upcasting - jtips exam

[Rosie Vogel]

Hello everyone,
This is from jtips.net, mock exam no. 1

class Superclass { 
   int x = 0; 
   int methodA() 
   { 
     return x; 
   } 
 } 
  
 class Subclass extends Superclass { 
   int x = 1; 
   int methodA() 
   { 
      return x; 
   } 
 } 
  
 class Q30 { 
  public static void main(String[] args) 
  { 
    Subclass sub = new Subclass(); 
    System.out.println("sub.methodA()= " + sub.methodA()); 
  
    Superclass sup = new Superclass(); 
    System.out.println("sup.methodA()=" + sup.methodA()); 
  
    Superclass sup1 = new Subclass(); 
    System.out.println("sup1.methodA()=" + sup1.methodA() + process(sup1)); 
  } 
  
   static int process(Superclass obj) 
   { 
     return obj.x; 
   } 
 } 

The output is:
sub.methodA() = 1
sup.methodA() = 0
sup1.methodA() = 10
I don't understand why process(sup1) leads to an output of zero.
sup1 is an object of Subclass, and x in Subclass is 1! I am obviously confused about the rules about upcasting. Can you help me?

[Rana Thakur]

Let me try to explain.
When you pass sup1 to the function process(Superclass obj), it is actually a Superclass reference pointing to a Subclas object. The value of variable x is picked up on the basis of reference type(Superclass) and not on the what the reference is actually pointing to (sup1). Hence the value returned is value of x in Superclass i.e. 0.
bye

[Rosie Vogel]

Thank you Rana, I think I understand it now.