可扩展类的哈希码(面向未来) [英] Hash code for expandable class (future proof)

问题描述

由于我的数学技能不高，我问你是否存在我将来可能会在课堂上使用的算法?

Since I don't have any great skills in math, I ask you if there exists any algorithm that I should use for a class which probably will change in the future.

请考虑以下情形:

角色"类具有以下字段:

Class "Roles" has following fields:

private boolean admin;
private boolean printer;

几周后，我决定添加一个来宾"角色:

After some weeks I decide to add a role "guest":

private boolean admin;
private boolean printer;
private boolean guest;

几周后，我决定删除打印机"角色；

After some weeks I decide to remove the role "printer";

private boolean admin;
private boolean guest;

由于我将哈希码保留在数据库中，因此我必须100％确保此类的所有版本都生成唯一的哈希码.

Since I will persist the hashcode in a database, I must be 100% sure that all versions of this class generates unique hashcodes.

也许这不是问题，我一直使用Eclispe IDE源代码生成器中提供的代码.

Maybe this is not a problem, I have always used the one provided in the Eclispe IDE source generator.

能否请您告诉我使用Eclipse IDE(Indigo)Java版本> = 6的方法是否安全，或者给我有关此主题的其他建议.我相信这是很普遍的事情.

Can you please tell me if I am safe with the Eclipse IDE (Indigo) Java version >= 6 method or give me some other advices regarding this topic. I am sure this is a very common thing.

预先感谢

推荐答案

因为我将哈希码保留在数据库中

Since I will persist the hashcode in a database

不要那样做. hashCode的结果不是 meant 要保留的.特别是从文档中:

Don't do that. The result of hashCode isn't meant to be persisted. In particular, from the docs:

从一个应用程序的执行到同一应用程序的另一执行，此整数不必保持一致.

This integer need not remain consistent from one execution of an application to another execution of the same application.

下一步:

我必须100％确保此类的所有版本都生成唯一的哈希码.

I must be 100% sure that all versions of this class generates unique hashcodes.

哈希码也不是唯一的……它们经常不会.好的，您的案例中只有5位数据，但通常情况并非如此...

Hash codes aren't meant to be unique, either... they very often won't be. Okay, you've only got 5 bits of data in your case, but in general that's not the case...

听起来您对Object.hashCode()的要求与正常要求不同-因此，您不应期望任何自动生成的实现都知道您的特殊要求.我建议您准确地说明您的要求，然后我们可以确定该怎么做...

It sounds like you have different requirements from the normal ones for Object.hashCode() - so you shouldn't expect any autogenerated implementation to know about your special requirements. I suggest you state exactly what your requirements are, and we can work out what to do...

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