Design question: When to use instance variables

Question

Looking for best practice/design advise.
I am working my way through an exercise where I convert a zip code to a postal barcode (and back), validate the format, and validate/generate a check digit. My code works fine. I posted comments and methods below for reference. In addition to the book I am using, I also read http://java.sun.com/docs/books/tutorial/java/javaOO/classvars.html.
My question is: In designing this class, should I have defined instance variables to hold the generated zipcode, barcode, and checkdigit and then used getter methods to get the zipcode or barcode?
I planned on creating a user interface to ask for a zipcode or barcode and present a conversion to the user. The only methods that need to be called are "convertBarcodeToZipcode" and "convertZipcodeToBarcode". All the other methods are utility methods. The strings that are returned by these two methods are presented to the user.
I could, easily enough, create and populate instance variables, but would I keep the return statements as well or remove them?
Thanks, Mike
ps: I am self-learning java, so I don't have an instructor to ask. :)
public class PostalBarCode {
     * Constructor.
     * Set up the barcode ArrayList
    public PostalBarCode() {
            barcodeUnits.add("||:::");
            barcodeUnits.add(":::||");
            barcodeUnits.add("::|:|");
            barcodeUnits.add("::||:");
            barcodeUnits.add(":|::|");
            barcodeUnits.add(":|:|:");
            barcodeUnits.add(":||::");
            barcodeUnits.add("|:::|");
            barcodeUnits.add("|::|:");
            barcodeUnits.add("|:|::");
     * Convert a barcode to a zipcode.
     * Assumes the input format is valid. Validates the check digit
     * @param theBarcode string to be converted
     * @return the zipcode as a String of empty string for error
    public String convertBarcodeToZipcode(String theBarcode) {}
     * Convert the Zipcode to a barcode
     * @param theZipcode string to be converted
     * @return the barcode as a string
    public String convertZipcodeToBarcode(String theZipcode) {}
     * Determines if theString is a barcode
     * @param theString
     * @return true if the input is a barcode, false otherwise.
    public boolean isBarcode (String theString) {}
     * Determines of theString is a zip code
     * @param theString the string to test for 5 digits
     * @return true for a zip code, false otherwise
    public boolean isZipcode (String theString) {}
     * Convert a barcode to a zipcode.
     * Assumes the input format is valid. Validates the check digit
     * @param theBarcode string to be converted
     * @return the zipcode as a String of empty string for error
    public String convertBarcodeToZipcode(String theBarcode) {}
     * Convert the Zipcode to a barcode
     * @param theZipcode string to be converted
     * @return the barcode as a string
    public String convertZipcodeToBarcode(String theZipcode) {}
     * Calculate the check digit from the zipcode
     * @param theZipcode as a String
     * @return the the check digit or -1 for error
    public int calculateCheckDigitFromZipcode(String theZipcode) {}
     * Calculate the check digit from the barcode
     * @param theZipcode as a String
     * @return the the check digit or -1 for error
    public int calculateCheckDigitFromBarcode(String theBarcode) {}
     * Validate the format of the barcode.
     * Should be 6 blocks of 5 symbols each.
     * Also check if frame bars are present and strip them.
     * @param theBarcode
     * @return true for proper format, false for improper format
    public boolean validateBarcodeFormat (String theBarcode){}
    private  ArrayList<String> barcodeUnits = new ArrayList<String>();
}

Answer 1

These are just my two cents. The various methods that determine whether something is a zip or bar code can easily remain static. After all, you probably don't want to create an instance and then call something like isValid(), although you could do so. The various other methods could easily be instance methods. You could have a toZipCode() or toBarCode() method in each object to convert from one to the other. I would personally throw something like ValidationException rather than returning -1 for your various check-digit methods.
- Saish