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对象比较的一般契约：equals（）和hashCode（）

[英]General contract for object comparision : equals() and hashCode()

原文 2013-01-28 07:54:07 3 3 java

There is a point in general contract of equals method, which says if You has defined equals() method then You should also define hashCode() method. 在equals方法的一般契约中有一点，它表示如果你已经定义了equals()方法，那么你还应该定义hashCode()方法。 And if o1.equals(o2) then this is must o1.hashCode() == o2.hashCode() . 如果o1.equals(o2)那么这必须是o1.hashCode() == o2.hashCode() 。

So my question is what if I break this contract? 所以我的问题是，如果我打破这份合同怎么办？ Where can bring fails the situation when o1.equals(o2) but o1.hashCode != o2.hashCode() ? 当o1.equals(o2)但是o1.hashCode != o2.hashCode()时，哪里可以带来失败？

3 个解决方案

它将导致基于散列的数据结构中的意外行为，例如： HashMap ，读取HashTable的工作原理

HashMap/HashTable/HashSet/etc will put your object into one of several buckets based on its hashCode , and then check to see if any other objects already in that bucket are equal . HashMap / HashTable / HashSet / etc会根据hashCode将你的对象放入几个桶中的一个桶中，然后检查该桶中已有的任何其他对象是否equal 。

Because these classes assume the equals/hashCode contract, they won't check for equality against objects in other buckets. 因为这些类假定等于/ hashCode合约，所以它们不会检查与其他存储桶中的对象的相等性。 After all, any object in another bucket must have a different hashCode, and thus (by the contract) cannot be equal to the object in quesiton. 毕竟，另一个桶中的任何对象必须具有不同的hashCode，因此（通过契约）不能等于quesiton中的对象。 If two objects are equal but have different hash codes, they could end up in different buckets, in which case the HashMap/Table/Set/etc won't have a chance to compare them. 如果两个对象equal但具有不同的哈希码，则它们最终会出现在不同的桶中，在这种情况下，HashMap / Table / Set /等将无法比较它们。

So, you could end up with a Set that contains two objects which are equal -- which it's not supposed to do; 所以，你最终可能会得到一个包含两个相等的对象的Set - 它不应该这样做; or a Map that contains two values for the same one key (since the buckets are by key); 或者包含同一个键的两个值的Map（因为桶是按键）; or a Map where you can't look up a key (since the lookup checks both the hash code and equality); 或者您无法查找键的Map（因为查找检查哈希码和相等）; or any number of similar bugs. 或任何数量的类似错误。

If you break the contract, your objects won't work with hash-based containers (and anything else that uses hashCode() and relies on its contract). 如果您违反合同，您的对象将无法使用基于散列的容器（以及使用hashCode()并依赖其合同的任何其他内容）。

The basic intuition is as follows: to see whether two objects are the same, the container could call hashCode() on both, and compare the results. 基本直觉如下：要查看两个对象是否相同，容器可以在两者上调用hashCode() ，并比较结果。 If the hash codes are different, the container is allowed to short-circuit by assuming that the two objects are not equal. 如果哈希码不同，则允许容器通过假设两个对象不相等而短路。

To give a specific example, if o1.equals(o2) but o1.hashCode() != o2.hashCode() , you'll likely be able to insert both objects into a HashMap (which is meant to store unique objects). 举一个具体的例子，如果o1.equals(o2)但是o1.hashCode() != o2.hashCode() ，你可能会将两个对象都插入HashMap （用于存储唯一对象）。