Which declarations will compile without warnings?.
Select the four correct answers.
(a) Map<Integer, Map<Integer, String>> map1
= new HashMap<Integer, HashMap<Integer, String>>();
(b) Map<Integer, HashMap<Integer, String>> map2
= new HashMap<Integer, HashMap<Integer, String>>();
(c) Map<Integer, Integer> map3 = new HashMap<Integer, Integer>();
(d) Map<? super Integer, ? super Integer> map4
= new HashMap<? super Integer, ? super Integer>();
(e) Map<? super Integer, ? super Integer> map5 = new HashMap<Number, Number>();
(f) Map<? extends Number, ? extends Number> map6
= new HashMap<Number, Number>();
(g) Map <?,?> map7 = new HashMap<?,?>();
(b), (c), (e), and (f)
In (b), (c), (e), and (f), the parameterized type in the object creation expression is a subtype of the type of the reference.
This is not the case in (a): just because HashMap<Integer, String> is a subtype of Map<Integer, String>, it does not follow that HashMap<Integer, HashMap<Integer, String>> is a subtype of Map<Integer, Map<Integer, String>>-there is no subtype covariance relationship between concrete parameterized types.
In (d) and (g), wild cards cannot be used to instantiate the class.