Question

This compiles (1.6)

List<? extends Object> l = new ArrayList<Date>();

But this does not

List<List<? extends Object>> ll = new ArrayList<List<Date>>();

with the error of

Type mismatch: cannot convert from ArrayList<List<Date>> to List<List<? extends Object>>

Could someone explain why?
Thanks

EDIT: edited for being consequent

Answer 1

Because it would break type safety:

List<List<Object>> lo = new ArrayList<List<Object>>();
List<List<? extends Object>> ll = lo;
List<String> ls = new ArrayList<String>();
ll.add(ls);
lo.get(0).add(new Object());
String s = ls.get(0); // assigns a plain Object instance to a String reference