HashMap<String, Integer> map = new HashMap<>();
map.put("zs", 18);
按照初始情况来分析
class HashMap{
transient Node<K,V>[] table;// Hashmap的底层数组
int threshold;// 阈值（数组初始长度 * 加载因子）
//（数组初始长度16，加载因子0.75，则阈值12）（数组初始长度32，加载因子1，则阈值32）
static final float DEFAULT_LOAD_FACTOR = 0.75f;
final float loadFactor;// 加载因子 默认0.75
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // 默认的初始容量16
//构造方法中没有对数组进行初始化，一定在第一次添加的时候进行初始化
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR;
}
static final int hash(Object key) {
int h = 0;
// (h = key.hashCode()) ^ (h >>> 16);
// 让高位右移，和低位异或（高位也起作用，充分散列）
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
// 添加方法 "zs"
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) {
// hash : key经过计算hash值
// key
// value
Node<K,V>[] tab;
Node<K,V> p;
int n, i;
// table : // Hashmap的底层数组
// tab = table == null: 真 构造方法中没有对数组初始化
if ((tab = table) == null || (n = tab.length) == 0)
// resize(): 扩容方法
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
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))))
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else {
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
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;
}
}
if (e != null) {
// existing mapping for key
V oldValue = e.value;
if (!onlyIfAbsent || oldValue == null)
e.value = value;
afterNodeAccess(e);
return oldValue;
}
}
++modCount;
//添加过程大于阈值12就要扩容
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
final Node<K,V>[] resize() {
// oldTab = table === null
Node<K,V>[] oldTab = table;
// oldCap = 0 旧容量
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// threshold = 0 阈值(没有初始化)
// oldThr = 0
int oldThr = threshold;
int newCap, newThr = 0; //定义新容量、新阈值
//不大于0，不会执行（第一次添加的时候）
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
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;
//走到这里
else {
// zero initial threshold signifies using defaults
//
// DEFAULT_INITIAL_CAPACITY = 16
// 新容量newCap = 16
newCap = DEFAULT_INITI AL_CAPACITY;
// 新阈值newThr = 16 * 0.75 = 12
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//不会执行
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
// 阈值threshold = 12
threshold = newThr;
// 创建一个长度为16 的数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
// hashmap的底层数组, 变成一个长度为16 的数组（这里就说明了数组的默认初始容量为16）
//TIME : 16:22
//接着分析扩容问题
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
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;
}
}
按照扩容的情况来分析 resize()
class HashMap{
static final int MAXIMUM_CAPACITY = 1 << 30;
// 在数组超过阈值扩容: 假如底层数组长度为16情况
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table;
// oldCap = oldTab.length = 16
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// 阈值oldThr = 12
int oldThr = threshold;
int newCap, newThr = 0;
if (oldCap > 0) {
//不大于1 << 30
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
//执行的是这里
// newCap = 2 oldCap ---> 新容量= 旧容量* 2
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
// newThr = 12 * 2 = 24
newThr = oldThr << 1; // double threshold
}
//else if和else就不会执行了
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
else {
// zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//不执行
if (newThr == 0) {
float ft = (float)newCap * loadFactor;
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
// threshold = 24，阈值变24了
threshold = newThr;
// 创建一个长度为32的数组，赋给底层的table
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
table = newTab; //说明了：数组扩容（默认扩为原来的2倍）
//至此，数组初始容量和扩容问题分析完了
//接下来，看第2个构造方法（文件Demo3）
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
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;
}
}
构造方法2
class HashMap{
public HashMap(int initialCapacity) {
//调用下面的构造方法
// 0.75f（float类型）
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap(int initialCapacity, float loadFactor) {
//3个if是在做参数检验
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;
// initialCapacity = 14
// 初始长度为14
// 给了threshold为16（），有点问题？对于这种初始化，resize()是一种新的情况，走的是新的逻辑
// threshold = 16
this.threshold = tableSizeFor(initialCapacity);// 不是很合理，没有用一个新定义的值记录：大于给定值的2个幂次（用阈值记录不合理）
}
//1000 --> 1111101000 //cap = 1000
// n |= n >>> 1;
// n = 1111100111 //n = cap - 1
// 0111110011 //n右移一位 >>>这个是无符号右移
// n = 1111110111 //或运算的结果（有一个为1则1）（区别于异或运算：不同则结果为1）
// n |= n >>> 2;
// 0011111101
// n = 1111111111
// 10000000000 -> 1024
static final int tableSizeFor(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
//return n + 1 1024
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
//对于这种初始化，resize()是一种新的情况，走的是新的逻辑
//新的扩容方式，来分析一下
final Node<K,V>[] resize() {
Node<K,V>[] oldTab = table; //null
//oldCap= 0 上面的初始化中只赋了2个值：
//this.loadFactor = loadFactor;（0.75）
//this.threshold = tableSizeFor(initialCapacity)(14到16)
int oldCap = (oldTab == null) ? 0 : oldTab.length;
// oldThr = 16
int oldThr = threshold;
int newCap, newThr = 0;
//不执行
if (oldCap > 0) {
if (oldCap >= MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << 1) < MAXIMUM_CAPACITY &&
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << 1; // double threshold
}
//执行
else if (oldThr > 0) // initial capacity was placed in threshold
//走到这里，新容量等于旧阈值
// newCap = 16
newCap = oldThr;
else {
// zero initial threshold signifies using defaults
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
//执行
if (newThr == 0) {
// ft = 16 * 0.75 = 12
float ft = (float)newCap * loadFactor;
// newThr = (int)ft = 12
newThr = (newCap < MAXIMUM_CAPACITY && ft < (float)MAXIMUM_CAPACITY ?
(int)ft : Integer.MAX_VALUE);
}
// threshold = 12
threshold = newThr;
// 创建一个长度为16 的数组
Node<K,V>[] newTab = (Node<K,V>[])new Node[newCap];
//第2个构造方法讲完了
//第3个构造方法不讲了（同样的道理）
//第4个构造方法不讲了（同样的道理）
table = newTab;
if (oldTab != null) {
for (int j = 0; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - 1)] = e;
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;
}
}