Yeah, what is wrong is that your Mark variable isn't initialized, and since there is no input going to your ExamMark yet it goes to your two bottom ifs and those aren't assigning Mark to anything. This is making the variable null. So you must initialize Mark in your Display class.
In other words do something like this at line 14 of Display class.
public static String Mark = "(Enter something here)";
ok thanks i managed to do it thanks for all your help.