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

Question

Possible Duplicate:
java generics covariance

I am trying to make sense of the fact that List<String> is not a subtype of List<Object>.

In effective Java, Josh Bloch notes that although it may seem counter-intuitive,it does make sense. The reason he stated is that you can put any Object in List<Object>, but can only put String in List<String>. I am not sure how this justifies why the String list is not a subtype of Object list.

Maybe I am confused by the term subtype. What I think it means is that when S is a subtype of T, an instance of S is an instance of T. Therefore, for List<String> to be a subtype of List<Object>, Object has to be a super class of String, which it technically is. Any idea where my reasoning went wrong?

Answer 1

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

Answer 2

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.