1、HashMap介绍
HashMap为Map接口的一个实现类，实现了Map所有的操作。
HashMap除了允许key、value为null值和非线程安全外，其他实现几乎和HashTable一致。
HashMap使用散列存储的方式保存kay-value键值对，因此其不支持数据保存的顺序。如果想要使用有序容器可以使用LinkedHashMap。
在性能上当HashMap中保存的key的哈希算法能够均匀的分布在每个bucket中的时候，HashMap在基本的get和set操作的的时间复杂度都是O(n)。
在遍历HashMap的时候，其遍历节点的个数为bucket（位桶）的个数+HashMap中保存的节点个数。因此当遍历操作比较频繁的时候需要注意HashMap的初始化容量不应该太大。 这一点其实比较好理解：当保存的节点个数一致的时候，bucket越少，遍历次数越少。
另外HashMap在resize的时候会有很大的性能消耗，因此当需要在HashMap中保存大量数据的时候，传入适当的默认容量以避免resize，可以很大的提高性能。 具体的resize操作请参考下面对此方法的分析。
HashMap采用位桶+链表+红黑树（自平衡二叉查找树）实现，当链表长度超过阈值（8）时，将链表转换为红黑树，这样大大减少了查找时间.
2、默认设置
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; //默认初始化容量 static final int MAXIMUM_CAPACITY = 1 << 30; //最大容量 static final float DEFAULT_LOAD_FACTOR = 0.75f; //默认加载因子 static final int TREEIFY_THRESHOLD = 8; //当put一个元素到某个位桶，如果其链表长度达到8时将链表转换为红黑树 static final int UNTREEIFY_THRESHOLD = 6; // static final int MIN_TREEIFY_CAPACITY = 64; // transient Node<K,V>[] table; //存储元素数组 transient Set<Map.Entry<K,V>> entrySet; // transient int size; //元素个数 transient int modCount; // int threshold; //扩容阈值 final float loadFactor; //加载因子
3、构造方法
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() { //无参数构造方法；默认初始容量16；加载因子0.75f；
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted//loadFactor = 0.75f：加载因子。其他成员默认。
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) { //初始容量initialCapacity；默认加载因子0.75f
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
public HashMap(int initialCapacity, float loadFactor) { //初始容量initialCapacity；初始加载因子：loadFactor
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
/**
* Returns a power of two size for the given target capacity. * 计算初始化容量，
*/
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1; //n右移1；按位或赋值给n
n |= n >>> 2; //n右移2；按位或赋值给n
n |= n >>> 4; //n右移4；按位或赋值给n
n |= n >>> 8; //n右移8；按位或赋值给n
n |= n >>> 16; //n右移16；按位或赋值给n
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
4、数据存与取
存数据：
/**
* Implements Map.put and related methods.
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don‘t change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
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) //判断添加第一个元素
n = (tab = resize()).length; //添加一地个元素
if ((p = tab[i = (n - 1) & hash]) == null) //判断table的在（n-1）&hash索引值是否空，如果空创建一个节点插入次位置
tab[i] = newNode(hash, key, value, null); //创建新的节点
else { //发生冲突时处理
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k)))) //判断第一个Node是否是要找的值
e = p;
else if (p instanceof TreeNode) //红黑树处理
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value); //
else {
for (int binCount = 0; ; ++binCount) { //累积hash冲突数量
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null); //
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash); //treeifyBin首先判断HashMap长度，如果小于64进行resize扩容，如果大于64存储结构转换为红黑树
break;
}
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k)))) //如果key已经存在，停止遍历
break;
p = e;
}
}
if (e != null) { // existing mapping for key //查到key已存在，更新对应value值
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value; //节点赋值新值
afterNodeAccess(e);
return oldValue; //返回旧值
}
}
++modCount;
if (++size > threshold) //判断当前元素容量是否大于阈值，如果是需要进行扩容
resize(); //扩容
afterNodeInsertion(evict);
return null;
}
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
*/
final Node<K,V>[] resize() { //扩容
Node<K,V>[] oldTab = table;
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) { //容量大于等于最大容量
threshold = Integer.MAX_VALUE; //阈值等于int最大值
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY && oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold //容量扩容一倍
}
else if (oldThr > 0) // initial capacity was placed in threshold //使用阈值初始化容量
newCap = oldThr;
else { // zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY; //初始化默认容量：16
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY); //初始化默认阈值：加载因子（0.75f） * 初始化容量（16）
}
if (newThr == 0) { //如果阈值=0，初始化大小
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ? (int)ft : Integer.MAX_VALUE);
}
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap]; //创建新的节点数组，数组长度为扩容后的长度
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) { //根据旧数组长度，进行遍历
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e; //将就数组元素赋值给新数组
else if (e instanceof TreeNode)
((TreeNode<K,V>)e).split(this, newTab, j, oldCap);
else { // preserve order //
Node<K,V> loHead = null, loTail = null;
Node<K,V> hiHead = null, hiTail = null;
Node<K,V> next;
do { //遍历链表
next = e.next;
if ((e.hash & oldCap) == 0) {
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else {
if (hiTail == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
}
} while ((e = next) != null);
if (loTail != null) { //
loTail.next = null;
newTab[j] = loHead;
}
if (hiTail != null) { //
hiTail.next = null;
newTab[j + oldCap] = hiHead;
}
}
}
}
}
return newTab;
}
/**
* Basic hash bin node, used for most entries. (See below for
* TreeNode subclass, and in LinkedHashMap for its Entry subclass.)
*/
static class Node<K,V> implements Map.Entry<K,V> { //链表数据结构
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;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
//判断两个Node节点是否相同
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
/**
* Entry for Tree bins. Extends LinkedHashMap.Entry (which in turn
* extends Node) so can be used as extension of either regular or
* linked node.
*/
static final class TreeNode<K,V> extends LinkedHashMap.Entry<K,V> {
TreeNode<K,V> parent; //父节点
TreeNode<K,V> left; //左子树
TreeNode<K,V> right; //右子树
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red; //颜色属性
TreeNode(int hash, K key, V val, Node<K,V> next) {
super(hash, key, val, next);
}
/**
* Returns root of tree containing this node. * 返回根节点
*/
final TreeNode<K,V> root() {
for (TreeNode<K,V> r = this, p;;) {
if ((p = r.parent) == null)
return r;
r = p;
}
}
。。。省略。。。
}
取数据：
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {
if (first.hash == hash && // always check first node
((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
if (first instanceof TreeNode) return ((TreeNode<K,V>)first).getTreeNode(hash, key); //红黑树查询结果
do { //遍历链表查询结果
if (e.hash == hash && ((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}
5、demo程序，查看数据结构（散列数组+链表）
package com.javabasic.map;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;
/**
* 源码分析
*
*
* 允许使用null键和null值；
* 线程不同步，效率高；
*
*
* @author wangymd
*
*/
public class HashMapTest2 {
public static void main(String[] args) {
Stu stu1 = new Stu(1,"xiaojia");
Stu stu2 = new Stu(1,"xiaoyi");
Stu stu3 = new Stu(1,"xiaobing");
Stu stu4 = new Stu(1,"xiaoding");
Map<Stu, String> hm = new HashMap<Stu, String>();
hm.put(stu1, "xiaojia");
hm.put(stu2, "xiaoyi");
hm.put(stu3, "xiaobing");
hm.put(stu4, "xiaoding");
Set<Entry<Stu, String>> entrySet = hm.entrySet(); //断点，查看数据结构；如下图
for (Entry<Stu, String> entry : entrySet) {
Stu key = entry.getKey();
String value = entry.getValue();
System.out.println(key.getName() + "----" + value);
}
}
}
class Stu{
Integer id;
String name;
public Stu() {}
public Stu(Integer id, String name) {
this.id = id;
this.name = name;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
原文地址：https://www.cnblogs.com/wangymd/p/11750194.html
时间： 2024-10-08 16:48:24