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16static 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64static 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(zhǎng)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1024static 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)2public HashMap(int initialCapacity) {this(initialCapacity, DEFAULT_LOAD_FACTOR);}//構(gòu)造函數(shù)3public 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;}@Overridepublic int hashCode() {// 如果返回1// return 1return i;}// object作為key存map里，必須實(shí)現(xiàn)equals方法@Overridepublic 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;}