LinkedHashMap源码

TreeMap是一颗红黑树做Map。HashMap是数组+链表+红黑树。HashTable是数组+链表。

LinkedHashMap底层存储结构与HashMap一样，不同的是LinkedHashMap增加了一个双向链表的头节点，插入的数据除了插入HashMap，还会插入链表中，因而可以保存插入节点的顺序

LinkedHashMap的节点在HashMap节点的基础上增加了前后节点的引用

LinkedHashMap相比HashMap在查找值和删除值时效率要高

LinkedHashMap还可以设置按插入顺序排序或是按访问顺序排序，默认是按插入顺序排序

LinkedHashMap没有put方法，而是覆写了afterNodeAccess方法和afterNodeInsertion方法。当插入的数据已经存在时，会调用afterNodeAccess方法看是否需要将数据插入到链表末尾；当插入的数据是新数据时，会通过afterNodeInsertion方法来根据设置删除近期使用最少的节点

LinkedHashMap可以用来实现LRU算法。首先需要用可以设置accessOrder的构造函数设置accessOrder为true，也就是按照节点访问顺序排序；

而LinkedHashMap还拥有HashMap一样的结构，原因在于其继承自HashMap，因此HashMap中用于存储元素的table也被继承过来，于是乎元素的存储也具备和HashMap一样的规则，所有元素存储在table中，table下的每个索引在原HashMap的结构下也是一个单向链表，同时每个元素中额外增加的before和after引用将table中的元素互连起来。

HashMap链表节点：next，

HashMap红黑树节点TreeNode：parent，left，right，prev，next， 

LinkedHashMap链表节点：before，after，next， 

LinkedHashMap跟HashMap一样，只不过增加了head，tail把所有元素串联起来了。

public class testLinkedHashMap1 {
    @SuppressWarnings({ "rawtypes", "unchecked" })
    public static void main(String[] args) {
        LinkedHashMap1<Integer,String> hh = new LinkedHashMap1<Integer,String>();
        
        //链表添加
        hh.put(0, "0");//放到LinkedHashMap1的table[0]中，此时是链表（节点有next没有after，before）
        hh.put(16, "16");
//        hh.remove(4);//链表删除
//        hh.put(4, "4");
        hh.put(32, "32");
        hh.put(48, "48");//0位置转成红黑树
        hh.put(54, "54");
        hh.put(60, "60");
        hh.put(76, "76");
        hh.put(1, "1");//1位置链表添加
        hh.put(2, "2");
        hh.put(3, "3");
        
        hh.get(1);
        
        Set s = hh.keySet();//[0, 16, 32, 48, 54, 60, 76, 1, 2, 3]
        Iterator i = s.iterator();
        if(i.hasNext()) System.out.println(i.next());
//        i.remove();
        if(i.hasNext()) System.out.println(i.next());
        
    }
}

//extends HashMap1,table也被继承过来，数组+链表+红黑树。
public class LinkedHashMap1<K, V> extends HashMap1<K, V> implements Map1<K, V> {
    private static final long serialVersionUID = 3801124242820219131L;
    transient LinkedHashMap1.Entry<K, V> head;// 双向链表的头节点和尾节点
    transient LinkedHashMap1.Entry<K, V> tail;
    final boolean accessOrder;// true，使用节点之后就把节点放到最末尾。false就不放。用于删除第一个最不常用的元素。

    private void linkNodeLast(LinkedHashMap1.Entry<K, V> p) {// 链表末尾添加,有before，after
        LinkedHashMap1.Entry<K, V> last = tail;//保留原来的tail
        tail = p;//修改tail
        if (last == null)// tail=null
            head = p;// tail = head = p
        else {
            p.before = last;
            last.after = p;
        }
    }

    // dst替换src
    private void transferLinks(LinkedHashMap1.Entry<K, V> src, LinkedHashMap1.Entry<K, V> dst) {
        LinkedHashMap1.Entry<K, V> b = dst.before = src.before;
        LinkedHashMap1.Entry<K, V> a = dst.after = src.after;
        if (b == null)// 替换的src是第一个节点
            head = dst;
        else
            b.after = dst;
        if (a == null)// 替换的src是最后一个节点
            tail = dst;
        else
            a.before = dst;
    }

    public void reinitialize() {
        super.reinitialize();// table = null; entrySet = null; keySet = null; values = null;
        head = tail = null;
    }
    //生成一个节点（链表中的节点，不是红黑树中的节点，有hash, key, value, next没有before，after）
    public Node<K, V> newNode(int hash, K key, V value, Node<K, V> e) {// 
        LinkedHashMap1.Entry<K, V> p = new LinkedHashMap1.Entry<K, V>(hash, key, value, e);
        linkNodeLast(p);// 生成新的节点并插入到尾部
        return p;
    }

    public Node<K, V> replacementNode(Node<K, V> p, Node<K, V> next) {
        LinkedHashMap1.Entry<K, V> q = (LinkedHashMap1.Entry<K, V>) p;
        LinkedHashMap1.Entry<K, V> t = new LinkedHashMap1.Entry<K, V>(q.hash, q.key, q.value, next);
        transferLinks(q, t);// t替换q
        return t;
    }

    public TreeNode<K, V> newTreeNode(int hash, K key, V value, Node<K, V> next) {
        TreeNode<K, V> p = new TreeNode<K, V>(hash, key, value, next);
        linkNodeLast(p);// 末尾添加p
        return p;
    }

    public TreeNode<K, V> replacementTreeNode(Node<K, V> p, Node<K, V> next) {
        LinkedHashMap1.Entry<K, V> q = (LinkedHashMap1.Entry<K, V>) p;
        TreeNode<K, V> t = new TreeNode<K, V>(q.hash, q.key, q.value, next);
        transferLinks(q, t);// t替换q
        return t;
    }

    public void afterNodeRemoval(Node<K, V> e) { // 删除节点e
        LinkedHashMap1.Entry<K, V> p = (LinkedHashMap1.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;
    }

    public void afterNodeInsertion(boolean evict) { // 添加新的节点时候，有可能删除最老的元素
        LinkedHashMap1.Entry<K, V> first;
        if (evict && (first = head) != null && removeEldestEntry(first)) {
            K key = first.key;// 删除第一个元素，每次使用元素之后，就会把元素放到最末尾，第一个元素就是最少使用的元素。
            removeNode(hash(key), key, null, false, true);// hashMap方法，
        }
    }

    public void afterNodeAccess(Node<K, V> e) { // 节点e被修改后，e不是尾节点（e是尾节点就不动），把e节点移到最后。
        LinkedHashMap1.Entry<K, V> last;
        if (accessOrder && (last = tail) != e) {
            LinkedHashMap1.Entry<K, V> p = (LinkedHashMap1.Entry<K, V>) e, b = p.before, a = p.after;
            p.after = null;// 多个属性指向同一地址值，只是修改其中一个的地址值，其余不变。

            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;
        }
    }

    public void internalWriteEntries(java.io.ObjectOutputStream s) throws IOException {
        for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after) {
            s.writeObject(e.key);
            s.writeObject(e.value);
        }
    }

    public LinkedHashMap1(int initialCapacity, float loadFactor) {
        super(initialCapacity, loadFactor);// hashMap方法，构造hashMap的table，一个数组+链表+红黑树
        accessOrder = false;// true，使用节点之后就把节点放到最末尾。false就不放。
    }

    public LinkedHashMap1(int initialCapacity) {
        super(initialCapacity);// hashMap方法，构造hashMap的table，一个数组+链表+红黑树
        accessOrder = false;
    }

    public LinkedHashMap1() {
        super();// hashMap方法，构造hashMap的table，一个数组+链表+红黑树
        accessOrder = false;
    }

    public LinkedHashMap1(Map1<? extends K, ? extends V> m) {
        super();// hashMap方法，构造hashMap的table，一个数组+链表+红黑树
        accessOrder = false;
        putMapEntries(m, false);
    }

    public LinkedHashMap1(int initialCapacity, float loadFactor, boolean accessOrder) {
        super(initialCapacity, loadFactor);// hashMap方法，构造hashMap的table，一个数组+链表+红黑树
        this.accessOrder = accessOrder;
    }

    public boolean containsValue(Object value) {
        for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after) {
            V v = e.value;
            if (v == value || (value != null && value.equals(v)))
                return true;
        }
        return false;
    }

    public V get(Object key) {
        Node<K, V> e;
        if ((e = getNode(hash(key), key)) == null)// hashMap方法
            return null;
        if (accessOrder)
            afterNodeAccess(e);// 把e节点移到最后。
        return e.value;
    }

    public V getOrDefault(Object key, V defaultValue) {
        Node<K, V> e;
        if ((e = getNode(hash(key), key)) == null)// hashMap方法
            return defaultValue;
        if (accessOrder)// 把e节点移到最后。
            afterNodeAccess(e);
        return e.value;
    }

    public void clear() {
        super.clear();
        head = tail = null;
    }

    // 添加一个元素时候是否删除最老的。 map是缓存有用。比如到达100之后，添加一个元素就删除最老的元素。要重写。
    protected boolean removeEldestEntry(Map1.Entry<K, V> eldest) {
        return false;
    }

    public Set<K> keySet() {
        Set<K> ks = keySet;
        if (ks == null) {
            ks = new LinkedKeySet();
            keySet = ks;
        }
        return ks;
    }

    final class LinkedKeySet extends AbstractSet<K> {
        public final int size() {
            return size;
        }

        public final void clear() {
            LinkedHashMap1.this.clear();
        }

        public final Iterator<K> iterator() {
            return new LinkedKeyIterator();
        }

        public final boolean contains(Object o) {
            return containsKey(o);
        }

        public final boolean remove(Object key) {
            return removeNode(hash(key), key, null, false, true) != null;// hashMap方法
        }

        public final Spliterator<K> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED | Spliterator.DISTINCT);
        }

        public final void forEach(Consumer<? super K> action) {
            if (action == null)
                throw new NullPointerException();
            int mc = modCount;
            for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after)
                action.accept(e.key);
            if (modCount != mc)
                throw new ConcurrentModificationException();
        }
    }

    public Collection<V> values() {
        Collection<V> vs = values;
        if (vs == null) {
            vs = new LinkedValues();
            values = vs;
        }
        return vs;
    }

    final class LinkedValues extends AbstractCollection<V> {
        public final int size() {
            return size;
        }

        public final void clear() {
            LinkedHashMap1.this.clear();
        }

        public final Iterator<V> iterator() {
            return new LinkedValueIterator();
        }

        public final boolean contains(Object o) {
            return containsValue(o);
        }

        public final Spliterator<V> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED);
        }

        public final void forEach(Consumer<? super V> action) {
            if (action == null)
                throw new NullPointerException();
            int mc = modCount;
            for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after)
                action.accept(e.value);
            if (modCount != mc)
                throw new ConcurrentModificationException();
        }
    }

    public Set<Map1.Entry<K, V>> entrySet() {
        Set<Map1.Entry<K, V>> es;
        return (es = entrySet) == null ? (entrySet = new LinkedEntrySet()) : es;
    }

    final class LinkedEntrySet extends AbstractSet<Map1.Entry<K, V>> {
        public final int size() {
            return size;
        }

        public final void clear() {
            LinkedHashMap1.this.clear();
        }

        public final Iterator<Map1.Entry<K, V>> iterator() {
            return new LinkedEntryIterator();
        }

        public final boolean contains(Object o) {
            if (!(o instanceof Map1.Entry))
                return false;
            Map1.Entry<?, ?> e = (Map1.Entry<?, ?>) o;
            Object key = e.getKey();
            Node<K, V> candidate = getNode(hash(key), key);// hashMap方法
            return candidate != null && candidate.equals(e);
        }

        public final boolean remove(Object o) {
            if (o instanceof Map1.Entry) {
                Map1.Entry<?, ?> e = (Map1.Entry<?, ?>) o;
                Object key = e.getKey();
                Object value = e.getValue();
                return removeNode(hash(key), key, value, true, true) != null;// hashMap方法
            }
            return false;
        }

        public final Spliterator<Map1.Entry<K, V>> spliterator() {
            return Spliterators.spliterator(this, Spliterator.SIZED | Spliterator.ORDERED | Spliterator.DISTINCT);
        }

        public final void forEach(Consumer<? super Map1.Entry<K, V>> action) {
            if (action == null)
                throw new NullPointerException();
            int mc = modCount;
            for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after)
                action.accept(e);
            if (modCount != mc)
                throw new ConcurrentModificationException();
        }
    }

    public void forEach(BiConsumer<? super K, ? super V> action) {
        if (action == null)
            throw new NullPointerException();
        int mc = modCount;
        for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after)
            action.accept(e.key, e.value);
        if (modCount != mc)
            throw new ConcurrentModificationException();
    }

    public void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
        if (function == null)
            throw new NullPointerException();
        int mc = modCount;
        for (LinkedHashMap1.Entry<K, V> e = head; e != null; e = e.after)
            e.value = function.apply(e.key, e.value);
        if (modCount != mc)
            throw new ConcurrentModificationException();
    }

    public static class Entry<K, V> extends HashMap1.Node<K, V> {
        public Entry<K, V> before, after;// hash, key, value, next

        public Entry(int hash, K key, V value, Node<K, V> next) {
            super(hash, key, value, next);// before和after引用节点以支持双向链表
        }
    }

    abstract class LinkedHashIterator {//头结点head开始一路after到末尾
        LinkedHashMap1.Entry<K, V> next;
        LinkedHashMap1.Entry<K, V> current;
        int expectedModCount;

        LinkedHashIterator() {
            next = head;// 初始化时下一个是head
            expectedModCount = modCount;
            current = null;
        }

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

        final LinkedHashMap1.Entry<K, V> nextNode() {
            LinkedHashMap1.Entry<K, V> e = next;
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            if (e == null)
                throw new NoSuchElementException();
            current = e;
            next = e.after;// before和after遍历链表
            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);// hashMap方法
            expectedModCount = modCount;
        }
    }

    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<Map1.Entry<K, V>> {
        public final Map1.Entry<K, V> next() {
            return nextNode();
        }
    }

}