Пример использования хранилища файлов

<?xml version="1.0" encoding="utf-8"?>
<shape 
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle" >

    <!-- View background color -->
    <solid
        android:color="@color/gray_light" >
    </solid>

    <!-- View border color and width -->
    <stroke
        android:width="1dp"
        android:color="@color/black" >
    </stroke>

    <!-- The radius makes the corners rounded -->
    <corners
        android:radius="3dp"   >
    </corners>
</shape>

<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">

    <TextView
     android:id="@+id/tvEditor"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:layout_marginStart="8dp"
     android:layout_marginTop="8dp"
     android:labelFor="@id/etmEditor"
     android:text="@string/tv_editor"
     android:textStyle="bold"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toTopOf="parent" />

    <EditText
     android:id="@+id/etmEditor"
     android:layout_width="380dp"
     android:layout_height="128dp"
     android:layout_marginStart="8dp"
     android:layout_marginTop="8dp"
     android:background="@drawable/border_white"
     android:ems="10"
     android:gravity="start|top"
     android:importantForAutofill="no"
     android:inputType="textMultiLine"
     android:lines="10"
     android:maxLines="10"
     android:padding="4dp"
     android:text="@string/multiple_text"
     android:textAlignment="viewStart"
     android:textSize="14sp"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/tvEditor"
     tools:ignore="DuplicateSpeakableTextCheck" />

    <TextView
     android:id="@+id/textView"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:layout_marginStart="8dp"
     android:layout_marginTop="16dp"
     android:text="@string/tv_viewer"
     android:textStyle="bold"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/etmEditor"/>

    <EditText
     android:id="@+id/etmViewer"
     android:layout_width="380dp"
     android:layout_height="128dp"
     android:layout_marginStart="8dp"
     android:layout_marginTop="8dp"
     android:background="@drawable/border_gray"
     android:cursorVisible="false"
     android:ems="10"
     android:focusable="false"
     android:gravity="start|top"
     android:importantForAutofill="no"
     android:inputType="textMultiLine"
     android:lines="10"
     android:longClickable="false"
     android:maxLines="10"
     android:padding="4dp"
     android:textSize="14sp"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/textView"
     tools:ignore="SpeakableTextPresentCheck" />

    <Button
     android:id="@+id/btnRead"
     android:layout_width="160dp"
     android:layout_height="56dp"
     android:layout_marginStart="8dp"
     android:layout_marginTop="16dp"
     android:onClick="onBtnReadClick"
     android:text="@string/btn_read"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/rbGroup"/>

    <Button
     android:id="@+id/btnSave"
     android:layout_width="160dp"
     android:layout_height="56dp"
     android:layout_marginStart="8dp"
     android:layout_marginTop="16dp"
     android:onClick="onBtnSaveClick"
     android:text="@string/btn_save"
     app:layout_constraintStart_toEndOf="@+id/btnRead"
     app:layout_constraintTop_toBottomOf="@+id/rbGroup"/>

    <RadioGroup
     android:id="@+id/rbGroup"
     android:layout_width="320dp"
     android:layout_height="56dp"
     android:layout_marginStart="16dp"
     android:layout_marginTop="8dp"
     android:background="@drawable/border_white"
     android:orientation="horizontal"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/tvLocation">

       <RadioButton
        android:id="@+id/rbIn"
        android:layout_width="160dp"
        android:layout_height="wrap_content"
        android:text="@string/radio_inner"
        app:layout_constraintStart_toStartOf="parent"/>

       <RadioButton
        android:id="@+id/rbOut"
        android:layout_width="160dp"
        android:layout_height="wrap_content"
        android:text="@string/radio_cd" />
    </RadioGroup>

    <TextView
     android:id="@+id/tvLocation"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
     android:layout_marginStart="16dp"
     android:layout_marginTop="16dp"
     android:text="@string/location"
     app:layout_constraintStart_toStartOf="parent"
     app:layout_constraintTop_toBottomOf="@+id/etmViewer"/>

</androidx.constraintlayout.widget.ConstraintLayout>

package com.example.store.file;

import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.EditText;
import android.widget.RadioButton;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity
{
    private EditText       tvEdit    = null;
    private EditText       tvView    = null;

    private RadioButton    rbIn      = null;

    private  final  String  FILENAME = "myFile";
    private  final  String  LOG_TAG  = "DEVELOPMENT";
    private         String  EXTERNAL_DIR;
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        tvEdit = findViewById(R.id.etmEditor);
        tvView = findViewById(R.id.etmViewer);
        rbIn   = findViewById(R.id.rbIn     );
        // Внешнее хранилище 
        EXTERNAL_DIR = Environment.DIRECTORY_MUSIC;
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    private void readExternalFile()
    {
        Log.d(LOG_TAG, "ExternalStorageState : " + 
                    Environment.getExternalStorageState());
        File dir= getExternalFilesDir(EXTERNAL_DIR);
        if (dir == null) {
            Log.d(LOG_TAG, "dir = NULL");
            return;
        }
        File file;
        file = new File(dir.getAbsolutePath(), FILENAME);
        Log.d(LOG_TAG, "file : " + file.getAbsolutePath());
        Log.d(LOG_TAG, "file exists : " + file.exists());
        if (file.exists()) {
            try {
                FileReader     fr = new FileReader(file);
                BufferedReader br = new BufferedReader(fr);
                String str;
                StringBuilder text = new StringBuilder();
                // читаем содержимое
                while ((str = br.readLine()) != null) {
                    if (!str.contains(":"))
                        str = "    " + str.trim();
                    text.append(str).append("\n");
                }
                tvView.setText(text.toString());
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    void readLocalFile()
    {
        try {
            FileInputStream   fis ;
            InputStreamReader isr ;
            BufferedReader    br  ;
            StringBuilder     text;

            fis  = openFileInput(FILENAME);
            isr  = new InputStreamReader(fis);

            br   = new BufferedReader(isr);
            text = new StringBuilder();
            // читаем содержимое
            String str;
            while ((str = br.readLine()) != null) {
                if (!str.contains(":"))
                    str = "    " + str.trim();
                text.append(str).append("\n");
            }
            tvView.setText(text.toString());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    void writeExternalFile()
    {
        Log.d(LOG_TAG, "ExternalStorageState : " +
                Environment.getExternalStorageState());
        File dir= getExternalFilesDir(EXTERNAL_DIR);
        try {
            File file;
            file= new File(dir.getAbsolutePath(),FILENAME);
            Log.d(LOG_TAG, file exists : "+file.exists());

            String txt="file :\n" +file.getAbsolutePath()
                      + "\ntext :\n" + tvEdit.getText()
                                             .toString();
            FileWriter     fw = new FileWriter(file);
            BufferedWriter bw = new BufferedWriter(fw);
            // пишем данные
            bw.write(txt);
            // закрываем поток
            bw.close();
            Log.d(LOG_TAG, "Файл записан");
            Log.d(LOG_TAG, "file exists : " + file.exists());
            Log.d(LOG_TAG, "file path : " + 
                            file.getAbsolutePath());
        } catch (IOException e) {
            e.printStackTrace();
        } catch (NullPointerException e) {
            e.printStackTrace();
        }
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    void writeLocalFile()
    {
        File internalStorageDir = getFilesDir();
        File file = new File(internalStorageDir, FILENAME);
        String txt = "file :\n" + file.getAbsolutePath()
               +"\ntext :\n" + tvEdit.getText().toString();
        try {
            FileOutputStream    fos;
            OutputStreamWriter  osw;
            fos = openFileOutput(FILENAME, MODE_PRIVATE);
            osw = new OutputStreamWriter(fos);
            BufferedWriter bw = new BufferedWriter(osw);
            bw.write(txt);
            bw.close();
            Log.d(LOG_TAG, "Файл записан");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    public void onBtnReadClick(View v)
    {
        if (rbIn.isChecked())
            readLocalFile();
        else
            readExternalFile();
    }
    //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    public void onBtnSaveClick(View v)
    {
        if (rbIn.isChecked())
            writeLocalFile();
        else
            writeExternalFile();
    }
}