đọc ghi nội dung file text trong android

package com.spx.docghifile;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.Toast;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {
    EditText ed_content;

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

        ed_content = findViewById(R.id.ed_content);

    }

    public void SaveFile(View view){
        String noidung = ed_content.getText().toString();
        String file_name = "vidu.txt";

        try {
            // mở luồng tạo file
            FileOutputStream fileOutputStream = openFileOutput(file_name, Context.MODE_PRIVATE);
            // ghi dữ liệu vào file
            fileOutputStream.write(noidung.getBytes());
            // đóng luồng
            fileOutputStream.close();

            Toast.makeText(getBaseContext(),"Đã ghi vào file", Toast.LENGTH_SHORT).show();

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void ReadFile(View view){
        String file_name = "vidu.txt";
        //tạo đối tượng string buff để xây dựng chuỗi dữ liệu
        StringBuffer stringBuffer = new StringBuffer();

        try {

            // luồng dữ liệu file
            FileInputStream fileInputStream = openFileInput(file_name);
            // khai báo luồng đọc dữ liệu
            InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream);
            // tạo biến đệm cho quá trình đọc
            BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

            String line;
            while ( ( line = bufferedReader.readLine() ) != null     ){
                stringBuffer.append(line);
            }


            Toast.makeText(getBaseContext(), stringBuffer.toString(),Toast.LENGTH_SHORT).show();


        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }


    }
}

============ file layout ==============================
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:layout_width="match_parent"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/ed_content"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Nội dung bất kỳ"
        />

    <Button
        android:id="@+id/btn_save"
        android:onClick="SaveFile"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Save to file"/>

    <Button
        android:id="@+id/btn_read"
        android:onClick="ReadFile"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Read from file"/>
</LinearLayout>