为什么hashmaps中的不可变对象如此有效？ [英] Why are immutable objects in hashmaps so effective?

问题描述

所以我阅读了 HashMap 。有人指出：

不可变性还允许缓存不同密钥的哈希码，这使整个检索过程非常快速，由Java Collection API提供的字符串和各种包装类（例如 Integer ）非常好 HashMap 键。 p>

我不太明白...为什么？

解决方案

String＃hashCode ：

  private int散列; 
 
 ... 
 
 public int hashCode（）{
 int h = hash; 
 if（h == 0& count> 0）{
 int off = offset; 
 char val [] = value; 
 int len = count; 
 
 for（int i = 0; i< len; i ++）{
 h = 31 * h + val [off ++]; 
} 
 hash = h; 
} 
 return h; 
 
 
 
 $ b 
由于 String 永远不会改变，这个类的制造者选择在它被计算一次之后缓存哈希值。这样，时间不会浪费，重新计算相同的值。
 
So I read about HashMap. At one point it was noted:

  
"Immutability also allows caching the hashcode of different keys which makes the overall retrieval process very fast and suggest that String and various wrapper classes (e.g., Integer) provided by Java Collection API are very good HashMap keys."
I don't quite understand... why?
解决方案
String#hashCode:
private int hash;

...

public int hashCode() {
    int h = hash;
    if (h == 0 && count > 0) {
        int off = offset;
        char val[] = value;
        int len = count;

        for (int i = 0; i < len; i++) {
            h = 31*h + val[off++];
        }
        hash = h;
    }
    return h;
}
Since the contents of a String never change, the makers of the class chose to cache the hash after it had been calculated once. This way, time is not wasted recalculating the same value.

                        
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