V> next() {
            return nextNode();
        }
    }

    abstract class LinkedHashIterator {
        LinkedHashMap.Entry<K,V> next;
        LinkedHashMap.Entry<K,V> current;
        int expectedModCount;

        LinkedHashIterator() {
            next = head;
            expectedModCount = modCount;
            current = null;
        }

        public final boolean hasNext() {
            return next != null;
        }

        final LinkedHashMap.Entry<K,V> nextNode() {
            LinkedHashMap.Entry<K,V> e = next;
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            current = e;
            next = e.after;
            return e;
        }

        public final void remove() {
            Node<K,V> p = current;
            if (p == null)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            current = null;
            K key = p.key;
            removeNode(hash(key), key, null, false, false);
            expectedModCount = modCount;
        }
    }