Why does this use of Java Generics not compile?

Question

For the life of me I cannot understand why the compiler won't let me do the following...

import java.util.HashMap;
import java.util.Map;

public class TestMap {
   private final Map<Integer, ? extends Number> map = new HashMap<Integer, Number>();

   public void put(Integer key, Long item) {
      this.map.put(key, item);
   }
}

Why does line this.map.put(key, item) cause a compile error?

I know that I can change the declaration of map to use Number rather than ? extends Number to make it work but it seems to me that what I am doing is perfectly legal, and I would prefer to not allow Number objects in the map. I am using Java 1.6.0_13.

Answer 1

You can't insert into collections that use wildcard types. This is because, while a List<Float> can be passed to a method that accepts List<? extends Number>, it isn't safe to insert a Long into it. In your case, one might expect the compiler to know better as the definition of the collection is so visible, but it doesn't.

Answer 2

This is related to generics covariance.

When you declare,

Map<Integer, ? extends Number> map

you can't insert anything to the map because you can't guarantee that ? extends Number is a Long, for instance.

Imagine this situation:

   Map<Integer, ? extends Number> map = new HashMap<Integer, Integer>();

   public void put(Integer key, Long item) {
      this.map.put(key, item);
   }

Here, Integer != Long and yet both obey ? extends Number.

Answer 3

Supplying the wildcard to a generic type effectively makes it read-only.