java - StringBuffer 可以用作 HashMap 中的键吗？

Question

StringBuffer 可以作为 HashMap 中的键吗？
如果是这样，使用 String 和 StringBuffer 作为键有什么区别？

Answer 1

Can a StringBuffer as a key in a HashMap?

不，因为 StringBuffer既不覆盖 equals 也不覆盖 hashCode，因此它不适合作为 HashMap 键(回想一下 HashMap 依赖于那些判断给定键是否存在于映射中的两种方法)。

除此之外，StringBuffer 是可变的，您通常希望 Map 键是不可变的。来自 Map :

Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.