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上节遗留的问题
- Node的hash属性何时赋值，如何赋值
- Node对象的next属性何时赋值
- 扩容的触发时机
由上节的扩容函数resize()继续讲解
接下来就看看哪里使用到了resize(）函数
/**
* 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;
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) {
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;
}
不难找出putVal()函数使用了resize()函数，接下来我们来分析下这个函数的作用，首先这个函数使用了全局变量有TREEIFY_THRESHOLD，modCount，size，我们先来看看这些全局变量干嘛用的
/**
* The bin count threshold for using a tree rather than list for a
* bin. Bins are converted to trees when adding an element to a
* bin with at least this many nodes. The value must be greater
* than 2 and should be at least 8 to mesh with assumptions in
* tree removal about conversion back to plain bins upon
* shrinkage.
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* The number of times this HashMap has been structurally modified
* Structural modifications are those that change the number of mappings in
* the HashMap or otherwise modify its internal structure (e.g.,
* rehash). This field is used to make iterators on Collection-views of
* the HashMap fail-fast. (See ConcurrentModificationException).
*/
transient int modCount;
/**
* The number of key-value mappings contained in this map.
*/
transient int size;
可以简单的看出size为表容量大小，modCount为修改的次数
接下来细细品味源码
if ((tab = table) == null || (n = tab.length) == 0)
n = (tab = resize()).length;
if (++size > threshold) resize();
从这两段代码不难看出，扩容触发的时机有两个，一个就是table值为null或者长度为0的时候，也就是初始化执行一次，第二个就是当table容量大于需要调整的容量是，会触发扩容（遗留的问题）
从函数参数来看，hash是形参，咱们先去看看实参的值
/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
return putVal(hash(key), key, value, false, true);
}
不难看出，是put()调用putVal()然后以key为参数调用hash()函数来获取hash值，咱们就来分析分析这个hash函数
/**
* 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);
}
可以看出当key为null的时候hashcode值为0，生成hash的时候使用了异或操作符，也就是二进制计算，相同为0不同为1，然后异或两边的值一个是以key生成的hashcode，一个是该值向右移位16也就是该值的二进制数向右边移动16位，左边补充0，这样生成hash主要是从速度、功效、质量来考虑的，这么做可以在数组table的length比较小的时候，也能保证考虑到高低Bit都参与到Hash的计算中，同时不会有太大的开销。(遗留问题)
然后就来深入分析这个putVal()函数，由于可读性，直接在代码上进行注释
/**
* 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) {
//创建两个Node变量tab，p和两个int变量n，i
Node<K,V>[] tab; Node<K,V> p; int n, i;
if ((tab = table) == null || (n = tab.length) == 0)
//tab为null或者容量为0，初始化扩容
n = (tab = resize()).length;
if ((p = tab[i = (n - 1) & hash]) == null)
//如果这个位置的值为null则直接赋值
tab[i] = newNode(hash, key, value, null);
else {//如果该位置有值的话，就以链表的形式进行扩充值
//这里也就相当于栈设计，让后添加的元素处于栈顶，先进后出
//接下来就是将新的值放在栈顶
//创建了一个Node对象和一个key泛型对象
Node<K,V> e; K k;
if (p.hash == hash &&
((k = p.key) == key || (key != null && key.equals(k))))
e = p;//取出旧的栈顶元素
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)//扩容
resize();
afterNodeInsertion(evict);
return null;
}