activity_main.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>

MainActivity.java

package com.example.internalstorage;

import androidx.appcompat.app.AppCompatActivity;

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

import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.InputStreamReader;

public class MainActivity extends AppCompatActivity {

    private final String FILENAME="testfile.txt";
    EditText mEditText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mEditText = (EditText) findViewById(R.id.editText);
    }

    public void writeFile(View view) {
        try {
            FileOutputStream fileOutputStream = openFileOutput(FILENAME, Context.MODE_PRIVATE);
            fileOutputStream.write(mEditText.getText().toString().getBytes());
            fileOutputStream.close();
        } catch (java.io.IOException e) {
            e.printStackTrace();
        }
    }

    public void readFile(View view) {
        StringBuilder stringBuilder = new StringBuilder();
        try {
            InputStream inputStream = openFileInput(FILENAME);
            if (inputStream != null) {
                InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
                BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
                String newLine = null;
                while ((newLine = bufferedReader.readLine()) != null) {
                    stringBuilder.append(newLine + "\n");
                }
                inputStream.close();
            }
        } catch (java.io.IOException e) {
            e.printStackTrace();
        }
        mEditText.setText(stringBuilder);
    }
}