Flight Cities problem

import java.util.ArrayList;

public class Main {
/* So the thing is I'm supposed to create a new method for
FlightDatabase class which will return (not print) the list
of all cities included in FlightDatabase constructor, without
duplicates. I got a hint to use reversed boolean (!=), and
Arraylist.contains (object o) method.
On the bottom of this paste is what I came up with, doesn't work,
IntelliJ's saying that this method's return is never used.*/

    public static void main ( String[] args ) {

        FlightDatabase flightDatabase = new FlightDatabase ();{
//            flightDatabase.checkIfFlightExists ("Wroclaw", "Berlin");
//            flightDatabase.displayFlightsFromCity ( "Warsaw" );
//            flightDatabase.displayFlightsToCity ( "Rotterdam" );
              flightDatabase.getCities ();


        }
    }
}
public class Flight{

    String departure;
    String arrival;

    public Flight (String departure, String arrival){
        this.departure = departure;
        this.arrival = arrival;
    }
    public String getInfo(){

        return "\n\nFlight from: " + this.departure + "\n\nFlight to: " + this.arrival;
    }

}
import java.util.ArrayList;

public class FlightDatabase {
    ArrayList<Flight> flights = new ArrayList<Flight> ();

    public FlightDatabase ( ) {
        this.flights.add ( new Flight ( "Wroclaw", "Berlin" ) );
        this.flights.add ( new Flight ( "Warsaw", "Poznan" ) );
        this.flights.add ( new Flight ( "Krakow", "Vienna" ) );
        this.flights.add ( new Flight ( "Wroclaw", "Rotterdam" ) );
        this.flights.add ( new Flight ( "Warsaw", "Moscow" ) );
        this.flights.add ( new Flight ( "Wroclaw", "Frankfurt" ) );
        this.flights.add ( new Flight ( "Warsaw", "London" ) );
        this.flights.add ( new Flight ( "Krakow", "Paris" ) );
    }

    public void checkIfFlightExists ( String departure, String arrival ) {
        for (int i = 0; i < flights.size (); i++) {
            Flight check = flights.get ( i );
            if (departure.equals ( check.departure ) && arrival.equals ( check.arrival )) {
                System.out.println ( "there is such flight" );
                return;
            }
        }
        System.out.println ( "No such flight sorry" );
    }

    public ArrayList<Flight> getFlightsFromCity ( String departure ) {
        ArrayList<Flight> results = new ArrayList<Flight> ();
        for (int i = 0; i < this.flights.size (); i++) {
            Flight check = this.flights.get ( i );
            if (departure.equals ( check.departure )) {
                results.add ( check );

            }
        }
        return results;
    }

    public ArrayList<Flight> getFlightsToCity ( String arrival ) {
        ArrayList<Flight> results = new ArrayList<Flight> ();
        for (int i = 0; i < this.flights.size (); i++) {
            Flight check = this.flights.get ( i );
            if (arrival.equals ( check.arrival )) {
                results.add ( check );
            }
        }
        return results;
    }

    public void displayFlights ( ArrayList<Flight> results ) {
        if (results.isEmpty ()) {
            System.out.println ( "There is no such flight" );
        }
        for (int i = 0; i < results.size (); i++) {
            Flight check = results.get ( i );
            System.out.println ( check.getInfo () );
        }
    }

    public void displayFlightsFromCity ( String departure ) {
        ArrayList<Flight> results = getFlightsFromCity ( departure );
        displayFlights ( results );
    }

    public void displayFlightsToCity ( String arrival ) {
        ArrayList<Flight> results = getFlightsToCity ( arrival );
        displayFlights ( results );
    }

    public ArrayList<String> getCities ( ) {
        ArrayList<String> cities = new ArrayList<String> ();
        for (Flight check : flights) {
            if (!flights.contains ( check.arrival )) cities.add ( check.arrival );
            if (!flights.contains ( check.departure )) cities.add ( check.departure );

        }
        return (cities);

    }
}