public class HashMap<K,V> extends AbstractMap<K,V>
implements Map<K,V>, Cloneable, Serializable {
private static final long serialVersionUID = 362498820763181265L;
//容量必須是2的指數次冪
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
//hashMap的最大容量爲2^30次方
static final int MAXIMUM_CAPACITY = 1 << 30;
//默認的負載因子爲0.75
static final float DEFAULT_LOAD_FACTOR = 0.75f;
//當鏈表中元素的數量到達8的時候，鏈表會轉換成紅黑樹結構
static final int TREEIFY_THRESHOLD = 8;
//當bucket中的元素個數小於6的時候,紅黑樹結構轉換成鏈表結構
static final int UNTREEIFY_THRESHOLD = 6;
//當HashMap中存在紅黑樹時，HashMap中數組（table）的最小容量爲64
static final int MIN_TREEIFY_CAPACITY = 64;
//hash表中存儲的每一個Node元素
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;//元素的hash值
final K key;
V value;
Node<K,V> next;//該node節點的下一個節點
Node(int hash, K key, V value, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.value = value;
this.next = next;
}
public final K getKey() { return key; }
public final V getValue() { return value; }
public final String toString() { return key + "=" + value; }
public final int hashCode() {
return Objects.hashCode(key) ^ Objects.hashCode(value);
}
public final V setValue(V newValue) {
V oldValue = value;
value = newValue;
return oldValue;
}
public final boolean equals(Object o) {
if (o == this)
return true;
if (o instanceof Map.Entry) {
Map.Entry<?,?> e = (Map.Entry<?,?>)o;
if (Objects.equals(key, e.getKey()) &&
Objects.equals(value, e.getValue()))
return true;
}
return false;
}
}
/**
key的hash值右移16位的原因：從putVal方法中可以看出，hashMap添加元素時，確認數組索引是用(p = tab[i = (n - 1) & hash])，
又因爲n必須是2的整數次冪，所以n的表現形式只能是01111這種形式，所以當（n-1）與hash進行 與 運算時，起決定性作用的永遠是hash。
01010010101000100101001100101010101
01111
------------------------------------
並且如上圖所示，hash的32位二進制數中，能夠起到運算作用的也只有後幾位，這樣計算出來的hash值重複率比較高。
所以需要讓key.hashCode()右移16位，然後與自身進行 異或 運算。這樣做可以降低hash值的重複率。
**/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
/**
* Returns x's Class if it is of the form "class C implements
* Comparable<C>", else null.
*/
static Class<?> comparableClassFor(Object x) {
if (x instanceof Comparable) {
Class<?> c; Type[] ts, as; Type t; ParameterizedType p;
if ((c = x.getClass()) == String.class) // bypass checks
return c;
if ((ts = c.getGenericInterfaces()) != null) {
for (int i = 0; i < ts.length; ++i) {
if (((t = ts[i]) instanceof ParameterizedType) &&
((p = (ParameterizedType)t).getRawType() ==
Comparable.class) &&
(as = p.getActualTypeArguments()) != null &&
as.length == 1 && as[0] == c) // type arg is c
return c;
}
}
}
return null;
}
/**
* Returns k.compareTo(x) if x matches kc (k's screened comparable
* class), else 0.
*/
@SuppressWarnings({"rawtypes","unchecked"}) // for cast to Comparable
static int compareComparables(Class<?> kc, Object k, Object x) {
return (x == null || x.getClass() != kc ? 0 :
((Comparable)k).compareTo(x));
}
//該方法的作用是初始化臨界值的大小----->threshold,返回大於initialCapacity的最小的二次冪數值,
//如果在構造方法中傳入的不是2的整數次冪，該方法會強制轉換成2的整數次冪
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 < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
//存儲元素的數組，總是2的冪次倍---->就是hash表中的數組
transient Node<K,V>[] table;
/**
* Holds cached entrySet(). Note that AbstractMap fields are used
* for keySet() and values().
*/
transient Set<Map.Entry<K,V>> entrySet;
//HashMap中所有key-value對的數量
transient int size;
//它是一個計數器，HashMap的結構每修改一次，modCount++
transient int modCount;
//它是一個臨界值，當size變量達到這個臨界值的時候，HashMap會自動擴容。threshold = capacity * loadFactor
int threshold;
//負載因子，默認爲0.75
final float loadFactor;
//*--------------------------------------------------HashMap的構造方法------------------------------------------------------------------------*//
public HashMap(int initialCapacity, float loadFactor) {
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;
//強制將初始化容量裝換成2的n次冪
this.threshold = tableSizeFor(initialCapacity);
}
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
//該函數的作用是將m集合中的所有元素全部放入到該HashMap實例中
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > 0) {
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + 1.0F;
int t = ((ft < (float)MAXIMUM_CAPACITY) ?
(int)ft : MAXIMUM_CAPACITY);
if (t > threshold)
threshold = tableSizeFor(t);
}
else if (s > threshold)
resize();
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
K key = e.getKey();
V value = e.getValue();
putVal(hash(key), key, value, false, evict);
}
}
}
//返回當前<key,value>的數量
public int size() {
return size;
}
public boolean isEmpty() {
return size == 0;
}
//提供的對外訪問的方法，內部調用的getNode();
public V get(Object key) {
Node<K,V> e;
return (e = getNode(hash(key), key)) == null ? null : e.value;
}
/*
getNode的過程：1：首先判斷當前HashMap是否爲空並且根據key的Hash值判斷當前索引處是否爲空，若不爲空，繼續查找，爲空，返回null
2:然後判斷當前索引處的第一個元素是否與要找的元素相等，如果相等，返回第一個數據，若不等，繼續查找
3：判斷當前節點是否是紅黑樹結構，若是，遍歷紅黑樹。若不是，遍歷鏈表
*/
final Node<K,V> getNode(int hash, Object key) {
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
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) {
if (first instanceof TreeNode)//判斷是否爲紅黑樹
return ((TreeNode<K,V>)first).getTreeNode(hash, key);
do {
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k))))
return e;
} while ((e = e.next) != null);//遍歷鏈表
}
}
return null;
}
//判斷HashMap中是否包含當前的key,實際調用getNode();
public boolean containsKey(Object key) {
return getNode(hash(key), key) != null;
}
//對外提供的向HashMap中添加元素的方法，內部實際調用的是putVal();
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
/*
putVal的過程：（1）首先將node[] table 復值給一個空的數組 tab 判斷tab[]是否爲空，爲null則進行擴容操作，不爲null，利用key的Hash&（n-1）確定該元素在數組中的位置。
（2）判斷該位置是否已經存在元素，若爲null，則直接添加，不爲null,判斷當前元素的key與已有元素的key的hash和equals是否相等，若相等，則進行覆蓋，返回oldVal
(3)若不相等，繼續判斷要加入的NOde是否屬於紅黑樹結構，若屬於，則以紅黑樹的方式進行添加操作，若不屬於，則遍歷後面的鏈表。
（4）若與鏈表中的某個元素相等，則進行覆蓋，若不等，則添加到鏈表的最後面。
（5）添加完以後判斷當前鏈表中元素的個數，有沒有達到鏈表轉換爲紅黑樹的臨界值（默認爲8），若達到，則轉換。
（6）最後判斷hash表中的元素個數size，有沒有達到擴容的臨界值，若達到，則擴容。threshold = loadFactor * capacity
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent,
boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
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) {//遍歷鏈表
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null);
if (binCount >= TREEIFY_THRESHOLD - 1) // -1 for 1st
treeifyBin(tab, hash);//轉成紅黑樹
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;
if (++size > threshold)
resize();//擴容
afterNodeInsertion(evict);
return null;
}
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold.
* Otherwise, because we are using power-of-two expansion, the
* elements from each bin must either stay at same index, or move
* with a power of two offset in the new table.
*
* @return the table
初始化或者翻倍表大小。
* 如果表爲null，則根據存放在threshold變量中的初始化capacity的值來分配table內存
* （這個註釋說的很清楚，在實例化HashMap時，capacity其實是存放在了成員變量threshold中，
* 注意，HashMap中沒有capacity這個成員變量）
* 。如果表不爲null，由於我們使用2的冪來擴容，
* 則每個bin元素要麼還是在原來的bucket中，要麼在2的冪中
*/
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;
}
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;
}