1. 파일 생성과 파일에 데이터 저장하기
------------------------------------------------------------------------------------
    MainActivity.kt
------------------------------------------------------------------------------------
class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        var outFs = openFileOutput("output.txt", Context.MODE_PRIVATE)
        var str = "내가 아름다운 애너벨 리의 꿈을 꿀 때만\n" +
                "달도 빛을 비췄고\n" +
                "내가 아름다운 애너벨 리의 빛나는 눈을 느낄 때만\n" +
                "별들도 떠올랐습니다 "
        outFs.write(str.toByteArray())
        outFs.close()
    }
}

2. EditText와 Button을 이용한 파일 생성과 파일에 데이터 저장하기
------------------------------------------------------------------------------------
      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/dataEditText"
        android:layout_width="400dp"
        android:layout_height="500dp"
        android:layout_marginStart="20dp"
        android:layout_marginTop="10dp"
        android:layout_marginEnd="20dp"
        android:layout_marginBottom="220dp"
        android:background="#BBDCFA"
        android:ems="10"
        android:text=""
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/SaveButton"
        android:layout_width="257dp"
        android:layout_height="59dp"
        android:layout_marginStart="8dp"
        android:layout_marginTop="213dp"
        android:layout_marginEnd="8dp"
        android:layout_marginBottom="24dp"
        android:background="#1E88E5"
        android:text="Save"
        android:textColor="#FDD835"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/dataEditText" />

</androidx.constraintlayout.widget.ConstraintLayout>

------------------------------------------------------------------------------------
    MainActivity.kt
-----------------------------class MainActivity : AppCompatActivity() {
    private lateinit var DataET: EditText
    private lateinit var saveButton: Button

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        DataET= findViewById(R.id.dataEditText)
        saveButton = findViewById(R.id.SaveButton)

        saveButton.setOnClickListener  {
            var outFs = openFileOutput("output.txt", Context.MODE_PRIVATE)
            var str = DataET.text.toString()
            outFs.write(str.toByteArray())
            outFs.close()

        }

    }
}-------------------------------------------------------