Calling a Activity method from BroadcastReceiver in Android

try this code :

your broadcastreceiver class for internet lost class :

public class InternetLostReceiver extends BroadcastReceiver{
@Override
public void onReceive(Context context, Intent intent) {
    context.sendBroadcast(new Intent("INTERNET_LOST"));
}
}

in your activity add this for calling broadcast:

public class TestActivity  extends Activity{

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

    registerReceiver(broadcastReceiver, new IntentFilter("INTERNET_LOST"));
}

BroadcastReceiver broadcastReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        // internet lost alert dialog method call from here...
    }
};

@Override
protected void onDestroy() {
    super.onDestroy();
    unregisterReceiver(broadcastReceiver);
}
}

INTERFACE: Keep BroadCastReceiver and Activity code separate!

You can make a CallBackListener interface. The interface will work as a bridge between BroadcastReceiver and Activity.

1) Create a CallbackListener

interface ConnectionLostCallback{

      public void connectionLost();

} 

2) Provide ConnectionLostCallback in your BroadcastReceiver

public class MyBroadcastReceiver extends BroadcastReceiver{

     private ConnectionLostCallback listener;

     public MyBroadcastReceiver(ConnectionLostCallback listener ){

           this.listener = listener     //<-- Initialze it

     }

     @Override
     public void onReceive(Context context, Intent intent) {

           listener.connectionLost();

     }
}

3) Implement the ConnectionLostCallback in your Activity and override the method

YourActvity extends AppcompatActivity implements ConnectionLostCallback{

    // Your Activity related code //
      //    new MyBroadcastReceiver(this);  <-- create instance

    private void showAlertMessage(){
       AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);
    } 


    @Override 
    public void connectionLost(){

         showAlertMessage();          //<--- Call the method to shoe alert dialog

    }


}

Relevant link:

If you want to know how to make a BroadcastReceiver independent of any activity ie How can you reuse the same BroadCastReceiver with different Activities? Then READ THIS

Add a boolean variable in you activity from where you are open alertdialog

boolean isDialogOpened = false;

// in broadcast recever check 
if(isDialogOpened) {
    alertDialog();
}

And replace your code for alertdialog with this one

public void alertDialog() {
    AlertDialog.Builder alertDialog = new AlertDialog.Builder(this);

    alertDialog.setMessage("Network not found.");
    alertDialog.setPositiveButton("Check Setting",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                }
            });
    alertDialog.setNegativeButton("Cancel",
            new DialogInterface.OnClickListener() {
                public void onClick(DialogInterface dialog, int which) {
                }
            });

    alertDialog.setOnDismissListener(new OnDismissListener() {
        @Override
        public void onDismiss(DialogInterface dialog) {
            isDialogOpened = false;
        }
    });

    alertDialog.setOnCancelListener(new OnCancelListener() {
        @Override
        public void onCancel(DialogInterface dialog) {
            isDialogOpened = false;
        }
    });

    alertDialog.show();
}

Pass your Activity's context to BroadcastReceiver's contructor.

public class ResponseReceiver extends BroadcastReceiver{

    MainActivity ma; //a reference to activity's context

    public ResponseReceiver(MainActivity maContext){
        ma=maContext;
    }

    @Override
    public void onReceive(Context context, Intent intent) {
        ma.brCallback("your string"); //calling activity method
    }

}

and in your MainActivity

public class MainActivity extends AppCompatActivity {
    ...
    public void onStart(){
        ...        
    ResponseReceiver responseReceiver = new ResponseReceiver(this); //passing context
    LocalBroadcastManager.getInstance(this).registerReceiver(responseReceiver,null);
        ...
    }

    public void brCallback(String param){
        Log.d("BroadcastReceiver",param);
    }
}

hope it helps