java集合HashMap的put方法
阿新 • • 發佈:2019-02-05
首先我們需要對hashmap初始換容器大小
HashMap< String, String > hashMap = new HashMap<>(16);
如果我們的寫的數字小於MAXIMUM_CAPACITY = 1 << 30的值,
那麼用我寫的值,負責使用MAXIMUM_CAPACITY。值得注意的是我們在初始化大小的時候,容器的大小都是以2的冪次方增長,具體程式碼實現如下: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; }
實現初始化;
初始化演算法說明:
cap = 12;
n = 11;
|=執行的是或的演算法
n = 11= 1011, 11 >>> 1 = 101
n = 1011 | 0101 = 1111;
n = 1111 , n >>> 2 = 0011
n = 1111 | 0011 = 1111
下面同上
最後 n= 1111 = 15, n + 1 = 16
在我們呼叫put方法的時候,首先我們將會使用
(key == null) ? 0 : (h = key.hashCode()) ^ (h >>> 16)來生成一個hash值,然後使用(n - 1) & hash獲取table[]的下表,進行值的操作,具體程式碼如下:
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;
}
值得注意的是再jdk1.8以後加入後紅黑樹的儲存,TREEIFY_THRESHOLD = 8為閥值;