class EmptyListException extends RuntimeException {}

public class List<T> {
  private final T head;
  private final List<T> tail;

  private List(T head, List<T> tail) {
    this.head = head;
    this.tail = tail;
  }

  public static <E> List<E> nil() {
    return new List<E>(null, null);
  }

  private boolean isNil() {
    return tail == null;
  }

  public T head() {
    if (isNil()) throw new EmptyListException();
    return head;
  }

  public List<T> tail() {
    if (isNil()) throw new EmptyListException();
    return tail;
  }

  public List<T> add(T o) {
    return new List<T>(o, this);
  }

  public static char char1(List<String> l) {
    return l.head().charAt(0);
  }

  public static void main(String[] args) {
    List<String> l = List.<String>nil().add("Banana");
    System.out.println(char1(l));
  }

}