Question

I am creating a flight controller application. A bit of functionality that i want is to be able to tell the user what the next flight is according to a specific airline. I have a hash map which stores strings and planes. In my plane class i am implementing Comparable and i have the compareTo method. Could anyone help me achieve using the compareTo method to arrange the planes in descending order to show the next flight. I want to arrange the flights by the variable overdue.

This is the case in the MainApp that i have to use the compareTo on

switch (nextChoice) 
                    {
                    case 1:
                        airlineMap.printAirline("Aer Lingus");
                        break;
                    case 2:
                        airlineMap.printAirline("Brittish Airways");
                        break;
                    case 3:
                        airlineMap.printAirline("Eithad");
                        break;
                    case 4:
                        airlineMap.printAirline("Iberia");
                        break;
                    case 5:
                        airlineMap.printAirline("Quantas");
                        break;

I hope to add the descending order to a airline print: airlineMap.printAirline(“Aer Lingus”);

Here is my Plane class:

import java.util.LinkedList;

public class Plane implements Comparable
{   
    private String flightNumber;
    public String airlineName;
    private double fuelRemaining;
    private int overdue;
    private int passengerNumber;
    private AIRPLANETYPE planeType;

    public enum AIRPLANETYPE
    {
        AIRBUS("1"), CORPORATE("2"), PRIVATE("3");

        private String planeName;

        private AIRPLANETYPE(String planeName)
        {
            this.planeName = planeName;
        }

        public String getPlaneName()
        {
            return this.planeName;
        }
    }

    public Plane(String flightNumber, String airlineName, double fuelRemaining, int overdue, int passengerNumber, AIRPLANETYPE planeType) {
        this.flightNumber = flightNumber;
        this.airlineName = airlineName;
        this.fuelRemaining = fuelRemaining;
        this.passengerNumber = passengerNumber;
        this.overdue = overdue;
        this.planeType = planeType;
    }


    public String getAirlineName() {
        return airlineName;
    }

    public void setAirlineName(String airlineName) {
        this.airlineName = airlineName;
    }


    public void setOverdue(int overdue) {
        this.overdue = overdue;
    }

    public int getOverdue(){
        return overdue;
    }

    public String getFlightNumber() {
        return flightNumber;
    }

    public void setFlightNumber(String flightNumber) {
        this.flightNumber = flightNumber;
    }

    public double getFuelRemaining() {
        return fuelRemaining;
    }

    public void setFuelRemaining(double fuelRemaining) {
        this.fuelRemaining = fuelRemaining;
    }

    public int getPassengerNumber() {
        return passengerNumber;
    }

    public void setPassengerNumber(int passengerNumber) {
        this.passengerNumber = passengerNumber;
    }

    public AIRPLANETYPE getPlaneType() {
        return planeType;
    }

    public void setPlaneType(AIRPLANETYPE planeType) {
        this.planeType = planeType;
    }

    public int compareTo(Object arg0) {
        if((arg0 != null) && (arg0 instanceof Plane))
        {
            Plane p = (Plane) arg0;
            return (int)Math.ceil(this.overdue - p.getOverdue());
        }
        return 0;
    }
    public String toString() {
        return "Plane: flightNumber=" + flightNumber + "."
                + " airlineName=" + airlineName + "."
                + " fuelRemaining=" + fuelRemaining + " litres."
                + " overdue=" + overdue + " minutes."
                + " passengerNumber="+ passengerNumber + "."
                + " airplaneType=" + planeType + ".\n";
    }
}

Answer 1

Since HashMap is unordered, you have two ways of going about sorting your planes:

• Put them into a sorted container, or
• Put them into an ArrayList<Plane> or an array Plane[], and sort that list or array

The first approach can be achieved with a TreeSet<Plane>: put your planes into the set, and iterate them in the “natural” order (i.e. the order consistent with their compareTo method).

The second approach requires copying the planes into a separate container or an array, and then using the sort method (or the Arrays.sort static method if it is an array) to order your planes in accordance with the order set by their compareTo implementation.

EDIT : (based on a comment) One way to deal with a problem of storing planes in a specific order inside a hash map is to make a hash map of tree sets, like this:

Map<String,TreeSet<Plane>> airlineMap = new HashMap<String,TreeSet<Plane>>();

Once you add the planes to each airline, they would be maintained in the order based on your compareTo implementation. with a TreeSet<Plane> in hand, you can easily find the next or the prior Plane by calling higher or lower.

You should use Math.signum rather than Math.ceil in your compareTo method.