public class Book {
 
    private String author;
    private String title;
    private String refNumber;
    private int pages;
   
    public Book(String bookAuthor, String bookTitle, String bookRefNumber, int bookPages) {
        author = bookAuthor;
        title = bookTitle;
        pages = bookPages;
        refNumber = "";
    }
   
    public String getAuthor() {
        return author;
    }
    public String getTitle() {
        return title;
    }
   
    public int getPages() {
        return pages;
    }
   
    public String getRefNumber() {
        return refNumber;
    }
   
    public void setRefNumber(String refNumber) {
        this.refNumber = refNumber;    
    }
   
    public void printAuthor() {
        System.out.println(author);
    }
   
    public void printTitle() {
        System.out.println(title);
    }
   
    public void printDetails() {
        String refNumPrint = refNumber;
       
        System.out.println("Title: " + title + ", Author: " + author + ", Pages: " + pages);
       
        if(refNumPrint.length() < 0) {
            refNumPrint = "ZZZ";
        }
        System.out.println("Reference number:  " + refNumPrint);
    }
}