Thread: NullPointerException - don't know why

hello,
I am writing a program of a library.
I have a class of magazine that contains an arrayList of papers and looks like that:

public class Magazine extends LibraryCollection {
 
    private ArrayList<Paper> papersCollection; //a collection of all the papers
 
 
    //constructor to initialize all of the fields
    public Magazine(String name, int number, ArrayList<Paper> papers) {
        super(name, number); //calling the superclass constructor to initialize his fields
        papersCollection = new ArrayList<Paper>(papers);
    }

I also have this method in the class:

//method to get the array list    public ArrayList<Paper> getPapersCollection() {
        return papersCollection;
    }

I have a library class with this method:

//method to return all of the magazine papers
    public String getMagazinePapers(Magazine magazine)
    {
        ArrayList<Paper> papers = new ArrayList<Paper>();
        papers = magazine.getPapersCollection();
        if(magazines.contains(magazine))
        {
            String magazinePapers = new String();
            magazinePapers = papers.toString();
            return magazinePapers;
        }
        else
        {
            return null;
        }
    }

And this are the releveant lines from main:

private static Library LibrarySys;
.
.
.
if (command.equals("addMagazine")) {
                String magazineName = input.next();
                ArrayList<Paper> listOfPapers = new ArrayList<Paper>();
                // build papers list
                listOfPapers.add(new Paper("paper1", Topic.CS, new Author("author5Name", "author5LastName", Topic.CS), PaperValue.CONFERENCE,"Haifa university" ));
 
                listOfPapers.add(new Paper("paper2", Topic.MATH,new Author("author6Name", "author6LastName", Topic.MATH), PaperValue.MAGAZINE,"MIT university" ));
                listOfPapers.add(new Paper("paper3", Topic.CHEMISTRY,new Author("author7Name", "author7LastName", Topic.CHEMISTRY), PaperValue.WORK_SHOP,"Oxford university" ));
                listOfPapers.add(new Paper("paper4", Topic.SOCIOLOGY,new Author("author8Name", "author8LastName", Topic.SOCIOLOGY), PaperValue.CONFERENCE,"Tel Aviv university" ));
                listOfPapers.add(new Paper("paper5", Topic.CS,new Author("author5Name", "author5LastName", Topic.CS), PaperValue.MAGAZINE,"haifa university" ));
                listOfPapers.add(new Paper("paper6", Topic.PHYSICS,new Author("author9Name", "author9LastName",  Topic.PHYSICS), PaperValue.CONFERENCE,"Yael university" ));
 
                int id = 2;
                //create magazine
                Magazine magazine = new Magazine(magazineName, id, listOfPapers); 
 
 
                // add to library
                if (LibrarySys.addMagazine(magazine)) { // if adding successfully, then true returned,
                    // the following message is written to the output file
                    MyFileLogWriter.writeToFileInSeparateLine("Successfully adding magazine: " + magazineName );
                } else {
                    MyFileLogWriter.writeToFileInSeparateLine("Failed to add magazine review");
                }
 
 
 
            }
// ================ Building Command ================================================
            if (command.equals("getMagazinePapers")) {
                String magazineName = input.next();
                Magazine magazine = LibrarySys.getMagazine(magazineName);
 
 
                if (magazine == null)
                    MyFileLogWriter.writeToFileInSeparateLine("no magazine with this name");
                else
                    MyFileLogWriter.writeToFileInSeparateLine(LibrarySys.getMagazinePapers(magazine).toString());
            }

I have the NullPointerException in the lines: magazinePapers = papers.toString(); and: MyFileLogWriter.writeToFileInSeparateLine(LibraryS ys.getMagazinePapers(magazine).toString());
I would love to know why I am getting this exception.

The papers array list initalized in the main with this lines:

                listOfPapers.add(new Paper("paper1", Topic.CS, new Author("author5Name", "author5LastName", Topic.CS), PaperValue.CONFERENCE,"Haifa university" ));
                listOfPapers.add(new Paper("paper2", Topic.MATH,new Author("author6Name", "author6LastName", Topic.MATH), PaperValue.MAGAZINE,"MIT university" ));
                listOfPapers.add(new Paper("paper3", Topic.CHEMISTRY,new Author("author7Name", "author7LastName", Topic.CHEMISTRY), PaperValue.WORK_SHOP,"Oxford university" ));
                listOfPapers.add(new Paper("paper4", Topic.SOCIOLOGY,new Author("author8Name", "author8LastName", Topic.SOCIOLOGY), PaperValue.CONFERENCE,"Tel Aviv university" ));
                listOfPapers.add(new Paper("paper5", Topic.CS,new Author("author5Name", "author5LastName", Topic.CS), PaperValue.MAGAZINE,"haifa university" ));
                listOfPapers.add(new Paper("paper6", Topic.PHYSICS,new Author("author9Name", "author9LastName",  Topic.PHYSICS), PaperValue.CONFERENCE,"Yael university" ));

this is how the papers class looks:

public class Paper extends LibraryItem {
 
	private PaperValue value; //the value of the paper
	private String universityName; //the university name of the paper
 
	//constructor to initialize all of the fields
	public Paper(String name, Topic topic, Author auther, PaperValue valueOfPaper,
			String university) {
		super(name, topic, auther); //calling the superclass constructor to initialize his fields
		value = valueOfPaper;
		universityName = university;
	}

And this is the how the libraryIten class looks:

//this class represents a item in the library
public class LibraryItem {
 
	protected String itemName; //the name of the item
	protected Topic itemTopic; //the topic of the item from the Topic class
	protected Author authorOfItem; //the author of the item
	protected ArrayList<Review> itemReviews; //array list that contains all of the reviews of the item
}

All of the papers and the magazine are initalized in the main class

It prints papers=null but I don't understand why is is empty beacuse there are papers in the magazine

I added prints and I faund out that the problem is with the addMagazine method.
This is the method:

//method to add a magazine to a library
	public boolean addMagazine(Magazine magazine)
	{
		boolean exist = magazines.contains(magazine); //checks if the magazine already exist
		if(exist) //if the magazine exist
		{
			return false; //doesn't add the magazine
		}
		else //if the magazine doesn't exist
		{
			magazines.add(magazine); //add it to the library
			return true; //return true if was added successfully
		}
	}

I don't understand what is the problem with it.

I put a printing line in the main after calling the addMagazine method so I think the problem is in the addMagazine and that it doesn't add the magazine properly. Maybe this also why the getMagazinePapers doesn't working.

I added a printing line and it prints the magazine this way: magazine name: HaifaMagazine
The problem is there is an array list of magazines in library and I don't know why it doesn't add the magazine to the array properly.