奇怪的出错信息,哪位高手给解答一下!!!!!

[wangzhisheshou]

class SubStr{

public static String sub(String src, int num){

char[] c = src.toCharArray();
int i = 0, n = 0;
for( ; n <= num; i++){
if(c[i] > 128)
n += 2;

else
n++;

if(n == num)
return new String(c, 0, i+1);
else if(n - num == 1)
return new String(c, 0, i);
}//在这里提示缺少返回值!!!!!!!!!!!!!!不明白.


}
public static void main(String[] args){

System.out.println(sub("电脑a的dsfdfi", 6));

}

}