Java interface Method

Question

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(); 

    }

}

Answer 1

General rule here is: using interface reference that is referencing implementing class object you can call only those methods which have been overridden by the implementing class.

To make your code work do:

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

PS: Another problem here is, By default all members of interface are public. so when overriding fun() in classes B and C, the visibility of this method is getting reduced to default level from public; this action is not valid in Java. Use public access specifier whenever overriding interface methods.

Answer 2

You don't have fun2 in A so you can't call to it by using object which reference type is A. To fix this put fun2 abstract method in A interface.

Answer 3

To be able to access the method fun2 from the class C you should make a cast to the class C, like this: ((C) b).fun2(); instead of b.fun2();

It is always reasonable to test whether or not your object is an instance of the class C before making the cast, this way not generating any error. An example:

if(b instanceof C) {
     ((C) b).fun2();
}

Answer 4

Well fun2 is not part of the interface - you're assigning to an A which does not have that method. You can do

C b = new C();

then you can call fun2 on that.

Or add fun2 to the interface:

interface A {
    void fun();
    void fun2();
}

Or extend the interface to keep A itself unchanged:

interface A2 extends A {
    void fun2();
}
class C implements A2 {
}

A2 b = new C(); 
b.fun2();

Answer 5

As fun2 is not declared in interface A. You cannot call the fun2 defined in class C using the reference of type A. To call the fun2 using A type reference you have to downcast the reference to child class C. You can tweak this code to call fun2 using reference of type A.

if(b instanceOf C)
   ((C)b).fun2();

Answer 6

Define fun2() in the interface A, but override it in your implementing classes.

    public interface A {
        void fun();
        void fun2();
     }

    public class C implements A{
        void fun(){
       // implementation code
        }
        void fun2(){
      // implementation code
       }
     }