nullpointerexception error in java? What am I doing wrong here?

(100%) 4:34 PM Eile Edit View Navigate Source Refactor Bun Debug Profile Team Iools Window Help Search (CErt+ list; 18 public BestSellers(SortedArrayCollection Book> books) 20 SortedArrayCollection Book> list-books; BestSellers - Navigator x Members 23 24 25 26 27 public void addBooks() Bestsellers(SortedArrayCollectionBook bo o addBooks0 O displayAscendingDates0 O displayAuthor O displayByPublisher0 O displayByTitle0 O displayDescendingDates0 O displayFiction0 O displayMonthandYear(int month, int year) O displayNonFiction O displayYears(int year1, int year2) File neEilenew File( "BSUpdated.txt"); Output - books (run) x Press 3 for .. Press 4 for... Press 5 for .. Exception in thread "main" java.lang.NullPointerException at books.BestSellers.printAll (BestSellers.java:31) at books.application.main(application.java:88) Java returned: 1 /home/kevin/ .cacheet beans/8.2/executor-snippets/ run.XM1:53 : BUILD FAILED (total time: G seconds) 20:6 INS