SCJP Exam Question

[CornedBee]

Because at compile-time it doesn't reference anything. Here's an example:

class Base {}
class A extends Base {}
class B extends Base {}
class C extends Base {}

public class Core {
  public static void main(String[] args) {
    int i = (int)(Math.random()*3.0);
    Base b;
    switch(i) {
      case 0:
        b = new A();
        break;

      case 1:
        b = new B();
        break;

      case 2:
        b = new C();
        break;
    }
    A a = b; // !!!
  }
}

How would the compiler know whether the marked assignment is valid? If i is 0 then it is, otherwise it isn't.