activity_layout.xml

<?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">

    <EditText
        android:id="@+id/editText"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textMultiLine"
        app:layout_constraintBottom_toTopOf="@+id/buttonRead"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />


    <Button
        android:id="@+id/buttonRead"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="readFile"
        android:text="Read"
        app:layout_constraintBottom_toTopOf="@+id/buttonWrite"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />

    <Button
        android:id="@+id/buttonWrite"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="writeFile"
        android:text="Write"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent" />


</androidx.constraintlayout.widget.ConstraintLayout>

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.externalstoragefile">
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity.java

package com.example.externalstoragefile;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;

import android.Manifest;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {
    private final String FILENAME="testfile.txt";
    private final String[] PERMISSIONS_STORAGE = {
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.WRITE_EXTERNAL_STORAGE
    };
    EditText mEditText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mEditText = findViewById(R.id.editText);
    }
    public boolean isExternalStorageWritable() {
        if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())) {
            return true;
        }
        return false;
    }

    public boolean isExternalStorageReadable() {
        if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) ||
                Environment.MEDIA_MOUNTED_READ_ONLY.equals(Environment.getExternalStorageState())) {
            return true;
        }
        return false;
    }

    public void checkStoragePermission() {
        int permission = ActivityCompat.checkSelfPermission(this,
                Manifest.permission.WRITE_EXTERNAL_STORAGE);

        if (permission != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, PERMISSIONS_STORAGE, 101);
        }
    }

    public void writeFile(View view) {
        if (isExternalStorageWritable()) {
            checkStoragePermission();
            try {
                File textFile = new File(Environment.getExternalStorageDirectory(), FILENAME);
                FileOutputStream fileOutputStream = new FileOutputStream(textFile);
                fileOutputStream.write(mEditText.getText().toString().getBytes());
                fileOutputStream.close();
            } catch (java.io.IOException e) {
                e.printStackTrace();
                Toast.makeText(this, "Error writing file", Toast.LENGTH_LONG).show();
            }
        } else {
            Toast.makeText(this, "Cannot write to External Storage", Toast.LENGTH_LONG).show();
        }
    }

    public void readFile(View view) {
        if (isExternalStorageReadable()) {
            checkStoragePermission();
            StringBuilder stringBuilder = new StringBuilder();
            try {
                File textFile = new File(Environment.getExternalStorageDirectory(), FILENAME);
                FileInputStream fileInputStream = new FileInputStream(textFile);
                if (fileInputStream != null) {
                    InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
                    BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
                    String newLine = null;
                    while ((newLine = bufferedReader.readLine()) != null) {
                        stringBuilder.append(newLine + "\n");
                    }
                    fileInputStream.close();
                }
                mEditText.setText(stringBuilder);
            } catch (java.io.IOException e) {
                e.printStackTrace();
                Toast.makeText(this, "Error reading file", Toast.LENGTH_LONG).show();
            }
        } else {
            Toast.makeText(this, "Cannot read External Storage",
                    Toast.LENGTH_LONG).show();
        }
    }
}