Add a method length to the LispList interface of Exercise •• P16.9 that returns the length of the list. Implement the method in the EmptyList and NonEmptyList classes.

Data from exercise  P16.9 The LISP language, created in 1960, implements linked lists in a very elegant way.
You will explore a Java analog in this set of exercises. Conceptually, the tail of a list—
that is, the list with its head node removed—is also a list. The tail of that list is again a list, and so on, until you reach the empty list. Here is a Java interface for such a list:

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public interface LispList { } boolean empty(); Object head(); LispList tail();