java - 为什么 HashMap 中的不可变对象(immutable对象)如此有效？

Question

所以我读到了 HashMap .有人指出:

"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."

我不太明白……为什么？

Answer 1

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;
}

由于 String 的内容永远不会改变，类的创建者选择在计算一次哈希后缓存它。这样，就不会浪费时间重新计算相同的值。