public class Comparators {

public static class CompareByNumber implements Comparator<Flight>, Serializable {

        private static final long serialVersionUID = -2382790006724690579L;

        @Override
        public int compare(Flight f1, Flight f2) {
            return f1.getNumber().compareTo(f2.getNumber());
        }

    }
}

String search = request.getParameter("search");
        if ("number".equals(search)){
            Collections.sort(flight, new Comparators.CompareByNumber());
            Collections.binarySearch(flight, "", new Comparators.CompareByNumber());
        }