這兩天在看HashMap的時候，被負載因子float loadFactor搞得很暈，經過一天的研究，最后理出了自己的一點個人見解。
在HashMap的底層存在着一個名字為table的Entry數組，在實例化HashMap的時候，會輸入兩個參數，一個是 int initCapacity（初始化數組大小，默認值是16），一個是float loadFactor（負載因子，默認值是0.75），首先會根據initCapacity計算出一個大於或者等於initCapacity且為2的冪的值capacity，例如initCapacity為15，那么capacity就會為16，還會算出一個臨界值threshold，也就是capacity * loadFactor，貼出源代碼
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and load factor.
*
* @param initialCapacity the initial capacity
* @param loadFactor the load factor
* @throws IllegalArgumentException if the initial capacity is negative
* or the load factor is nonpositive
*/
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);
// Find a power of 2 >= initialCapacity
int capacity = 1;
while (capacity < initialCapacity)
capacity <<= 1;
this.loadFactor = loadFactor;
threshold = (int)(capacity * loadFactor);
table = new Entry[capacity];
init();
}
創建完HashMap之后，下面來看put方法，首先會根據key值計算出其HashCode值，然后在通過一個indexFor方法計算出此元素該存放於table數組的哪個數組之中（我猜想可能是通過對table.length的值取余的操作計算出來的），再檢測此table的此坐標位置的entry鏈是否存在此key或者此key值，若存在，則更新此元素的value值。源代碼如下：
/**
* 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) {
if (key == null)
return putForNullKey(value);
int hash = hash(key.hashCode());
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
modCount++;
addEntry(hash, key, value, i);
return null;
}
下面來看看addEntry方法，參數bucketIndex就是當前元素應該插入到entry數組的下標，先取出放在此位置的entry，然后把當前元素放入該數組中，當前元素的next指向之前取出元素，形成entry鏈表。（描述的不是很清楚，大概就是把新加入的entry當成頭放入到數組當中，然后指向之前的鏈表），放入之后就去判斷當前的size是否達到了threshold極限值，若達到了，將會進行擴容。源代碼如下：
/**
* Adds a new entry with the specified key, value and hash code to
* the specified bucket. It is the responsibility of this
* method to resize the table if appropriate.
*
* Subclass overrides this to alter the behavior of put method.
*/
void addEntry(int hash, K key, V value, int bucketIndex) {
Entry<K,V> e = table[bucketIndex];
table[bucketIndex] = new Entry<K,V>(hash, key, value, e);
if (size++ >= threshold)
resize(2 * table.length);
}
下面來看resize方法，方法比較簡單就是生成一個新的table數組（entry數組），然后根據新的Capacity和負載因子去生成新的臨界值。重點是里面有個transfer方法。源代碼如下：
/**
* Rehashes the contents of this map into a new array with a
* larger capacity. This method is called automatically when the
* number of keys in this map reaches its threshold.
*
* If current capacity is MAXIMUM_CAPACITY, this method does not
* resize the map, but sets threshold to Integer.MAX_VALUE.
* This has the effect of preventing future calls.
*
* @param newCapacity the new capacity, MUST be a power of two;
* must be greater than current capacity unless current
* capacity is MAXIMUM_CAPACITY (in which case value
* is irrelevant).
*/
void resize(int newCapacity) {
Entry[] oldTable = table;
int oldCapacity = oldTable.length;
if (oldCapacity == MAXIMUM_CAPACITY) {
threshold = Integer.MAX_VALUE;
return;
}
Entry[] newTable = new Entry[newCapacity];
transfer(newTable);
table = newTable;
threshold = (int)(newCapacity * loadFactor);
}
因為table數組的容量增加了，那么相應的table的length也增加了，那么之前存儲的元素的位置也就不一樣了，比如之前的length是16，現在的length是32，那么hashCode模16和HashCode模32的結果很有可能會不一樣，所以就只有重新去計算新的位置，方法是遍歷數組，在遍歷數組上的entry鏈。（此時就是所謂的rehash？？）
/**
* Transfers all entries from current table to newTable.
*/
void transfer(Entry[] newTable) {
Entry[] src = table;
int newCapacity = newTable.length;
for (int j = 0; j < src.length; j++) {
Entry<K,V> e = src[j];
if (e != null) {
src[j] = null;
do {
Entry<K,V> next = e.next;
int i = indexFor(e.hash, newCapacity);
e.next = newTable[i];
newTable[i] = e;
e = next;
} while (e != null);
}
}
}
總結：當負載因子較大時，去給table數組擴容的可能性就會少，所以相對占用內存較少（空間上較少），但是每條entry鏈上的元素會相對較多，查詢的時間也會增長（時間上較多）。反之就是，負載因子較少的時候，給table數組擴容的可能性就高，那么內存空間占用就多，但是entry鏈上的元素就會相對較少，查出的時間也會減少。所以才有了負載因子是時間和空間上的一種折中的說法。所以設置負載因子的時候要考慮自己追求的是時間還是空間上的少。
注意：設置initCapacity的時候，盡量設置為2的冪，這樣會去掉計算比initCapactity大，且為2的冪的數的運算。
疑問：感覺transfer方法會相當的耗時，是不是不去擴容會比較好？