Interface questions

Question

Suppose that I have interface MyInterface and 2 classes A, B which implement MyInterface.
I declared 2 objects: MyInterface a = new A() , and MyInterface b = new B().
When I try to pass to a function - function doSomething(A a){} I am getting an error.

This is my code:

public interface MyInterface {}

public class A implements MyInterface{}

public class B implements MyInterface{}

public class Tester {

    public static void main(String[] args){
        MyInterface a = new A();
        MyInterface b = new B();
        test(b);
    }

    public static void test(A a){
        System.out.println("A");
    }

    public static void test(B b){
        System.out.println("B");
    }

}

My problem is that I am getting from some component interface which can be all sorts of classes and I need to write function for each class.
So one way is to get interface and to check which type is it. (instance of A)

I would like to know how others deal with this problem??

Thx

Answer 1

Can you not just have a method on the interface which each class implements? Or do you not have control of the interface?

This would provide both polymorphism and avoid the need to define any external methods. I believe this is the intention of an interface, it allows a client to treat all classes implementing it in a non type specific manner.

If you cannot add to the interface then you would be best introducing a second interface with the appropriate method. If you cannot edit either the interface or the classes then you need a method which has the interface as a parameter and then check for the concrete class. However this should be a last resort and rather subverts the use of the interface and ties the method to all the implementations.

Answer 2

It sounds like you are after something like this:

public static void test(MyInterface obj){
  if(obj instanceof A) {
     A tmp = (A)obj;
  } else if(obj instanceof B) {
     B tmp = (B)obj;
  } else {
     //handle error condition
  }
}

But please note this is very bad form and indicates something has gone seriously wrong in your design. If you don't have control of the interface then, as suggested by marcj, adding a second interface might be the way to go. Note you can do this whilst preserving binary compatibility.