以下程序的输出结果为 ( ) 。 class A { double f(double x, double y) { return x * y; } } class B extends A { double f(double x, double y) { return x + y; } } public class Test { public static void main(String args[]) { B obj = new B(); System.out.println("The program output is " + obj.f(4, 6)); } }