java中如何生成一个接口，可以让别人调用这个接口获取响应的数据，求速解大神（希望能有个例子）

public interface X{ public double f();}public class A implements X{ public double f(){ int sum = 0; for(int i = 1; i < 10; i+=2){ int tmp = 1; for(int j = 1; j <= i; j++){ tmp *= j; } sum += tmp; } return sum; }}public class B implements X{ public double f(){ int sum = 0; for(int i = 2; i < 11; i+=2){ int tmp = 1; for(int j = 1; j <= i; j++){ tmp *= j; } sum += tmp; } return sum; }}public class Main { public static void main(String[] args){ X a = new A(); System.out.println(a.f()); X b = new B(); System.out.println(b.f()); }}