How to use interface to communicate between two activities

I would suggest to create a model class. Let me give you an example:

Model class:

public class CustomModel {    public interface OnCustomStateListener {        void stateChanged();    }    private static CustomModel mInstance;    private OnCustomStateListener mListener;    private boolean mState;    private CustomModel() {}    public static CustomModel getInstance() {        if(mInstance == null) {            mInstance = new CustomModel();        }        return mInstance;    }    public void setListener(OnCustomStateListener listener) {        mListener = listener;    }    public void changeState(boolean state) {        if(mListener != null) {            mState = state;            notifyStateChange();        }    }    public boolean getState() {        return mState;    }    private void notifyStateChange() {        mListener.stateChanged();    }}

And here's how you would use this:

// Importspublic class MainActivity extends Activity implements OnCustomStateListener {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        CustomModel.getInstance().setListener(this);        boolean modelState = CustomModel.getInstance().getState();        Log.d(TAG, "Current state: " + String.valueOf(modelState));        Intent intent = new Intent(this, SecondActivity.class);        startActivity(intent);    }    @Override    public void stateChanged() {        boolean modelState = CustomModel.getInstance().getState();        Log.d(TAG, "MainActivity says: Model state changed: " +             String.valueOf(modelState));    }}

Changing the member state in second activity:

// Importspublic class SecondActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        CustomModel.getInstance().changeState(true);    }}

LogCat output:

Current state: false

MainActivity says: Model state changed: true

Have you considered using LocalBroadcastManager?

In Act1's onCreate:

act2InitReceiver= new BroadcastReceiver()    {        @Override        public void onReceive(Context context, Intent intent)        {            // do your listener event stuff        }    };LocalBroadcastManager.getInstance(this).registerReceiver(act2InitReceiver, new IntentFilter("activity-2-initialized"));

In Act1's onDestroy:

LocalBroadcastManager.getInstance(this).unregisterReceiver(act2InitReceiver);

In Act2's onCreate:

LocalBroadcastManager.getInstance(this).sendBroadcast(new Intent("activity-2-initialized"));

Give me a comment if the code doesn't compile, I'm writing it by hand.

The best, shortest and the easiest way to do this is to use static variables, like this:

class Main extends Activity {  static String message = "Hi";}class Another extends Activity {  public onCreate() {     Log.i(Main.message); // implementation of the message, 'Hi'   }}