Why is this HashMap declaration using generics wrong?

Question

HashMap<String, ArrayList<? extends Serializable>> map = new HashMap<String, ArrayList<ArrayList>>();

This does not compile. To the best of my knowledge on Java Generics, it should. And this:

ArrayList<? extends Serializable> c = new ArrayList<ArrayList<String>>();

successfully compiles.

Can anyone say why the above wouldn't compile?

Answer 1

Why do you think that it should? A HashMap<String, Apple> is never assignable from a HashMap<String, Orange> for any possible unequal Apple and Orange, as long as both Apple and Orange are not wildcard-types.

And ArrayList<? extends Serializable> is not the same thing as ArrayList<ArrayList<?>>.

What you probably meant:

HashMap<String, ? extends ArrayList<? extends Serializable>> map = 
  new HashMap<String, ArrayList<ArrayList<?>>>();

Now it compiles, because indeed:

? extends Serializable // can be assigned from
          ArrayList<?>

and

? extends ArrayList<? extends Serializable> // can be assigned from
          ArrayList<          ArrayList<?>>