How to implement interface into application class?

Question

I have this application:

public class App extends Application {

    private AppInterface appInterface;

    @Override
    public void onCreate() {
        super.onCreate();

        appInterface = (AppInterface) this;
    }

    public void something() {
        appInterface.result();
    }
}

Using this interface:

public interface AppInterface {
    void result();
}

And I am trying to call some method inside app and get result from interface:

public class MainActivity extends AppCompatActivity implements AppInterface {

    @Override
    public void result() {
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final App app = (App) getApplication();
        app.something();
    }
}

But I am getting this error:

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.something, PID: 13803 java.lang.RuntimeException: Unable to create application com.something.App: java.lang.ClassCastException: com.something.App cannot be cast to com.something.AppInterface at android.app.ActivityThread.handleBindApplication(ActivityThread.java:5406) at android.app.ActivityThread.-wrap2(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1545) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:154) at android.app.ActivityThread.main(ActivityThread.java:6119) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) Caused by: java.lang.ClassCastException: com.msd.test.App cannot be cast to com.msd.test.AppInterface

What I am doing wrong?

First, calling getApplicationContext() inside an Application is pointless. Just use this. Second, App contains an AppInterface instance. App itself is not an AppInterface instance. — CommonsWare
– CommonsWare, Commented Feb 18, 2017 at 14:45
Ok, replace getApplicationContext() by "this" is clear, but how to fix it? Can you show me? — Michalsx
– Michalsx, Commented Feb 18, 2017 at 14:52

Abhishek Verma · Accepted Answer · 2017-02-18 14:52:12Z

6

Try Interface as following :

In App.java :

    public class App extends Application {

        private AppInterface appInterface;

        @Override
        public void onCreate() {
            super.onCreate();
        }

        public void something(AppInterface appInterface) {
            this.appInterface = appInterface;


//save this for future use, and whenver you want to pass data , use this method like-
        appInterface.result();
    }
}

In MainActivity.java

public class MainActivity extends AppCompatActivity implements AppInterface {

    @Override
    public void result() {
      Log.d("result", "called");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final App app = (App) getApplication();
        // pass you reference from here
        app.something(MainActivity.this);
    }
}

answered Feb 18, 2017 at 14:52

Abhishek Verma

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3 Comments

Michalsx Over a year ago

Thank you very much, now I see my mistake :)

NehaK Over a year ago

Hey @Michalsx you should mark this answer right, So other can also get their answer if they also get this issue..

Michalsx Over a year ago

To all, I marked answer as right, you should see that :)

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