初学接口自编的有点小问题

interface Computable
{
final int MAX=100;
void speak(String s);
int f(int x);
float g(float x,float y);

}



class China implements Computable
{
int xuehao;
public int f(int x)
{int i;
int sum=0;
for(i=1;i<=x;i++)
sum=sum+1;
return sum;
}
public float g(float x,float y)
{

return y-x;
}
public void speak(String s)
{
System.out.println("中国人:"+s);
}

}



class Japan implements Computable
{
int xuehao;
public int f(int x)
{
return x;
}
public float g(float x,float y)
{
return x+y;
}
public void speak(String s)
{
System.out.println("日本人:"+s);
}
}


public class Text
{
//China Li;
//Japan Henlu;

public static void main(String args[])
{
ChinaLi=new China();
JapanHenlu=new Japan();
Li.xuehao=991898;
Henlu.xuehao=941448;


System.out.println(Li.f(5));//这一句输出的是5,不是15.我想求从一加到五
System.out.println(Henlu.f(10));
System.out.println(Li.g(1.0f,2.0f));
System.out.println(Henlu.g(3.0f,4.0f));
System.out.println(Li.MAX+Li.xuehao+"\t"+Li.f(100));
System.out.println(Henlu.MAX+Henlu.xuehao+"\t"+Henlu.g(2.0f,3.0f));
Li.speak("杀光日本仔");
Henlu.speak("我们都是猪");
}
}

sum=sum+i not sum=sum+1