通过Android的openFileInput、openFileOutput来获取一个文件输入输出流，从而实现写入/读取文件。主界面只有一个EditText，其中的内容在活动销毁时会自动保存到文件。

主布局文件：

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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:hint="这里的内容会自动保存"
        android:textSize="25sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</android.support.constraint.ConstraintLayout>

主活动类：

public class MainActivity extends AppCompatActivity {

    private EditText editText;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        editText = findViewById(R.id.edittext);
        String data = load();
        if (!TextUtils.isEmpty(data)) {//这里会同时判断两种空值状态 null 和 ""
            editText.setText(data);
            editText.setSelection(data.length());//设置光标位置到输入框的最后
        }
    }

    //    从文件里引导数据
    private String load() {
        FileInputStream fileInputStream = null;
        BufferedReader bufferedReader = null;
        StringBuilder stringBuilder = new StringBuilder();
        try {
            fileInputStream = openFileInput("data.txt");
            bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));
            String line = null;
            while ((line = bufferedReader.readLine()) != null) {
                stringBuilder.append(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (bufferedReader != null) {
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return stringBuilder.toString();
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        save();
    }

    private void save() {
        String data = editText.getText().toString();
        FileOutputStream fileOutputStream;
        BufferedWriter bufferedWriter = null;
        try {
            fileOutputStream = openFileOutput("data.txt", Context.MODE_PRIVATE);
            bufferedWriter = new BufferedWriter(new OutputStreamWriter(fileOutputStream));
            bufferedWriter.write(data);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (bufferedWriter != null) {
                try {
                    bufferedWriter.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

运行截图：