JDK 1.8 HashMap 数据结构优化

JDK 1.7 中的 HashMap 数据结构是数组 + 链表:

JDK 1.8 中的 HashMap 数据结构是数组大专栏 JDK 1.8 HashMap 数据结构优化 + 链表或红黑树

在 JDK 1.7 中之所以存在链表结构是因为当哈希碰撞时，不同的元素需要存放在相同的数组位置上，故数组同一个位置需要可以保存不同的元素，JDK 1.7 中选择的是链表，当哈希碰撞剧烈，导致链表较长时，影响了 HashMap 的查找性能。

故 JDK 1.8 中 HashMap 中 Entry 是一个接口，存在两种实现类：

static class <K,V> implements Map.Entry<K,V> {
    final int hash;
    final K key;
    V value;
    Node<K,V> next;

    Node(int hash, K key, V value, Node<K,V> next) {
        this.hash = hash;
        this.key = key;
        this.value = value;
        this.next = next;
    }
}

static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
    TreeNode<K,V> parent;  
    TreeNode<K,V> left;
    TreeNode<K,V> right;
    TreeNode<K,V> prev;    // needed to unlink next upon deletion
    boolean red;
    TreeNode(int hash, K key, V val, Node<K,V> next) {
        super(hash, key, val, next);
    }
}

TreeNode 是新增的红黑树节点，有父亲，左右孩子，前一个元素的节点，还有颜色值

因为新型红黑树节点的出现，HashMap 中多了三个关键属性：

/**
 * The bin count threshold for using a tree rather than list for a
 * bin.  Bins are converted to trees when adding an element to a
 * bin with at least this many nodes. The value must be greater
 * than 2 and should be at least 8 to mesh with assumptions in
 * tree removal about conversion back to plain bins upon
 * shrinkage.
 */
static final int TREEIFY_THRESHOLD = 8;

/**
 * The bin count threshold for untreeifying a (split) bin during a
 * resize operation. Should be less than TREEIFY_THRESHOLD, and at
 * most 6 to mesh with shrinkage detection under removal.
 */
static final int UNTREEIFY_THRESHOLD = 6;

/**
 * The smallest table capacity for which bins may be treeified.
 * (Otherwise the table is resized if too many nodes in a bin.)
 * Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
 * between resizing and treeification thresholds.
 */
static final int MIN_TREEIFY_CAPACITY = 64;

TREEIFY_THRESHOLD : 链表节点转红黑树节点的阈值，当节点数量大于8个时，链表节点转化为红黑树节点
UNTREEIFY_THRESHOLD : 红黑树节点还原链表节点的阈值，当扩容时，元素数量小于这个值，红黑树节点转化为链表节点
MIN_TREEIFY_CAPACITY : 哈希表最小树形化容量

JDK 1.8 HashMap 数据结构优化

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