默认大小=16；
默认加载因子=0.75；
阈值=16*0.75；
JDK7
HashMap底层使用哈希表/散列也就是数组+链表的形式存储数据；
构造方法
//指定数组长度和加载因子
public HashMap(int initialCapacity, float loadFactor) {
//数组长度不可小于0
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
//数组长度不可大于最大容量
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
//加载因子不可小于=0 并且不能非数字
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//赋值
this.loadFactor = loadFactor;
threshold = initialCapacity;
//给其实现类使用不用管
init();
}
添加方法
public V put(K key, V value) {
//判断该数值是否为空，若为空则需初始化
if (table == EMPTY_TABLE) {
//初始化数组
//传入new对象时定义好的阈值
inflateTable(threshold);
}
//如果key为null则插入数据
if (key == null)
//插入key为null的数据
return putForNullKey(value);
//key通过算法返回一个hash值
int hash = hash(key);
//再通过&返回一个下标
int i = indexFor(hash, table.length);
//通过下标获取到一个链表
//再通过链表循环获取每一个值
//获取到值后判断hash值&&key值是否相等
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
//如若相等进行替换则不需要插入
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
//如若不等则插入数据
addEntry(hash, key, value, i);
return null;
}
//添加值
void addEntry(int hash, K key, V value, int bucketIndex) {
//size为添加数据的个数
//若个数大于或等于阈值&&当前下标的数组不为null则进入
if ((size >= threshold) && (null != table[bucketIndex])) {
//判断是否扩容
resize(2 * table.length);
//判断key是否为null,如果是进行设置
hash = (null != key) ? hash(key) : 0;
bucketIndex = indexFor(hash, table.length);
}
//添加
createEntry(hash, key, value, bucketIndex);
}
//添加
void createEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<>(hash, key, value, e);
size++;
}
初始化数组
private void inflateTable(int toSize) {
//返回一个大于或等于该值的2的次方数
int capacity = roundUpToPowerOf2(toSize);
//重新定义阈值
threshold = (int) Math.min(capacity * loadFactor, MAXIMUM_CAPACITY + 1);
//初始化数组长度
table = new Entry[capacity];
//设置hashSeed值
initHashSeedAsNeeded(capacity);
}
插入key为null的数据
private V putForNullKey(V value) {
//如果table有数据，判断是否已经插入过为null的key，若有则替换为值，并返回替换的值
for (Entry<K,V> e = table[0]; e != null; e = e.next) {
if (e.key == null) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
//若table为null则添加值、
//hash值，key值，value值，下标值
addEntry(0, null, value, 0);
return null;
}
返回一个哈希值
final int hash(Object k) {
int h = hashSeed;
if (0 != h && k instanceof String) {
//一种新的获取hash值的算法
return sun.misc.Hashing.stringHash32((String) k);
}
//获取hash值并使用异或运算
h ^= k.hashCode();
//打散确保散列性
//这个函数确保在每个位位置上只相差
//常数倍数的hashCodes有一个有界的
//冲突数(默认负载因数大约为8)
h ^= (h >>> 20) ^ (h >>> 12);
return h ^ (h >>> 7) ^ (h >>> 4);
}
返回一个下标
static int indexFor(int h, int length) {
// 这也是为什么要用2的次方来做增长的原因;
//应为使用2的次方可以保证下标不越界
return h & (length-1);
}
扩容
//初始化扩容
void resize(int newCapacity) {
//将值放入另一个容器中
Entry[] oldTable = table;
//判断数组长度是否达到上限
int oldCapacity = oldTable.length;
//如果达到了上限则不允许扩容了
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
//创建一个新的数组容器
Entry[] newTable = new Entry[newCapacity];
//transfer(数组, 设置hashSeed用于获取哈希码);
transfer(newTable, initHashSeedAsNeeded(newCapacity));
table = newTable;
threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
}
//扩容
void transfer(Entry[] newTable, boolean rehash) {
int newCapacity = newTable.length;
for (Entry<K,V> e : table) {
while(null != e) {
Entry<K,V> next = e.next;
if (rehash) {
e.hash = null == e.key ? 0 : hash(e.key);
}
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
}
}
}
JDK7在扩容的时候如果是多线程同时操作会出现死循环。
当两个线程同时进入扩容方法，并切都同时拥有了局部变量e,和next时。
替换时会出现死锁。
get方法
public V get(Object key) {
if (key == null)
//获取key=null的值，已知key=null的值存放再下标为0的位置，所以该方法查询更快
return getForNullKey();
//获取
Entry<K,V> entry = getEntry(key);
//返回值
return null == entry ? null : entry.getValue();
}
//获取
final Entry<K,V> getEntry(Object key) {
if (size == 0) {
return null;
}
//获取可key的哈希码
int hash = (key == null) ? 0 : hash(key);
//通过哈希码和数组长度获取该key的下标并获取到该值
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
//判断hash,key是否都一致，并返回
Object k;
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
}
return null;
}
JDK8
构造方法
public HashMap(int initialCapacity, float loadFactor) {
//数组 长度不可小于0
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
//数组长度不可大于最大长度
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
//加载因子不可不可小于或等于0，并且不可非数值
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
//赋值
this.loadFactor = loadFactor;
//该方法返回的是一个大于或=该值的2的次方数
this.threshold = tableSizeFor(initialCapacity);
}
put方法
public V put(K key, V value) {
//hash(key)表示：通过key返回一个hash值
return putVal(hash(key), key, value, false, true);
}
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,boolean evict) {
//创建一个Node数组，和一个Node对象，以及一个int类型的变量n=数组长度；
Node<K,V>[] tab; Node<K,V> p; int n, i;
//判断当前对象是否为空
if ((tab = table) == null || (n = tab.length) == 0)
//如果为空初始化数组
//resize()初始数组长度和扩容
n = (tab = resize()).length;
//通过与运算获取到hash值对应的下标下的值是否为null
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
//如果当前下标不为null
Node<K,V> e; K k;
//则判断当前当前对象下的hash，key和存入的hash,key是否一致，
//如果一致将p赋给e
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;
//判断p节点是否为一个树节点
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
//遍历链表
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
//判断当前链表是否超过阈值如果超过则该便存储方式为红黑树
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
//如果e不为null则替换value值
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;
}
//只有初始化和超过阈值时才会调用该方法
final Node<K,V>[] resize() {
//获取当前对象
Node<K,V>[] oldTab = table;
//当前数组长度
int oldCap = (oldTab == null) ? 0 : oldTab.length;
//获取阈值
int oldThr = threshold;
//定义一个新的接收初始化长度，和阈值的变量
int newCap, newThr = 0;
//判断数组是否初始化
if (oldCap > 0) {
//如果大于0，则说明已经初始化了
//判断数组长度是否，超过最大长度大小
if (oldCap >= MAXIMUM_CAPACITY) {
//如果超过则给阈值赋予新值，并返回当前数组对象。
threshold = Integer.MAX_VALUE;
return oldTab;
}
//判断老数组长度扩容一次后要小于能承受的最大长度 && 老数组大于或等于默认的数组长度
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
//如果都为true则阈值翻倍
newThr = oldThr << 1; // double threshold
}
//判断阈值是否大于0
else if (oldThr > 0)
//如果大于0则将老阈值赋予，新数组长度
newCap = oldThr;
else {
初始化容量和阈值
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//判断新阈值是否=0
if (newThr == 0) {
//新数组长度*加载因数
float ft = (float)newCap * loadFactor;
//赋予新的阈值
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
//将新的阈值赋给当前对象的阈值
threshold = newThr;
@SuppressWarnings({"rawtypes","unchecked"})//表示屏蔽一些无关紧要的警告
//创建一个新的数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
//并将当前容器指针指向它
table = newTab;
//判断容器内值是否为null
if (oldTab != null) {
//循环变量数组
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
//判断当前数组是否为null
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
//判断e是否为一个树节点
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 {
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;
}
红黑树添加
final void treeifyBin(Node<K,V>[] tab, int hash) {
int n, index; Node<K,V> e;
if (tab == null || (n = tab.length) < MIN_TREEIFY_CAPACITY)
resize();
else if ((e = tab[index = (n - 1) & hash]) != null) {
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)
hd.treeify(tab);
}
}
红黑树扩容替换
final void split(HashMap<K,V> map, Node<K,V>[] tab, int index, int bit) {
TreeNode<K,V> b = this;
// Relink into lo and hi lists, preserving order
TreeNode<K,V> loHead = null, loTail = null;
TreeNode<K,V> hiHead = null, hiTail = null;
int lc = 0, hc = 0;
for (TreeNode<K,V> e = b, next; e != null; e = next) {
next = (TreeNode<K,V>)e.next;
e.next = null;
if ((e.hash & bit) == 0) {
if ((e.prev = loTail) == null)
loHead = e;
else
loTail.next = e;
loTail = e;
++lc;
}
else {
if ((e.prev = hiTail) == null)
hiHead = e;
else
hiTail.next = e;
hiTail = e;
++hc;
}
}
if (loHead != null) {
if (lc <= UNTREEIFY_THRESHOLD)
tab[index] = loHead.untreeify(map);
else {
tab[index] = loHead;
if (hiHead != null) // (else is already treeified)
loHead.treeify(tab);
}
}
if (hiHead != null) {
if (hc <= UNTREEIFY_THRESHOLD)
tab[index + bit] = hiHead.untreeify(map);
else {
tab[index + bit] = hiHead;
if (loHead != null)
hiHead.treeify(tab);
}
}
}
jdk7和jdk8的区别
1.jdk7底层采用数组和链表 jdk8采用数组链表加红黑树，红黑树只有在链表达到8时才会采用。
2.插入节点的方式改变了，jdk7在链表头部，jdk在尾部，解决了死循环的问题
3.获取hash值的算法改变了，jdk7算的要跟散列一点。
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