java covariant return type

Question

Why does below code prints "1" ?

class A {
    int x = 1;
}

class B extends A {
    int x = 2;
}

class Base {

    A getObject() {
        System.out.println("Base");
        return new B();
    }
}

public class CovariantReturn extends Base {

B getObject() {
   System.out.println("CovariantReturn");
   return new B(); 
}
/**
 * @param args
 */
public static void main(String[] args) {
    Base test = new CovariantReturn();
    System.out.println(test.getObject() instanceof B);
    System.out.println(test.getObject().x);
}
}

Answer 1

Because you are referring to fields, which are not affected by polymorphism. If you instead used getX(), it would've returned 2.

What you are asking is, the value of field x defined in class A (because Base.getObject() returns A). Even though CovariantReturn overrides the method to return B, you are not referring to your object as CovariantReturn.

To expand a bit on how fields are not affected by polymorphism - field access is realized at compile time, so whatever the compiler sees, that's what's accessed. In your case the method defines to return A and so A.x is accessed. On the other hands methods are invoked based on the runtime type. So even if you define to return A but return an instance of B, the method you invoke will be invoked on B.