class IntListTest {

  static final IntList oneElt  = new Cons(1, Empty.ONLY);
  static final IntList twoElts = new Cons(5, oneElt);
  static final IntList threeElts = new Cons(-10, twoElts);

  /** Given the the answer ans, computes the sum() on this and
   *  compares it against ans.  If the values disagree, it throws an
   *  exception with an embedded error message; otherwise it
   *  silently returns.
   */

  void test(IntList input, int ans) { 
    int result = input.sum(); 
    if (result != ans) throw new 
      RuntimeException("Test FAILURE: computed answer is: " + result + 
                       " correct answer is: " + ans);
    }

  void sumTest() {
    test(Empty.ONLY, 0);
    test(oneElt, 1);
    test(twoElts, 6);
    test(threeElts, -4);
  }
}