What is the class type of a superclass ref pointing to a subclass object?

I have the following codes:

1. public class Tester
2. {
3.      public static void main(String[] args)
4.      {
5.          A a = new B();
6.          System.out.println(a.getClass());    //Prints class B       
7.          System.out.println(a instanceof A);  //Prints true
8.          System.out.println(a instanceof B);  //Prints true
9.          System.out.println(a.valA);          //Prints 1
10.         System.out.println(a.valB);          //Compilation error
11.
12.     }
13. }
14.
15. class A
16. {
17.     int valA=1;
18. }
19.
20. class B extends A
21. {
22.     int valB=2;
23. }

At line 6 it shows that a is of type class B. However when it reaches line 10, the compiler produces an error: location: variable a of type A.

So my question is: What exactly is the class type of a now? Why getClass() shows that it is of type class B, yet the compiler complains it as type A during compilation?

Further more, since a instanceof B is true, why can't I access valB?

To make things clearer:

EDIT: I ran this statement: System.out.println(a); and the output was B@36d98810 which somehow proves that the toString() method of class B was executed. Since variable a can access the toString() method within class B, why can't it access valB which also resides in class B?