在Android 7设备的SD卡中写入文件

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

String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
    // SD卡可用
    ...
} else {
    // SD卡不可用
    ...
}

File dir = new File(Environment.getExternalStorageDirectory(), "my_folder");
if (!dir.exists()) {
    dir.mkdirs();
}

File file = new File(dir, "my_file.txt");
String data = "This is some sample data.";
try {
    FileOutputStream fos = new FileOutputStream(file);
    fos.write(data.getBytes());
    fos.close();
} catch (IOException e) {
    e.printStackTrace();
}