/**
* This map usually acts as a binned (bucketed) hash table, but
* when bins get too large, they are transformed into bins of
* TreeNodes, each structured similarly to those in
* java.util.TreeMap. Most methods try to use normal bins, but
* relay to TreeNode methods when applicable (simply by checking
* instanceof a node). Bins of TreeNodes may be traversed and
* used like any others, but additionally support faster lookup
* when overpopulated. However, since the vast majority of bins in
* normal use are not overpopulated, checking for existence of
* tree bins may be delayed in the course of table methods.
*/
其中
but when bins get too large, they are transformed into bins of TreeNodes
這句說明了當bin過大時就會采用紅黑樹進行存儲。
/**
* 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 = ;
/**
* The bin count threshold for untreeifying a (split) bin during a
* resize operation. Should be less than TREEIFY_THRESHOLD, and at
* most 6 to mesh with shrinkage detection under removal.
*/
static final int UNTREEIFY_THRESHOLD = ;
/**
* The smallest table capacity for which bins may be treeified.
* (Otherwise the table is resized if too many nodes in a bin.)
* Should be at least 4 * TREEIFY_THRESHOLD to avoid conflicts
* between resizing and treeification thresholds.
*/
static final int MIN_TREEIFY_CAPACITY = ;
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable {
// 預設的初始容量是16，必須是2的幂
static final int DEFAULT_INITIAL_CAPACITY = << ;
// 最大容量
static final int MAXIMUM_CAPACITY = << ;
// 預設的填充因子
static final float DEFAULT_LOAD_FACTOR = f;
// 當桶(bucket)上的結點數大于這個值時會轉成紅黑樹
static final int TREEIFY_THRESHOLD = ;
// 當桶(bucket)上的結點數小于這個值時樹轉連結清單
static final int UNTREEIFY_THRESHOLD = ;
// 桶中結構轉化為紅黑樹對應的table的最小大小
static final int MIN_TREEIFY_CAPACITY = ;
// 存儲元素的數組，總是2的幂
transient Node<k,v>[] table;
// 存放具體元素的集
transient Set<map.entry<k,v>> entrySet;
// 存放元素的個數，注意這個不等于數組的長度。
transient int size;
// 每次擴容和更改map結構的計數器
transient int modCount;
// 臨界值 當實際大小(容量*填充因子)超過臨界值時，會進行擴容
int threshold;
// 填充因子
final float loadFactor;
}
函數詳解
構造函數
HashMap一共提供了四個構造函數。
/**
* Constructs an empty <tt>HashMap</tt> with the default initial capacity
* (16) and the default load factor (0.75).
*/
public HashMap() {
this.loadFactor = DEFAULT_LOAD_FACTOR; // all other fields defaulted
}
/**
* Constructs an empty <tt>HashMap</tt> with the specified initial
* capacity and the default load factor (0.75).
*
* @param initialCapacity the initial capacity.
* @throws IllegalArgumentException if the initial capacity is negative.
*/
public HashMap(int initialCapacity) {
this(initialCapacity, DEFAULT_LOAD_FACTOR);
}
/**
* 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 < )
throw new IllegalArgumentException("Illegal initial capacity: " +
initialCapacity);
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
if (loadFactor <= || Float.isNaN(loadFactor))
throw new IllegalArgumentException("Illegal load factor: " +
loadFactor);
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}
/**
* Constructs a new <tt>HashMap</tt> with the same mappings as the
* specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
* default load factor (0.75) and an initial capacity sufficient to
* hold the mappings in the specified <tt>Map</tt>.
*
* @param m the map whose mappings are to be placed in this map
* @throws NullPointerException if the specified map is null
*/
public HashMap(Map<? extends K, ? extends V> m) {
this.loadFactor = DEFAULT_LOAD_FACTOR;
putMapEntries(m, false);
}
/**
* Returns a power of two size for the given target capacity.
*/
static final int tableSizeFor(int cap) {
int n = cap - ;
n |= n >>> ;
n |= n >>> ;
n |= n >>> ;
n |= n >>> ;
n |= n >>> ;
return (n < ) ? : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + ;
}
最後一個構造函數是把一個已有的Map對象最為參數，其中主要是調用
putMapEntries
方法，這個函數中最後是調用了
putVal
,這個方法在下面介紹。
/**
* Implements Map.putAll and Map constructor
*
* @param m the map
* @param evict false when initially constructing this map, else
* true (relayed to method afterNodeInsertion).
*/
final void putMapEntries(Map<? extends K, ? extends V> m, boolean evict) {
int s = m.size();
if (s > ) {
if (table == null) { // pre-size
float ft = ((float)s / loadFactor) + F;
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);
}
}
}
PUT
有了對象，就要開始往裡面放資料，現在就看下
put
這個方法。
/**
* 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);
}
這個方法隻是
putVal
方法的封裝
/**
* 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) == ) //如果原來的table為空則resize
n = (tab = resize()).length;
if ((p = tab[i = (n - ) & hash]) == null) //對應的位置中沒有值，則直接放進去
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)))) //如果hash值相同、key相同（equals判斷）則進行更新操作
e = p;
else if (p instanceof TreeNode)
e = ((TreeNode<K,V>)p).putTreeVal(this, tab, hash, key, value);
else { //hash沖突處理
for (int binCount = ; ; ++binCount) {
if ((e = p.next) == null) {
p.next = newNode(hash, key, value, null); //如果為目前連結清單的最後一個元素則直接将新元素放到最後
if (binCount >= TREEIFY_THRESHOLD - ) // -1 for 1st //超過轉為黑紅樹的上限時轉換為黑紅樹
treeifyBin(tab, hash);
break;
}
if (e.hash == hash &&
((k = e.key) == key || (key != null && key.equals(k)))) //連結清單中有hash值相同、key相同（equals判斷）則進行更新操作
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;
}
/**
* Initializes or doubles table size. If null, allocates in
* accord with initial capacity target held in field threshold. 初始化或者翻倍table大小，當原始table為空時使用threshold中存儲的值初始化table。
* 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) ? : oldTab.length;
int oldThr = threshold;
int newCap, newThr = ;
if (oldCap > ) {
if (oldCap >= MAXIMUM_CAPACITY) { // 超過最大值就不再擴充了
threshold = Integer.MAX_VALUE;
return oldTab;
}
else if ((newCap = oldCap << ) < MAXIMUM_CAPACITY && //容量擴充到原來的兩倍
oldCap >= DEFAULT_INITIAL_CAPACITY)
newThr = oldThr << ; // double threshold // 臨界值擴充到原來的兩倍
}
else if (oldThr > ) //當原來的table長度為0時，并且指定了初始容量（HashMap(int initialCapacity, float loadFactor)），進行到這裡
newCap = oldThr;
else { // 沒有指定初始容量時采用預設
newCap = DEFAULT_INITIAL_CAPACITY;
newThr = (int)(DEFAULT_LOAD_FACTOR * DEFAULT_INITIAL_CAPACITY);
}
if (newThr == ) {
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 = ; j < oldCap; ++j) {
Node<K,V> e;
if ((e = oldTab[j]) != null) {
oldTab[j] = null;
if (e.next == null)
newTab[e.hash & (newCap - )] = 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) == ) { //索引為低位的元素
if (loTail == null)
loHead = e;
else
loTail.next = e;
loTail = e;
}
else { //索引為高位的元素
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;
}
方法的文檔說明中有這樣一句話
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.
