应用 Collections.sort() 实现生成随机字符串

    public void testSort4(){

        List<StringBuilder> StringIntegerList = new ArrayList<StringBuilder>();
       //保存所需要的字符
        String a = "abcdefghijklmnopqrstuvwxyz0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        Random random = new Random();
        int k = random.nextInt(10);
        StringBuilder str = new StringBuilder();

        for(int j = 0;j < 10;j++){
            for(int i = 0;i < k;i++){
            int number = random.nextInt(a.length());
            str.append(a.charAt(number));
        }
        StringIntegerList.add(str);
        System.out.println("将要添加字符串：" + "'" + str +"'"); 
        //清空str里的字符串
         str.delete(0, k);
        }
    }

但是collections的sort的方法好像对StringBuilder不适用。