摇奖程序和随机生成验证码

//摇奖程序
        Random r1=new Random();
        Set<Integer> s=new HashSet<Integer>();
        while(s.size()<10)
        {
            int i1=r1.nextInt(20);
            if(!s.contains(i1))
            {
                s.add(i1);
            }
        }
        System.out.println("从20里面随机抽取10个数：");
        for(int t:s)
        {
            System.out.print(" "+t);
        }
        System.out.println();
        //产生随机验证码
         String str1 = "0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM";
            Random yan=new Random();
            for(int i2=0; i2<4; i2++)
                 {
                 int j = yan.nextInt(str1.length()-1);
                 
                 String s1 = (str.substring(j, j+1));
                 
                 System.out.print(s1);
                 }