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

    <DatePicker
            android:id="@+id/datePicker1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:calendarViewShown="false"/>
    <EditText
            android:id="@+id/edtDiary"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="#00ff00"
            android:lines="8" />
    <Button
            android:id="@+id/btnWrite"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:enabled="false"
            android:text="Save"/>
</LinearLayout>

package com.example.myapplication2;

import android.annotation.SuppressLint;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Calendar;

public class MainActivity extends AppCompatActivity {

    DatePicker dp;
    EditText edtDiary;
    Button btnWrite;
    String fileName;

    @SuppressLint("MissingInflatedId")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        setTitle("일기장");

        dp = (DatePicker) findViewById(R.id.datePicker1);
        edtDiary = (EditText) findViewById(R.id.edtDiary);
        btnWrite = (Button) findViewById(R.id.btnWrite);

        Calendar cal = Calendar.getInstance();
        int cYear = cal.get(Calendar.YEAR);
        int cMonth = cal.get(Calendar.MONTH);
        int cDay = cal.get(Calendar.DAY_OF_MONTH);

        // 현재 날짜에 해당하는 파일 이름 생성
        fileName = Integer.toString(cYear) + "_" + Integer.toString(cMonth + 1) + "_" + Integer.toString(cDay) + ".txt";

        // 현재 날짜에 해당하는 일기 읽어오기
        String str = readDiary(fileName);
        edtDiary.setText(str);
        btnWrite.setEnabled(true);

        dp.init(cYear, cMonth, cDay, new DatePicker.OnDateChangedListener() {
            @Override
            public void onDateChanged(DatePicker datePicker, int year, int monthOfYear, int dayOfMonth) {
                fileName = Integer.toString(year) + "_" + Integer.toString(monthOfYear + 1) + "_" + Integer.toString(dayOfMonth) + ".txt";
                String str = readDiary(fileName);
                edtDiary.setText(str);
                btnWrite.setEnabled(true);
            }
        });

        btnWrite.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                try {
                    FileOutputStream outFs = openFileOutput(fileName, MODE_PRIVATE);
                    String str = edtDiary.getText().toString();
                    outFs.write(str.getBytes());
                    outFs.close();
                    Toast.makeText(getApplicationContext(), fileName + " 이 저장됨", Toast.LENGTH_SHORT).show();
                } catch (IOException e) {
                }
            }
        });
    }

    private String readDiary(String fileName) {
        String diaryStr = null;
        FileInputStream inFs;
        try {
            inFs = openFileInput(fileName);
            byte[] txt = new byte[500];
            inFs.read(txt);
            inFs.close();
            diaryStr = (new String(txt)).trim();
            btnWrite.setText("수정하기");
        } catch (IOException e) {
            edtDiary.setHint("일기 없음");
            btnWrite.setText("새로 저장");
        }

        return diaryStr;
    }
}