Question

In Java, In general, some collection implementations allow null elements and some don’t.
Has it something to do with garbage collection?

Answer 1

In a broad sense I think it’s just basically up to the implementation to decide whether null can be a valid element of the collection (‘logically’) or not. I really doubt there’s more to it although I could be proven wrong.

Relevant bits from the documentation:

Some collection implementations have restrictions on the elements that
they may contain. For example, some implementations prohibit null
elements, and some have restrictions on the types of their elements.
Attempting to add an ineligible element throws an unchecked exception,
typically NullPointerException or ClassCastException. Attempting to
query the presence of an ineligible element may throw an exception, or
it may simply return false; some implementations will exhibit the
former behavior and some will exhibit the latter. More generally,
attempting an operation on an ineligible element whose completion
would not result in the insertion of an ineligible element into the
collection may throw an exception or it may succeed, at the option of
the implementation. Such exceptions are marked as “optional” in the
specification for this interface.