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?
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<?>>
ArrayList<String>doesn't extendSerializable?Aextends (or implements)Bdoesn't imply thatHashMap<K,A>extendsHashMap<K,B>. After all, aHashMap<K,B>is something that you can put aBin, but aHashMap<K,A>isn't. Equivalent questions have been asked before.