在 Java 中向 HashMap 添加空键或值有什么用?

问题描述

HashMap 允许一个空键和任意数量的空值.有什么用?

HashMap allows one null key and any number of null values. What is the use of it?

推荐答案

我不是很肯定你在问什么，但如果你正在寻找一个何时想要使用空键的例子，我用它们通常在映射中表示默认情况(即如果给定键不存在则应使用的值):

I'm not positive what you're asking, but if you're looking for an example of when one would want to use a null key, I use them often in maps to represent the default case (i.e. the value that should be used if a given key isn't present):

Map<A, B> foo;
A search;
B val = foo.containsKey(search) ? foo.get(search) : foo.get(null);

HashMap 专门处理空键(因为它不能在空对象上调用 .hashCode())，但空值没什么特别的，它们'像其他任何东西一样重新存储在地图中

HashMap handles null keys specially (since it can't call .hashCode() on a null object), but null values aren't anything special, they're stored in the map like anything else

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