public class Main {

    public static void main(String[] args) throws IOException {

        URL otodom = new URL("https://www.otodom.pl/sprzedaz/mieszkanie/opole/");
        BufferedReader in = new BufferedReader(new InputStreamReader(otodom.openStream()));
        List<String> links = new ArrayList<>();
        String line;

        while ((line = in.readLine()) != null){
            if (line.contains("https://www.otodom.pl/oferta/")) {
                int index = line.indexOf("https://www.otodom.pl/oferta/");
                links.add(line.substring(index, line.indexOf(".html")));
            }
        }
        links.forEach(s -> System.out.println(s));
        in.close();
    }
}