文章目录 HashMap HashTable HashMap与HashTable的区别 HashMap 线程不安全 底层实现：数组+链表或者红黑树 Node节点的定义(链表) 初始容量为16的原因 put方法 //底层实现
文章目录
- HashMap
- HashTable
- HashMap与HashTable的区别
HashMap
//保存的数组，初始化16个
transient Node<K,V>[] table;
//为entrySet和value提供一个缓存
transient Set<Map.Entry<K,V>> entrySet;
//元素的数量
transient int size;
//初始容量
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
//数组递增的策略 当size > capacity*loadFacotor的时候递增
final float loadFactor;
//Node节点的定义(链表)
static class Node<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;
}
//初始容量为16的原因
//hash算法，保证哈希值平均分布，只有当为16的时候才可以最大程度的保证平均分布
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
//put方法
//创建一个HashMap对象，并且设定它的加载因子为0.75（超过则扩容，）
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//执行put方法
public V put(K key, V value) {
//key通过hash算法计算一个index
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;
//第一次进入为null,所以执行初始化容器大小
if ((tab = table) == null || (n = tab.length) == 0)
//此时返回的就是初始化容器以后的大小即16
n = (tab = resize()).length;
//计算下标，如果等于null，直接赋值
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//如果该数组刚好有值，则采用链表或者红黑树的方式添加数据节点
Node<K,V> e; K k;
//判断两个节点是否相等
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//判断当前节点是否属于红黑树节点
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//如果不是直接进行链表连接
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
//将当前节点的下一个节点设置为新的实体节点
p.next = newNode(hash, key, value, null);
//如果此时的节点容量为7那么将链表转换为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
//判断新传入的实体和当前绑定节点的子节点是否相同，如果相同直接退出
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
//进入这个子节点
p = e;
}
}
if (e != null) { // existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
//修改次数
++modCount;
//查看当前容器的容量是否大于threshold ,如果大于增加数组容量为原来的一倍
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
//初始化容器大小
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
//旧容量为0
int oldCap = (oldTab == null) ? 0 : oldTab.length;
int oldThr = threshold;
//设置当前容器的递增为0
int newCap, newThr = 0;
//此时的oldCap=0 , newThr = 0 直接else执行
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//扩容，将数组的容量和扩容因子变为原来的一倍
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else { // zero initial threshold signifies using defaults
//初始化容器为默认16
newCap = DEFAULT_INITIAL_CAPACITY;
//初始化阙值
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == 0) {
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];
//初始化存储容器数组
table = newTab;
}
HashTable
private transient Entry<?,?>[] table;//存储数组
private transient int count;//容器中数据多少
private int threshold;//容器容量达到次数以后进行修改
private transient int modCount = 0;//修改次数
//Hash函数
int hash = key.hashCode(); //在这两行
int index = (hash & 0x7FFFFFFF) % tab.length; //在这两行
//初始化。在构造方法中初始化。初始化指为11
public Hashtable(int initialCapacity, float loadFactor) {
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: "+
initialCapacity);
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal Load: "+loadFactor);
if (initialCapacity==0)
initialCapacity = 1;
this.loadFactor = loadFactor;
table = new Entry<?,?>[initialCapacity];
threshold = (int)Math.min(initialCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
}
//put方法
public synchronized V put(K key, V value) {
// Make sure the value is not null
if (value == null) {
throw new NullPointerException();
}
// Makes sure the key is not already in the hashtable.
Entry<?,?> tab[] = table;
//hash函数计算一个index
int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;
@SuppressWarnings("unchecked")
Entry<K,V> entry = (Entry<K,V>)tab[index];
for(; entry != null ; entry = entry.next) {
if ((entry.hash == hash) && entry.key.equals(key)) {
V old = entry.value;
entry.value = value;
return old;
}
}
addEntry(hash, key, value, index);
return null;
}
//增加实体
private void addEntry(int hash, K key, V value, int index) {
Entry<?,?> tab[] = table;
if (count >= threshold) {
// Rehash the table if the threshold is exceeded
rehash();
tab = table;
hash = key.hashCode();
index = (hash & 0x7FFFFFFF) % tab.length;
}
// Creates the new entry.
@SuppressWarnings("unchecked")
Entry<K,V> e = (Entry<K,V>) tab[index];
tab[index] = new Entry<>(hash, key, value, e);
count++;
modCount++;
}
HashMap与HashTable的区别
对比项
HashTable
HashMap
底层时间
哈希表+链表
哈希表+链表+红黑树
初始化时间及大小
构造方法初始化，大小为11
put方法初始化，大小为16
线程安全
安全
不安全
Hash值
直接使用了hashcode
重新计算
扩容
二倍+1
二倍