BoatRide.java

public class BoatRide implements Chargeable {
  private double baseFare;
  private double distance;
  
  public BoatRide(double baseFare, double distance) {
    this.baseFare = baseFare;
    this.distance = distance;
  }
  
  public double cost() {
    return baseFare * distance;
  }
  
}

// IS-A

Chargeable.java

public interface Chargeable {
  public double cost();
}

PlaneRide.java

public class PlaneRide implements Chargeable, Comparable<PlaneRide> {
  private double fare;
  private double discount;
  
  public PlaneRide(double fare, double discount) {
    this.fare = fare;
    this.discount = discount;
  }
  
  public double cost() {
    return fare - (fare * discount);
  }
  
  public int compareTo(PlaneRide o) {
    return fare > o.fare ? 1 : (fare < o.fare ? -1 : 0);
    /*
    if (fare > o.fare) {
      return 1; 
    } else if (fare < o.fare) {
      return -1;
    } else {
      return 0; 
    }
    */
  }
  
  public String toString() {
    return String.valueOf(fare); 
  }
}

TravelItinerary.java

public class TravelItinerary {
  public static double cost(Chargeable[] items) {
    double total = 0;
    for (Chargeable item : items) {
      total += item.cost();
    }
    return total;
  }
}