hashmap是一個Node的陣列,以下為原始碼分析： 陣列大小是而2的冪，初始大小16：
/**
* The default initial capacity - MUST be a power of two.
*/
static final int DEFAULT_INITIAL_CAPACITY = 1 << 4; // aka 16
複製程式碼如果傳入的構造引數不是2的冪，以下方法取該數字的下一個2的冪。通過無符號右移和或運算，讓每一個二進位制位都變成1，
/**
* Returns a power of two size for the given target capacity.
*/
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;
}
複製程式碼陣列長度為2的冪的優點：
- 1.hash效率高。 put階段，要判斷該node放到陣列的哪個桶裡，採用的方式是table[（n-1）&hash(key)],n是2的冪，n-1的二進位制位上全部是1，這樣和hash值位與運算，可以更高效率的求出該hash值對應的陣列下標。
- 2.resize效率高。（具體程式碼實現在hashmap.resize（）中，）resize階段，擴容2倍，陣列大小n的二進位制位數增加1，在rehash的時候，hash值得最高位是0，那麼該node不需要移動。只要在hash值最高位是1的時候，rehash的值才會發生變化，而且是增加2的n-1次方。這樣擴容的效率更高。
/**
* Computes key.hashCode() and spreads (XORs) higher bits of hash
* to lower. Because the table uses power-of-two masking, sets of
* hashes that vary only in bits above the current mask will
* always collide. (Among known examples are sets of Float keys
* holding consecutive whole numbers in small tables.) So we
* apply a transform that spreads the impact of higher bits
* downward. There is a tradeoff between speed, utility, and
* quality of bit-spreading. Because many common sets of hashes
* are already reasonably distributed (so don't benefit from
* spreading), and because we use trees to handle large sets of
* collisions in bins, we just XOR some shifted bits in the
* cheapest possible way to reduce systematic lossage, as well as
* to incorporate impact of the highest bits that would otherwise
* never be used in index calculations because of table bounds.
*/
static final int hash(Object key) {
int h;
return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);
}
複製程式碼hash(Object key)方法是對key求hash雜湊值，比直接用hashcode更好，因為h無符號右移16位後得到高位，
V Get（Object key）方法
通過getNode(hash(key), key)獲得。程式碼邏輯：
final Node<K,V> getNode(int hash, Object key) {
//陣列tab，首節點first，陣列大小n，
Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
//三個條件判斷，&&具有短路功能，注意先後順序。
//1.陣列非空，2.陣列長度大於零，3，node節點非空.or 直接return null；
if ((tab = table) != null && (n = tab.length) > 0 && (first = tab[(n - 1) & hash]) != null) {
//判斷是否是first：hash值是否相同&& key引用相同或者equals.(k)
if (first.hash == hash && ((k = first.key) == key || (key != null && key.equals(k))))
return first;
if ((e = first.next) != null) {
//判斷是紅黑樹or連結串列
if (first instanceof TreeNode)
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);
}
}
return null;
}
複製程式碼map的key可以是null
以上get程式碼可以發現，如果key是null，hash(null)=0，固定在陣列的0下標處。null==null位true。
put(key,value)
使用如下方法：
/**
* Implements Map.put and related methods
*
* @param hash hash for key
* @param key the key
* @param value the value to put
* @param onlyIfAbsent if true, don't change existing value
* @param evict if false, the table is in creation mode.
* @return previous value, or null if none
*/
final V putVal(int hash, K key, V value, boolean onlyIfAbsent boolean evict) {
Node<K,V>[] tab; Node<K,V> p; int n, i;
//如果陣列為空，則通過resize（）初始化陣列。
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
// 陣列裡面，還沒有node物件則新建；否則新建Node物件插入該Node。
//插入桶，分為兩種情況，一種是紅黑樹。一種是連結串列。
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;
if (++size > threshold)
resize();
afterNodeInsertion(evict);
return null;
}
複製程式碼以上程式碼用的重要方法為resize，treeifyBin。以下為原始碼：
/**
* 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
*/
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;
}
複製程式碼還有個treeifyBin方法，把該桶裡面的單項鍊錶轉換為雙向連結串列，然後再轉換為紅黑樹。關於紅黑樹的具體實現，這邊文章先不分析。
remove方法
用到的final Node<K,V> removeNode方法是final，不能被子類重寫，只能重新afterNodeRemoval方法來增加邏輯。 removeNode原始碼主要考慮了單節點, 連結串列和樹三種情況，主要邏輯是（每一步都要考慮三種情況）：
- 1，先找到key這個節點，
- 2，然後再刪除這個節點。
keys（） values（），entrySet（）都提供了一個該hashmap的視窗。
computeIfAbsent
java8之前。從map中根據key獲取value操作可能會有下面的操作
Object key = map.get("key");
if (key == null) {
key = new Object();
map.put("key", key);
}
java8之後。上面的操作可以簡化為一行，若key對應的value為空，會將第二個引數的返回值存入並返回
Object key2 = map.computeIfAbsent("key", k -> new Object());
複製程式碼