Exception in thread "main" java.lang.NullPointerException \ at com.example.myproject.Book.getTitle(Book.java:16) \ at com.example.myproject.Author.getBookTitles(Author.java:25) \ at com.example.myproject.Bootstrap.main(Bootstrap.java:14) Exception in thread "main" java.lang.NullPointerException \ at com.example.myproject.Book.getTitle(Book.java:16) \ at com.example.myproject.Author.getBookTitles(Author.java:25) \ at com.example.myproject.Bootstrap.main(Bootstrap.java:14) Hello world! Exception in thread "main" java.lang.NullPointerException \ at com.example.myproject.Book.getTitle(Book.java:16) \ at com.example.myproject.Author.getBookTitles(Author.java:25) \ at com.example.myproject.Bootstrap.main(Bootstrap.java:14)