Interface and Implementation

[Lucky Singh]

Hi, I have a package ABC. In that package, I have an interface, an implementation for the interface and a Driver that uses tests the application.

public interface Greetings {
public void sayHi();
public void sayBye();
}

public class Implementation {
public void sayHi() {
//do something
}

public void sayBye() {
//do something
}

public boolean check() {
//return a bolean value
}
}

public class Driver {
public static void main(String args[])
{
Greetings obj = new Impl();
boolean ret = obj.check(); ---------ERROR
obj.sayHi();
obj.sayBye();
}
}

The object obviously does not recognize the method check. How do I overcome this problem so that I could use check() in the Driver but not include it in my interface?

[Paul Sturrock]

There are two ways, either use you implementation directly:

Impl obj = new Impl();

or cast the object before using it:

Greetings obj = new Impl();
boolean ret = ((Impl)obj).check(); 

[Lucky Singh]

Thanks for your response. Solution 1 seems fine.
I would however like to use soultion 2. When I tried it, I got an error-
Cannot convert IMPL to type boolean. Is there any way, I can use solution 2 or something similar?

[Tony Morris]

Originally posted by Lucky Singh:
Hi, I have a package ABC. In that package, I have an interface, an implementation for the interface and a Driver that uses tests the application.

public interface Greetings {
     public void sayHi();
     public void sayBye();
}
 
public class Implementation {
    public void sayHi() {
         //do something
     }
 
     public void sayBye() {
         //do something
     }
 
     public boolean check() {
         //return a bolean value
     }
}
 
public class Driver {
    public static void main(String args[])
    {
        Greetings obj = new Impl();
         boolean ret = obj.check(); ---------ERROR
         obj.sayHi();
         obj.sayBye(); 
     }
}

The object obviously does not recognize the method check. How do I overcome this problem so that I could use check() in the Driver but not include it in my interface?

Simply using interface inheritance, or horribly using a cast to a concrete type.

interface Checkable {
    void check();
}
 
interface CheckableGreetings extends Checkable, Greetings {
    // never declare an interface with no methods, 
    // except for the case where it has two or more supertypes.
}

Now your implementation should implement the CheckableGreetings interface (if that indeed what it should do). Note that your provided source code sample contains errors, so I won't try to fix them based on assumptions. I hope the point is clear. Casting can always be avoided with a more approopriate abstraction.
[ August 24, 2005: Message edited by: Tony Morris ]