/**
 *  Data Definition: IntList := new Empty() + new Cons(int, IntList)
 */

abstract class AObjList {

    /**
    * throws an IllegalArgumentException on Empty
    * returns first element of non-empty list
    */
    abstract Object getFirst();

    /**
    * given the elements of this appear in non-descendin order
    * returns this with e inserted in nonde
    */
    abstract AObjList getRest();

    abstract String toStringHelp();
    
    abstract int sum();
    
    abstract int sumHelp(int acc);
    
}

/**
 * Represents the unique empty list using the singleton pattern.
 * This is also an example of the Null Object pattern.
 * Note the use of exceptions to express programming errors in getFirst() and
 * getRest().
 */
class EmptyObjList extends AObjList {

    /**
    * Singleton pattern.
    */
    public static EmptyObjList only = new EmptyObjList();
    private EmptyObjList() {}

    Object getFirst() {
        throw new IllegalArgumentException("An Empty IntList has no first.");
    }

    AObjList getRest() {
        throw new IllegalArgumentException("An Empty IntList has no rest.");
    }

    public String toString() {
        return "()";
    }

    String toStringHelp() {
        return "";
    }
    
    int sum(){
      return 0;
    }
    
    int sumHelp(int acc){
      return acc;
    }
         
      
}

/**
 * Represents a non-empty list.
 * Note the use of the toStringHelp() "helper" method to "help" compute the
 * String representation of the rest.
 */
class ConsObj extends AObjList {
    Object first;
 private AObjList rest;

    ConsObj(Object f, AObjList r) {
        first = f;
        rest = r;
    }

    Object getFirst() {
        return first;
    }

    AObjList getRest() {
        return rest;
    }

    /**
     * no leading space before first
     */
    public String toString() {
        return "(" + first + rest.toStringHelp() + ")";
    }

    /**
     * leading space before each elt
     */
    String toStringHelp() {
        return " " + first + rest.toStringHelp();
    }
    
    int sum(){
      return rest.sumHelp(  ((Integer)first).intValue() );
    }
    
    int sumHelp(int acc){
      return rest.sumHelp(acc + ((Integer)first).intValue());
    }
    
}