在 Java 中 String hashCode() 的旧实现中跳过字符背后的想法是什么

Question

What is the idea of skipping some characters from a String in old versions of Java's String hashCode() implementation:

public int hashCode() {
   int hash = 0;
   int skip = Math.max(1, length()/8);
   for (int i = 0; i < length(); i += skip)
      hash = (hash * 37) + charAt(i);
   return hash;
}

In the current version there is no skipping and the prime number is 31 instead of 37

Answer 1

Probably to fast up the hashCode() computation but as consequence it had more potential collisions.
The new version favors less collisions but requires more computations.

But in the facts, String s are immutable, so in more recent versions of hashCode() , that is computed once :

public int hashCode() {
    int h = hash; 
    if (h == 0 && value.length > 0) {
        hash = h = isLatin1() ? StringLatin1.hashCode(value)
                              : StringUTF16.hashCode(value);
    }
    return h;
}

So in a some way it makes sense to favor this way as it reduces the collision number and not skipping some characters in the hashCode() computation is not so expensive as the result is cached.