Generics cannot convert error java

Question

List<? extends Number> ls = new ArrayList<Number>(); //(1)
Map <String,List<? extends Number>> mp = new HashMap<String,List<Number>>(); //(2)
Map <String,List<? extends Number>> mp = new HashMap<String,List<? extends Number>>(); //(3)

I can do (1) and (3) but not (2).

If I can do (1) why I am getting an error when doing (2)

If you're using java 7+ you can just do `new HashMap<>();` – BitNinja Sep 02 '14 at 18:45 — BitNinja, Sep 02 '14 at 18:45

score 1 · Accepted Answer · edited May 23 '17 at 12:12

Even though a List<Number> is a List<? extends Number>, a HashMap<String, List<Number>> is not a Map<String, List<? extends Number>, because Java's generics are invariant. This is the same thing that makes a List<Dog> not assignable to a List<Animal>, even if Dog extends Animal. Here, List<Number> plays the part of the Dog and List<? extends Number> plays the part of the Animal.

List<Number> can't match List<? extends Number> when used as a type parameter.

You can get (2) to compile by introducing a wildcard: Change

Map <String,List<? extends Number>> mp = new HashMap<String,List<Number>>();

to

Map <String, ? extends List<? extends Number>> mp = new HashMap<String,List<Number>>();

Generics cannot convert error java

1 Answers1