Communication between fragments via interfaces

Question

The fragments are a part of the bottom navigation bar, so far so good on the navigation part. But when i try to pass data from fragment1 to fragment2 the app is crashing. Also I am using the example given by google at this following link

When I use the following code android studio gives me deprecated warning!

@Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

So I am using CONTEXT instead of ACTIVITY below is my code that crashes :

MainActivity

import android.support.annotation.NonNull;
import android.support.design.widget.BottomNavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;
import com.br.tron.bottombar.RadioFragment;
import com.br.tron.bottombar.StreamFragment;
import com.br.tron.bottombar.InfoFragment;

public class MainActivity extends AppCompatActivity implements RadioFragment.OnNameSetListener {

    BottomNavigationView bottomNavigationView;
    private Fragment fragment;
    private FragmentManager fragmentManager;

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

        fragmentManager = getSupportFragmentManager();
        fragment = new RadioFragment();
        final FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.add(R.id.main_container, fragment).commit();

        bottomNavigationView = (BottomNavigationView) findViewById(R.id.bottomNavigationBar);
        bottomNavigationView.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener(){
            @Override
            public boolean onNavigationItemSelected(@NonNull MenuItem item) {

                switch (item.getItemId()) {
                    case R.id.nav_button_one:
                        fragment = new RadioFragment();
                        break;
                    case R.id.nav_button_two:
                        fragment = new StreamFragment();
                        break;
                    case R.id.nav_button_three:
                        fragment = new InfoFragment();
                        break;
                }
                final FragmentTransaction transaction = fragmentManager.beginTransaction();
                transaction.replace(R.id.main_container, fragment).commit();
                return true;
            }
        });
    }

    public void performStreamClick(){
        View view = bottomNavigationView.findViewById(R.id.nav_button_two);
        view.performClick();
    }

    @Override
    public void setUrl(String url) {
        StreamFragment frag=(StreamFragment) getSupportFragmentManager().findFragmentByTag("frag");
        frag.getUrl(url);
    }
}

RadioFragment(Fragment1 where the data exist)

import android.app.Activity;
import android.content.Context;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

public class RadioFragment extends Fragment implements Button.OnClickListener  {

    Button buttonman;
    View rootView;
    String url;
    Activity a;
    OnNameSetListener onNameSetListener;

    @Override
    public void onAttach(Context context) {
        super.onAttach(context);
        if (context instanceof Activity) {
            a = (Activity) context;
            try{
                onNameSetListener=(OnNameSetListener) context;
            }
            catch (Exception e){}
        }
    }

    public RadioFragment(){
    };

    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container, Bundle savedInstanceState) {

        rootView = inflater.inflate(R.layout.fragment_player, container, false);
        buttonman = (Button)rootView.findViewById(R.id.buttonman);
        buttonman.setOnClickListener(this);
        return rootView;
    }

    @Override
    public void onClick(View v) {
        /*Fragment fragment = new StreamFragment();
        FragmentManager fragmentManager = getActivity().getSupportFragmentManager();
        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.replace(R.id.main_container, fragment);
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();*/
        url="www.idontknow.com";
        onNameSetListener.setUrl(url);
        ((MainActivity)a).performStreamClick();
    }

    public interface OnNameSetListener
    {
        public void setUrl(String url);
    }
}

StreamFragment(fragment2 where I want to send the data -from fragment1)

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

public class StreamFragment extends Fragment {

    TextView textViewStream;

    public StreamFragment(){};

    @Override
    public View onCreateView(final LayoutInflater inflater,final ViewGroup container,final Bundle savedInstanceState) {
        return  inflater.inflate(R.layout.fragment_stream, container, false);

    }

    public void getUrl(String url)
    {
        textViewStream.setText(url);
    }

}

LOGCAT

01-06 20:10:23.023 3744-3744/com.br.tron.bottombar E/AndroidRuntime: FATAL EXCEPTION: main Process: com.br.tron.bottombar, PID: 3744 java.lang.NullPointerException: Attempt to invoke virtual method 'void com.br.tron.bottombar.StreamFragment.getUrl(java.lang.String)' on a null object reference at com.br.tron.bottombar.MainActivity.setUrl(MainActivity.java:64) at com.br.tron.bottombar.RadioFragment.onClick(RadioFragment.java:61) at android.view.View.performClick(View.java:5201) at android.view.View$PerformClick.run(View.java:21209) at android.os.Handler.handleCallback(Handler.java:739) at android.os.Handler.dispatchMessage(Handler.java:95) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5525) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) 01-06 20:10:24.663 3744-3744/? I/Process: Sending signal. PID: 3744 SIG: 9

Answer 1

You should initialize your textViewStream in the StreamFragment fragment.

Answer 2

You never register the frag tag you later search for.

In your MainActivity modify:

final FragmentTransaction transaction = fragmentManager.beginTransaction();
transaction.add(R.id.main_container, fragment, "frag").commit();
...
final FragmentTransaction transaction = fragmentManager.beginTransaction();
transaction.replace(R.id.main_container, fragment, "frag").commit();
...

In addition to that within your StreamFragment class you never set the TextViewStream variable after you inflate your layout.

@Override
public View onCreateView(final LayoutInflater inflater,final ViewGroup container,final Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_stream, container, false);
    textViewStream = (TextView) view.findViewById(R.id.ID_GOES_HERE);
    return view;
}

Edit: Further Errors

You're getting a ClassCastException because you're not checking to see whether the fragment you get in MainActivity.setUrl is a StreamFragment. And it makes sense that it's failing because you register the frag tag for all three types of custom Fragments. Here's a further solution:

        fragment = new RadioFragment();
        final FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.add(R.id.main_container, fragment, "radio_fragment").commit();
        ...

        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {

            switch (item.getItemId()) {
                String tag = "";
                case R.id.nav_button_one:
                    fragment = new RadioFragment();
                    tag = "radio_fragment";
                    break;
                case R.id.nav_button_two:
                    fragment = new StreamFragment();
                    tag = "stream_fragment";
                    break;
                case R.id.nav_button_three:
                    fragment = new InfoFragment();
                    tag = "info_fragment";
                    break;
            }
            final FragmentTransaction transaction = fragmentManager.beginTransaction();
            transaction.replace(R.id.main_container, fragment, tag).commit();
            return true;
        }

Make the appropriate change in the Tag string in setUrl:

StreamFragment frag=(StreamFragment) getSupportFragmentManager().findFragmentByTag("stream_fragment");
if (frag != null) frag.getUrl(url);

Answer 3

Both of the current answers key in on your crash problem. I expect that will take care of your primary question. @asadmshah seems to have taken care of your next problem. Make sure to up vote his answer & accept it.

Regarding the warning from using Google's example code, here is what AS will currently give you if you tell it to create a new fragment.

@Override
public void onAttach(Context context) {
    super.onAttach(context);
    if (context instanceof OnFragmentInteractionListener) {
        mListener = (OnFragmentInteractionListener) context;
    } else {
        throw new RuntimeException(context.toString()
                + " must implement OnFragmentInteractionListener");
    }
}

The point of this is to make sure that the Activity you're attaching to implements the interface you've defined in your fragment.

And on the subject of the interface, all fragment to activity comms should be done through it. It would be better to find a way to call performStreamClick() through the interface as well. (I just realized I made this same mistake on an app I started several months ago but am just finishing up now :).

Lastly, just an FYI, you can get the attached activity of a fragment with getActivity().

EDIT: 1st, it might help to see your current code. I'm going to assume you have what Asad has put in his answer, more or less. Also, what is the expected behavior? I believe that you make a selection in RadioFragment then replace it with StreamFragment. That is, both fragments take the whole nav bar, they don't exist at the same time. If this is wrong, let me know.

If you're still calling setUrl() and performStreamClick() in the same order as before, you're getting null because you've never created a fragment by that name. You can't find a fragment you haven't already created.

Without actually setting this up to test it myself, I believe what you need to do would be to save the url in a global variable in setUrl(), then in your click listener, pass that to the StreamFragment when you create it.

See the "Deliver a Message to a Fragment" section of the link you've sited for doing that

        Bundle args = new Bundle();
        args.putInt(ArticleFragment.ARG_POSITION, position);
        newFragment.setArguments(args);

(obviously, handling your url string instead of the int in the example)

Also, it's not a bad idea to handle null when searching for a fragment that should exist already

    StreamFragment frag=(StreamFragment) getSupportFragmentManager().findFragmentByTag("stream_frag");
    if(frag == null) {
        final FragmentTransaction transaction = fragmentManager.beginTransaction();
        StreamFragment fragment = new StreamFragment();
        transaction.replace(R.id.main_container, fragment, "stream_frag").commit();
    }