太平洋来的风

IOpenDoor = interface
  function Open: boolean
end;

THome = class(IOpenDoor)
  function Open
end;

TCar = class(IOpenDoor)
  function Open;
end;

var
  AOpen: IOpenDoor;
begin
  if xxx then AOpen := THome.Create
  else AOpen := TCar.Create;
  AOpen.Open();
end;

这个很像：

var
  a:Tstrings
begin
  a:=Tstringlist.create
end;