A linked list must end with a null value. Without such a marker at the end of the list, a routine cannot “see” the end of the data structure. This assignment of a null value at the end of the list is often taken care of when a new node is packaged or through the use of a constructor.

When a reference variable is null, it is a programming error to invoke one of its methods or to try to access one of its instance variables. For example, a program may maintain a reference to the first node of a linked list, as follows:

ListNode first;

Initially, the list is empty and first is the null reference. At this point, it is a programming error to invoke one of the first’s methods. The error would occur as a NullPointerException.