CODEWITHSUNDEEP
javainterface
Dishonered
Dishonered

Reputation: 8841

Check if subclass implements an interface in parent class?

I know how to check if an object implements an interface. But how do I check in the parent class if the subclass implements the interface?

Suppose 

class A {

   //What can I do to check here?

}

and 

class B extends A implements Listener{


  }

and 

class C extends A {


  }

Here B implements the interface Listener but C does not. How do I check it?

Upvotes: 2

Views: 1870

Answers (4)

davidxxx
davidxxx

Reputation: 131346

this or Object.getClass() returns the reference respectivly the Class object at runtime.
So you can use them to check whether the object or the class implements a specific interface.

If you manipulate classes : 

if (Listener.class.isAssignableFrom(C.class)){
   ...
}

If you manipulate instances :

if (this instanceof Listener){
   ...
}

Upvotes: 5

Sina Hartung
Sina Hartung

Reputation: 26

What I can think of right now is creating an object of that class and checking with 

    object instanceof interface

Or

     object.getClass().getInterfaces()

Upvotes: 0

Joachim Sauer
Joachim Sauer

Reputation: 308021

You can just have a method like this in A:

public void maybeAddListener() {
    if (this instanceof Listener) {
        somethingOrAnother.addListener((Listener) this);
    }
}

Whether or not that's a good idea depends on what exactly you want to achieve with this. If subclasses of A are likely to implement Listener then a better idea is to just implement it in A with a do-nothing implementation and let subclasses override the appropriate methods.

Upvotes: 1

Lino
Lino

Reputation: 19926

You can check in the parent class if the this variable implements your Listener-interface, because the this-variable references to the actual subclass object:

public class A{
    public void doSomething(){
         if(this instanceof Listener){
             // do something if it is a listener
         }
    }
}

So above code block will be executed for the subclass B but not for C

Upvotes: 2

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