src\main\java\com\example\myapplication\MainActivity.java

package com.example.myapplication;

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;

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

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * src\main\AndroidManifest.xml
 * <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
 * <application android:requestLegacyExternalStorage="true" ...></application>
 */
public class MainActivity extends AppCompatActivity {
    int PERMISSION_REQUEST_CODE = 1;
    String fileName = "content.txt";
    File file = new File(Environment.getExternalStorageDirectory().getAbsolutePath(), fileName);
    String text = "Test string";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }


    public void onWithoutPermissions(View view) {
        writeToFile();
    }

    public void onWithPermissions(View view) {
        if (checkPermission()) {
            writeToFile();
        } else {
            requestPermission();
        }
    }

    private void requestPermission() {
        if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.WRITE_EXTERNAL_STORAGE)) {
            Toast.makeText(this, "Permissions should be granted", Toast.LENGTH_LONG).show();
        } else {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, PERMISSION_REQUEST_CODE);
        }
    }

    private boolean checkPermission() {
        int result = ContextCompat.checkSelfPermission(getApplicationContext(), Manifest.permission.WRITE_EXTERNAL_STORAGE);
        if (result == PackageManager.PERMISSION_GRANTED) {
            return true;
        } else {
            return false;
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults)
    {
        super.onRequestPermissionsResult(requestCode,
                permissions,
                grantResults);

        if (requestCode == PERMISSION_REQUEST_CODE) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                Toast.makeText(MainActivity.this, "Permission granted", Toast.LENGTH_SHORT) .show();
                writeToFile();
            }
            else {
                Toast.makeText(MainActivity.this, "Permission denied", Toast.LENGTH_SHORT) .show();
            }
        }
    }

    private void writeToFile() {
        try
        {
            file.createNewFile();
            FileOutputStream fos = new FileOutputStream(file);
            fos.write(text.getBytes());
            fos.close();
            Toast.makeText(this, "Текстовый файл успешно сохранён!", Toast.LENGTH_SHORT).show();
        } catch (FileNotFoundException e)
        {
            e.printStackTrace();
            Toast.makeText(this, "Файл не найден! " + e.getMessage(), Toast.LENGTH_SHORT).show();
        } catch (IOException e)
        {
            e.printStackTrace();
            Toast.makeText(this, "Ошибка сохранения файла! " + e.getMessage(), Toast.LENGTH_SHORT).show();
        }
    }
}