Why the application does not work on API 17?

I want to save image in gallery, this code works, but only for above API 17? Have you any idea?

public class MainActivity extends AppCompatActivity {

Bitmap anImage;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Drawable myDrawable = ContextCompat.getDrawable(this, R.drawable.tapeta1);
    anImage = ((BitmapDrawable) myDrawable).getBitmap();
}

public void addImageToGallery(View view) {
    int e = view.getId();

    switch (e) {
        case R.id.button:
            saveImageToExternalStorage(anImage);
            if (ContextCompat.checkSelfPermission(this,
                    Manifest.permission.WRITE_EXTERNAL_STORAGE)
                    != PackageManager.PERMISSION_GRANTED) {

                // Should we show an explanation?
                if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                        Manifest.permission.WRITE_EXTERNAL_STORAGE)) {

                    // Show an explanation to the user *asynchronously* -- don't block
                    // this thread waiting for the user's response! After the user
                    // sees the explanation, try again to request the permission.

                } else {

                    // No explanation needed, we can request the permission.

                    ActivityCompat.requestPermissions(this,
                            new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
                            1);

                    // MY_PERMISSIONS_REQUEST_READ_CONTACTS is an
                    // app-defined int constant. The callback method gets the
                    // result of the request.
                }
            }

            break;
    }
}

private void saveImageToExternalStorage(Bitmap finalBitmap) {
    String root = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES).toString();
    File myDir = new File(root + "/saved_images_1");
    myDir.mkdirs();
    Random generator = new Random();
    int n = 10000;
    n = generator.nextInt(n);
    String fname = "Image-" + n + ".jpg";
    File file = new File(myDir, fname);
    if (file.exists())
        file.delete();
    try {
        FileOutputStream out = new FileOutputStream(file);
        finalBitmap.compress(Bitmap.CompressFormat.JPEG, 90, out);
        out.flush();
        out.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

    MediaScannerConnection.scanFile(this, new String[]{file.toString()}, null,
            new MediaScannerConnection.OnScanCompletedListener() {
                public void onScanCompleted(String path, Uri uri) {
                    Log.i("ExternalStorage", "Scanned " + path + ":");
                    Log.i("ExternalStorage", "-> uri=" + uri);
                }
            });
    Toast.makeText(getApplicationContext(), "Saved successfully, Check gallery", Toast.LENGTH_SHORT).show();

}

}