if else

Jeg får da godt nok livet noget op i denne java tråd ;-)

Den næste opgave hedder:

Modify your printDetails method to include printing the reference
number. However, the method should only print the reference number if it has been set - that is, the refNumber string has a non-zero length. If it has not been set, then print the string, "ZZZ" instead. Hint: Use a conditional statement whose test calls the length method on the refNumber string.

__________________________________________________________

Koden ser sådan ud:

/**
* A class that maintains information on a book.
* This might form part of a larger application such
* as a library system, for instance.
*
* @author (Insert your name here.)
* @version (Insert today's date here.)
*/
class Book
{
    // The fields.
    private String author, title, refNumber;
    private int pages;

    /**
    * Set the author and title fields when this object
    * is constructed.
    */
    public Book(String bookAuthor, String bookTitle, String refNumber, int pages)
    {
        author = bookAuthor;
        title = bookTitle;
        this.pages = pages;
        refNumber = ("");

    }

    // Add the methods here ...
   
    public String getAuthor()
    {
        return author;
    }
   
    public String gettitle()
    {
        return title;
    }
   
    public void printAuthor()
    {
        System.out.println(author);
    }
   
    public void printTitle()
    {
        System.out.println(title);
    }
   
    public int getpages()
    {
        return pages;
    }
   
    public void printDetails()
    {
        System.out.println("Title:" + title);
        System.out.println("Author:" + author);
        System.out.println("pages:" + pages);     
    }
   
    public void setRefNumber(String ref)
    {
        refNumber = ref;
    }
   
    public String getRefNumber()
    {
        return refNumber;
    }
}
______________________________________________________

Jeg har prøvet at skrive dette:


public void printDetails()
    {
        System.out.println("Title:" + title);
        System.out.println("Author:" + author);
        System.out.println("pages:" + pages);
       
        if (refNumber.length() = 0)
        {
            System.out.println("Indtast min et tegn");
        }
        else
        {
            System.out.println("zzz");
        }
       
    }

Men får en fejl ved refNumber.length. Jeg er lidt i tvivl, fordi sådan som jeg har forstået det, skal det hentes fra constructoren, hvor der nu står:
refnumber = "()";
Men der får jeg en fejl?

Mvh