public class HashMap<k,v> extends AbstractMap<k,v> implements Map<k,v>, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;
// 默認(rèn)的初始容量是16
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;
// 最大容量
static final int MAXIMUM_CAPACITY = 1 << 30;
// 默認(rèn)的填充因子(以前的版本也有叫加載因子的)
static final float DEFAULT_LOAD_FACTOR = 0.75f;
// 這是一個(gè)閾值，當(dāng)桶(bucket)上的鏈表數(shù)大于這個(gè)值時(shí)會(huì)轉(zhuǎn)成紅黑樹，put方法的代碼里有用到
static final int TREEIFY_THRESHOLD = 8;
// 也是閾值同上一個(gè)相反，當(dāng)桶(bucket)上的鏈表數(shù)小于這個(gè)值時(shí)樹轉(zhuǎn)鏈表
static final int UNTREEIFY_THRESHOLD = 6;
// 看源碼注釋里說是：樹的最小的容量，至少是 4 x TREEIFY_THRESHOLD = 32 然后為了避免(resizing 和 treeification thresholds) 設(shè)置成64
static final int MIN_TREEIFY_CAPACITY = 64;
// 存儲(chǔ)元素的數(shù)組，總是2的倍數(shù)
transient Node<k,v>[] table;
transient Set<map.entry<k,v>> entrySet;
// 存放元素的個(gè)數(shù)，注意這個(gè)不等于數(shù)組的長度。
transient int size;
// 每次擴(kuò)容和更改map結(jié)構(gòu)的計(jì)數(shù)器
transient int modCount;
// 臨界值 當(dāng)實(shí)際大小(容量*填充因子)超過臨界值時(shí)，會(huì)進(jìn)行擴(kuò)容
int threshold;
// 填充因子
final float loadFactor;

// 指定初始容量和填充因子的構(gòu)造方法
public HashMap(int initialCapacity, float loadFactor) {
// 指定的初始容量非負(fù)
if (initialCapacity < 0)
throw new IllegalArgumentException(Illegal initial capacity: +
initialCapacity);
// 如果指定的初始容量大于最大容量,置為最大容量
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
// 填充比為正
if (loadFactor <= 0 || Float.isNaN(loadFactor))
throw new IllegalArgumentException(Illegal load factor: +
loadFactor);
this.loadFactor = loadFactor;
// 指定容量后，tableSizeFor方法計(jì)算出臨界值，put數(shù)據(jù)的時(shí)候如果超出該值就會(huì)擴(kuò)容，該值肯定也是2的倍數(shù)
// 指定的初始容量沒有保存下來，只用來生成了一個(gè)臨界值
this.threshold = tableSizeFor(initialCapacity);
}
// 該方法保證總是返回大于cap并且是2的倍數(shù)的值，比如傳入999 返回1024
static final int tableSizeFor(int cap) {
int n = cap - 1;
// 向右做無符號(hào)位移
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
// 三目運(yùn)算符的嵌套
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
//構(gòu)造函數(shù)2
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
//構(gòu)造函數(shù)3
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}

static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}

public class MutableKeyTest {
public static void main(String args[]){
class MyKey {
Integer i;
public void setI(Integer i) {
this.i = i;
}
public MyKey(Integer i) {
this.i = i;
}
@Override
public int hashCode() {
// 如果返回1
// return 1
return i;
}
// object作為key存map里，必須實(shí)現(xiàn)equals方法
@Override
public boolean equals(Object obj) {
if (obj instanceof MyKey) {
return i.equals(((MyKey)obj).i);
} else {
return false;
}
}
}
// 我機(jī)器配置不高，25000的話正常情況27毫秒，可以用2500萬試試，如果hashCode()方法返回1的話，250萬就卡死
Map<MyKey,String> map = new HashMap<>(25000,1);
Date begin = new Date();
for (int i = 0; i < 20000; i++){
map.put(new MyKey(i), "test " + i);
}
Date end = new Date();
System.out.println("時(shí)間(ms) " + (end.getTime() - begin.getTime()));

public V get(Object key) {
Node<k,v> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
final Node<k,v> getNode(int hash, Object key) {
Node<k,v>[] tab; Node<k,v> first, e; int n; K k;
// hash & (length-1)得到紅黑樹的樹根位置或者是鏈表的表頭
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) {
// 如果是樹，遍歷紅黑樹復(fù)雜度是O(log(n))，得到節(jié)點(diǎn)值
if (first instanceof TreeNode)
return ((TreeNode<k,v>)first).getTreeNode(hash, key);
// else是鏈表結(jié)構(gòu)
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);
}
}
return null;
}

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;
// 如果tab為空或長度為0，則分配內(nèi)存resize()
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// (n - 1) & hash找到put位置，如果為空,則直接put
if ((p = tab[i = (n - 1) & hash]) == null)
tab[i] = newNode(hash, key, value, null);
else {
Node<k,v> e; K k;
// 第一節(jié)節(jié)點(diǎn)hash值同，且key值與插入key相同
if (p.hash == hash &&((k = p.key) == key || (key != null && key.equals(k))))
e = p;
else if (p instanceof TreeNode)
// 紅黑樹的put方法比較復(fù)雜，putVal之后還要遍歷整個(gè)樹，必要的時(shí)候修改值來保證紅黑樹的特點(diǎn)
e = ((TreeNode<k,v>)p).putTreeVal(this, tab, hash, key, value);
else {
// 鏈表
for (int binCount = 0; ; ++binCount) {
if ((e = p.next) == null) {
// e為空，表示已到表尾也沒有找到key值相同節(jié)點(diǎn)，則新建節(jié)點(diǎn)
p.next = newNode(hash, key, value, null);
// 新增節(jié)點(diǎn)后如果節(jié)點(diǎn)個(gè)數(shù)到達(dá)閾值，則將鏈表轉(zhuǎn)換為紅黑樹
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);
break;
}
// 容許空key空value
if (e.hash == hash &&((k = e.key) == key || (key != null && key.equals(k))))
break;
p = e;
}
}
// 更新hash值和key值均相同的節(jié)點(diǎn)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;
}

final Node<K,V>[] resize() {
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) {
threshold = Integer.MAX_VALUE;
return oldTab;
}
// 這一句比較重要，可以看出每次擴(kuò)容是2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
newCap = DEFAULT_INITIAL_CAPACITY;
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];
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;
}