分享Java小白的第一个实用程序，其中因为android权限问题差点挠破头，最后通过查找各类教程终于成功。下面是源代码：

package com.example.timemachine;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;import android.Manifest;
import android.annotation.SuppressLint;
import android.content.pm.PackageManager;
import android.os.Build;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.TextView;
import android.widget.Toast;import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.text.SimpleDateFormat;
import java.util.Date;
public class MainActivity extends AppCompatActivity {private static final int REQUEST_EXTERNAL_STORAGE = 1;private static String[] PERMISSIONS_STORAGE = {Manifest.permission.READ_EXTERNAL_STORAGE,Manifest.permission.WRITE_EXTERNAL_STORAGE};private String fileName;// 记录目录路径private String fileDir;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {//android 6.0以上//进入程序，申请读写权限int permission = ActivityCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE);if (permission != PackageManager.PERMISSION_GRANTED) {ActivityCompat.requestPermissions(this, PERMISSIONS_STORAGE, REQUEST_EXTERNAL_STORAGE);}}}public void TimeC1(View view){@SuppressLint("SimpleDateFormat") SimpleDateFormat format = new SimpleDateFormat("yyyyMM");String y= format.format(new Date());TextView textView1=(TextView) findViewById(R.id.TimeY);textView1.setText(y);}public void TimeC2(View view){@SuppressLint("SimpleDateFormat") SimpleDateFormat format1=new SimpleDateFormat("ddHHmm");String d=format1.format(new Date());TextView textView2=(TextView) findViewById(R.id.TimeD);textView2.setText(d);}public void TimeC(View view){String y1;String d1;String date;TextView textView1=(TextView) findViewById(R.id.TimeY);TextView textView2=(TextView) findViewById(R.id.TimeD);y1=textView1.getText().toString();d1=textView2.getText().toString();String yd;yd=(y1+"\ndayTime "+d1);String head;{head=("default 0\nmres 24\nphoto_size 0\nsplit 222\nyearMonth");}date=(head+" "+yd);TextView textView3=(TextView) findViewById(R.id.textView);textView3.setText(date);}private void createFile(){String mStrPath = Environment.getExternalStorageDirectory().getPath() + "/default.ini";//String SYS_savePath=Environment.getExternalStorageDirectory().toString()+"/SYS";//File SFile=new File(SYS_savePath);File mFile=new File(mStrPath);TextView textView=(TextView) findViewById(R.id.textView2);//if (SFile.exists()){//    SFile.delete();//}//try {//    SFile.createNewFile();//    textView.setText("文件创建成功");//}catch (IOException e){//    e.printStackTrace();//    textView.setText("文件创建失败");//}if (mFile.exists()){mFile.delete();}try {mFile.createNewFile();textView.setText("文件创建成功");}catch (IOException e){e.printStackTrace();textView.setText("文件创建失败");}OutputStream ou = null;String y1;String d1;String date;TextView textView1=(TextView) findViewById(R.id.TimeY);TextView textView2=(TextView) findViewById(R.id.TimeD);y1=textView1.getText().toString();d1=textView2.getText().toString();String yd;yd=(y1+"\ndayTime "+d1);String head;{head=("default 0\nmres 24\nphoto_size 0\nsplit 222\nyearMonth");}date=(head+" "+yd);try {ou = new FileOutputStream(mFile);byte[] buffer = date.getBytes();ou.write(buffer);ou.flush();Toast.makeText(this,"写入成功",Toast.LENGTH_LONG).show();} catch (Exception e) {e.printStackTrace();} finally {try {if (ou != null) {ou.close();}} catch (Exception e) {e.printStackTrace();}}}public void YingY(View view){createFile();}}

读写文件要在AndroidManifest.xml里加入权限：

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/><!-- 允许创建和删除外部存储设备的文件 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"tools:ignore="ProtectedPermissions" /><applicationandroid:requestLegacyExternalStorage="true"

最重要一点是在build.gradle里更改targetSdk版本为28：

 targetSdk 28

下面是功能截图：

本app生成的default.ini文件用于更改华容录像手电日期时间，可获取系统当前时间也可自定义时间。