Question

I have a homework assignment which requires input from the user and stores it in various data structures (arrays of linked-lists, stacks, etc.). However, I’ve been writing the main class of my previous homework assignments and this one in a very similar fashion. I have a very tiny main method. All the main method does is instantiate a new object which I don’t want to be destroyed and then loop through the program forever until the user chooses otherwise. I then have a menu() method which prints a list of selections and reads the user’s selection. And then from there, I pass that selection to another method which interprets the selection and performs accordingly.

The problem I have been having in the past assignments and now I’ve never really gotten a good answer for. The problem seems to lie with my menu() method and more specifically, the Scanner object. There always seems to be some junk left in the stream after I call the nextLine() method on a scanner object. So the next time the menu() method is called, it reads in that junk and loops though the rest of the program with that junk until menu() is called a third time. In the past, I would remedy this by calling the next() method right after I received my input and ignoring it. However, I seem to be having issues with that now as well.

In this program in particular, I have a method which request’s a user enter a city name. Now, city names can be more than one word (Palm City, West Palm Beach, Satellite Beach, New York, etc.). Now, when the scanner reads in one of those multi-word cities, it does the same thing as before, reads in some junk the next time the menu() method is called and goes though the whole program with it until menu() is called again. In this case, it prints an string “Invalid Selection” and then prints the menu again. I can’t for the life of me figure out what’s going on. any help would be appreciated.

import java.util.Scanner;


public class CSAir
{
    public static Scanner input = new Scanner(System.in);

    public static void main(String[] args)
    {
        CityList flightLog = new CityList();
        boolean loop = true;

        while(loop)
        {
            loop = actions(menu(),flightLog);
        }

    }

    private static String menu()
    {
        System.out.print("Please Make a Selection\n" +
            "I) Insert a city\n" +
            "A)Add a flight path (One Way)\n" +
            "R) Request a flight\n" +
            "L) Load from a text file\n" +
            "Q) Quit\n" +
            "\nSelection: ");

        String in = input.next();
        //input.next();
        System.out.println();
        return in;
    }

    private static boolean actions(String selection, CityList flightLog)
    {
        if(selection.equalsIgnoreCase("I"))
        {
            insert(flightLog);
            return true;
        }

        else if(selection.equalsIgnoreCase("A"))
        {
            add(flightLog);
            return true;
        }

        else if(selection.equalsIgnoreCase("R"))
        {
            request(flightLog);
            return true;
        }

        else if(selection.equalsIgnoreCase("L"))
        {
            return true;
        }

        else if(selection.equalsIgnoreCase("Q")) return false;

        else 
        {
            System.out.println("Invalid Selection!\n");
            return true;
        }
    }

    private static void request(CityList flightLog)
    {
        System.out.print("Origin: ");
        String origin = input.nextLine();
        System.out.print("\nDestination: ");
        try
        {
            flightLog.isPath(origin, input.next());
        }
        catch (AllDestinationsVisitedException e)
        {
            e.getMessage();
        }
    }

    private static void add(CityList flightLog)
    {
        System.out.print("Origin: ");
        String origin = input.nextLine();
        System.out.print("\nDestination: ");
        flightLog.addPath(origin, input.next());
    }

    private static void insert(CityList flightLog)
    {
        System.out.print("Enter a City: ");

        try
        {
            flightLog.addCity(input.next());
        }
        catch(Exception e)
        {
            System.out.println(e.getMessage());
        }
        System.out.println();
    }

}

Answer 1

Default Deliminator for scanner is white space. So when you enter New York since it has white space now scanner treats it as two tokens if you call next().

A better option would be to use nextLine() method for reading such values.