HashMap调整表大小

Question

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？

Answer 1

最好总是看一下源代码：

 final Node<K,V>[]  [More ...] resize() {      
         Node<K,V>[] 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) < MAXIMUM_CAPACITY &&   
                      oldCap >= 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<K,V>[] newTab = (Node<K,V>[])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);
}