class EmptyListException extends RuntimeException {}

public class List {
  private final Object head;
  private final List tail;

  private List(Object head, List tail) {
    this.head = head;
    this.tail = tail;
  }

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

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

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

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

  public List add(Object o) {
    return new List(o, this);
  }

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

  public static void main(String[] args) {
    List ls = List.nil().add("Banana");
    System.out.println(char1(ls));
    List li = List.nil().add(1);
    System.out.println(char1(li));
  }

}