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)
