hashCode唯一性 [英] hashCode uniqueness

问题描述

Object的两个实例是否可以具有相同的hashCode()?

Is it possible for two instances of Object to have the same hashCode()?

从理论上讲，对象的hashCode是从其内存地址派生的，所以所有hashCodes都应该是唯一的，但是如果对象在GC中移动，该怎么办?

In theory an object's hashCode is derived from its memory address, so all hashCodes should be unique, but what if objects are moved around during GC?

推荐答案

给出合理的对象集合，很可能会有两个具有相同的哈希码.在最好的情况下，它成为生日问题，与数以万计的对象发生冲突.在实践中，对象创建时使用的哈希码池相对较小，并且仅数千个对象就很容易发生冲突.

Given a reasonable collection of objects, having two with the same hash code is quite likely. In the best case it becomes the birthday problem, with a clash with tens of thousands of objects. In practice objects a created with a relatively small pool of likely hash codes, and clashes can easily happen with merely thousands of objects.

使用内存地址只是获取稍微随机数的一种方法. Sun JDK源有一个开关，可以使用安全随机数生成器或常数.我相信IBM(曾经使用过?)使用快速随机数生成器，但它一点也不安全.文档中提到的内存地址似乎具有历史性(大约十年前，具有固定位置的对象句柄并不罕见).

Using memory address is just a way of obtaining a slightly random number. The Sun JDK source has a switch to enable use of a Secure Random Number Generator or a constant. I believe IBM (used to?) use a fast random number generator, but it was not at all secure. The mention in the docs of memory address appears to be of a historical nature (around a decade ago it was not unusual to have object handles with fixed locations).

这是我几年前写的一些代码，用来展示冲突:

Here's some code I wrote a few years ago to demonstrate clashes:

class HashClash {
    public static void main(String[] args) {
        final Object obj = new Object();
        final int target = obj.hashCode();
        Object clash;
        long ct = 0;
        do {
            clash = new Object();
            ++ct;
        } while (clash.hashCode() != target && ct<10L*1000*1000*1000L);
        if (clash.hashCode() == target) {
            System.out.println(ct+": "+obj+" - "+clash);
        } else {
            System.out.println("No clashes found");
        }
    }
}

RFE来澄清文档，因为这太经常出现了: CR 6321873

RFE to clarify docs, because this comes up way too frequently: CR 6321873

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