0x00.新老HashMap区别
本文使用jdk7（1.7.0_79）与 jdk8（1.8.0_45）进行对比，主要学习数据结构区别
数据结构
jdk7内部数据结构为数组+链表，通过key的hash值计算数据所在数组下标，多个key的hash相同或hash计算的数组下标相同，但是key值不同时，往链表尾追加Entry。
transient Entry<K,V>[] table = (Entry<K,V>[]) EMPTY_TABLE;
static class Entry<K,V> implements Map.Entry<K,V> {
final K key;
V value;
Entry<K,V> next;
int hash;
}
jdk8内部数据结构为数组+(链表 或 红黑树)，通过key的hash值计算数据所在数组下标，多个key的hash相同或hash计算的数组下标相同，但是key值不同时，检查节点是否为树节点，是树节点则往树节点添加，如果是普通节点则往链表尾追加Entry，当链表长度大于8时，则将链表转为红黑树。
transient Node<K,V>[] table;
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
V value;
Node<K,V> next;
}
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
TreeNode<K,V> parent; // red-black tree links
TreeNode<K,V> left;
TreeNode<K,V> right;
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red;
}
//LinkedHashMap.Entry
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);
}
}
0x01. HashMap.put源码阅读
源码学习，边看源码边加注释，边debug，边理解。
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
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)
//当HashMap的内部table为空时，触发resize()
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//当通过key的hash值计算数据所在数组下标值为null时直接生成新节点放入桶
tab[i] = newNode(hash, key, value, null);
else {
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
//key完全相同
e = p;
else if (p instanceof TreeNode)
//桶内对象已经时红黑树
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//桶内已经存在普通节点则遍历链表，往链表尾部添加节点
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) { //第1个循环是获取第2个节点了（0->root.next）
//binCount为0时插入的第二个节点
p.next = newNode(hash, key, value, null);
//如果链表内的数据已经超过8个则将链表转成红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
//binCount等于7时插入的第9个节点
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
//检查tab的长度是否大于等于64，小于则扩容
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
//将Node对象转为TreeNode
TreeNode<K,V> hd = null, tl = null;
do {
TreeNode<K,V> p = replacementTreeNode(e, null);
if (tl == null)
hd = p;
else {
p.prev = tl;
tl.next = p;
}
tl = p;
} while ((e = e.next) != null);
if ((tab[index] = hd) != null)
//将TreeNode链表转成红黑树
hd.treeify(tab);
}
}
final void treeify(Node<K,V>[] tab) {
TreeNode<K,V> root = null;
for (TreeNode<K,V> x = this, next; x != null; x = next) {
next = (TreeNode<K,V>)x.next;
x.left = x.right = null;
if (root == null) {
//root为空时直接第一个元素存入root
x.parent = null;
x.red = false;
root = x;
}
else {
K k = x.key;
int h = x.hash;
Class<?> kc = null;
//往二叉树中插入当前节点
for (TreeNode<K,V> p = root;;) {
int dir, ph;
K pk = p.key;
if ((ph = p.hash) > h)//优先直接比较hash值
dir = -1;
else if (ph < h)//优先直接比较hash值
dir = 1;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) || //hash值相同则判断key是否实现Comparable接口
(dir = compareComparables(kc, k, pk)) == 0) //实现了Comparable接口则直接比较
//没有实现Comparable接口则用System.identityHashCode比较
dir = tieBreakOrder(k, pk);
TreeNode<K,V> xp = p;
//检查树节点是否为空，不为空继续比较，直到找到空的节点放入当前节点。
if ((p = (dir <= 0) ? p.left : p.right) == null) {
x.parent = xp;
if (dir <= 0)
xp.left = x;
else
xp.right = x;
//进行二叉树平衡操作
root = balanceInsertion(root, x);
break;
}
}
}
}
//将root接口放入table的第一个元素
moveRootToFront(tab, root);
}
static <K,V> TreeNode<K,V> balanceInsertion(TreeNode<K,V> root,
TreeNode<K,V> x) {
x.red = true;
for (TreeNode<K,V> xp, xpp, xppl, xppr;;) {
//判断当前节点的父节点为空
if ((xp = x.parent) == null) {
//则当前节点为root 直接将颜色设置为黑色
x.red = false;
return x;
}
else if (!xp.red || (xpp = xp.parent) == null)
//当前节点父节点为黑色节点 或者当前节点的父节点时root节点
return root;
if (xp == (xppl = xpp.left)) {//当前节点的父节点是左子节点（父节点为红色节点）
if ((xppr = xpp.right) != null && xppr.red) {
//当前节点的父节点以及父节点的兄弟节点都是红色 则颜色反转
xppr.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {
//当前节点为右节点 则先进行左旋
if (x == xp.right) {
root = rotateLeft(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
//父节点不为空 则变色 右旋
if (xp != null) {
xp.red = false;
if (xpp != null) {
xpp.red = true;
root = rotateRight(root, xpp);
}
}
}
}
else {//当前节点的父节点是右子节点
if (xppl != null && xppl.red) {
//当前节点的父节点以及父节点的兄弟节点都是红色 则颜色反转
xppl.red = false;
xp.red = false;
xpp.red = true;
x = xpp;
}
else {
//节点在左侧则右旋
if (x == xp.left) {
root = rotateRight(root, x = xp);
xpp = (xp = x.parent) == null ? null : xp.parent;
}
//父节点不为空 则变色 左旋
if (xp != null) {
xp.red = false;
if (xpp != null) {
xpp.red = true;
root = rotateLeft(root, xpp);
}
}
}
}
}
}
0x02. 测试代码
触发链表转红黑树的测试代码 可直接用来debug
import java.util.HashMap;
/**
* Created by qiyan on 2017/4/9.
*/
public class HashMapTest {
public static void main(String[] args) throws Exception {
//map内的数组容量大于等于64 且 链表数量大于8才会进行红黑树转换
HashMap map = new HashMap(64);
for (int i = 0; i <= 8; i++) {
map.put(new HashCodeOneObj(i), i);
}
System.out.println(map);
}
private static class HashCodeOneObj implements Comparable<HashCodeOneObj> {
private int val;
public HashCodeOneObj(int val) {
this.val = val;
}
public int getVal() {
return val;
}
@Override
public int hashCode() {
return 1; //让所有数据都存入一个桶
}
@Override
public int compareTo(HashCodeOneObj o) {
if (null == o) {
return -1;
}
return Integer.compare(this.getVal(), o.getVal());
}
}
}
链表转红黑树过程示意图：
红黑树v2(insert-0-8).png
HashMap中的红黑树
