JDK1.8 LinkedHashMap和LinkedHashSet有序分析

基于JDK1.8的源码分析

LinkedHashMap 有序原理分析

LinkedHashMap中的Entry重写了.

static class Entry<K,V> extends HashMap.Node<K,V> {

       //每个Entry增加了before,after,双向链表
       Entry<K,V> before, after;
       Entry(int hash, K key, V value, Node<K,V> next) {
           super(hash, key, value, next);
       }
   }

put方法

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

putVal方法

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)
            //重点是newNode这个方法
            tab[i] = newNode(hash, key, value, null);
            // 后续省略...
}

newNode方法

LinkedHashMap重写了该方法

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);
        //把新建的节点放入Entry链表尾部
        linkNodeLast(p);
        return p;
    }

linkNodeLast方法

//这里会把Entry按照insert顺序插入到链表尾部，所以实现了访问的有序性
  // link at the end of list
  private void linkNodeLast(LinkedHashMap.Entry<K,V> p) {
      LinkedHashMap.Entry<K,V> last = tail;
      tail = p;
      //如果第一次put元素
      if (last == null)
      // head = tail;
          head = p;
      else {
          // 新插入的节点的前驱节点指向之前的tail
          p.before = last;
          //之前的tail的后继节点指向新节点
          // oldTail->after = p;
          // p->before = oldTail
          // tail-> p
          last.after = p;
      }
  }

按照节点访问顺序来排序

void afterNodeAccess(Node<K,V> e) { // move node to last

    LinkedHashMap.Entry<K,V> last;
    // accessOrder 默认为false，按插入顺序来访问，否则把访问过的元素放在链表后面
    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;
    }
}

//默认 put(k,v) evict(剔除)为false
void afterNodeInsertion(boolean evict) { // possibly remove eldest
    LinkedHashMap.Entry<K,V> first;
    if (evict && (first = head) != null && removeEldestEntry(first)) {
        K key = first.key;
        removeNode(hash(key), key, null, false, true);
    }
}

LinkedHashSet 有序原理分析

首先LinkedHashSet继承自HashSet

查看LinkedHash构造器源码

LinkedHashSet构造器

/**
  * Constructs a new, empty linked hash set with the default initial
  * capacity (16) and load factor (0.75).
  */
 public LinkedHashSet() {
     // 调用父类Haset构造器
     super(16, .75f, true);
 }

HashSet构造器

HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        // map的实现使用了LinkedHashMap
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
}

因为LinkedHashSet中使用的map实现是LinkedHashMap，所以天然有序

具体见上文分析