Java HotSpot Virtual Machine

plz help!~

public class Student implements Comparable
{
	private long studentId;
	private String studentName;
	private String surname;

public int compareTo(Object other)
	{
	int order = 0;
	{
	if (surname.equals(((Student)other).surname))	
	order = studentName.compareTo(((Student)other).studentName);
                      else
	order = surname.compareTo(((Student)other).surname);
	return order;
	}

this is my code, when i run the program, i got a NullPointerException in this line : order = surname.compareTo(((Student)other).surname);

what's wrong?? what is it that is null? The surname? or the student object??
plz help me~~ really frustrated to see my assignment becomes like this...i always start early but still end up in this situation...