向sd卡读写数据

/data/data 是本地存储

/storage/ 是外部存储

SD卡存储

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

   public void writeToSdCard() throws IOException {
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, permissions, REQUEST_CODE);
        } else {
            if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
                String name = et_of_name.getText().toString();
                String content = et_of_content.getText().toString();
                String dir = getExternalFilesDir(null).getAbsolutePath();
                dir = dir + "/" + name;
                FileOutputStream fos = new FileOutputStream(dir);
                fos.write(content.getBytes("utf-8"));
                fos.close();
                Toast.makeText(this, "save to external sdcard", Toast.LENGTH_SHORT).show();
            } else {
                Toast.makeText(this, "without external sdcard", Toast.LENGTH_SHORT).show();
            }
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        switch (requestCode) {
            case REQUEST_CODE:
                if (grantResults.length>0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                    try {
                        writeToSdCard();
                    } catch (IOException e) {
                        Toast.makeText(this, "failed save to external sdcard", Toast.LENGTH_SHORT).show();
                    }
                } else {
                    Toast.makeText(this, "Write External SDCARD Permission Not Granted", Toast.LENGTH_SHORT).show();
                }
                break;
            default:
                break;
        }
    }

    public void read(View v) throws IOException {
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            String name = et_of_name.getText().toString();
            String dir = getExternalFilesDir(null).getAbsolutePath();
            dir = dir + "/" + name + "txt";
            InputStream fis = new FileInputStream(dir);
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            int len = -1;
            byte[] bytes = new byte[1024];
            while ((len = fis.read(bytes)) != -1) {
                bos.write(bytes);
            }
            String content = bos.toString();
            et_of_content.setText(content);
        } else {
            Toast.makeText(this, "without external sdcard", Toast.LENGTH_SHORT).show();
        }
    }

判断文件夹是否存在

File file = new File(dir);if(!file.exists()) {    file.mkdir();}

String dir =

 context.getExternalFilesDir(null).getAbsolutePath();

/storage/emulated/0/Android/data/com.example.storage/files

Environment.getExternalStorageDirectory().getAbsolutePath()

/storage/emulated/0