protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_mostrar_datos);

    new BackgroundTask()
            .execute("https://i.ytimg.com/vi/AyM5sjRpwGk/hqdefault.jpg");

}

@Override
    protected Bitmap doInBackground(String... urls) {
        Bitmap bm = null;
        try {
            URL _url = new URL(urls[0]);
            URLConnection con = _url.openConnection();
            con.connect();
            InputStream is = con.getInputStream();
            BufferedInputStream bis = new BufferedInputStream(is);
            bm = BitmapFactory.decodeStream(bis);
            bis.close();
            is.close();
        } catch (Exception e) {
            Log.e(MainActivity.class.toString(), e.getMessage());
        }
        return bm;
    }

    @Override
    protected void onPostExecute(Bitmap result) {
            MostrarDatos.this.imagen.setImageBitmap(result);
            MostrarDatos.this.imagen.setVisibility(View.VISIBLE);

String imagen = "https://i.ytimg.com/vi/AyM5sjRpwGk/hqdefault.jpg";

new BackgroundTask()
        .execute(imagen);

public class mICLASE extends AppCompatActivity {

    String imagen = "https://i.ytimg.com/vi/AyM5sjRpwGk/hqdefault.jpg";

    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mostrar_datos);

        new BackgroundTask()
                .execute(imagen);

    }

}