ClassException casting Interface

Question

I have a fragment, which is being used inside a MainActivity, actually it is used inside a ViewPager in a MainActivity.

public class Myfragment extends Fragment implements MySingleton.ResponseInterface{


    public static Myfragment newInstance() {
        final Myfragment mf = new Myfragment();
        return mf;
    }

    public Myfragment() {
    }

@Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        //.....
        final MySingleton mysingleton = MySingleton.getInstance(getContext());

        //I have a button in the fragment that I use like this

        myButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            mysingleton.getSomeResponse();
        }
    });

}

@Override
    public void onResponseGiven(String response) {
        Log.d("response", response);
    }

}

I have a Singleton class to be used for different actions, the singleton includes an interface:

public class MySingleton{


    private static MySingleton mInstance;
    private static Context mContext;
    public ResponseInterface responseInterface;

    private MySingleton(Context context){
        mContext = context;
        this.responseInterface = (ResponseInterface) context;

    }

    public static synchronized MySingleton getInstance(Context context){
        if(mInstance == null){
            mInstance = new MySingleton(context);
        }
        return mInstance;
    }

    public void getSomeResponse(){
       responseInterface.onResponseGiven("send response");
    }


    public interface ResponseInterface{
        void onResponseGiven(String response);
    }
}

Why do I get ClassCastException telling that MainActivity cannot be casted to MySingleton.ResponseInterface??

would like to be able to call the singleton method `getSomeResponse` from any fragment, and get the response back inside the fragment that made the call. — codeKiller, Sep 15 '17 at 11:15

KeLiuyue · Accepted Answer · 2017-09-15T11:44:39.257

1

In your MySingleton class

public class MySingleton{


private static MySingleton mInstance;
private static Context mContext;
public ResponseInterface responseInterface;

private MySingleton(Context context){
    mContext = context;
}

public static synchronized MySingleton getInstance(Context context){
    if(mInstance == null){
        mInstance = new MySingleton(context);
    }
    return mInstance;
}

public void getSomeResponse(ResponseInterface responseInterface){
    this.responseInterface = responseInterface;
    responseInterface.onResponseGiven("send response");
}


public interface ResponseInterface{
    void onResponseGiven(String response);
}
}

And in your Fragment

final MySingleton mysingleton = MySingleton.getInstance(getActivity());
// edited here
mysingleton.getSomeResponse(Myfragment.this);

edited Sep 15 '17 at 11:44

answered Sep 15 '17 at 09:59

KeLiuyue

8,149
4
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are you sure?...no NullPointerException with that? – codeKiller Sep 15 '17 at 10:02
You can check my answer.@eddie – KeLiuyue Sep 15 '17 at 10:25
`mysingleton.getSomeResponse(this);`is being called inside OnClickListener, so `this` won`t work – codeKiller Sep 15 '17 at 10:59
You can try again . @eddie – KeLiuyue Sep 15 '17 at 11:44
I think your answer is THE ONLY one that is working according to expected. Lots of thanks. – codeKiller Sep 15 '17 at 11:53

crgarridos · Answer 2 · 2017-09-15T10:09:27.260

0

If you are casting MainActivity to MySingleton.ResponseInterface, you have to be sure that is implementing it.

In you actual code, MyFragment is implementing MySingleton.ResponseInterface.
I guess MainActivity doesn't

When you are calling MySingleton.getResponse(), you are sending the message to the responseInterface in your singleton. (I guess your activity, given it is a context when you create your singleton's instance).

public void getSomeResponse(){
   responseInterface.onResponseGiven("send response");
}

If you want to get the answer to your fragment, you have to ensure tha the singleton is calling to your fragment instance.

Anyway, there is a very serieus design issue. attaching a fragment/activity instance to your singleton can lead to memeory leaks if you don't free when them are destroyed. read this

Also your singleton isn't really a singleton, you create a new instance after you call getIntance every time.

edited Sep 15 '17 at 10:09

answered Sep 15 '17 at 09:57

crgarridos

8,758
3
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certainly not, but I want to get the `response` on `MyFragment`, If I make `MainActivity` implement `Mysingleton.ResponseInterfce` the response goes to MainActivity, right? – codeKiller Sep 15 '17 at 09:59
implementing the interface, means you have a compatible interface instance. It won't call it magically in your singleton through all implementations. Read my edit – crgarridos Sep 15 '17 at 10:11
I think, with my singleton implementation, if the singleton instance already exists, no new one is created. – codeKiller Sep 15 '17 at 11:32

score 0 · Answer 3 · answered Sep 15 '17 at 09:59

0

You should pass this to the getInstance method:

 MySingleton.getInstance(this);

And try to cast it to MySingleton.ResponseInterface instead of MySingleton.

answered Sep 15 '17 at 09:59

Sammekl

501
4
20

Rahul · Answer 4 · 2017-09-15T10:29:58.613

You are getting class cast exception because in the line final MySingleton mysingleton = MySingleton.getInstance(getContext());, you are passing activity context and you are trying to cast that context to ResponseInterface. ResponseInterface is implemented by Myfragment.

To solve it: You can change MySingeton class constructor as below

private MySingleton(Context context, ResponseInterface responseInterface){
    mContext = context;
    this.responseInterface = responseInterface;

}

and the calling part (in MyFragment OnCreateView) as

final MySingleton mysingleton = MySingleton.getInstance(getContext(), this);

and in getInstance method as

 public static synchronized MySingleton getInstance(Context context, ResponseInterface responseInterface){
    if(mInstance == null){
        mInstance = new MySingleton(context, responseInterface);
    }
    return mInstance;
}

that gives an error on the getInstance method of the Singleton — codeKiller, Sep 15 '17 at 10:06
I think that solution might work, only that, I would always get the response on the same fragment, and I would like to get the response on the fragment that called `getSomeResponse` method — codeKiller, Sep 15 '17 at 11:31

ClassException casting Interface

4 Answers4