TripPlanner.java

import java.util.Scanner;

public class TripPlanner {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        // Pass a Scanner object instead of creating a Scanner object in every single method!
        greeting(in);
        travelTimeAndBudget(in);
        timeDifference(in);
        countryArea(in);
        howFar(in);
    }

    public static void greeting(Scanner input) {
        System.out.println("Welcome to Vacation Planner!");
        System.out.print("What is your name? ");
        String name = input.nextLine();
        System.out.print("Nice to meet you " + name + ", where are you travelling to? ");
        String place = input.nextLine();
        System.out.println("Great! " + place + " sounds like a great trip!");
    }

    public static void travelTimeAndBudget(Scanner input) {
        final int HOURS_IN_DAY = 24;
        final int MINUTES_IN_DAY = HOURS_IN_DAY * 60;

        System.out.print("How many days are you going to spend travelling? ");
        int days = Integer.parseInt(input.nextLine());
        while (days <= 0) {
            System.out.println("Input must be positive!");
            System.out.print("How many days are you going to spend travelling? ");
            days = Integer.parseInt(input.nextLine());
        }
        System.out.print("How much money, in USD, are you planning to spend on your trip? ");
        double money = Double.parseDouble(input.nextLine());
        while (money < 0) {
            System.out.println("Input must be positive!");
            System.out.print("How much money, in USD, are you planning to spend on your trip? ");
            money = Double.parseDouble(input.nextLine());
        }

        System.out.print("What is the three letter currency symbol for your travel destination? ");
        String currency = input.nextLine();

        System.out.print("How much " + currency + " are there in 1 USD? ");
        double conversion = Double.parseDouble(input.nextLine());
        while (conversion < 0) {
            System.out.println("Input must be positive!");
            System.out.print("How much " + currency + " are there in 1 USD? ");
            conversion = Double.parseDouble(input.nextLine());
        }

        // NO NEED TO CONVERT ANY FACTOR TO AN INT!
        double budgetPerDayInUSD = (100 * money / days) / 100.0;
        double budgetPerDay = ((100 * conversion * money / days) / 100.0);

        System.out.println(); // prints whitespace for readability!
        System.out.printf("If you are travelling for %,d days, that is the same as %,d hours or " +
                "%,d minutes.\n", days, (days * HOURS_IN_DAY), days * MINUTES_IN_DAY);
        System.out.printf("If you are going to spend $%,.2f USD, that means per day you can " +
                "spend up to $%,.2f USD.\n",money, budgetPerDayInUSD);
        System.out.printf("Your total budget in %s is %,.2f, which per day is %,.2f\n",
                currency, (conversion * money), budgetPerDay);
    }

    public static void timeDifference(Scanner input) {
        System.out.print("What is the difference, in hours, between your home and your destination? ");
        int hourDifference = Integer.parseInt(input.nextLine());

        // WHAT HAPPENS IF THE int entered by the user is negative???
        while (hourDifference < 0) {
            System.out.println("Hours must e positive!");
            System.out.print("What is the difference, in hours, between your home and your destination? ");
            hourDifference = Integer.parseInt(input.nextLine());
        }
        System.out.println(); // prints whitespace for readability!
        System.out.println("That means that when it is midnight at home, it will be "
                + hourDifference + ":00 in your travel destination and when it's " +
                "noon at home, it will be " + (hourDifference + 12) + ":00");
    }

    public static void countryArea(Scanner input) {
        System.out.print("What is the square area of your destination country in km2? ");
        double area = Double.parseDouble(input.nextLine());
        while (area <= 0) {
            System.out.println("Area must be positive and > 0!");
            System.out.print("What is the square area of your destination country in km2? ");
            area = Double.parseDouble(input.nextLine());
        }
        double areaInSqMi = (38.6102 * area) / 100.0;
        System.out.println(); // prints whitespace for readability!
        System.out.printf("In square miles that is %,.2f\n", areaInSqMi);
    }

    public static void howFar(Scanner input) {
        final double R = 6367.51;
        System.out.print("What are the latitude coordinates in degrees of your destination city? " +
                "(positive for north, negative for south) ");
        double latDest = Double.parseDouble(input.nextLine());
        System.out.print("What are the longitude coordinates in degrees of your destination city?" +
                "(positive for east, negative for west) ");
        double longDest = Double.parseDouble(input.nextLine());
        System.out.print("What are the latitude coordinates in degrees of your home city? " +
                "(positive for north, negative for south) ");
        double latHome = Double.parseDouble(input.nextLine());
        System.out.print("What are the longitude coordinates in degrees of your home city? " +
                "(positive for east, negative for west) ");
        double longHome = Double.parseDouble(input.nextLine());

        System.out.println(); // prints whitespace for readability!

        latDest *= Math.PI / 180;
        longDest *= Math.PI / 180;
        latHome *= Math.PI / 180;
        longHome *= Math.PI / 180;
        double havLat = (1 - Math.cos(latDest - latHome)) / 2;
        double havLong = (1 - Math.cos(longDest - longHome)) / 2;
        double haversine = havLat + Math.cos(latHome) * Math.cos(latDest) * havLong;
        double distance = 2 * R * Math.asin(Math.sqrt(haversine));
        double distanceBetCityAndDest = (distance * 100) / 100.0;

        System.out.printf("The distance between your home city and your destination is %,.2f km" +
                ".\n",distanceBetCityAndDest);
    }
}