layout_text_to_speech.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">
    
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:hint="Enter text here!"
        android:id="@+id/textToSpeak"/>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" 
        android:layout_centerHorizontal="true"
        android:layout_below="@id/textToSpeak"
        android:id="@+id/btnSpeak"/>

</RelativeLayout>

AndroidTextToSpeechActivity.java

public class AndroidTextToSpeechActivity extends Activity implements
        TextToSpeech.OnInitListener {

    EditText textToSpeak = null;
    Button btnSpeak = null;
    TextToSpeech tts;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        textToSpeak = findViewById(R.id.textToSpeak);
        btnSpeak = findViewById(R.id.btnSpeak);
        btnSpeak.setEnabled(false);
        tts = new TextToSpeech(this, this);
        btnSpeak.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    speakOut();
                }
            });
    }

    @Override
    public void onDestroy() {
        // Don't forget to shutdown tts!
        if (tts != null) {
            tts.stop();
            tts.shutdown();
        }
        super.onDestroy();
    }

    @Override
    public void onInit(int status) {
        if (status == TextToSpeech.SUCCESS) {
            int result = tts.setLanguage(Locale.US);

            if (result == TextToSpeech.LANG_MISSING_DATA
                    || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                Log.e("TTS", "This Language is not supported");
            } else {
                btnSpeak.setEnabled(true);
                speakOut();
            }
        } else {
            Log.e("TTS", "Initilization Failed!");
        }
    }

    private void speakOut() {
        String text = textToSpeak.getText().toString();
        if(text == null || text.isEmpty())
            return;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            String utteranceId=this.hashCode() + "";
            tts.speak(text, TextToSpeech.QUEUE_FLUSH, null, utteranceId);
        } else {
            tts.speak(text, TextToSpeech.QUEUE_FLUSH, null);
        }
    }
}

The language to be spoken can be set by providing a Locale to the setLanguage() method:

tts.setLanguage(Locale.CHINESE); // Chinese language

The number of supported languages varies between Android levels. The method isLanguageAvailable() can be used to check if a certain language is supported:

tts.isLanguageAvailable(Locale.CHINESE);

The speech pitch level can be set by using the setPitch() method. By default, the pitch value is 1.0. Use values less than 1.0 to decrease the pitch level or values greater than 1.0 to increase the pitch level:

tts.setPitch(0.6);

The speech rate can be set using setSpeechRate(). The default speech rate is 1.0. The speech rate can be doubled by setting it to 2.0 or made half by setting it to 0.5:

tts.setSpeechRate(2.0);

Cold observable implementation, emits true when TTS engine finishes speaking, starts speaking when subscribed. Notice that API level 21 introduces different way to perform speaking:

public class RxTextToSpeech {

@Nullable RxTTSObservableOnSubscribe audio;

WeakReference<Context> contextRef;

public RxTextToSpeech(Context context) {
    this.contextRef = new WeakReference<>(context);
}

public void requestTTS(FragmentActivity activity, int requestCode) {
    Intent checkTTSIntent = new Intent();
    checkTTSIntent.setAction(TextToSpeech.Engine.ACTION_CHECK_TTS_DATA);
    activity.startActivityForResult(checkTTSIntent, requestCode);
}

public void cancelCurrent() {
    if (audio != null) {
        audio.dispose();
        audio = null;
    }
}

public Observable<Boolean> speak(String textToRead) {
    audio = new RxTTSObservableOnSubscribe(contextRef.get(), textToRead, Locale.GERMANY);
    return Observable.create(audio);
}


public static class RxTTSObservableOnSubscribe extends UtteranceProgressListener
        implements ObservableOnSubscribe<Boolean>,
        Disposable, Cancellable, TextToSpeech.OnInitListener {

    volatile boolean disposed;
    ObservableEmitter<Boolean> emitter;
    TextToSpeech textToSpeech;
    String text = "";
    Locale selectedLocale;
    Context context;

    public RxTTSObservableOnSubscribe(Context context, String text, Locale locale) {
        this.selectedLocale = locale;
        this.context = context;
        this.text = text;
    }

    @Override public void subscribe(ObservableEmitter<Boolean> e) throws Exception {
        this.emitter = e;
        if (context == null) {
            this.emitter.onError(new Throwable("nullable context, cannot execute " + text));
        } else {
            this.textToSpeech = new TextToSpeech(context, this);
        }
    }

    @Override @DebugLog public void dispose() {
        if (textToSpeech != null) {
            textToSpeech.setOnUtteranceProgressListener(null);
            textToSpeech.stop();
            textToSpeech.shutdown();
            textToSpeech = null;
        }
        disposed = true;
    }

    @Override public boolean isDisposed() {
        return disposed;
    }

    @Override public void cancel() throws Exception {
        dispose();
    }

    @Override public void onInit(int status) {

        int languageCode = textToSpeech.setLanguage(selectedLocale);

        if (languageCode == android.speech.tts.TextToSpeech.LANG_COUNTRY_AVAILABLE) {
            textToSpeech.setPitch(1);
            textToSpeech.setSpeechRate(1.0f);
            textToSpeech.setOnUtteranceProgressListener(this);
            performSpeak();
        } else {
            emitter.onError(new Throwable("language " + selectedLocale.getCountry() + " is not supported"));
        }
    }

    @Override public void onStart(String utteranceId) {
        //no-op
    }

    @Override public void onDone(String utteranceId) {
        this.emitter.onNext(true);
        this.emitter.onComplete();
    }

    @Override public void onError(String utteranceId) {
        this.emitter.onError(new Throwable("error TTS " + utteranceId));
    }

    void performSpeak() {

        if (isAtLeastApiLevel(21)) {
            speakWithNewApi();
        } else {
            speakWithOldApi();
        }
    }

    @RequiresApi(api = 21) void speakWithNewApi() {
        Bundle params = new Bundle();
        params.putString(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, "");
        textToSpeech.speak(text, TextToSpeech.QUEUE_ADD, params, uniqueId());
    }

    void speakWithOldApi() {
        HashMap<String, String> map = new HashMap<>();
        map.put(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, uniqueId());
        textToSpeech.speak(text, TextToSpeech.QUEUE_ADD, map);
    }

    private String uniqueId() {
        return UUID.randomUUID().toString();
    }
}

public static boolean isAtLeastApiLevel(int apiLevel) {
    return Build.VERSION.SDK_INT >= apiLevel;
}

}