我是一个初学者，努力让这个工作今天。

下面是我最终选择的类。它起作用了，但我想知道我的解决方案有多不完美。不管怎样，我希望你们中一些更有经验的人可能愿意看看我的IO课程，并给我一些建议。干杯!

public class HighScore {
    File data = new File(Environment.getExternalStorageDirectory().getAbsolutePath() + File.separator);
    File file = new File(data, "highscore.txt");
    private int highScore = 0;

    public int readHighScore() {
        try {
            BufferedReader br = new BufferedReader(new FileReader(file));
            try {
                highScore = Integer.parseInt(br.readLine());
                br.close();
            } catch (NumberFormatException | IOException e) {
                e.printStackTrace();
            }
        } catch (FileNotFoundException e) {
            try {
                file.createNewFile();
            } catch (IOException ioe) {
                ioe.printStackTrace();
            }
            e.printStackTrace();
        }
        return highScore;
    }

    public void writeHighScore(int highestScore) {
        try {
            BufferedWriter bw = new BufferedWriter(new FileWriter(file));
            bw.write(String.valueOf(highestScore));
            bw.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

我们可以使用这段代码将String写入文件

public static void writeTextToFile(final String filename, final String data) {
    File file = new File(filename);
    try {
        FileOutputStream stream = new FileOutputStream(file);
        stream.write(data.getBytes());
        stream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

然后在Main代码中，我们使用这个

writeTextToFile(getExternalFilesDir("/").getAbsolutePath() + "/output.txt", "my-example-text");

之后，检查Android/data/<package-name>/files文件。

我们需要的第一件事是AndroidManifest.xml中的权限

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

所以在asyncTask Kotlin类中，我们处理文件的创建

    import android.os.AsyncTask
    import android.os.Environment
    import android.util.Log
    import java.io.*
    class WriteFile: AsyncTask<String, Int, String>() {
        private val mFolder = "/MainFolder"
        lateinit var folder: File
        internal var writeThis = "string to cacheApp.txt"
        internal var cacheApptxt = "cacheApp.txt"
        override fun doInBackground(vararg writethis: String): String? {
            val received = writethis[0]
            if(received.isNotEmpty()){
                writeThis = received
            }
            folder = File(Environment.getExternalStorageDirectory(),"$mFolder/")
            if(!folder.exists()){
                folder.mkdir()
                val readME = File(folder, cacheApptxt)
                val file = File(readME.path)
                val out: BufferedWriter
                try {
                    out = BufferedWriter(FileWriter(file, true), 1024)
                    out.write(writeThis)
                    out.newLine()
                    out.close()
                    Log.d("Output_Success", folder.path)
                } catch (e: Exception) {
                    Log.d("Output_Exception", "$e")
                }
            }
            return folder.path

    }

        override fun onPostExecute(result: String) {
            super.onPostExecute(result)

            if(result.isNotEmpty()){
                //implement an interface or do something
                Log.d("onPostExecuteSuccess", result)
            }else{
                Log.d("onPostExecuteFailure", result)
            }
        }

    }

当然，如果你使用的是Api 23以上的Android，你必须处理请求以允许写入设备内存。就像这样

    import android.Manifest
    import android.content.Context
    import android.content.pm.PackageManager
    import android.os.Build
    import androidx.appcompat.app.AppCompatActivity
    import androidx.core.app.ActivityCompat
    import androidx.core.content.ContextCompat

    class ReadandWrite {
        private val mREAD = 9
        private val mWRITE = 10
        private var readAndWrite: Boolean = false
        fun readAndwriteStorage(ctx: Context, atividade: AppCompatActivity): Boolean {
            if (Build.VERSION.SDK_INT < 23) {
                readAndWrite = true
            } else {
                val mRead = ContextCompat.checkSelfPermission(ctx, Manifest.permission.READ_EXTERNAL_STORAGE)
                val mWrite = ContextCompat.checkSelfPermission(ctx, Manifest.permission.WRITE_EXTERNAL_STORAGE)

                if (mRead != PackageManager.PERMISSION_GRANTED) {
                    ActivityCompat.requestPermissions(atividade, arrayOf(Manifest.permission.READ_EXTERNAL_STORAGE), mREAD)
                } else {
                    readAndWrite = true
                }

                if (mWrite != PackageManager.PERMISSION_GRANTED) {
                    ActivityCompat.requestPermissions(atividade, arrayOf(Manifest.permission.WRITE_EXTERNAL_STORAGE), mWRITE)
                } else {
                    readAndWrite = true
                }
            }
            return readAndWrite
        }
    }

然后在一个活动中执行调用。

  var pathToFileCreated = ""
    val anRW = ReadandWrite().readAndwriteStorage(this,this)
    if(anRW){
        pathToFileCreated =  WriteFile().execute("onTaskComplete").get()
        Log.d("pathToFileCreated",pathToFileCreated)
    }

Kotlin的方法是在文件上使用内置的扩展函数

写:yourFile.writeText (textFromEditText) 读:yourFile.readText ()

对于那些寻找读取和写入字符串到文件的一般策略的人:

首先，获取一个文件对象

您需要存储路径。对于内部存储，使用:

File path = context.getFilesDir();

外置存储(SD卡)使用:

File path = context.getExternalFilesDir(null);

然后创建你的文件对象:

File file = new File(path, "my-file-name.txt");

写入一个字符串到文件中

FileOutputStream stream = new FileOutputStream(file);
try {
    stream.write("text-to-write".getBytes());
} finally {
    stream.close();
}

或者用谷歌番石榴

字符串内容=文件。toString(文件、StandardCharsets.UTF_8);

将文件读入字符串

int length = (int) file.length();

byte[] bytes = new byte[length];

FileInputStream in = new FileInputStream(file);
try {
    in.read(bytes);
} finally {
    in.close();
}

String contents = new String(bytes);   

或者如果你用谷歌番石榴

String contents = Files.toString(file,"UTF-8");

为了完整起见，我要提一下

String contents = new Scanner(file).useDelimiter("\\A").next();

它不需要库，但基准测试比其他选项慢50% - 400%(在我的Nexus 5上的各种测试中)。

笔记

对于这些策略，您将被要求捕获IOException。

Android上默认的字符编码是UTF-8。

如果你使用外部存储，你需要添加到你的清单:

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

or

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

写权限意味着读权限，所以不需要两者兼备。

public static void writeStringAsFile(final String fileContents, String fileName) {
    Context context = App.instance.getApplicationContext();
    try {
        FileWriter out = new FileWriter(new File(context.getFilesDir(), fileName));
        out.write(fileContents);
        out.close();
    } catch (IOException e) {
        Logger.logError(TAG, e);
    }
}

public static String readFileAsString(String fileName) {
    Context context = App.instance.getApplicationContext();
    StringBuilder stringBuilder = new StringBuilder();
    String line;
    BufferedReader in = null;

    try {
        in = new BufferedReader(new FileReader(new File(context.getFilesDir(), fileName)));
        while ((line = in.readLine()) != null) stringBuilder.append(line);

    } catch (FileNotFoundException e) {
        Logger.logError(TAG, e);
    } catch (IOException e) {
        Logger.logError(TAG, e);
    } 

    return stringBuilder.toString();
}