Java中的Collections.shuffle

我这里有一个示例程序，该程序使用Collections并对数组进行混洗以避免重复显示问题.但是它不起作用，仍然有问题重复.这里是代码，请帮助我检测我的程序中有什么错误.

//collections.shuffle的代码

I have here a sample program that use Collections and shuffle the array to avoid the repetition of questions to display.But it doesn''t work,there still a duplication of questions.Here is the code,please help me to detect what is the error in my program.

//Code for collections.shuffle

String cntq = request.getParameter("quest");
int totq = Integer.parseInt(cntq);
ArrayList<imgaddquestBEAN> aList = new ArrayList<imgaddquestBEAN>();
projectDAO pDAO1 = new projectDAO();
	
Random diceRoller = new Random();//Random number generator
			int i=0;
			int[] roll = new int[totq];
			int s = 50;
			try{
				//--- Initialize the array to the ints 0-49
				Collections.shuffle(Arrays.asList(roll));
				for (i=0; i< roll.length; i++) {
		    		roll[i] = i;
					roll[i] = diceRoller.nextInt(s);
					aList = pDAO1.displayquest(true, s);
					request.setAttribute("one", aList);
				}
				//--- Shuffle by exchanging each element randomly
				for (i=0; i< roll.length; i++) {
		    		int randomPosition = diceRoller.nextInt(roll.length);
		    		int temp = roll[i];
		    		roll[i] = roll[randomPosition];
		    		roll[randomPosition] = temp;
					}
			}catch(SQLException ex){
				ex.printStackTrace();
			}
			request.setAttribute("one", aList);
			ServletContext sc = this.getServletContext();
			RequestDispatcher rd = sc.getRequestDispatcher("/test1.jsp");
			rd.forward(request, response);
		}
	}

}