Android : Android Http ContentLength always -1

I need a method that determines the size of a file on the Internet. This is my method:

    private int getSize(String url){
    try {
        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        //connection.setConnectTimeout(TIMEOUT);
        //connection.setReadTimeout(TIMEOUT);
        //connection.setRequestMethod("HEAD");
        int responseCode = connection.getResponseCode();
        int size = connection.getContentLength();

        if(responseCode != 200){
            Log.e(LOG_TAG, "Fehlercode: " + responseCode + " beim Ermitteln der Dateigroesse von " + url);
            return -1;
        }

        return size;            
    } catch (IOException e) {
        Log.e(LOG_TAG, "Fehler beim Ermitteln der Dateigroesse von " + url, e);
        return -1;
    }
}

With Java (desktop) the method works - on Android it doesn't work. What's wrong with that?

Thanks for the help!