According to the code specified below, b.fun2() is not allowed in main method, I know why it is not working. I wonder that how can I solve these kind of problem. According to the solid programming principle, I need to write an interface for my problem. But this problem prevent me to define an interface.

```
public interface A{
   void fun();
}
public class B implements A{
     void fun(){
          // some code
     }
}
public class C implements A{
     void fun(){
          // some code
     }
     void fun2(){
          // some code
     }
}
public class Main {
    public static void main(String[] args){

             A b = new C();
             b.fun2(); 

    }

}

```