public class Relation {

   protected ListOfObjects assoc;

   public Relation() {
      assoc = new ListOfObjects();
   }

   public void map(Object d, Object r) {
      DomainRange found = locate(d);
      if (found == null) {
         assoc.append(new DomainRange(d, r));  // placement is arbitrary
      }
      else found.setRange(r);                  // replace what was there
   }

   public Object lookup(Object d) {
      DomainRange dr = locate(d);
      if (dr != null) return dr.getRange();
      else return null;
   }

   public Object drop(Object d) {
      Object ans = null;
      if (locate(d) != null) { // NB:  count on marker positioned correctly
         DomainRange dropped = (DomainRange) assoc.delete();
         ans = dropped.getRange();
      }
      return ans;
   }

   private DomainRange locate(Object d) {
      assoc.reset();
      while (!assoc.atEnd()) {
         DomainRange dr = (DomainRange) assoc.getItem();
         if (dr.matchesDomain(d)) return dr;
         assoc.next();
      }
      return null;
   }

   public static void selfTest() {
      Object d1 = new String("d1");
      Object d2 = new String("d2");
      Object d3 = new String("d3");
      Object r1  = new String("r1");
      Object r2  = new String("r2");
      Relation r = new Relation();
      r.map(d1, r2);
      r.map(d2, r2);
      r.map(d3, r1);
      if (r.lookup(d1) == r2) System.out.println("d1,r2 correct");
      else System.out.println("incorrect");
      if (r.lookup(d2) == r2) System.out.println("d2,r2 correct");
      else System.out.println("incorrect");
      if (r.lookup(d3) == r1) System.out.println("d3,r1 correct");
      else System.out.println("incorrect");
      r.map(d3,r2);
      r.drop(d1);
      if (r.lookup(d3) == r2) System.out.println("d3,r2 correct");
      else System.out.println("incorrect");
      if (r.lookup(d1) == null) System.out.println("d3 dropped correct");
      else System.out.println("incorrect");
      
   }

   public static void main(String args[]) {
     selfTest();
   }
      
}