写出程序结果: interface A { double f(double x, double y); } class B implements A { public double f(double x, double y) { return x * y; } int g(int a, int b) { return a + b; } } public class E { public static void main(String[] args) { A a = new B(); System.out.println(a.f(3, 5)); B b = (B) a; System.out.println(b.g(3, 5)); } }