Java中的equals（）和hashCode（）契约 [英] equals() and hashCode() contract in Java

问题描述

Bert Bates和Kathy Sierra的SCJP 6学习指南在第554页（以及其他要求中）指出 x.hashCode（）！= y.hashCode（）要求x.equals（y）== false 。

The SCJP 6 Study Guide from Bert Bates and Kathy Sierra states on page 554 (among other requirements) that x.hashCode() != y.hashCode() requires that x.equals(y) == false.

但是Javadoc for Object没有明确提到这样的要求。引用：

如果两个对象根据equals（Object）方法相等，那么在两个对象中的每一个上调用hashCode方法必须产生相同的整数结果。

But the Javadoc for Object doesn't mention such requirement explicitly. Quote:
If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

我应该采用Javadoc所说的物质含义，例如 eq - > hc ？然后这两个来源之间不会发生冲突。

Should I take what Javadoc says as a material implication, such as eq -> hc? Then there would be no conflict between these two sources.

推荐答案

正如z5h所说，这些陈述是等价的。

As z5h says, the statements are equivalent.

对于逻辑条件x和y，x暗示y与！y暗示！x相同。

For logical conditions x and y, "x implies y" is the same as "!y implies !x".

如果有东西是公共汽车，它的红色在逻辑上等同于如果某些东西不是红色，那它不是公共汽车。

"If something is a bus, it's red" is logically equivalent to "if something isn't red, it's not a bus."

这是对位。

我应该采取Javadoc所说的物质含义，例如eq - > hc。

Should I take what Javadoc says as a material implication, such as eq -> hc.

是的，这正是它所说的：两个物体相等在等于下暗示他们的哈希码必须相等。

Yes, that's exactly what it's saying: two objects being equal under equals implies their hashcodes must be equal.

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