Why does this use of Java Generics not compile?
generics, java
Solution
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.
Problem
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.