How does groupingBy work when classifier returns a List?

Consider this SO answer

I have a few questions:

1. How does it work? Does groupingBy() check if the returned value from the classifier is a List and compare each and every item? That doesn’t sound like a typical Java function to me
2. How can List contain different variable types? Does the compiler treat it like a List<Object>?
3. Is it anti-pattern to use List?

>Solution :

What is happening here is that the value returned by the classifier is compared using the equals()-method, which in case of a List is specified:

List.equals():

Returns true if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal.

Actually, the comparing and equals-invoking is done by the underlying Map implementation.