//
// COMP102
// Example X:
//
// Paul E. Dunne 8/11/99
//
public class Records
  {
  //******************************************************
  // Embed a RecordCell Class as earlier.                *
  //******************************************************
  private class RecordCell
    {
    private Object Key;
    private Object Datum;
    private RecordCell Link;
    //
    private RecordCell(Object KeyVal, Object head, RecordCell next_cell)
      {
      Key=KeyVal; Datum = head; Link=next_cell; 
      }
  }
  //******************************************************
  // Records Fields                                      *
  //******************************************************
  private RecordCell Head;         
  private RecordCell Tail;         
  private int CellCount;         // The number of list cells in this instance.
  //******************************************************
  // Constructor                                         *
  //******************************************************
  public Records()
    {
    Head = null;
    Tail = null;      
    CellCount=0; // Initiates the empty record set;
    }
  //******************************************************
  // Instance Methods                                    *
  //******************************************************
  //
  //  Test if this instance is Empty
  //
  public boolean IsEmpty()
    {
    return (CellCount==0);       // `Safer' than testing for null reference.
    }
  //
  //************************************************************
  //  Add a new RecordCell as the first cell in this           *
  //************************************************************
  //
  public void AddHead ( Object Datum, Object KeyVal )
    {
    if (IsEmpty())               // If it's currently empty;
      {
      Head = new RecordCell(KeyVal,Datum,Head); 
      Tail = null;                       // The link is still `null'
      CellCount=1;                       // and this list now contains 1 cell.
      }
    else                                 // If it's not empty
      {
      Head = new RecordCell(KeyVal,Datum,Head);   // Add the new head Datum,
      Tail = Head.Link;                  // Reset the `Tail' of the list.
      CellCount++;                       // and this list now has 1 extra cell.
      };
    }
  //
  //************************************************************************
  //  Remove the RecordCell at the start of this instance                  *
  //************************************************************************
  //
  public void RemoveHead()
    {
    if (CellCount<=1)                    // There is an argument for raising
      {                                  // an exception if CellCount=0
                                         // however, we adopt the convention
                                         // that RemoveHead() applied to
                                         // empty leaves empty.
      Head = null;
      CellCount=0;
      }
    else
      {
      Head = Tail;                      // Tail is never the null reference here.
      CellCount--;                      // and this instance has one fewer record.
      if (CellCount==1)
        Tail=null;                      // If there's only a single cell then its Link field
                                        // must point to empty (i.e. null reference)
      else
        Tail = Head.Link;               // Otherwise we can update Tail without any problem.
      };
    }
  //
  //**********************************************************************
  //  Obtain the Object in the Datum field of the RecordCell at          *
  //  the start of this instance.                                        *
  //**********************************************************************
  //
  public Object GetHeadDatum()
    {
    if (!(CellCount==0))                // If there's anything to return
      return Head.Datum;                // then return it.
    else
      return null;                      // otherwise return a null reference.
    }
  //
  //**********************************************************************
  //  Obtain the Object in the Key field of the RecordCell at            *
  //  the start of this instance.                                        *
  //**********************************************************************
  //
  public Object GetHeadKey()
    {
    if (!(CellCount==0))                // If there's anything to return
      return Head.Key;                // then return it.
    else
      return null;                      // otherwise return a null reference.
    }
  //
  //**********************************************************************
  //  Obtain the (reference) to the Records for the Link field of the    *
  //  RecordCell at the start of this instance.
  //  This method will not change the current instantiation
  //**********************************************************************
  //
  public Records GetTail()
    {
    Records temporary= new Records();
    temporary.Head=Head; temporary.Tail=Tail;
    temporary.CellCount = CellCount;
    temporary.RemoveHead();
    return temporary;
    }     
  }