if else if出错

if     else if失效，所以只用if if就好了

if(oldbooks !=null)
     {
      Iterator it2 = oldbooks.iterator();
      while (it2.hasNext())
      {
       Book bk2 = (Book)it2.next();
       System.out.println(bk2.getB_id()+" "+bk2.getB_name()+" "+bk2.getFirst_time());
      }
      System.out.println("- - - - - - ");
     }
     if(newbooks !=null)
     {
      Iterator it1 = newbooks.iterator();
      while (it1.hasNext())
      {
       Book bk1 = (Book)it1.next();
       System.out.println(bk1.getB_id()+" "+bk1.getB_name()+" "+bk1.getFirst_time());
      }
      System.out.println("- - - - - - ");
     }