Java中hashCode和equals方法的关系
我在很多地方都看到在Java中重写equals方法时，也应该重写hashCode方法，否则就是违反合同".
I read in many places saying while override equals method in Java, should override hashCode method too, otherwise it is "violating the contract".
但到目前为止，如果我只覆盖 equals 方法而不是 hashCode 方法，我还没有遇到任何问题.
But so far I haven't faced any problem if I override only equals method, but not hashCode method.
合同是什么?为什么我在违反合同时没有遇到任何问题?在什么情况下，如果我没有覆盖 hashCode 方法，我会遇到问题吗?
What is the contract? And why am I not facing any problem when I am violating the contract? In which case will I face a problem if I haven't overridden the hashCode method?
您将遇到的问题是根据 .equals() 和 计算元素唯一性的集合.hashCode()，例如HashMap 中的键.
The problem you will have is with collections where unicity of elements is calculated according to both .equals() and .hashCode(), for instance keys in a HashMap.
顾名思义，它依赖于哈希表，而哈希桶是对象的.hashCode()的函数.
As its name implies, it relies on hash tables, and hash buckets are a function of the object's .hashCode().
如果你有两个 .equals() 对象，但是哈希码不同，你就输了！
If you have two objects which are .equals(), but have different hash codes, you lose!
这里合同中重要的部分是:.equals() 的对象必须具有相同的 .hashCode().
The part of the contract here which is important is: objects which are .equals() MUST have the same .hashCode().
这一切都记录在 对象代码>.Joshua Bloch 说你必须在 有效的 Java.说的够多了.
This is all documented in the javadoc for Object. And Joshua Bloch says you must do it in Effective Java. Enough said.