Java invoke subclass and superclass

Question

public class prelim {
   public static void main(String[] args) {
    A a = new A();
    System.out.println(a.x);
    System.out.println(a.m());
    B b = new B();
    System.out.println(b.x);
    System.out.println(b.m());
    a = new B();
    System.out.println(a.x);
    System.out.println(a.m());
    b = (B)a;
    System.out.println(b.x);
    System.out.println(b.m());
} 
}
class A {
    int x;
    A() { 
         this(1); 
    }
    A(int x) {
             this.x = x; 
    }
    int m() {
             return x; 
    }
}
class B extends A {
     int x;
     B() { 
       this(2); 
     }
     B(int x) {
       super(x+1);
       this.x = super.x + 1;
     }
     int m() {
        return x; 
     }
  }

As for a = new B(); and b = (B)a; What is the type of a and b since it is not declared. And the output was 3 4 4 4. I think a is the type B. So ax and am() should be 4 4.

I don't know exactly the procedure of invoking subclass and its methods.

Answer 1

Variable "a" has type A, which is what it is declared with. The type of the variable never changes. Variables in Java are references. A variable of type A can point to an object of type A or one of its subclasses (or null ).

Likewise variable "b" has type B. It can only point to an object of type B or null since B has no subclasses.

The correct printout (removing linebreaks for brevity) is:

1 1 4 4 3 4 4 4

The types of the objects for the four printouts are

A B B B

In the third printout ax accesses the "x" variable for the A class, which exists, even though the object has class B.