//--------- file Bottles.dpr ----------
program Bottles;

{$APPTYPE CONSOLE}

uses
  SysUtils,
  Singers in 'Singers.pas';

var
  Singer: ISinger ;

begin
  Singer := TBottlesSinger.Create ;

  While Singer.CanSing do
    WriteLn(Singer.Sing) ;

  FreeAndNil(Singer) ;
end.  //end of file Bottles.dpr

//--------- file Singers.pas ----------
unit Singers;

interface

uses
  SysUtils, Classes;

type
  ISinger = interface(IUnknown)
    function Sing: String ;
    function CanSing: boolean ;
  end ;

  TAbstractSinger = class(TInterfacedObject,ISinger)
  private
    FLine: String ;
    FCanSing: boolean ;
    function GetLine: String ; virtual ;
    procedure SetLine(aLine: String) ; virtual ;
  public
    function CanSing: boolean ; virtual ;
    property Line: String read GetLine write SetLine ;
    function Sing: String ; virtual ; abstract ;
  end ;

  TBottlesSinger = class(TAbstractSinger