【java源码一带一路系列】之HashMap.putAll()

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public void putAll(Map<? extends K, ? extends V> m) {

putMapEntries(m, true); // ↓

}

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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; // ft即table此时所需capacity，“+1.0F”为什么，个人理解弥补float与int转换、比较精度弥补，加二也未尝不可?

int t = ((ft < (float)MAXIMUM_CAPACITY) ?

(int)ft : MAXIMUM_CAPACITY);

if (t > threshold)

threshold = tableSizeFor(t); // ↓

}

else if (s > threshold)

resize(); // ↓

// 笔者疑问：原map加上m后可能需要扩容的判断在putVal中，在此是不是更佳呢？答：因为除此之外还有其他函数调用了putVal

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);

}

}

}

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// 找到大于等于cap的最小的2的幂（3的最小2的幂=4；4->4；5->8...）

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;

}

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// hash table扩容至原来2倍，并将原table数据重新映射到新table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; // 清空原table

if (e.next == null) // 哈希表只有一个节点，直接赋值

newTab[e.hash & (newCap - 1)] = e;

else if (e instanceof TreeNode)

((TreeNode<K,V>)e).split(this, newTab, j, oldCap); // 红黑树情况

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;

}

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static final int hash(Object key) {

int h;

return (key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16);

}