SpeechToText

Add the below code in activity_main.xml. Here an image view for the mic icon and a textview to show the text that is converted from the speech is added. 

<?xml version="1.0" encoding="utf-8"?>

<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    tools:context=".MainActivity">

    <ImageView

        android:id="@+id/iv_mic"

        android:layout_width="60dp"

        android:layout_height="60dp"

        android:layout_alignParentTop="true"

        android:layout_alignParentEnd="true"

        android:layout_marginTop="204dp"

        android:src="@drawable/ic_mic"

        app:layout_constraintEnd_toEndOf="parent"

        app:layout_constraintHorizontal_bias="0.498"

        app:layout_constraintStart_toStartOf="parent"

        app:layout_constraintTop_toTopOf="parent" />

    <TextView

        android:id="@+id/tv_speech_to_text"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_alignParentTop="true"

        android:layout_alignParentEnd="true"

        android:layout_centerHorizontal="true"

        android:layout_marginTop="44dp"

        android:textSize="30sp"

        android:padding="10dp"

        android:text="@string/tap_mic_and_speek"

        app:layout_constraintEnd_toEndOf="@+id/iv_mic"

        app:layout_constraintHorizontal_bias="0.489"

        app:layout_constraintStart_toStartOf="@+id/iv_mic"

        app:layout_constraintTop_toBottomOf="@+id/iv_mic" />

</androidx.constraintlayout.widget.ConstraintLayout>

Add the below code in MainActivity.java. Here onClickListener is added with the mic icon so when the user clicks on the icon(image) of mic it is invoked.  RecognizerIntent.ACTION_RECOGNIZE_SPEECH is used in the listener that starts an activity that prompts the user for speech and send it through a speech recognizer. The results will be returned via activity results in the onActivityResult() method, when the intent is started using startActivityForResult(). In onActivityResult() method a list of strings is returned and the text is replaced with it in the textview. 

class MainActivity extends AppCompatActivity(){

    private ImageView iv_mic;

    private TextView tv_Speech_to_text;

    private static final int REQUEST_CODE_SPEECH_INPUT = 1;

    @Override 

      protected void onCreate(Bundle savedInstanceState){

        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        iv_mic = findViewById(R.id.iv_mic);

        tv_Speech_to_text = findViewById(R.id.tv_speech_to_text);

        iv_mic.setOnClickListener(new View.OnClickListener() {

            @Override

            public void onClick(View v)

            {

                Intent intent

                    = new Intent(RecognizerIntent.ACTION_RECOGNIZE_SPEECH);

                intent.putExtra(RecognizerIntent.EXTRA_LANGUAGE_MODEL,

                                RecognizerIntent.LANGUAGE_MODEL_FREE_FORM);

                intent.putExtra(RecognizerIntent.EXTRA_LANGUAGE,

                                Locale.getDefault());

                intent.putExtra(RecognizerIntent.EXTRA_PROMPT, "Speak to text");

                try {

                    startActivityForResult(intent, REQUEST_CODE_SPEECH_INPUT);

                }

                catch (Exception e) {

                    Toast

                        .makeText(MainActivity.this, " " + e.getMessage(),

                                  Toast.LENGTH_SHORT)

                        .show();

                }

            }

        });

    }

    @Override

    protected void onActivityResult(int requestCode, int resultCode,

                                    @Nullable Intent data)

    {

        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == REQUEST_CODE_SPEECH_INPUT) {

            if (resultCode == RESULT_OK && data != null) {

                ArrayList<String> result = data.getStringArrayListExtra(

                    RecognizerIntent.EXTRA_RESULTS);

                tv_Speech_to_text.setText(

                    Objects.requireNonNull(result).get(0));

            }

        }

    }

}