HashMap調整其表格大小

Question

我知道默認情況下HashMap的大小是16，我們也可以提供一些其他值。如果我已經初始化大小爲5，負載因子爲0.8f然後我添加是它的第五個元素。它是否增長到10或16？一旦閾值違約發生2值的非冪次，它會跳到2的冪次方嗎？

Answer 1

它始終是最好有一個看看source code：

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，電流容量和閾值調整大小時翻了一番。

但是，構建一個初始容量不是2的冪的HashMap對象是不可能的！構造函數將初始容量轉換爲2的冪：

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); 
}