为什么我会收到未经检查的作业警告？

Question

I have the following interface and class definitions...

abstract interface I{}

class Foo implements I{}

abstract class A<T extends I> {
    List<T> list;
}

class B extends A<Foo> {}

All the definitions work fine. I then want to do the following:

A b = new B();
List<? extends I> iList = b.list;

The compiler will indeed give me an Unchecked Assignment warning... but why? Won't all of A 's lists be of type <? extends I> <? extends I> ? b.list always has elements that extend I, so I am having trouble seeing why there would be an error

Answer 1

The variable A b is of a raw type. Perhaps you meant to use A<Foo> b instead.