作者:ian | 来源:互联网 | 2023-02-07 00:08
我知道默认情况下HashMap的大小为16,我们还可以为其提供其他值。如果我用0.8f的加载因子将大小初始化为5,然后向其中添加第五个元素,该怎么办?它会增长到10或16?一旦阈值突破发生(非2的幂),它会跳到2的幂吗?
1> Miljen Mikic..:
最好总是看一下源代码:
final Node[] [More ...] resize() {
Node[] 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) = DEFAULT_INITIAL_CAPACITY)
newThr = oldThr <<1; // double threshold
}
else if (oldThr > 0) // initial capacity was placed in threshold
newCap = oldThr;
...
// The capacity of the inner data structure is doubled
Node[] newTab = (Node[])new Node[newCap];
table = newTab;
...
因此,当前容量和阈值在调整大小时会加倍。
但是,无法构造初始容量不是2的幂的HashMap对象!构造函数将初始容量转换为2的幂:
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;
}
public [More ...] HashMap(int initialCapacity, float loadFactor) {
...
this.loadFactor = loadFactor;
this.threshold = tableSizeFor(initialCapacity);
}