Adding new method to interface

Question

I am using an interface in my code which has been there for a long time and many classes have implemented it. Now i have to add a new method to this interface for a new class [old classes dont need the new method]. So as has been suggested at many places that i can extend that old interface and create a new one with new method. Now my problem is that app launcher which uses the interface implementation is only having reference to base interface and using base interface i can't call method in new interface.

baseInterface is extended by newInterface

Class Applauncher{

  baseInterface b;

}

So as can be seen i can't call new method in "newInterface" in Applauncher class.

I want a solution which will not shake up my old implementation.

Why do you need to add this method in your interface if there is only one class using it? — Rob
– Rob, Commented Mar 8, 2013 at 12:38
At present just one and it has to implement baseInterface + new method. — Lokesh
– Lokesh, Commented Mar 8, 2013 at 12:38
The question is a bit confusing - are you saying that your Applauncher class cannot be changed, and holds a reference to baseInterface - but the actual reference being stored is to a newInterface? If so, I think you might be able to handle your problem with casting. So newInterface asNew = (newInterface) b, where b is a variable of type baseInterface which actually stores a reference to newInterface. — devrobf
– devrobf, Commented Mar 8, 2013 at 12:39
Why don't you simply add the method you need to the new class without adding it to the interface? — assylias
– assylias, Commented Mar 8, 2013 at 12:44
Yes Applaunch class is not supposed to be changed. I need to amke a call "newInterface.newMethod()" in AppLauncher but with current implementation i can't do that. You are doing down casting i.e base class to child class, java should throw exception, isn't it? — Lokesh
– Lokesh, Commented Mar 8, 2013 at 12:45

pushy · Accepted Answer · 2013-03-08 12:51:32Z

6

Basically at the point where you need to call the new methods in your applauncher, you could do something like this:

if (b instanceof NewInterface) {
  ((NewInterface)b).newMethod();
}

Would that be a solution for you?

edited Mar 8, 2013 at 12:51

answered Mar 8, 2013 at 12:39

pushy

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2 Comments

Lokesh Over a year ago

I agree, i would look for a more neat solution but if i dont get any other way then your approach will suit me most.

pushy Over a year ago

Yeah, this is not a solution you want to use too much, but in cases like yours it makes sense...

Michael Berry · Accepted Answer · 2013-03-08 12:44:41Z

4

Admittedly not yet released, but when Java 8 comes out you'll have a nice solution to this general issue:

public interface MyInterface {
   Calendar myMethod();
   Object myMeth2() default null;
}

The important point there is the default null bit - the default keyword has a new usage from 8 onwards which allows you to specify default return values (meaning you don't need to implement this method in all implementations.)

answered Mar 8, 2013 at 12:44

Michael Berry

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Comments

Riyafa Abdul Hameed · Accepted Answer · 2022-09-06 08:09:01Z

0

Java 8 on wards provides a facility to create default methods inside the interface. Methods which are defined inside the interface and tagged with default are known as default methods. These methods are non-abstract methods. They allow adding new methods to interfaces without breaking the interface for those that already implement it.

interface MyInterface {    
    // default method    
    default void method1(){    
        System.out.println("Hello, this is default method");    
    }    
    // Already existing Abstract method    
    void method2(String msg); 
}

answered Sep 6, 2022 at 8:09

Riyafa Abdul Hameed

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