为什么懒惰生成Java String哈希码？

Question

它出现在java.lang.String.java中,Java只会在调用hashcode()之后生成哈希码,然后存储它,但为什么不在构造函数中创建哈希码呢？

相关代码:

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;
}

大部分都可以放在构造函数中.

Answer 1

为什么要花时间生成最不可能使用的哈希码？大多数字符串都是构造,使用,然后在没有调用hashcode()的情况下进行垃圾收集.

Answer 2

约书亚·布洛赫 (Joshua Bloch) 称这种做法为“活泼的单一检查”。

Jeremy Manson has excellent explanation of why it's done and why it'safe: on his blog

In essence, at construction time you save some time by skipping calculating hash code. In multithreaded environment you'll pay for this back because multiple thread potentially could do the same calculation.