Why is List not a subtype of List<Object>? [duplicate]

Answer 1

This goes back to what it means for A to be a subtype of B. The formal name for this is the Liskov substitution principle, which says basically that A is a subtype of B if and only if you can take any valid program that has in it something of type B, swap in something of type A and it would still be a valid program. The effect of this is that if you can use an A wherever you could use a B, then A is a subtype of B.

So in this case, since this is part of a valid (valid meaning "compiling") program:

public static void doThing(List<Object> x) {
  x.add(new Object());
}

Then, by the Liskov substitution principle, if List<String> were a subtype of List<Object> this would be part of a valid program too:

public static void doThing(List<String> y) {
  y.add(new Object());
}

But clearly that second snippet can't compile. Therefore, that second snippet is not part of a valid program, and therefore List<String> is not a subtype of List<Object>.

Likewise, the other way around is also not true: List<Object> is not a subtype of List<String>. Finding the program snippet to prove that is left as an exercise for the reader.

Answer 2

List<String> s = new ArrayList<String>();
List<Object> o = s;
o.add(new Object());
String first = s.get(0);  // boom