Android：在外部存储设备上写入/读取文件时出现问题

Question

I'm trying to write/read the external storage, after reading some questions here and on internet, exmeple etc. But i have problem in creating file in the SD, and it doesn't creates and loads, and the mkdirs give me the error that it will be ignored:

public EditText et;
public Button buttonSave,buttonLoad;
public TextView tv;

public String myData = "";

public File root = android.os.Environment.getExternalStorageDirectory();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.requestWindowFeature(Window.FEATURE_NO_TITLE);
    this.getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_main);

    et = findViewById(R.id.inserimento);
    buttonSave = findViewById(R.id.save);
    buttonLoad = findViewById(R.id.load);
    tv = findViewById(R.id.tv);

    File dir = new File(root.getAbsolutePath() + "/eliminaCoda");
    dir.mkdirs();
    final File file = new File(dir, "Lista.txt");

    buttonSave.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            try {
                FileOutputStream fos = new FileOutputStream(file,true);
                fos.write(et.getText().toString().getBytes());
                fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            et.setText("");
        }
    });

    buttonLoad.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            try {
                FileInputStream fis = new FileInputStream(file);
                DataInputStream in = new DataInputStream(fis);
                BufferedReader br =
                        new BufferedReader(new InputStreamReader(in));
                String strLine;
                while ((strLine = br.readLine()) != null) {
                    myData = myData + strLine;
                }
                in.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            tv.setText(myData);
        }
    });
}

Answer 1

You have to add write uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" in AndroidManifest.xml file. Also, you have to request for runtime permission.

Please check this demo for runtime permission Runtime Permissions in Marshmallow