Why does ArrayList<ArrayList<?>> list = new ArrayList<ArrayList<String>>() not compile? [duplicate]

Question

Why does this code compile

final ArrayList<?> dp1 = new ArrayList<String>();

But this doesn't

final ArrayList<ArrayList<?>> dp2 = new ArrayList<ArrayList<String>>();

Answer 1

In

final ArrayList<?> dp1 = new ArrayList<String>();

The type argument ? is a wildcard which is a superset (not super-type) of String. So, ArrayList<?> is super type of ArrayList<String>.

But in

final ArrayList<ArrayList<?>> dp2 = new ArrayList<ArrayList<String>>();

The type argument ArrayList<?> (a parameterized type where ? just stands for some unkown type, and doesn't have anything to do with String) is not a wildcard, the wildcard would be ? extends ArrayList<?>, with an upper bound ArrayList<?>, which actually is a supertype of ArrayList<String>.

You can read about the rules regarding super/sub set/type in parameterized type here.

Answer 2

It's quite complicated to understand, but in summary in your first code, String extends ? but the second one doesn't compile because ArrayList<String> does not directly inherit from ArrayList<?> you can look here if you want all the details. If you want your second example to compile you have to modify it to this :

final ArrayList<? extends ArrayList<?>> dp2 = new ArrayList<ArrayList<String>>();