# Passing an array of objects to a class

**URL:** <https://forum.kirupa.com/t/passing-an-array-of-objects-to-a-class/299182>\
**Category:** programming\
**Created:** [October 31, 2009, 5:45am UTC](https://forum.kirupa.com/t/passing-an-array-of-objects-to-a-class/299182 "2009-10-31T05:45:46Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![koolkrasher](https://avatars.discourse-cdn.com/v4/letter/k/c6cbf5/32.png) [@koolkrasher](https://forum.kirupa.com/u/koolkrasher)\
**Post date:** [October 31, 2009, 5:45am UTC](https://forum.kirupa.com/t/passing-an-array-of-objects-to-a-class/299182/1 "2009-10-31T05:45:46Z")

</div>

Hello guys, i seem to have a problem. For some reason I can not pass an array of object to a class. Passing a single object works just fine but when i try to pass in an array i get a Null Pointer Exception.

Here is the scenario.  
I have a person class that stores the first middle and last name.

i have a book class that stores the following  
Author Name - Person object(s)  
Book title  
Book pages  
Book year  
Book isbn

I have 3 constructors set up.  
1 -default constructor with default values  
2 -parametrized constructor that accepts a single person object  
3 -parametrized constructor that accepts a array of person objects

Here is my Book Class

```auto

import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class Book {
//begin Book Class
    
    //private Person authors []; //store author objects as an array
    private Person author; // store author object
    private String title; // store book title
    private int pages; // store book pages
    private String isbn; // store book isbn
    private int year; // store book year
    

    public Book()
    {// Book default constructor
        
        //authors[0] = new Person();
        setTitle("Default Book Title"); // Set default book title
        setIsbn("0000000000"); // set default book isbn
        setPages(1); // set default book pages
        setYear(2009); // set default book year
        
    }
    public Book(Person userAuthor, String userTitle, String userIsbn, int userPages, int userYear)
    {
        this(); // call default constructor if test return false.
        
        setAuthor(userAuthor); //set authers name to user author
        userTitle.trim(); //trim the book title
        
        if(userTitle.equals("")==false)
        {// check to see if the title is empty
            setTitle(userTitle);//set title to userTitle
        }
        
        if(isbnValidator(userIsbn)==true)
        {// check to see if isbn is 10 digits long
            
            String sequence1 = userIsbn.substring(0, 3);// extract first 3 numbers from isbn number
            String sequence2 = userIsbn.substring(3, 10);// extract final 7 digits from isbn number
            String newIsbn = sequence1+"-"+sequence2; // compile new string consisting of 3digits - 7digits
            setIsbn(newIsbn); // assign newIsbn to isbn
            
        }
        
        if(userPages>1)
        {// check to see if user pages is greater than 1
            setPages(userPages);// set pages to userPages
        }
        
        if((userYear>1900)&&(userYear<=2010))
        {// check to see if year is greater than 1900 and less than 2010
            setYear(userYear); // set year to userYear
        }
        
        
    }
    
    public Book(Person userAuthors[], String userTitle, String userIsbn, int userPages, int userYear)
    {
        this();
        
        setAuthors(userAuthors);
        
        userTitle.trim();
        
        if(userTitle.equals("")==false)
        {
            setTitle(userTitle);
        }
        
        if(isbnValidator(userIsbn)==true)
        {
            String sequence1 = userIsbn.substring(0, 3);
            String sequence2 = userIsbn.substring(3, 10);
            String newIsbn = sequence1+"-"+sequence2;
            setIsbn(newIsbn);
        }
        
        if(userPages>1)
        {
            setPages(userPages);
        }
        
        if((userYear>1900)&&(userYear<=2010))
        {
            setYear(userYear);
        }
        
        
    }

    //mutators
    public void setAuthor(Person userAuthor)
    {
        //authors[0] = userAuthor;
        author = userAuthor;
    }
    
    public void setAuthors(Person userAuthors[]) 
    {
        for(int i = 0; i > userAuthors.length; i++)
        {
        // authors* = userAuthors*;
        }
    
    }
    
    public void setTitle(String userTitle) {
        title = userTitle;
    }
    
    public void setIsbn(String userIsbn) {
        isbn = userIsbn;
    }
    
    public void setPages(int userPages) {
        pages = userPages;
    }
    
    public void setYear(int year) {
        this.year = year;
    }

    //acessors
    
    /*
    public Person [] getAuthors() {
        return authors;
    }
    */
    
    public String getTitle() {
        return title;
    }
    
    public String getIsbn() {
        return isbn;
    }
    
    public int getPages() {
        return pages;
    }
    
    public int getYear() {
        return year;
    }
    
    //isbn validator
    private static boolean isbnValidator(String userIsbn)
    {
        Pattern p = Pattern.compile("^[0-9]{10}+$");
        Matcher matcher = p.matcher(userIsbn);
        return matcher.matches();
    
    }
    
    
    //toString method
    public String toString()
    {
        return getTitle() + " " + "( " + getYear() + " ).	" + author + ".	" + getPages() +" pages.	" + "ISBN " + getIsbn() + ".";
    }
}

```

I commented out all the blocks that call the author array  
any one wana give it a whirl?