How does "polymorphism" applies to members?

Hello!

I also got this question at a jobb interview, se code below.

I said that the output would be x.i = 10 since when polymorphism is used; it is the class of the object that matters and not the reference during execution.

But when I execute it it prints out: x.i = 5

class X
{
    public int i = 5;
}

class Y extends X
{
    public int i = 10;
}

public class Test1
{
public static void main( String[] args )
{
  X x = new Y();

  System.out.println( "x.i = " + x.i );
}
}

To me this is stange, too strange since I thougt I know this.
A bit embarrassing!

So if any one could explain how this works please let me know!

Best regards
Fredrik