Something

        for(int i = 0; i < entry.length; i++) {
            if(entry[i] != null) {
                int displayNumber = i++;
                System.out.println("\n" + displayNumber);
                System.out.print(". " + entry[i].toString());
            }
        }

Exception in thread "main" java.lang.NullPointerException
        at ApptBook.displayAll(ApptBook.java:58)
        at ApptBookDriver.main(ApptBookDriver.java:100)