public class Relation {

   protected DomainRangeList assoc;

   public Relation() {
      assoc = null;
   }

   public void map(Domain d, Range r) {
      DomainRangeList found = locate(d);
      if (found == null) {
         assoc = new DomainRangeList(new DomainRange(d, r), assoc);
      }
      else
         found.getDomainRange().setRange(r);
   }

   public Range lookup(Domain d) {
      DomainRangeList found = locate(d);
      if (found != null) return found.getDomainRange().getRange();
      else return null;
   }

   public void drop(Domain d) {
      DomainRangeList found = locate(d);
      if (found != null) {
         List first = assoc.find(1);
         first.exchange(found);
         assoc = assoc.getRest();
      }
   }

   protected DomainRangeList locate(Domain d) {
      DomainRangeList drl = assoc;
      while (drl != null) {
         DomainRange dr = drl.getDomainRange();
         if (d == dr.getDomain()) return(drl);
         drl = drl.getRest();
      }
      return null;
   }

   public static void selfTest() {
      Domain d1 = new Domain("d1");
      Domain d2 = new Domain("d2");
      Domain d3 = new Domain("d3");
      Range r1  = new Range("r1");
      Range r2  = new Range("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();
   }
      
}