"null object reference error" when trying to use TTS

I am trying to use Text-To-Speech but get the following error:

> 2020-07-26 22:58:51.876 15964-15964/ch.yourclick.kitt
> E/AndroidRuntime: FATAL EXCEPTION: main
> Process: ch.yourclick.kitt, PID: 15964
> java.lang.NullPointerException: Attempt to invoke virtual method 'int android.speech.tts.TextToSpeech.speak(java.lang.String, int,
> java.util.HashMap)' on a null object reference
> at ch.yourclick.kitt.MainActivity.speak(MainActivity.java:61)
> at ch.yourclick.kitt.fragments.GeneralFragment$1.onClick(GeneralFragment.java:59)

MainActivity.java

package ch.yourclick.kitt;

import android.os.Bundle;
import com.google.android.material.floatingactionbutton.FloatingActionButton;
import com.google.android.material.snackbar.Snackbar;
import com.google.android.material.tabs.TabLayout;
import androidx.viewpager.widget.ViewPager;
import androidx.appcompat.app.AppCompatActivity;
import android.speech.tts.TextToSpeech;
import android.util.Log;
import android.view.View;
import java.util.Locale;
import ch.yourclick.kitt.ui.main.SectionsPagerAdapter;

public class MainActivity extends AppCompatActivity {
    public TextToSpeech tts;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        SectionsPagerAdapter sectionsPagerAdapter = new SectionsPagerAdapter(this, getSupportFragmentManager());
        ViewPager viewPager = findViewById(R.id.view_pager);
        viewPager.setAdapter(sectionsPagerAdapter);
        TabLayout tabs = findViewById(R.id.tabs);
        tabs.setupWithViewPager(viewPager);
        FloatingActionButton fab = findViewById(R.id.fab);

        // Text to speech
        tts = new TextToSpeech(this, new TextToSpeech.OnInitListener() {
            @Override
            public void onInit(int status) {
                if (status == TextToSpeech.SUCCESS) { // <-- I never get into that if statement
                    int result = tts.setLanguage(Locale.getDefault());
                    // Language is not supported
                    if (result == TextToSpeech.LANG_MISSING_DATA || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                        Log.e("TTS", "Language not supported");
                    }
                }
                else {
                    Log.e("TTS", "" + status); // Returns -1
                    Log.e("TTS", "" + TextToSpeech.SUCCESS); // Returns 0
                }
            }
        });

        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    /**
     * Speak
     */
    public void speak() {
        String text = "Hello";
        tts.speak(text, TextToSpeech.QUEUE_ADD, null);
    }

    /**
     * Turn off
     */
    @Override
    public void onDestroy() {
        if (tts != null) {
            tts.stop();
            tts.shutdown();
        }
        super.onDestroy();
    }

}

GeneralFragment.java

package ch.yourclick.kitt.fragments;

import android.os.Bundle;
import androidx.fragment.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

import ch.yourclick.kitt.MainActivity;
import ch.yourclick.kitt.R;

/**
 * A simple {@link Fragment} subclass.
 * Use the {@link GeneralFragment#newInstance} factory method to
 * create an instance of this fragment.
 */
public class GeneralFragment extends Fragment {

    public GeneralFragment() {
        // Required empty public constructor
    }

    /**
     * Use this factory method to create a new instance of
     * this fragment using the provided parameters.
     *
     * @return A new instance of fragment General.
     */
    // TODO: Rename and change types and number of parameters
    public static GeneralFragment newInstance() {
        GeneralFragment fragment = new GeneralFragment();
        Bundle args = new Bundle();
        fragment.setArguments(args);
        return fragment;
    }

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

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View view = inflater.inflate(R.layout.fragment_general, container, false);
        Button hello = view.findViewById(R.id.hello);
        
        hello.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                MainActivity mainActivity = new MainActivity();
                mainActivity.speak();
            }
        });
        return view;
    }


}

It seems like tts returns null but why?

Consider the following two snippets from your post:

// Code found in onCreate(...) in MainActivity.java
tts = new TextToSpeech(this, new TextToSpeech.OnInitListener() {
// A bunch of code here
}

and

// Code found in an OnClickListener in GeneralFragment.java
MainActivity mainActivity = new MainActivity();
mainActivity.speak();

You're probably getting a NullPointerException due to the fact that tts is null when you call through to mainActivity.speak().

While your intentions were good, the reason for this not working as planned is that you're actually creating a new instance of MainActivity that hasn't gone through the appropriate lifecycle steps, meaning that onCreate(...) was never called and thus tts was never assigned... in that specific activity instance. Worth mentioning is that the "real" activity instance your fragment is running in however should have it set-up correctly.

So, what's the fastest way to get it working? Well, I'd say get the current activity, cast it to MainActivity if possible and try to call speak() on that instance instead:

hello.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
final Activity activity = getActivity();
if (activity instanceof MainActivity) {
((MainActivity) activity).speak();
}
}
});