code

package fuelefficiency;

import javax.swing.JFrame;
import javax.swing.JOptionPane;

/**
 * Jakub Kargul
 * Assessment: Fuel Efficiency program
 * 18/01/2018
 */

public class FuelEfficiency {

    // declare variables
    double gallonsPerLitre = 0.219969;
    String milesTraveled;
    double litresFuel;
    String airConTip = "If travelling at low speed opening the windows is more efficient. \nIf travelling at 50 miles per hour or above, closing the windows and using the air con will save you more.";
    String tyrePressureTip = "Under inflated tyres increase your fuel consumption and can be dangerous. Check them once a month and before long journeys.";

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        // TODO code application logic here
        FuelEfficiency fuelCalc = new FuelEfficiency();
        // declare username variable
        String username;
        // set username variable to user input
        username = JOptionPane.showInputDialog(null, "Please login with your username:");
        // call the first menu method
        fuelCalc.menu(username);
    }

    public void menu(String username) {
        // define string variable for the option
        String optionStr;
        // start try block to ensure no text is entered
        try {
            // set variable optionStr to user input, offering the user three options of basic MPG, advanced fuel consumption and exit
            optionStr = (JOptionPane.showInputDialog(null,
                                                "Enter 1 for BASIC MPG\nEnter 2 for Advance Fuel Consumption\nEnter 3 to exit",
                                                "Hello " + username + ". Please choose from the options below",
                                                JOptionPane.WARNING_MESSAGE));
            // start switch statement for optionStr
            switch (Integer.parseInt(optionStr)) {
                // if user picked 1 then call method basicMPG
                case 1: basicMPG();
                        break;
                // if user picked 2 then call method advancedMPG
                case 2: advancedMPG();
                        break;
                // if user picked 3 then call method exit(username)
                case 3: exit(username);
                        break;
                // if user picked anything other than 1, 2 or 3, display error message and initiate menu method again
                default: JOptionPane.showMessageDialog(null, "Invalid number, please try again");
                        menu(username);
            }
        // if the user inputs anything that's not a number, display error message and initiate menu method again
        } catch(NumberFormatException e) {
                JOptionPane.showMessageDialog(null, "You have entered an invalid option, please try again.");
                menu(username);
        }
    }

        public void basicMPG() {
            // define local variale called mpg
            double mpg;
            // set mpg to the return of method calculateMPG
            mpg = calculateMPG();
            // display message telling user his MPG
            JOptionPane.showMessageDialog(null, "Your MPG is " + mpg);
            // exit the program
            System.exit(0);
        }

        public double calculateMPG() {
            // define local double variable mpg
            double mpg;
            // define local string variable milesStr
            String milesStr;
            // define local double variable miles and set it to 0
            double miles = 0;

            // define local string variable litresFuelStr
            String litresFuelStr;

            // define local boolean variable inputAccepted
            boolean inputAccepted = false;

            // start while loop that runs while inputAccepted is false
            while(!inputAccepted) {
                // start try block to ensure input is a number
                try {
                    // set variable milesStr to user input
                    milesStr = JOptionPane.showInputDialog(null, "How many miles were driven?");
                    // set milesTraveled to milesStr
                    milesTraveled = milesStr;
                    // set miles to milesStr parsed to double
                    miles = Double.parseDouble(milesStr);
                    // set inputAccepted to true and subsequently end loop
                    inputAccepted = true;
                // if the user inputs anything that's not a number, display error message and restart the loop
                } catch(NumberFormatException e) {
                    JOptionPane.showMessageDialog(null, "You have entered an invalid input, please try again. Numbers only");
                }
            }

            // reset inputAccepted variable to ensure it's false for next input validation
            inputAccepted = false;

            // start while loop that runs while inputAccepted is false
            while(!inputAccepted) {
                // start try block to ensure input is a number
                try {
                    // set variable litresFuelStr to user input
                    litresFuelStr = JOptionPane.showInputDialog(null, "How many litres of fuel were used");
                    // set variable litresFuel to litresFuelStr parsed to double
                    litresFuel = Double.parseDouble(litresFuelStr);
                    // set inputAccepted to true and subsequently end loop
                    inputAccepted = true;
                // if the user inputs anything that's not a number, display error message and restart the loop
                } catch(NumberFormatException e) {
                    JOptionPane.showMessageDialog(null, "You have entered an invalid input, please try again. Numbers only");
                }
            }

            // declare local variable called gallons and set it to litresFuel multiplied by gallonsPerLitre
            double gallons = (litresFuel * gallonsPerLitre);
            // set mpg to miles divided by gallons
            mpg = miles / gallons;
            // use math function to round MPG to 2 decimal places and return it so it can be displayed
            return mpg = Math.round(mpg * 100.0) / 100.0;
        }

        public void advancedMPG() {
            // declare local double variable actualMPG and set it to the return of method calculateMPG
            double actualMPG = calculateMPG();
            // declare local variables
            double airConReduction = 0;
            double tyreReduction = 0;
            double fuelCostPL = 0;
            // declare three local boolean variables one for validation and two for tips
            boolean inputAccepted = false;
            boolean airTip = false;
            boolean tyreTip = false;

            // declare local variable N for user validation
            // set N to user input from a yes no confirm dialog
            int n = JOptionPane.showConfirmDialog(null, "Was air conditioning on?", "Calculating Advanced MPG", JOptionPane.YES_NO_OPTION);

            // if N equals 0 (which means user picked yes)
            if(n == 0){
                // set airConReduction to the actualMPG multiplied by 0.07 (reduce efficiency by 7%)
                airConReduction = actualMPG * 0.07;
                // set the airTip to true so the appropriate tip pops up at end of method
                airTip = true;
            }

            // set N to user input from a yes no confirm dialog
            n  = JOptionPane.showConfirmDialog(null, "Were the tyres under inflated?", "Calculating Advanced MPG", JOptionPane.YES_NO_OPTION);

            // if N equals 0 (which means user picked yes)
            if(n == 0){
                // set tyreReduction to the actualMPG multiplied by 0.25 (reduce efficiency by 25%)
                tyreReduction = actualMPG * 0.25;
                // set the tyreTip to true so the appropriate tip pops up at end of method
                tyreTip = true;
            }

            // calculate potentialMPG by adding the actual MPG plus any reductions if the user had air conditioning on or the tyres were inflated
            double potentialMPG = actualMPG + airConReduction + tyreReduction;
            // calculate potentialNumLitres by dividing miles traveled by potentialMPG and gallonsPerLitre
            double potentialNumLitres = ((Double.parseDouble(milesTraveled) / potentialMPG) / gallonsPerLitre);

            // start while loop that runs while inputAccepted is false
            while(!inputAccepted) {
                // start try block to ensure input is a number
                try {
                    // set fuelCostPL to input from user parsed into double
                    fuelCostPL = Double.parseDouble(JOptionPane.showInputDialog(null, "What was the cost of fuel per litre?"));
                    // set inputAccepted to true and end loop
                    inputAccepted = true;
                // if the user inputs anything that's not a number, display error message and restart the loop
                } catch(NumberFormatException e) {
                    JOptionPane.showMessageDialog(null, "You have entered an invalid input, please try again. Numbers only");
                }
            }

            // calculate the actual cost by multiplying how many litres of fuel the user used and the cost of fuel per litre
            double actualCost = litresFuel * fuelCostPL;
            // calculate the potential cost by multiplying the potential amount of litres of fuel the user used and the cost of fuel per litre
            double potentialCost = potentialNumLitres * fuelCostPL;

            // round all the results to two decimal places
            actualMPG = Math.round(actualMPG * 100.0) / 100.0;
            actualCost = Math.round(actualCost * 100.0) / 100.0;
            potentialMPG = Math.round(potentialMPG * 100.0) / 100.0;
            potentialCost = Math.round(potentialCost * 100.0) / 100.0;

            // display user's actual MPG along with his actual cost
            JOptionPane.showMessageDialog(null, "Your MPG is " + actualMPG + " and your cost is " + actualCost, "Actual", JOptionPane.INFORMATION_MESSAGE);

            // display user's potential MPG with his potential cost
            JOptionPane.showMessageDialog(null, "Your potential MPG is " + potentialMPG + " and your potential cost is " + potentialCost, "Potential", JOptionPane.INFORMATION_MESSAGE);

            // if the user had air conditioning on display tip regarding efficient air conditioning use
            if(airTip){
                JOptionPane.showMessageDialog(null, airConTip, "Top Tips", JOptionPane.INFORMATION_MESSAGE);
            }

            // if the user's tyres were inflated display tip regarding inflated tyres
            if(tyreTip){
                JOptionPane.showMessageDialog(null, tyrePressureTip, "Top tips", JOptionPane.INFORMATION_MESSAGE);
            }

            // end program
            System.exit(0);
        }

        public void exit(String username) {
            // declare local variable called confirmExit and set it to user input
            // ensure user wants to exit program
            int confirmExit = JOptionPane.showConfirmDialog(null, username + ", are you sure you want to exit?", "Exit", JOptionPane.YES_NO_OPTION);
            // if confirmExit equals 0 it means the user picked Yes
            if (confirmExit == 0) {
                // exit the program
                System.exit(0);
            } else {
                // otherwise initiate the menu method again
                menu(username);
            }
        }
}