为什么Java使用'equals'代替'=='来检查对象在Map中的存在？ [英] Why Java uses 'equals' instead of '==' to check an object's presence in a Map?

问题描述

假设我想将一个键及其值放在Map中。

Suppose if I want to put a key and its value in a Map.

我相信这就是Java的作用：

I believe it is what Java does:

1. 获取密钥的哈希码并检查地图中是否存在具有相同哈希码的密钥。

1. Get the Hashcode of the key and check whether there is a key in the map with the same hashcode.

如果没有具有相同Hashcode的密钥，则可以将密钥放入Map。

If there is no key with the same Hashcode then the key can be put into Map.

如果有一个具有相同Hashcode的密钥，则使用equals来确定密钥是否可以放入Map。

If there is a key with the same Hashcode then use equals to determine whether the key can be put into Map or not.

我的问题是为什么不使用 == 而不是等于在这个过程中？

My question is why not use == instead of equals in this process?

推荐答案

做你的建议会破坏<$ c $的整个概念c>等于 / hashCode tandem。 等于和 hashCode 都会变得无用，甚至不应该存在。

Doing what you suggest would break the entire concept of equals/hashCode tandem. Both equals and hashCode would be rendered useless and should not even exist.

Java允许程序员为他的类定义相等集;如果他不想这样做，他可以选择退出覆盖等于和 hashCode —并结束准确地说出你提出的语义。

Java allows the programmer to define the equality sets for his class; if he doesn't want to do it, he can simply opt out of overriding equals and hashCode—and end up with exactly the semantics you propose.

举一个具体的例子，根据你的提议，这将在地图中创建两个单独的条目：

To give a specific example, under your proposal, this would create two separate entries in the map:

Map<Integer, String> map = new HashMap<>();
map.put(10_000, "a");
map.put(10_000, "a");

这是因为文字10_000每次都会自动装入 Integer ，根据您的语义，它是两个单独的键。然后，语句

That is because the literal 10_000 will be autoboxed each time into a new instance of Integer, which, under your semantics, are two separate keys. And then, the statement

System.out.println(map.get(10_000));

将打印 null ，因为你当然正在使用第三个​​键来获取。事实上，按键检索任何地图值都是不可能的。

would print null, because of course you are using a third key to get. It would in fact be impossible to retrieve any map value by key.

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