请输入验证码优化版

 1            String str111 = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
 2            
 3            for(int x = 1; x <= 4; x++)
 4            {
 5                Random a = new Random();
 6               
 7                int m = a.nextInt(62);
 8               
 9                System.out.print(str111.charAt(m) + " ");
10            }
11            System.out.println();
12            System.out.println("请输入验证码：");