please help with these java questions thanks

CSSE2002/7023 Tute Week 5 1 XFiles The following class maintains a collection of strings all begining with X. class XFiles I public LinkedList files; public void add (String s) f if (s.charAt (0)--'X') I files .add (s); public void remove (String s) { files .remove (s); public List getWords ) return files; . Fix the above code, so it won't throw any exceptions [without using try/catch] Find two ways to break the invariant for this class 2 Black box testing The following class tracks how many passengers are on a bus public class Bus {[ public Bus (int capacity); int getCurrent (); int getAverageCount (); //average passengers between stops void stop (int on, int off); // Passenger changeover at a bus stop how many on the bus right nou //passengers get off before any new get on 1. What would you test? ie what situations and inputs would you test. You don't need to specify expected output