We could have used a val with a mutable type, like ListBuffer.

我们也可以使用val和一个可变的类型，比如ListBuffer。

youdao

Unlike strings, which are immutable, lists are a mutable type, i. e. it is possible to change their content.

不像字符串是不可变量，列表是可变类型，例如：它可以改变它本身某个元素的值。

youdao

Clearly, this approach is a pain but the solution is easy: Never use a mutable object type as a key in a HashMap.

很显然，这种方法很糟糕，但是解决方法也很简单：永远不要将可变对象类型用作hashmap中的键。

youdao