abstract class List {

  // Any List, of any subclass, can tell us its length.
  // However, the code must wait, since Lists can be either
  // an instance of Cons or Null.
  //
  abstract public int length();


  /* List.main
   * A test suite.
   * This method should be inside the class List.
   */
  public static void main( String[] args ) {
    List l1, l2;
    l1 = new Null();
    l2 = new Cons( 3, new Cons( 5, new Cons( 4, new Null() )));

    System.out.println( "l1 is " + l1 + ", with length " + l1.length() + "." );
    System.out.println( "l2 is " + l2 + ", with length " + l2.length() + "." );
    }
  }
  


class Null extends List {
  public int length() { 
    // --- you'll complete this function momentarily! ---
    return -99;
    }
  }

class Cons extends List {
  public int car;
  public List cdr;

  public Cons( int datum, List rest ) {    // The constructor for a Cons object
    car = datum;                           //   (this is what "new" calls).
    cdr = rest;
    }

  public int length() { 
    // --- you'll complete this function momentarily! ---
    return -99;
    }

  }