How to handle empty scanner input strings in java?

Question

I'm working on a .xml file generator. It works by asking the user to input 5 different values and then using them for generating a .xml data container. This is the code:

public class Generator {
public static void main(String[] args) throws InterruptedException {
    Scanner reader = new Scanner(System.in);
    while (true) {

        System.out.println("Input the furni ID (input only numbers here).");
        String furniID = reader.nextLine();

        System.out.println("Input the furni's file name (without '.swf'. You can input numbers, letters and underscores here).");
        String furniFileName = reader.nextLine();

        System.out.println("Input the furni's revision (input only numbers here).");
        String furniRevision = reader.nextLine();

        System.out.println("Input the furni's name (this will be the name that it will display when you click on the furni in a room or in your inventory).");
        String furniName = reader.nextLine();

        System.out.println("Input the furni's description (this will be displayed under the furni's name).");
        String furniDescription = reader.nextLine();

        System.out.println("Input the furni's furniline. This is just a name for a group of furnis that belong to the same collection. For example you can input 'custom' (without quotation marks).");
        String furniLine = reader.nextLine();

        System.out.println("Generating your furnidata...");
        System.out.println(" ");

        TimeUnit.SECONDS.sleep(1);

        System.out.println("<furnitype id=\"" + furniID + "\"" + " classname=\"" + furniFileName + "\"" + ">");
        System.out.println("<revision>" + furniRevision + "</revision>");
        System.out.println("<xdim>1</xdim>");
        System.out.println("<ydim>1</ydim>");
        System.out.println("</partcolors>");
        System.out.println("<name>" + furniName + "</name>");
        System.out.println("<description>" + furniDescription + "</description>");
        System.out.println("</adurl");
        System.out.println("<offerid>-1</offerid>");
        System.out.println("<buyout>0</buyout>");
        System.out.println("<rentofferid>-1</rentofferid>");
        System.out.println("<rentbuyout>0</rentbuyout>");
        System.out.println("<bc>0</bc>");
        System.out.println("<excludeddynamic>0</excludeddynamic>");
        System.out.println("<customparams/>");
        System.out.println("<specialtype>1</specialtype>");
        System.out.println("<canstandon>0</canstandon>");
        System.out.println("<cansiton>0</cansiton>");
        System.out.println("<canlayon>0</canlayon>");
        System.out.println("<furniline>" + furniLine + "</furniline>");
        System.out.println("</furnitype>");
        System.out.println(" ");

        System.out.println("Do you want to generate another furnidata? (YES/NO)");
        String confirmation = reader.nextLine();

        if (confirmation.equals("NO")) {
            System.out.println("Furnidata generator has been stopped.");
            break;
        } else if (confirmation.equals("no")) {
            System.out.println("Furnidata generator has been stopped.");
            break;
        } else if (confirmation.equals("No")) {
            System.out.println("Furnidata generator has been stopped.");
            break;
        }
    }
}

I want the program to detect when someone inputs an empty string by pressing the enter key with no written text and print a message asking the user to not input empty strings. I tried this with a try-catch statement but after printing the message the program executes the next line instead of re-executing the one where the user input an empty string. How can I achieve this?

Answer 1

Whenever you have a prompt for a User to supply specific data you should validate that data that was entered and if it is incorrect, permit the User to try again so as not to halt the application due to an error of some kind that can be avoided. There is no problem using Regular Expressions (regex) for this sort of thing.

A User entering nothing (by just hitting the ENTER key) is just one problem. At some prompts you are expecting the User to enter only numerical digits. A small regex within the String#matches() method is ideal for this, for example:

if (!userInput.matches("\\d+") {
   System.out.println("Invalid input! Numerical digits only!");
}

If the User input does not match all numerical digits then display "Invalid Input!".

Every prompt should be in its own do or while loop so that validation can be done and an opportunity for the User to re-enter the data can be accomplished. If you have lots of prompts then a class method can be created to handle all of them in order to remove duplicate coding. As an example, here is your code modified so that a method named askUser() can handle the prompts to the User:

public class Generator {

    public static void main(String[] args) {
        java.util.Scanner reader = new java.util.Scanner(System.in);
        while (true) {
            // Only a String repesentation of numerical digits will be accepted. 
            // 1 to 20 digits are acceptable)
            String furniID = askUser(reader, "Input the furni ID (input only numbers here).", 
                    "\\d{1,20}");

            /* Only characters A to Z in either upper or lower case, digits 
               from 0 to 9, whitespaces, and underscores will be accepted 
               within the Users input. The period (.) is not acceptable 
               therefore eliminating the possibility of a file name extension
               like .swf to be applied to the supplied file name. File name can 
               be 1 to 80 characters.                               */
            String furniFileName = askUser(reader, "Input the furni's file name (without "
                    + "'.swf'.\nYou can input numbers, letters and underscores here).", 
                    "(?i)[a-z0-9 _]{1,80}");
            
            // Only a String repesentation of numerical digits will be accepted. 
            // 1 to 20 digits are acceptable)
            String furniRevision = askUser(reader, "Input the furni's revision (input "
                    + "only numbers here).", "\\d{1,20}");
            
            /* Only characters A to Z in either upper or lower case, digits 
               from 0 to 9, whitespaces, and underscores will be accepted 
               within the Users input. The period (.) is not acceptable. The
               name can be 1 to 25 characters in length.                  */
            String furniName = askUser(reader, "Input the furni's name (this will be "
                    + "the name that will\nbe displayed when you click on the furni in "
                    + "a room or in\nyour inventory).", "(?i)[a-z0-9 _]{1,25}");

            /* Only characters A to Z in either upper or lower case, digits 
               from 0 to 9, whitespaces, and periods will be accepted within 
               the Users input. Description can be a minimum of 5 to a maximum 
               of 120 characters in length.  */
            String furniDescription = askUser(reader, "Input the furni's description ("
                    + "this will be displayed\nunder the furni's name).", "(?i)[a-z0-9. ]{5,120}");

            /* Only characters A to Z in either upper or lower case, digits 
               from 0 to 9, and whitespaces will be accepted within the Users 
               input. The period (.) is not acceptable. The furniline name can 
               be 1 to 25 characters in length.                      */   
            String furniLine = askUser(reader, "Input the furni's furniline. This is just "
                    + "a name for a\ngroup of furnis that belong to the same collection. "
                    + "For\nexample you can input 'custom' (without quotation marks).", 
                    "(?i)[a-z0-9 ]{1,25}");

            System.out.println("Generating your furnidata...");
            System.out.println(" ");
            System.out.println("<furnitype id=\"" + furniID + "\"" + " classname=\"" + furniFileName + "\"" + ">");
            System.out.println("<revision>" + furniRevision + "</revision>");
            System.out.println("<xdim>1</xdim>");
            System.out.println("<ydim>1</ydim>");
            System.out.println("</partcolors>");
            System.out.println("<name>" + furniName + "</name>");
            System.out.println("<description>" + furniDescription + "</description>");
            System.out.println("</adurl");
            System.out.println("<offerid>-1</offerid>");
            System.out.println("<buyout>0</buyout>");
            System.out.println("<rentofferid>-1</rentofferid>");
            System.out.println("<rentbuyout>0</rentbuyout>");
            System.out.println("<bc>0</bc>");
            System.out.println("<excludeddynamic>0</excludeddynamic>");
            System.out.println("<customparams/>");
            System.out.println("<specialtype>1</specialtype>");
            System.out.println("<canstandon>0</canstandon>");
            System.out.println("<cansiton>0</cansiton>");
            System.out.println("<canlayon>0</canlayon>");
            System.out.println("<furniline>" + furniLine + "</furniline>");
            System.out.println("</furnitype>");
            System.out.println(" ");

            // Only y or n can be supplied in any letter case. 
            // Anything else generates an Invalid Input message.
            String confirmation = askUser(reader, "Do you want to generate another furnidata? (y/n)", 
                    "(?i)[yn]", "Invalid Choice! Either 'y' for Yes or 'n' for No!");
            if (confirmation.equalsIgnoreCase("n")) {
                System.out.println("Furnidata generator has been stopped.");
                break;
            }
        }
    }
    
    /**
     * Displays the supplied prompt within the Console Window and permits the User 
     * to enter a response. User input is validated based on the regex argument 
     * supplied to the <b>validationRegEx</b> parameter.<br><br>
     * 
     * @param inputReader (Scanner) Scanner object to use for User Input.<br>
     * 
     * @param promptString (String) The prompt to display within the console window.<br>
     * 
     * @param validationRegEx (String) A regular expression that will be used 
     * within a <b>String#matches()</b> method to validate the User's input.<br>
     * 
     * @param errorString (Optional - String) Default validation error message is:<pre>
     * 
     *     "Invalid input! Try again..."</pre><br>
     * 
     * Here you can optionally supply your own message for non-valid entries.<br>
     * 
     * @return (String) The valid response from User.
     */
    public static String askUser(java.util.Scanner inputReader, String promptString, String validationRegEx, String... errorString) {
        String userInput = "";
        String errString = "Invalid input! Try again...";
        if (errorString.length > 0 && !errorString[0].trim().isEmpty()) {
            errString = errorString[0];
        }
        
        while (userInput.isEmpty()) {
            System.out.println();
            System.out.println(promptString);
            userInput = inputReader.nextLine();
            if (!userInput.matches(validationRegEx)) {
                System.err.println(errString);
                userInput = "";
            }
        }
        return userInput;
    }
    
}

Answer 2

You might want to use.

 String furniID = reader.nextLine();
 furniID = furniID.trim();
 if(furniID.equals("")){
     System.out.println("Empty Here. ");
 }

As suggested in comments .readLine() returns an empty string if you supply a blankline or simply hit enter.

The above regex expression will take care of empty string "" or whitespaces that user might enter.

edit 1 As suggested in comments regex could have been over-kill here. The string.trim() will take care of tabs, trailing-leading whitespaces and empty string.